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Bob Chill

Mid Atlantic Met Class Thread

87 posts in this topic

It can get complicated real quick, so I'll try my best. You are right in that the strength of the low is in part, due to how strong the vortmax is, embedded in the shortwave trough. This is one of several factors in determining storm strength and position, but it's the most important one. 500mb is an important height level because in general this separates the atmosphere where about half the airmass lies below it and half above it..more or less. It's this level where features drive the surface pattern in terms of low and high pressure. If you want a low to form, you want convergence of air below this level, and the air to diverge or evacuate at that level and above. After all, in order to get low pressure to form, air must continuously be evacuated away from the area, so that more air rises in its place. This is a broad brushed idea, but I hope you understand what I mean.

So getting back to the original question, a strong vortmax embedded in the shortwave trough will really help kick things off. The basic idea, is that these features move at a certain phase speed....we'll say 40kts or so. Now the air moving through it, may be 60-80 kts. So, the air moving through this feature is moving faster than the actual feature itself. As the air leaves this area of spin, it begins to slow down and like a skater moving their arms away from their body...begins to diverge. So, if the air is diverging in the mid and upper levels, buy definition..air must converge below in order to help fill this void of air. So the tighter and stronger the vortmax..usually the stronger a storm will be. Now there are other feedback factors that start to come into play, but I'm just focusing on 500mb right now.

Here is an example of a powerful shortwave trough and associated strong vorticity maxima. Lets look at the Jan 26-27 2011 storm. Here is the 500mb prog for 06z on the 27th.

post-33-0-80956500-1329834739.gif

Here is the surface low depiction. Notice that it's very close, if not under the 500mb vortmax. This is an indication of a mature low that is getting ready to occlude.

post-33-0-20931500-1329834801.gif

Now remember we talked about rising air out ahead of the vorticity maxima? Look at this water vapor animation. Notice the higher cloud tops over SNE spiraling out ahead of the vortmax well to the south of SNE. This is an indication of rapidly rising air, and indeed there were 5" per hour snows in CT and we all know about your TSSN that you had in the evening.

post-33-0-56127200-1329834978.gif

Funny thing about this storm...NAM was by far (if I remember correctly) one fo the best guidance in terms of rapidly bombing that surface low and developing that warm sector moist convection and wrapping it into the deform band. That storm had a trop fold I believe.

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It can get complicated real quick...(snip)

Coastal, extremely helpful post, thank you. You have a great way of explaining things in "dumbed" down terms and the images really bring the point home.

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I'm not positive on this, but I don't actually think it is the vertical stacking of the lows that causes the "eye" - or it is, but not always. When lows vertically stack it signals the occlusion process happening and the ceasing of much further strengthening. All cyclones eventually vertically stack and occlude, but clearly not all cyclones have "eyes". I think the eye formation process has more to do with the frontal structure and occlusion process of some cyclones vs. others. Shapiro-Keyser style cyclones are much more likely to develop an eye (and are also more likely to develop over water, and hence affect the EC region with snow) due to the progression of the warm front. Anyway, this may be a technicality from what you wrote, so I apologize if I'm simply repeating what you've said.

http://weatherfaqs.org.uk/node/98

In general yes, but this can be deceiving because certain storms can exhibit what seems to be a vertically stacked config but are actualy undergoing rapid cyclogenesis. Instant occlusion storms are an example where the surface cyclone can undergo rapid intensification well after the storm has stacked/occulded at the surface.as the main surface low "bends" westward. Intense marine cyclones like the Shapiro-Keyser model with seclusions.

A great example is vertical stretching of a deep PV anomaly interacting with a moist/warm low level baroclinic zone. This type of cyclogenesis happens often on the lee side of the Rockies as deep PV's can stretch vertically rapidly intensify given the right low level thermal environment.

See the April 14-15th 2011 severe weather outbreak/blizzard.

post-999-0-19025100-1329869404.gif

post-999-0-97888800-1329869402.png

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Coming from a software engineering background, I am curious what programming language most models are developed in? Also, what kind of computers are they using to process all of the algorithms?

Almost exclusively FORTRAN (some C++). The NCEP operational computer as well as those at both the UKMet Office and ECMWF are IBM power clusters (6 for us, and 7 for them, I believe). NCEP is getting a new supercomputer in 2013, but the contract was just recently awarded and the details haven't been made available.

