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AMZ8990

December 2018 Pattern And Forecast Discussion

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1 hour ago, nrgjeff said:

Looking for a couple things this morning, still 6 days out. Need vort max to track south of here. Need high press (HP) north anchored and still spilling in somewhat. I am less worried about the surface low which looks good. 

First the 500 mb vort max should track to our south for snow. Right overhead is not good enough, especially this case. If HP is very strong, anchoring cold, and the vort max goes overhead, WAA may cause ice. That's like in Ferris Bueller where faking a fever lands you in the doctor's office. Wrong outcome, lol!

Next HP has to be firmly anchored in the right place, and even still building in. Agree 100% with John's snapshot above. Also I'm looking for HP/cold to keep spilling in from the Plains and Midwest. Down the East Coast won't work here, though the Carolinas love it. You can almost feel the right pattern on the good model runs; isobars and thickness lines are packed tight - all wound up and energetic. Reverse is true on bad model runs with HP already starting to retreat with loose isobars/thickness lines. HP must anchor very firmly, especially this case. Midwest / OV air is not particularly cold or dry, so evaporative cooling will only be a slight help. Again HP/cold still spilling in (not retreating) would make it all work out.

I mention the surface low pressure (LP) looks good. LP has been shown well south consistently across model runs (NWP). I would call it safely south, from a pro-snow perspective.

In conclusion, the concern is the temperature profile. Gee, what's new down South? 500 mb vort max should be south to take care of the mid-levels. HP should firmly hold, and continue to supply low level cold. Both will depend on confluence (of the jet branches) location and timing, per previous page. Quick check: 500 mb vort max and surface HP look & feel.

Finally, I think somebody in the sub-forum Region will get it. Odds are best I-40 north IMO. Perhaps even Kentucky, but NWP verbatim is I-40. Most of my pessimism / nit-picking is to get it into MSL-HSV-CHA, always a huge chore.

Nice write up Jeff, I think it sums up a lot about what we need to keep an eye for the couple of days. A lot is hinged on the High.

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The 12z ICON just isn't quite there to support a winter storm for us but notably takes a step in the right direction giving us some snow as the system leaves. Last night where it had the 540 line around Chicago it now has it along the Ohio River for Sunday.

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GFS is looking good thru hr132, HP a little further west this run which really gets the snow cranking in Arkansas and west tn by 132.  Solid track so far on the 12zGFS, by 144 the LP is off the coast of SC.

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Great post Flash! If the storm fails, either the pool option or drive a nice car off a cliff, LOL! Oh the ICON, make that a German luxury car.

25 minutes ago, *Flash* said:

As one whose name is Cameron Fry...the Ferris Bueller metaphors speaks to me. If trends go south in a hurry, instead of cliff-diving...I may have to fall in a pool and pretend to drown. ;)

...my concern goes back to where Lucy often lies...and that is between 700-850 mb. Even if WAA/warm noses are tempered...I'm not sold BNA/surrounding areas will have the appropriate surface-500 mb column max throughout primary QPF duration (hate to admit). ..

Flash is being straight forward, frank, and brutally honest here. Great analysis. Temp profile has problems. Main cause is lack of really cold air north. I do think if the 500 low really goes south it will be at least a little colder here. Note the GFS Para/FV3 has been colder than the Op. However the 500 mb rule still must be met. ECMWF has made perfect sense, regardless of track; temp profile and 500 mb track line up.

Even the better models runs start as rain. Cold air comes in as winds back to the north and with the comma head. Agree that method risks failure in the South. Nashville suffers from what I believe is under-rated downslope off the Plateau. Many on this sub-forum will attest to it. Chattanooga definitely has a downslope issue off the Apps. We simply must have cold air well anchored with continued low-level CAA.

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12 minutes ago, nrgjeff said:

Great post Flash! If the storm fails, either the pool option or drive a nice car off a cliff, LOL! Oh the ICON, make that a German luxury car.

Flash is being straight forward, frank, and brutally honest here. Great analysis. Temp profile has problems. Main cause is lack of really cold air north. I do think if the 500 low really goes south it will be at least a little colder here. Note the GFS Para/FV3 has been colder than the Op. However the 500 mb rule still must be met. ECMWF has made perfect sense, regardless of track; temp profile and 500 mb track line up.

Even the better models runs start as rain. Cold air comes in as winds back to the north and with the comma head. Agree that method risks failure in the South. Nashville suffers from what I believe is under-rated downslope off the Plateau. Many on this sub-forum will attest to it. Chattanooga definitely has a downslope issue off the Apps. We simply must have cold air well anchored with continued low-level CAA.

We can get downslope winds in a myriad of ways in Kingsport.  I never even knew that existed until I started reading this forum many years ago!  Interesting downslope comment about Nashville.  As you noted early, my favorite weather event is the Cumberland Plateau induced CAD.  We could probably give that a specific name and make some money off it.  

