42 replies to this topic

[phil882]

Hello everyone. I just wanted to go ahead and post a thread in the general weather forecast and discussion forum since this post is not geared towards one particular region over the other in the East coast, but rather the threat as a whole. I want to discuss synoptically what is causing the GFS to be super amplified, and why this goes in contrast with the ECMWF solution. Finally, I'm going to highlight the biggest features we need to watch.

So lets start with the GFS and its crazy amplified solutions. In order to understand how each model run is evolving, lets look at the main players involved that make the solution happen:

THE PLAYERS

1. The Multiple Jet Interactions:

First up is the subtropical jet and the associated jet streak that is currently occurring over the central and eastern part of the United States. The jet streak in particular is an important feature, because it provides the necessary wind flow to evacuate air in the upper levels away from the developing surface cyclone center which allows for strong vertical motion that produces convection and precipitation. You can have a potent shortwave that little to no convection and precpitation because the orientation of the jet streak doesn't favor suface cyclogenesis. On the flip side, you can have a weak shortwave that has an explosion of deep convection and precipitation just because the jet dynamics were favorable to allow a large area of rising motion which favored condensation of air parcels and eventual rain/snow/thunderstorms.

So with that said, lets see what we have in store? Instead of looking at the 18z GFS solution, lets first focus on where we have been, showing the last suite of forecast solutions ending at 12z today. This is a d(prog)/dt map which highlights how the model has evolved for a given forecast interval over the past 7 days.

d(prog)/dt of 250 hPa Isotachs / 1000-500 hPa Thickness / MSLP

Here is a quick .gif animation (click the loop above to get better resolution) of the last 72 hours of model changes ending at the analysis time


Figure 1. 250 hPa isotachs / 1000-500 hPa Thickness / MSLP of GFS for the 12z 2/15 Forecast.

Starting from about 132 hours up to the model analysis, you notice a pretty distinct trend in the isotachs (shaded region of winds at 250 hPa) that represent the jet streak. We started out at around 50-60 m/s max (~120 knots) for a jet streak maximum centered over the gulf coastline. Note that over time as we get closer to the analysis, the jet streak intensifies quite a bit while also shifting further north and developing anticyclonic curvature. These two items are very key concepts in understanding how mid-latitude baroclinc lows are able to develop and intensify. Many of you on here understand that you want to have a surface cyclone in the left exit region or right entrance region of a jet in order to have favorable divergence aloft to support cyclogenesis. However, the curvature of a jet streak also plays a major role in modifying these regions. The right entrance region of a anticyclonic jet streak becomes the quadrant that experiences maximum divergence. This is where we will end up seeing our surface cyclone develop on the GFS. However, I just wanted to point out that the trend over the past 10+ model cycles has been to intensify the jet streak to 80-90 m/s max (~180 knots!) while making the flow more anticyclonic.

But WAIT, there is far more than one jet interaction going on. In fact, this system has the benefit of having three jet streak interactions, all in a seemly favorable interaction.


Figure 2. Same as figure one, except for 12 hour forecast at 6z on Feb 16th.

Thanks in large part to the current active MJO, we actually see the East Pacific is rather active convectively. This is pretty unusual for a La Nina regime this time of the year, but its actually going to play in this storms favor. Thanks to the active convection ongoing in the EPac we are experiencing a diabatic response with enhanced westerlies north of the convection. Part of this is also related to a dangling trough which has also helped to generate 100 knot westerlies aloft which is supporting the southernmost jet streak. This tropical connection is joined by two polar jet steam jet streaks bookending each side of the shortwave disturbance. Fast forward to 72 hours and we see how the jet streaks have combined together to aid in the rapid cyclogenesis that occurs over the southeast. This surface bombing rivals anything we have seen over the past year since the Boxing Day blizzard.


Figure 3. Same as figure 1 except for 72 hour forecast at 18z Feb 18th.

Of course the big question is how does this compare to the ECMWF? Generally they do look similar, but the GFS is notably stronger with almost all of the pieces of energy at 200 hPa by a factor of 5-10 m/s.

2-3. The Shortwaves and Anticyclonic Wave Breaking (AWB):

Another key factor we see with pretty much every major east coast snowstorm is the phasing between two shortwaves in a larger longwave trough. The interaction between two shortwaves that ride along two different jet streams is especially exciting. On rare occasions, we see a tripple jet interaction (March 1993 is one of the more extreme examples) that produces a historic storm. Now I could show you 500 hPa vorticity maps and heights, but that has already been done plenty of times before. This time, lets look at the dynamic tropopause (a measure of where in pressure the tropopause in the atmosphere is located) with winds plotted at that level. The animation below will go from 12-108 hours at 24 hour intervals.


Figure 4. Dynamic Tropopause and Wind. Relative vorticity contoured in fine black lines.