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PVA is usually colocated with sthe strongest height falls. At least from what I estimate on the map. Sometimes there is a stray mesovortex that doesn't lower heights much.

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Funny thing about this storm...NAM was by far (if I remember correctly) one fo the best guidance in terms of rapidly bombing that surface low and developing that warm sector moist convection and wrapping it into the deform band. That storm had a trop fold I believe.

I think you are thinking of the Jan 11-12 storm perhaps?

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During severe season, what is meant by CAPE?

CAPE is an acronym for Convective Available Potential Energy, and is used as a metric when identifying the potential for convection (thunderstorm activity). It must be stressed that CAPE is the end all for determining severe weather risk, but is certainly important. Remember that air must have buoyancy (ability to rise) and then condense to form clouds, rain, hail, etc and CAPE provides a measure of this buoyancy. The higher this buoyancy, the quicker air will rise and begin the formation of thunderstorms. The CAPE value is measured in Joules per kilogram (J/Kg) and, generally, you need a minimum of 1,000j/kg of CAPE to see severe weather (Blanchard, 1998). CAPE can reach values of up to 5,000J/Kg, but that is reserved for rare events, such as large scale severe weather outbreaks. Here in the Mid Atlantic a good day is when CAPE gets to ~2,000J/Kg. There are two other types of CAPE talked about in forecast discussions and severe weather outlooks: MLCAPE (Mixed Layer CAPE) and SBCAPE (Surface-Based CAPE). Unfortunately I am not adept at explaining these two terms, but if you were to Google: SPC + MLCAPE or SPC + SBCAPE there are several good snippets of information. Hope this helps!

Reference:

Blanchard, D. O. (1998). Assessing the Vertical distribution of Convective Available Potential Energy. American Meteorological Society, 13, 870 - 877.:

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I don't have much to add other than repeating how good this thread is, and thank you for the information.

Coming from a software engineering background, I am curious what programming language most models are developed in? Also, what kind of computers are they using to process all of the algorithms?

The computing systems are really a system of systems with Supercomputers doing a lot of the heavy lifting and mainframes and client/server stuff taking a significant amount of local load. Here's a high-level diagram of the overall architecture, with some stuff redacted out:

post-109-0-94155400-1329916053.png

Here's a slide deck that details most of the infrastructure and its tie in to numerical weather prediction modelling. There are some interesting computing systems links on slide 68.

https://skydrive.liv...B544483E3%21258

The local office stuff is typical of a remote or regional office. Local, high-res models with smaller domains are run at these sites so that 1) the main computing systems are not impeded and 2) it offers the local offices more flexibility in ad-hoc runs for significant or hyper-local forecasting needs. Here's a pic of our local office:

http://www.erh.noaa....r/computers.htm

Here's a draft roadmap for the NextGen exascale computing systems that will likely power all sorts of government-facing numerical and graphical predictive analysis systems, including those in NOAA, going forward. I only have the draft on this workstation. Ping me via PM if you want the latest and I'll dig it up when I can.

https://skydrive.liv...B544483E3%21257

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Speaking of CAPE and following up about the skew-T discussion, the amount of CAPE can be directly calculated from the skew-T by adding up the area between the parcel trajectory (along a moist adiabat) and the environmental temperature profile. So, if there's a lot of room between the two, you can know the CAPE is high before looking at the calculated parameters directly.

The "opposite" of CAPE is CIN (convective inhibition) and is also measured in J/kg. CIN occurs when the parcel trajectory will lead it to be colder than the environmental temperature. If the parcel is colder, it's not buoyant in the free atmosphere and needs to be forced to a height in the atmosphere where it is bouyant before it can freely begin to rise. When people speak of a "cap", there means the atmospheric profile has a certain amount of CIN to overcome.

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Speaking of CAPE and following up about the skew-T discussion, the amount of CAPE can be directly calculated from the skew-T by adding up the area between the parcel trajectory (along a moist adiabat) and the environmental temperature profile. So, if there's a lot of room between the two, you can know the CAPE is high before looking at the calculated parameters directly.