I am just now getting back in the loop this AM.   Looks like you all have it covered.  I think the biggest thing with any system is to learn from it, even if we get the shaft.  I am still leaning pretty heavily on the ensembles and watching operational for trends.  Looks to me like we are still seeing a plethora of hits and misses which is to be expected from a system approaching at this angle and with questionable strength in high pressure over the top.  On a global scale, this storm has barely moved.  On a regional scale, it probably still has some surprises.   

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Just a quick glance at the 12z suite...

1.  The CMC snow axis is quite similar to the GFS.  Those are not bad tracks either.  It would mean someone would get snow.  

2.  When systems approach our forum area at this angle(basically parallel to it), even the slightest wiggle has big downstream implications.  Think about accuracy when a hurricane approaches a coast.  If it is a perpendicular strike, landfall accuracy is pretty good.  If the storm runs parallel to the coast, accuracy can be way off.  The southwest coast of Florida deals with that scenario often.

3.  There are things(probably stuff we don't know about or understand) working against us in early December in the valleys.  There is a reason snow is a rarity here prior to Christmas in the valleys.  There is a reason that mid-January and early February are better.  

4.  All of that said, I don't feel any better or worse after looking at the 12z suite.  I really like the tracks on the GFS and CMC.  Both have snow on their northern peripheries.  But as many have noted (with great discussion to echo Jeff's comments yesterday), the devil is in the details - and we don't know all of those yet.

5.  Lastly, we are tracking this one, because it is all that we have on the radar.  Hopefully by January, we will only glance at d7-10 storms.  These things wear me out following them for this long!  LOL.

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2 minutes ago, Carvers Gap said:

Just a quick glance at the 12z suite...

1.  The CMC snow axis is quite similar to the GFS.  Those are not a bad tracks either.  It would mean someone would get snow.  

2.  When systems approach our forum area at this angle(basically parallel to it), even the slightest wiggle has big downstream implications.  Think about accuracy when a hurricane approaches a coast.  If it is a perpendicular strike, landfall accuracy is pretty good.  If the storm runs parallel to the coast, accuracy can be way off.  The southwest coast of Florida deals with that scenario often.

3.  There are things(probably stuff we don't know about or understand) working against us in early December in the valleys.  There is a reason snow is a rarity here prior to Christmas in the valleys.  There is a reason that mid-January and early February are better.  

4.  All of that said, I don't feel any better or worse after looking at the 12z suite.  I really like the tracks on the GFS and CMC.  Both have snow on their northern peripheries.  But as many have noted (with great discussion to echo Jeff's comments yesterday), the devil is in the details - and we don't know all of those yet.

5.  Lastly, we are tracking this one, because it is all that we have on the radar.  Hopefully by January, we will only glance at d7-10 storms.  These things wear me out following them for this long!  LOL.

Good update, Carvers.

FV3 should be further north with more GOM ridging, less confluence in the northeast and a weaker vort around the MN region at hour 96.  If it would ever update we likely won't see the TN Valley crusher we saw at 6z.  Still a long way to go.  Euro............... you're up!

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9 minutes ago, tnweathernut said:

Good update, Carvers.

FV3 should be further north with more GOM ridging, less confluence in the northeast and a weaker vort around the MN region at hour 96.  If it would ever update we likely won't see the TN Valley crusher we saw at 6z.  Still a long way to go.  Euro............... you're up!

Spot on! The FV3 shifts the heaviest axis of snow to 40 north and Plateau/Mtns. Man, if we were getting this track in January we would be golden statewide!

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I m feeling increasingly optimistic with how consistent the euro/fv3 are.  I don't see the same run to run consistency in the gfs or cmc, or any reason to weight them anywhere close to the euro or fv3

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Euro should be entering it's wheelhouse (compared to other modeling) at 500 for day 5.  It will be interesting to see how it handles the northern stream and our southern wave.  While overall the 12z FV3 was a bit north from 6z the run was not much different as the wave came across.  It will be interesting to see if this is a TN Valley snowstorm or a mostly KY snowstorm in the end.  I'm also really interested to see how the FV3 does regarding thermals with this system as compared to the Euro.  Will be a fascinating battle to watch play out.

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20 minutes ago, Bango said:

Are these fv3 snowfall outputs remotely correct ha? This is dgex type algorithms

Screenshot_20181203-122531.png

College of Dupage site does a better job taking the sleet out of the accumulation maps for the FV3. 12z FV3 Kuchera Acc map:

Screenshot_20181203-124842.png

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19 minutes ago, Bango said:

Are these fv3 snowfall outputs remotely correct ha? This is dgex type algorithms

Screenshot_20181203-122531.png

Oh, the DGEX.  I about forgot about those days.  The only model that I looked at during my early years following this hobby was the navgem.  And yes, I think I saw where someone in the MA forum said to be wary of the snowfall output on the FV3.  I mean, when they get to beige(see North Carolina), it probably is having some issues.  I got downsloped and still managed to scrape together a foot on that run.  LOL.  Either way, I liked the overall setup.  