In the animation above, there are three key features which I attempted to show yesterday in some of the regional threads. The first shortwave is located in the southwestern United States and this is the main feature. Even without phasing this upper level feature is probably strong enough to generate a surface cyclone that will track across the Gulf Coast States. However, its the two more northern shortwaves that will be the real keys to properly forecast. The GFS shows that by 36 hours, the second shortwave (which is just entering the US near the MT/ND border) will be pinched off by an anticyclonic wave breaking event (a fancy term that signifies when a ridge aloft breaks off from the mean flow). This ridge will force both shortwaves further south between the 36-84 hour period. This is also where the greatest differences lie between the ECMWF and the GFS. The ECMWF is not nearly as strong with this ridge breaking off and thus the northern two shortwaves don't dig as much. The result is a southern stream storm with some snow on the northern end, but no significant storm that rides up the coast or inland. On the GFS, however, we see the first two shortwaves phase starting at 60 hours, and then by 84 hours the final shortwave phases. I want to note here that the AWB episode will allow for the shortwaves to drop southward as the jet level winds on the downstream side of the high will force southerly motion. This is one way we can get shortwaves to drop southward without a powerful west coast ridge, which is noticeably absent.

A three phasing solution should be sufficient, combined with favorable jet dynamics, to produce a meteorological "bomb" although this would likely mean a lot of the I-95 coordoor will have rain for this system. However, if we get shortwaves 1 and 2 to phase but not 3, we could still see a snowstorm for the I-95 crowd. If there is no phasing or weak phasing (which is closest to what the ECMWF shows) then we see mainly rain with a bit of snow on the northern end for folks in WNC and parts of VA.

THE OPTIONS

The Three Solutions:

This leads nicely into three possible solutions which I'll outline here. The jet configuration is such that we should see plenty of heavy precipitation break out. The biggest question though is where and how much, and will it be cold enough. Based on the maps and explanations above, here are three diagrams that outline what I think are three possible solutions.


Figure 5. Three potential surface low solutions based on the current synoptic pattern.

Solution one will most likely occur if we see a limited amount of phasing between shortwave 1 and 2 outlined in the animation above, and no interaction with shortwave three. While jet dynamics will still support widespread precipitation, the low track will remain suppressed and most of the big I-95 cities in the Northeast will remain dry.

Solution two will most likely occur if we see full phasing between shortwave 1 and 2, with limited to no phasing with shortwave 3. In this senerio we will see the low track move significantly inland with a large swath of heavy snow from DC to the extreme SNE. Note there will be lighter snows from the third shortwave across parts of new england and Upstate New York, but most of the heavy snow associated with the deepening surface cyclone will be missed.

Solution three will most likely occur if we see full phasing between shortwaves 1, 2, and 3. This will result in explosive cyclogenesis with surface pressure likely getting lower than 980 hPa at the peak of the systems lifespan. Unfortunately for snow-lovers, this will pull the 850 hPa 0 degree C isotherm further north and west and most of the I-95 cities (DC/PHL/NYC/BOS) may be spared the brunt of the snow. Right now its exceptionally difficult to tell which solution is more likely than the other. Considering the relative rarity of seeing a tripple phasing solution, intuition would say the extreme inland solution is the least likely of the three. However, the GFS's solution is certainly plausible given the 500 hPa and upper level setup for this system. If there was a system that had potential to bomb this winter, this is it.

THE UNCERTAINTY

Too many moving parts:


Figure 6. GEFS model spread of 250 hPa winds at initialization

Part of the reason why this forecast is so uncertain has a lot to do with the tiny pieces that have to come into play for a major phasing event to occur. Focusing first on aloft, we actually see the largest uncertainty in the GFS ensembles is actually the westerly flow coming from the tropics. A 10-14 m/s error could have some major ramifications down the road, especially in regards to the jet dynamics.

At 500 hPa, we see the largest uncertainty lies with the heights over the great lakes, not surprising considering this is where we are expecting to see several of the s/w to merge.


Figure 7. GEFS Model Spread of 500 hPa heights at 48 hours

The hardest part to forecast is the fact that These features are being stretched across the large ridge (the same one that will be breaking off from the mean flow) which makes it more difficult for models to handle the shortwave amplitude. Its harder to resolve waves when they are small and only have tiny changes horizontally.

So this leads us to now... the models are completely divergent because the small changes in the shortwave features are going to have huge repercussions downstream beyond the ridge. The anticyclonic ridge breaking allows for the shortwave features to dig down like the GFS suggests, and you don't necessarily have to have a ridge over the West Coast for this to occur. Still, the unusual nature of this pattern suggets that such deep amplification is rare. What happens from here on out is anyones guess, but I just wanted out outline the players and the options, and why its just so difficult to get a forecast solution 4-5 days out right now. It will be fun to watch, and I hope you will enjoy the roller coaster ride to come. After the boring weather across the East US the last month, its about time we have something exciting to talk about!

[burgertime]

Awesome write up Phil...wish we could get something for my neck of the woods but at this point it looks out of the cards. In this winter you can't hate on anyone that cashes in.

[donsutherland1]

Very thorough discussion. Great reading.

[NorEaster17]

Here's a video of the NHC Recon Mission explained for the upcoming storm ... and then Jim Cantore shows different Track senarios.

[Ottawa Blizzard]

I am so dissapointed with the latest run of the GFS as I really wanted a storm that gave moderate to heavy snow to the entire northeast/eastern lakes. Those types of storms are rare though. Usually it's either the lakes or the I-95 cities that get the heavy snow - rarely both with the same storm.

[Harry]

NorEaster17, on 15 February 2012 - 11:00 PM, said:

Here's a video of the NHC Recon Mission explained for the upcoming storm ... and then Jim Cantore shows different Track senarios.

They need to send something up into Canada as well. Models have struggled big time with stuff dropping almost due south out of Canada. In most cases it has been stronger and further southwest then what models had just a few days prior and tonights stuff out this way is another example of it. Last weeks sorta surprise snow event over this region was another.

[Harry]

Ottawa Blizzard, on 15 February 2012 - 11:52 PM, said:

I am so dissapointed with the latest run of the GFS as I really wanted a storm that gave moderate to heavy snow to the entire northeast/eastern lakes. Those types of storms are rare though. Usually it's either the lakes or the I-95 cities that get the heavy snow - rarely both with the same storm.

That Feb 10th 2010 snow storm was the only time i can ever recall seeing a snowstorm drop a foot in this area/region and then nailing DC/Pilly and or that part of i95. Thus yeah they are rare.

I personally would not get worked up yet on a track. Again my issue is that stuff dropping out of Canada which will probably play havoc with the models as we just saw with this latest system and the one last week. That is my opinion though so take it fwiw. lol

[weatherwiz]

Incredible write-up! Loaded with detail as well as images...perfect learning material.

[Harry]

weatherwiz, on 16 February 2012 - 12:28 AM, said:

Incredible write-up! Loaded with detail as well as images...perfect learning material.

Yep.. Huge kudos to Phil! Awesome discussion.

[phil882]

Thanks guys! It looks like the GFS at 00z is opting for option #2. We don't see all three pieces of energy phase, at least not while the energy remained onshore. There is still plenty of moisture to work with, however, especially given the jet connection with the tropics.

[TNE]

phil882, on 16 February 2012 - 12:51 AM, said:

Thanks guys! It looks like the GFS at 00z is opting for option #2. We don't see all three pieces of energy phase, at least not while the energy remained onshore. There is still plenty of moisture to work with, however, especially given the jet connection with the tropics.

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The NAM and the GFS all phase your number 3 with the Newfoundland Low. If the disturbance does manage to phase with your number 3 (it means there is more ridging coming toward the Great Lakes), hence there will be a stronger front and warmer temperatures pushing from the Atlantic (toward the Great Lakes).

The flow is coming from the southwest based on the 500mb. The highest PVA is over Tennessee at Hr 72, temperatures at H85 are above freezing. If the disturbance hits us head on, we will see most of the precipitation occurring with above freezing temperatures.

The upper trough gains a negative tilt in the Western Atlantic after the disturbance moves through at hr 99. This means that if any upstream energy does make it down from the poles, it will most likely phase with the disturbance in the Atlantic. The disturbance in my opinion will most likely have the best impact in the Atlantic and most of the activity prior to that will occur in the warm sector (given the latest American Models).

TNE

[baroclinic_instability]

Hey nice write-up Phil. I love seeing AWB on the DT in massive oceanic bombs. A nice way to see WAA/LHR processes having a massive effect on the upper troposphere and tropo heights.

Anyways, it seems to me the biggest factor in inhibiting a major bomb is the significant northern stream influence (which you hit on with all the multiple northern stream waves) which has a tendency to inhibit significant positive feedback processes including enhanced WAA processes, the shortwave ridge building process, etc. The warm frontal boundary has the look of many positive tilt configurations with an elongated front and the potential for significant LHR/convective processes along the front which would have a tendency to inhibit low level moisture transport near the center of the cyclone...a negative feedback process in terms of rapid cyclogenesis while also developing diabatic enhanced low level PV anomalies along that front.

I know this is not a valid reasoning...but seasonal trend alone w.r.t. the northern stream influence (models have been over-aggressive basically all winter with southern stream anomalies) would suggest the least aggressive northern stream model would have the most credence here.

[baroclinic_instability]

Harry, on 16 February 2012 - 12:06 AM, said:

They need to send something up into Canada as well. Models have struggled big time with stuff dropping almost due south out of Canada. In most cases it has been stronger and further southwest then what models had just a few days prior and tonights stuff out this way is another example of it. Last weeks sorta surprise snow event over this region was another.

I would tend to agree with this. Boxing Day was another example where northern stream short wave length features (in that case, phasing as well as equatorward upper level jet features) played a major role (including the infamous "initialization" errors). In this case, the mess of northern stream Pacific occlusions impinging on wave amplification after phase (including the potential for complete squashing of the upper low) does not incite much confidence towards an amped up solution.

[baroclinic_instability]

phil882, on 15 February 2012 - 10:06 PM, said:

Figure 3. Same as figure 1 except for 72 hour forecast at 18z Feb 18th.

Of course the big question is how does this compare to the ECMWF? Generally they do look similar, but the GFS is notably stronger with almost all of the pieces of energy at 200 hPa by a factor of 5-10 m/s.