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Tropical has issues with the FV3. Pivotal has a little better one from as closely as I can tell. The safest bet with most snowfall maps is to cut 33-40 percent of whatever they spit out and be extra happy if they actually verify.

The GFS is doing something I've seen it do before, and that's follow the Euro run from 12-24 hours before. I believe yesterday's 12z Euro had everything well south of 40. Today the GFS moves there. 

As Carvers just said, these situations where we potentially get an I-40 special to span the whole state, 50-75 mile shifts of track mean the difference between 12 inches or nothing sometimes.

This is a snow depth map from Pivotal and it may be more reflective of what the FV3 is putting out. Totals around 60 percent of what the Tropical map shows.

 

 snod.us_ov.png

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Comparing last night's 0z and watching the EURO roll in at 12z and it appears early on if there are small changes through hour 96.  The high pressure is a touch quicker and a bit deeper, but the southern stream is a little slower and deeper (looks like it wants to ...at least briefly.... close off).  We don't need to slow the southern stream down any more.  I hope that isn't a trend that continues.

By 120, the low is off the southeast TX gulf coast.  High is a little quicker.  Looks like the CAD regions are a touch stronger.  850s are warmer across the state of TN.  going to be close......  the HP is centered in northern IL at 1036 vs. 1039 in IA at 0z.  

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Watching the Euro like I'm watching a sports event.  Haha.  Looks like the 500mb is coming in a little deeper this run and closing off @144.  But it also looks like the HP is sliding east.

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It's slower vs yesterday's runs. It's 850 cold pool is further north than 12z yesterday when it was down in Georgia.  I'd think between 144 and 168 East Tennessee/Western NC get a pretty good dump of snow. But with the 24 hour frames. I'm not sure.

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Regarding the 12z Euro.  Surface low track from the northwestern gulf, to southern GA to off the outer banks of NC is a money track for the TN Valley.  Runs have been going back and forth with the depth of the cold and position of the high so expect more changes in the coming day.   That's the biggest thing missing this run (cold, lol). Hopefully we will revert back to a deeper cold and stronger high while keeping a similar track. 

 

It's what we do in the mid-south, walk the tight rope and hope for the best.

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2 minutes ago, tnweathernut said:

Regarding the 12z Euro.  Surface low track from the northwestern gulf, to southern GA to off the outer banks of NC is a money track for the TN Valley.  Runs have been going back and forth with the depth of the cold and position of the high so expect more changes in the coming day.   That's the biggest thing missing this run (cold, lol). Hopefully we will revert back to a deeper cold and stronger high while keeping a similar track. 

 

It's what we do in the mid-south, walk the tight rope and hope for the best.

I'll take my chances with that track any day. 

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Looks to be a similar scenario for most snowfall events here in middle TN.  Pray there's enough cold air and that it doesn't get scoured out by WAA....

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One thing that caught my eye about the run.  Sunday morning at 7am it shows a 1007mb low in southern GA, 24 hours later at 7am Monday morning it has rapidly deepened to 997.  24 hours later at 7am Tuesday morning, it has moved about 50 miles east, still off the outer banks of NC at 995mb. That's pretty impressive.

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Been out of pocket most of the day...just took a look at the 12z Euro.  I am like @Blue Ridge.  I would roll with that track.  I think the problem was the high was weak over the top, maybe like 1024 over the Lakes.  That hp fluctuates with each run due to being so far out.  The 12z GEFS is impressive w snowfall amounts.  Ensembles are important at this range as most know.  Someone, feel free to post it if you have it.  I am about out of space and time....

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17 minutes ago, Carvers Gap said:

Been out of pocket most of the day...just took a look at the 12z Euro.  I am like @Blue Ridge.  I would roll with that track.  I think the problem was the high was weak over the top, maybe like 1024 over the Lakes.  That hp fluctuates with each run due to being so far out.  The 12z GEFS is impressive w snowfall amounts.  Ensembles are important at this range as most know.  Someone, feel free to post it if you have it.  I am about out of space and time....

Here are a couple of GEFS images.

GEFS Ensembles United States MSLP 138.png

GEFS Ensembles undefined undefined 162.png

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12Z Monday runs have trended toward climo. I-40 north including Kentucky is reasonable this time of year. No sugar coating it for HSV-CHA. It is like watching prevent defense give up 300 yards. Still Day 6 so it could change. However the US-72 corridor needs a much colder solution in the next 24-36 hours. Day 5 in models will start moving less.

Like Carvers said (page 9) it remains something to watch even if it looks more local than regional. Still days out too. If this one fails it is not a biggie so early in the season. I'd rather lose in December than in February, whether we are talking snow or college basketball. Heck even in Kansas I did not expect real snow until January.

Yes I checked CAPE on a couple 12Z products. Gulf Coast MCS closes that door too. Weeklies come out in a few hours. ;)

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