A question regarding this image. This has the classic look, at this point, of taking on a highly favorable curved coupled upper level jet streak seen in basically all cyclogenetic bombs. One thing I am interested in (with a case out here I am beginning to write about), is what is really more important...that particular feature itself, or the way that feature develops. It seems that a hallmark of the greatest synoptic bombs include the coupling of low level WCB convective processes immediately ascending into the divergent region of the upper coupled jet. But it seems to me, in terms of forecasting, that the big question is always, does that feature even develop? One challenge in some of the more marginal cases usually seems to be both the onset of rapid cyclogenetic development (when does the feedback process begin..Boxing Day was a classic example of needing perfect timing w.r.t. onset of rapid cyclogenesis) as well as low level advective processes. The development of a favorable Polar anticyclonic curved jet streak is many times due in part to enhanced WAA poleward processes (ideally with a westward component as well in the NH) which influences (positively) the horizontal thermal gradient (enhancing the upper jet). With the more marginal cases (although cases like Boxing Day were extreme with extremely divergent solutions) it always seems minor differences in the low level advective processes (and potentially LHR processes owing to DMC) have the biggest contributing factor in determining whether a system is a bomb or flat, weak bust. In those cases tilt configuration, timing and onset of feedback processes (positive or negative), etc. are highly important.

[Srain]

Just so everyone understands, there are wide reaching aspects to this storm system. We down here in Texas are facing our first Regional Flash Flood Watch potential since September 2010. This will likely be a major news weather headline across a vast region of the US...

Morning e-mail from Jeff:

Heavy to excessive rainfall becoming likely over the region Friday-Saturday

Flash Flood Watches may be required

Very weak cool front trying to limp through the area this morning with dense fog south and east of the boundary. Strong sub-tropical jet stream aloft riding nearly parallel to this boundary is already helping to set off an area of widespread thunderstorms over S TX and the adjacent western Gulf waters. Moisture will be on a rapid increase today as ESE to SE winds above the surface begin to bring Gulf moisture back inland over the frontal slope. TX Tech meso model breaks out showers/thunderstorms across the counties counties by this evening in a region of enhanced lift from an approaching upper air disturbance and increasing isentropic lift over the coastal waters frontal boundary.

Will likely see a break in the action early Friday before the onset of the main event Friday afternoon-Saturday morning. Strong upper level trough over S CA this morning will dig SE into MX and then eject across TX early Saturday. This system will force a western Gulf of Mexico surface low with strong isentropic upglide over much of the southern half of TX by midday Friday. Expect widespread rains to quickly develop and spread NE during the day on Friday. NW Gulf surface boundary begins to back northward in response to NW Gulf surface low formation and this will add a surface boundary into the mix to help focus training of heavy rainfall. Current thinking is that this boundary will not move very far inland if at all and the threat for the really big totals may be out over the Gulf water however should this boundary move further north than expected, rainfall totals will need to be raised and the flash flood threat will be even higher. Even with the surface boundary remaining near the coast/offshore, there will be very strong lift coming to bear across the region along with the 850mb boundary in the region and both of these will favor some form of heavy rainfall inland. Could also see some strong elevated thunderstorms Friday night with the main threat being hail. System should clear the area by Saturday evening with another 24-36 hour break before the next system arrives Monday-Tuesday.

Rainfall Amounts/Hydro:
Upstream look at PW values shows a deep pool of 1.6-1.7 inch values over deep S TX into N MX and this moisture will rapidly spread northward over the next 24 hours. These moisture levels will be at or above the 200% levels for this time of year and when combined with some kind of stalled or slow moving boundary (surface front or 850mb front) really raises the warning flags when it comes to excessive rainfall and flooding. Additionally rainfall in the past 3 weeks has been well above average and frequent with little drying of the soil moisture between events. The top layers of soil are now saturated and will result in quicker run-off. Our northern counties are wetter than the coastal and SW counties as this has been where the greatest rainfall has been over the past month and this event looks to focus in the coastal areas.

Will go with widespread 1-3 inches of rainfall south of I-10 with isolated amounts of 4-5 inches possible especially under the strongest storms and where training develops. Locations most at risk for some of the heavier totals appear to be right along the coast northward to US 59 near/just north of the surface front and south of the 850mb front. North of I-10 totals will average .5-2.0 inches.

With grounds saturated or nearing saturation rainfall of this magnitude will result in large amounts of run-off. Significant rises on area creeks, bayous, and rivers will be likely if the rainfall forecast verify as forecasted.

HPC (Day 1-3 rainfall (QPF) Totals)

Attached Files

•  02162012 QPF d13_fill.gif   32.38K   2 downloads

[Ottawa Blizzard]

Harry, on 16 February 2012 - 12:14 AM, said:

That Feb 10th 2010 snow storm was the only time i can ever recall seeing a snowstorm drop a foot in this area/region and then nailing DC/Pilly and or that part of i95. Thus yeah they are rare.

I personally would not get worked up yet on a track. Again my issue is that stuff dropping out of Canada which will probably play havoc with the models as we just saw with this latest system and the one last week. That is my opinion though so take it fwiw. lol

Good post. It's interesting to see that southern and eastern Ontario might get some light snow on Saturday night ahead of the bigger system, as per the 6z GFS

[phil882]

baroclinic_instability, on 16 February 2012 - 07:00 AM, said:

A question regarding this image. This has the classic look, at this point, of taking on a highly favorable curved coupled upper level jet streak seen in basically all cyclogenetic bombs. One thing I am interested in (with a case out here I am beginning to write about), is what is really more important...that particular feature itself, or the way that feature develops. It seems that a hallmark of the greatest synoptic bombs include the coupling of low level WCB convective processes immediately ascending into the divergent region of the upper coupled jet. But it seems to me, in terms of forecasting, that the big question is always, does that feature even develop? One challenge in some of the more marginal cases usually seems to be both the onset of rapid cyclogenetic development (when does the feedback process begin..Boxing Day was a classic example of needing perfect timing w.r.t. onset of rapid cyclogenesis) as well as low level advective processes. The development of a favorable Polar anticyclonic curved jet streak is many times due in part to enhanced WAA poleward processes (ideally with a westward component as well in the NH) which influences (positively) the horizontal thermal gradient (enhancing the upper jet). With the more marginal cases (although cases like Boxing Day were extreme with extremely divergent solutions) it always seems minor differences in the low level advective processes (and potentially LHR processes owing to DMC) have the biggest contributing factor in determining whether a system is a bomb or flat, weak bust. In those cases tilt configuration, timing and onset of feedback processes (positive or negative), etc. are highly important.

Good questions and thoughts... the interplay between convection and jet dynamics are certainly inter-related and it can be very difficult to figure out which comes first. Do we get the deep convection which then forces strong outflow in the form of a powerful jet streak, or do we first get the jet streak which forces the deep convection. Honestly, I think it comes down to the particulars of each synoptic situation... in much of the same way that every severe weather outbreak is unique in terms of shear vs. instability. Some cases, the jet dynamics are just perfect that you are not as reliant on LHR to aid in creating the dual jet structure. In other cases, its more difficult to pin down, because just a little bit of additional heating could spur a positive feedback cycle that makes all the difference turning a marginal case into something exceptional.

Where this case falls is still debatable, but I think the jet structure is there to have this system to intensify still, just perhaps not as rapidly like the GFS was suggesting yesterday. I think we need the interaction with that northernmost shortwave to really see this system "bomb" out. What is looking more likely in the most recent guidance is a gradually intensifying surface low that remains a surface wave and does not occlude until after passing the East Coast by.

http://www.atmos.alb...amer_dprog.html

The missing piece that has changed is that we don't have the additional jet interaction from the northern stream shortwave... Here is a d(prog)/dt map for 2/19 at 18z. I want you to focus on the extra little jet streak appendage that occurs on the 114 hour, 102 hour and 96 hour panels (about the time the storm was "bombing out" on the 00z, 12z and 18z runs yesterday... note the 06z run is not in this set of products). You can see the extra jet located along the Maine/New Brunswick border that was associated with that extra shortwave phasing with the other two shortwave that had merged previously. This provided a jet streak directly to the north of the surface low to continue the cyclogenesis process. The deepening trough due to this interaction also increased the cyclonic curvature of the upstream jet streak as well which also gave the surface cyclone a boost in upper level divergence aloft. Here is a direct comparison of the 18z and 00z runs, and you can see the changes in both the anticyclonic jet streak (only one distinct streak that is displaced southward) and cyclonic jet streak (much more linear and not as displaced south compared to surface low) at 00z.



The other runs that show a more suppressed solution don't have this jet streak appendage, and I think its this combination of increasing baroclinicity and shortwave energy phasing that allow for a significantly stronger solution.

[phil882]

Srain, on 16 February 2012 - 08:11 AM, said:

Just so everyone understands, there are wide reaching aspects to this storm system. We down here in Texas are facing our first Regional Flash Flood Watch potential since September 2010. This will likely be a major news weather headline across a vast region of the US...

Bingo... this system has a lot of juice to work with, unlike pretty much every system we have seen for the last month across the Eastern United States. This will happen irregardless to any additional phasing that might occur further east and thus the flooding thread is very high for many areas across the deep south, as well as severe weather potential! There is a direct tropical connection which I highlight at the top of the post which does seem to have some MJO connections with a relatively active East Pacific. This is pretty unusual since most of the Pacific east of the dateline has been very inactive during this moderate La Nina.

[baroclinic_instability]

phil882, on 16 February 2012 - 10:51 AM, said:

Good questions and thoughts... the interplay between convection and jet dynamics are certainly inter-related and it can be very difficult to figure out which comes first. Do we get the deep convection which then forces strong outflow in the form of a powerful jet streak, or do we first get the jet streak which forces the deep convection. Honestly, I think it comes down to the particulars of each synoptic situation... in much of the same way that every severe weather outbreak is unique in terms of shear vs. instability. Some cases, the jet dynamics are just perfect that you are not as reliant on LHR to aid in creating the dual jet structure. In other cases, its more difficult to pin down, because just a little bit of additional heating could spur a positive feedback cycle that makes all the difference turning a marginal case into something exceptional.

Where this case falls is still debatable, but I think the jet structure is there to have this system to intensify still, just perhaps not as rapidly like the GFS was suggesting yesterday. I think we need the interaction with that northernmost shortwave to really see this system "bomb" out. What is looking more likely in the most recent guidance is a gradually intensifying surface low that remains a surface wave and does not occlude until after passing the East Coast by.

http://www.atmos.alb...amer_dprog.html

The missing piece that has changed is that we don't have the additional jet interaction from the northern stream shortwave... Here is a d(prog)/dt map for 2/19 at 18z. I want you to focus on the extra little jet streak appendage that occurs on the 114 hour, 102 hour and 96 hour panels (about the time the storm was "bombing out" on the 00z, 12z and 18z runs yesterday... note the 06z run is not in this set of products). You can see the extra jet located along the Maine/New Brunswick border that was associated with that extra shortwave phasing with the other two shortwave that had merged previously. This provided a jet streak directly to the north of the surface low to continue the cyclogenesis process. The deepening trough due to this interaction also increased the cyclonic curvature of the upstream jet streak as well which also gave the surface cyclone a boost in upper level divergence aloft. Here is a direct comparison of the 18z and 00z runs, and you can see the changes in both the anticyclonic jet streak (only one distinct streak that is displaced southward) and cyclonic jet streak (much more linear and not as displaced south compared to surface low) at 00z.



The other runs that show a more suppressed solution don't have this jet streak appendage, and I think its this combination of increasing baroclinicity and shortwave energy phasing that allow for a significantly stronger solution.

I am in full agreement here, and it seems strong positive feedback is simply lacking here...and much of it is related to the forecast setup and lack of any decisive northern stream features for a full phase. In this case, as you mentioned, the GFS bomb runs were highly dependent upon on latching onto one northern stream feature in this excessively "messy" flow pattern. From a pure standpoint of bomb cyclogenesis, this storm is just another series in positive tilt junk featuring an elongated warm front, quite a bit of DMC along that front, and junky looking eastward directed moisture transport vectors and LHR directed away from the cyclone. In this case one win for the MA is a loss for others (and vice versa if this was to be a bomb).

[baroclinic_instability]

One thing you highlight here that I would lvoe to see more usage in weather forecasting is dprog/dt w.r.t to significant features that can have massive impacts on forecast development w.r.t. intense cyclones. Small details make huge forecast differences, and I would like to see more consideration for these details without simply relying on the final model forecast.

[phil882]

baroclinic_instability, on 16 February 2012 - 11:19 AM, said:

One thing you highlight here that I would lvoe to see more usage in weather forecasting is dprog/dt w.r.t to significant features that can have massive impacts on forecast development w.r.t. intense cyclones. Small details make huge forecast differences, and I would like to see more consideration for these details without simply relying on the final model forecast.

d(prog)/dt maps are an exceptionally useful tool. I'm trying to work on a special kind of loop that will make it possible to be able to cycle forward in time while also cycling through different model cycles using the up and down arrow keys. Its still a work in progress and it will take some time to get it operational, but it should prove really useful for cases like this.

http://www.atmos.alb...amer_dprog.html

Looking at d(prog)/dt for 500 hpa though up to the last analysis time this program ran for the 06z GFS, notice how there have been significant structural changes to the southern stream s/w associated with the 500 hPa low over SW US. Even through the 48-24 hour period, there was a major shift in the strength of the s/w as an extra piece of shortwave energy which pretty much develops out of thin air helped to make the shortwave broader and is part of the reason why most model solutions are now showing the first two pieces of energy phasing in the 48-60 hour period.

We could easily see another shortwave that previously didn't show up well in the modeling pop up too, and there is also our third shortwave that we are following is still a nearly linear vorticity feature across the Beaufort Sea. This is in a very sparse area for radiosondes and plane observations. Geostationary satellites also have very poor viewing angles for identifying features, so oftentimes we are reliant on polar orbiting satellites to catch these features at such high latitudes. I think once this feature drops southward in the next 24 hours the model variance should decrease.

http://www.atmos.alb...namer_loop.html

[CoastalWx]

In winter, the entrance region of the jet is where its at. I know people think the left exit region is the most "fun" region and this may be true for severe wx, but in the winter..this jet region is where the party is at. This controls the behavior of the LLJ and as Phil and Baroclinic said, latent heat creates a feedback process that intensifies the jet and enhances curvature. Just look at PDII.

[Nic]

NorEaster17, on 15 February 2012 - 11:00 PM, said:

Here's a video of the NHC Recon Mission explained for the upcoming storm ... and then Jim Cantore shows different Track senarios.

That is the most interesting segment that has been of the weather channel for years.

[Ed Lizard]

CoastalWx, on 16 February 2012 - 12:41 PM, said:

In winter, the entrance region of the jet is where its at. I know people think the left exit region is the most "fun" region and this may be true for severe wx, but in the winter..this jet region is where the party is at. This controls the behavior of the LLJ and as Phil and Baroclinic said, latent heat creates a feedback process that intensifies the jet and enhances curvature. Just look at PDII.

If anyone hasn't read the KU snowstorm book, there is a primer in the front, understandable even to untrained amateurs like me, and gets into jet streaks, favored regions, self-development/reduced half lengths. It is serious good stuff. With pictures.

[CoastalWx]

Ed Lizard, on 16 February 2012 - 02:20 PM, said:

If anyone hasn't read the KU snowstorm book, there is a primer in the front, understandable even to untrained amateurs like me, and gets into jet streaks, favored regions, self-development/reduced half lengths. It is serious good stuff. With pictures.

You don't need to derive equations to understand this stuff, as long as you realize the physics as to why it happens. Good idea for others to read that, or even look online.

[baroclinic_instability]

It is beginning to become clear (at least I think) that the southern anomaly (as suggested by the guidance) is weaker and shallower than earlier progged, and will also be weakened substantially by latent heat release/strong diabatic processes as it passes near the GOM. In many ways it is a double whammy with both northern stream influence affecting upper height field processes/low level advective processes as well as a weaker southern anomaly which will prevent more vigorous warm sector advection as this system traverses the SE.

Tropo heights on the 18Z GFS look unimpressive, so I would have to think SNE/NYC are nearly completely out of the picture here. VA and farther S locations will have to watch and see what type of height falls there are as there will be a "late to the party" CCB given the positive tilt nature of this system (i.e., this storm will be dependent upon low level height falls for profile cooling).

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•  GFSTS_NA2012021618F072.gif   85.73K   3 downloads

[phil882]

Time for a quick update, with a lot more later tonight. It looks like a combination of tracks 1&2 appears to be most likely to pan out. The lack of the northern most shortwave phasing has actually acted as a double edged sword for folks north of DC, with suppression in the form of added confluence, combined with the lack of phasing. There will be a very sharp cutoff somewhere in VA/MD on where the heavy snow starts and little to no precipitation occurs. At this point, I don't see any major changes taking place, since most of the shortwave players are now within a dense radiosonde and aircraft network. Once again the ECMWF seems to be preforming admirably in comparison to the GFS.

More details later tonight...

[bluewave]

Nice 228 hr forecast.

[Inudaw]

Euro nailed this pretty far out pretty much. Hands down. Big kudos for it. Might be a bit premature but that is a great look from 228 hours out.

[phil882]

Inudaw, on 18 February 2012 - 12:43 PM, said:

Euro nailed this pretty far out pretty much. Hands down. Big kudos for it. Might be a bit premature but that is a great look from 228 hours out.

Indeed... it was also far more consistant than the majority of the rest of the guidance which was the key to me. Its easy to look at any one particular solution and say the ECMWF was king, but overall... it never came as far north as the other deterministic guidance and remained far more consistant. I think this would show up really well if somebody looked at a d(prog)/dt plot of the ECMWF.

[messenger]

In retrospect based on what we know at this stage, and based on what we have seen at the Euro at 228+ hours would we say that the G4 data helped? The Euro obviously did not have this data at D7 and still did pretty well from there on in. But the GFS actually seemed to be more erratic after data ingestion.

Curious as to the opinions. Personally I'm not sure this data helped the NCEP models much and I'd hate to think if it did help what the solutions would have been like otherwise.

Great writeup Phil.

[Srain]

messenger, on 18 February 2012 - 07:13 PM, said:

In retrospect based on what we know at this stage, and based on what we have seen at the Euro at 228+ hours would we say that the G4 data helped? The Euro obviously did not have this data at D7 and still did pretty well from there on in. But the GFS actually seemed to be more erratic after data ingestion.

Curious as to the opinions. Personally I'm not sure this data helped the NCEP models much and I'd hate to think if it did help what the solutions would have been like otherwise.

Great writeup Phil.

While some may think that the G-IV data was a waste of NOAA funds and offered no help to the almighty Euro, I suggest that the data did provide meaningful help for those of us beyond the I-95 corridor...

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•  02162012 12Z Euro f48.gif   93.46K   0 downloads
•  02092012 00Z Euro Geopotential3250032hPa32and32Temperature32at3285032hPa_North32America_216.gif   101.66K   0 downloads

[messenger]

Srain...but the Euro had kind of nailed it at 228 and really everywhere under that. Granted it did pin it better after the data.

What I'm more interested in figuring out is whether it helped the NCEP models. Given their variability I'd argue no, partly because one of the problematic s/w's for the NCEP models was coming from south central canada.

[dtk]

messenger, on 18 February 2012 - 07:55 PM, said:

Srain...but the Euro had kind of nailed it at 228 and really everywhere under that. Granted it did pin it better after the data.

What I'm more interested in figuring out is whether it helped the NCEP models. Given their variability I'd argue no, partly because one of the problematic s/w's for the NCEP models was coming from south central canada.

This will be impossible to know until someone can go back and run OSEs (and we simply don't have the resources to do it). These things always have to be evaluated on a per-case basis. I started to go on about this in a regional thread the other day, and plan to do a longer write-up about this when I get a chance. One thing about DA is that it is really statistical (and/or probabilistic) in nature. We are combining a lot of information, and every single bit of this information has errors associated with it (yes, even the observations). The nice thing about radiosondes/dropsondes is that they are more direct measurements of the model fields, so the representativeness error (errors associated with interpolation, transforming the observation into model space, etc.) isn't as extreme as most other observations types. The idea behind DA is to come up with a best estimate of the state, one that minimizes the errors associated with each bit of information.

For NWP, things are even more complicated, because the best analysis (i.e. the state that truly minimizes the errors) is NOT the best initial condition. This is because of model error/bias.

The one thing I want to try to hit home....because of all aforementioned, not all observations improve forecasts (or reduce forecast error). In fact, in terms of counts....only a small majority of observations (say between 51-60%) actually reduce forecast error (and yes, that implies that for any given cycle, as much as 49% of the observations actually can have a "negative" impact on the forecast). That is not to say that those observations are bad, just that they can cause things to happen through the assimilation that can act to increase forecast error (through representativeness or sensor errors, through a mismatch with something in the model climatology, through over/under extrapolation of information through the assimilation process, by incorrectly projecting the multi-variate information with in the assimilation process).

I'm going to show a figure, with one example, to highlight sort of what I'm talking about. We (the community, not NCEP, yet) actually have a means of estimating the actual impact every observation has in reducing short term forecast error (either through adjoint or ensemble based methods). Here is an example of the impact of assimilating dropsonde observations into the NASA General Circulation Model (they use the NCEP DA system) for the past month (figure is updated quasi-regularly):

Basically, anything negative means that the observations reduced the forecast error (i.e. had a positive impact on the system). Any cases for which the number is positive, that subset of observations actually increased the short term forecast error. First, note that the impact isn't always positive for dropsondes (in their system, for this estimation/metric). Again, it is not to say that the observations are actually bad for the cases where the impact was negative. Secondly, note how variable the amplitude of the impact can be (this is related to how the information projects on to growing/decaying modes, as well as the error growth characteristics of each case).

Lastly, even though "negative" information is regularly brought into the system....the net impact when summed up is actually positive as expected. Here it is broken down for the NASA system by observation type:

The relative amplitude and ranking is going to be different for each operational center, so you have to take the information for what it's worth. Also, the method is only an estimate of the impact (and doesn't require running actual data denial experiments).

[phil882]

dtk, on 19 February 2012 - 09:06 AM, said:

This will be impossible to know until someone can go back and run OSEs (and we simply don't have the resources to do it). These things always have to be evaluated on a per-case basis. I started to go on about this in a regional thread the other day, and plan to do a longer write-up about this when I get a chance. One thing about DA is that it is really statistical (and/or probabilistic) in nature. We are combining a lot of information, and every single bit of this information has errors associated with it (yes, even the observations). The nice thing about radiosondes/dropsondes is that they are more direct measurements of the model fields, so the representativeness error (errors associated with interpolation, transforming the observation into model space, etc.) isn't as extreme as most other observations types. The idea behind DA is to come up with a best estimate of the state, one that minimizes the errors associated with each bit of information.

For NWP, things are even more complicated, because the best analysis (i.e. the state that truly minimizes the errors) is NOT the best initial condition. This is because of model error/bias.

The one thing I want to try to hit home....because of all aforementioned, not all observations improve forecasts (or reduce forecast error). In fact, in terms of counts....only a small majority of observations (say between 51-60%) actually reduce forecast error (and yes, that implies that for any given cycle, as much as 49% of the observations actually can have a "negative" impact on the forecast). That is not to say that those observations are bad, just that they can cause things to happen through the assimilation that can act to increase forecast error (through representativeness or sensor errors, through a mismatch with something in the model climatology, through over/under extrapolation of information through the assimilation process, by incorrectly projecting the multi-variate information with in the assimilation process).

I'm going to show a figure, with one example, to highlight sort of what I'm talking about. We (the community, not NCEP, yet) actually have a means of estimating the actual impact every observation has in reducing short term forecast error (either through adjoint or ensemble based methods). Here is an example of the impact of assimilating dropsonde observations into the NASA General Circulation Model (they use the NCEP DA system) for the past month (figure is updated quasi-regularly):

Basically, anything negative means that the observations reduced the forecast error (i.e. had a positive impact on the system). Any cases for which the number is positive, that subset of observations actually increased the short term forecast error. First, note that the impact isn't always positive for dropsondes (in their system, for this estimation/metric). Again, it is not to say that the observations are actually bad for the cases where the impact was negative. Secondly, note how variable the amplitude of the impact can be (this is related to how the information projects on to growing/decaying modes, as well as the error growth characteristics of each case).

Lastly, even though "negative" information is regularly brought into the system....the net impact when summed up is actually positive as expected. Here it is broken down for the NASA system by observation type:

The relative amplitude and ranking is going to be different for each operational center, so you have to take the information for what it's worth. Also, the method is only an estimate of the impact (and doesn't require running actual data denial experiments).

Excellent Information... really interesting to see that dropsondes had quite a positive impact in some periods (especially between January 29th - Feb 5) while in other periods the impact was negligible. It is good to see that the vast majority of the time, the impact is positive as one would expect

Do you have a source link to the figures you have provided above, or is this information restricted to the public?