The "opposite" of CAPE is CIN (convective inhibition) and is also measured in J/kg. CIN occurs when the parcel trajectory will lead it to be colder than the environmental temperature. If the parcel is colder, it's not buoyant in the free atmosphere and needs to be forced to a height in the atmosphere where it is bouyant before it can freely begin to rise. When people speak of a "cap", there means the atmospheric profile has a certain amount of CIN to overcome.

The easiest way for me to understand cap was to think of the atmosphere as a pot with a lid on it. The lid being the cap that needs to come off before any storms can develop and grow in size.

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Speaking of CAPE and following up about the skew-T discussion, the amount of CAPE can be directly calculated from the skew-T by adding up the area between the parcel trajectory (along a moist adiabat) and the environmental temperature profile. So, if there's a lot of room between the two, you can know the CAPE is high before looking at the calculated parameters directly.

The "opposite" of CAPE is CIN (convective inhibition) and is also measured in J/kg. CIN occurs when the parcel trajectory will lead it to be colder than the environmental temperature. If the parcel is colder, it's not buoyant in the free atmosphere and needs to be forced to a height in the atmosphere where it is bouyant before it can freely begin to rise. When people speak of a "cap", there means the atmospheric profile has a certain amount of CIN to overcome.

Very good explanation. I found this skew diagram while reading through the links that trix posted. I don't think there could be a better visual to go along with your explanation.

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CAPE is an acronym for Convective Available Potential Energy, and is used as a metric when identifying the potential for convection (thunderstorm activity). It must be stressed that CAPE is the end all for determining severe weather risk, but is certainly important. Remember that air must have buoyancy (ability to rise) and then condense to form clouds, rain, hail, etc and CAPE provides a measure of this buoyancy. The higher this buoyancy, the quicker air will rise and begin the formation of thunderstorms. The CAPE value is measured in Joules per kilogram (J/Kg) and, generally, you need a minimum of 1,000j/kg of CAPE to see severe weather (Blanchard, 1998). CAPE can reach values of up to 5,000J/Kg, but that is reserved for rare events, such as large scale severe weather outbreaks. Here in the Mid Atlantic a good day is when CAPE gets to ~2,000J/Kg. There are two other types of CAPE talked about in forecast discussions and severe weather outlooks: MLCAPE (Mixed Layer CAPE) and SBCAPE (Surface-Based CAPE). Unfortunately I am not adept at explaining these two terms, but if you were to Google: SPC + MLCAPE or SPC + SBCAPE there are several good snippets of information. Hope this helps!

Reference:

Blanchard, D. O. (1998). Assessing the Vertical distribution of Convective Available Potential Energy. American Meteorological Society, 13, 870 - 877.:

MLCAPE and SBCAPE are just two different ways of initially lifting the parcel. With MLCAPE you average the temp and dewpoint in the lowest 100mb and lift from where they intersect. With SBCAPE, you lift from the surface temp and dewpoint to get your parcel trajectory. For example, in Bob's post above, the parcel looks to be SBCAPE where the traces start at the surface temp/dewpoint. If you try lifting from the average of the lowest 100mb temp/dewpoint, it looks like you would get slightly less CAPE.

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Ellinwood and other severe guys, could you provide a little tutorial on what you guys look for irt t-storms and severe? I have a handle on some of the basics but I thought it was pretty cool that you identified today's threat pretty easily in advance. I'm interested in which charts and models you use.

Thanks!

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Ellinwood and other severe guys, could you provide a little tutorial on what you guys look for irt t-storms and severe? I have a handle on some of the basics but I thought it was pretty cool that you identified today's threat pretty easily in advance. I'm interested in which charts and models you use.

Thanks!

Soundings, available moisture, dynamics and jet streaks are the most important things to look at... I'll get something drawn up this weekend but for now it's chase day!

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Soundings, available moisture, dynamics and jet streaks are the most important things to look at... I'll get something drawn up this weekend but for now it's chase day!

Excellent. No rush of course. Thread won't be going anywhere and I'm looking forward to expanding my brain this year. Winter analysis is getting kinda boring in these parts. lol

Bring your camera and don't pull a Dorothy and Oz on us!

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Sometimes i think the things that inhibit severe can be better to know; dry air, cap, stable marine LL BL, too much shear, lack of mid level lapse rates, no trigger/lift, no cold pool aloft, bad timing, no curved hodo signature, no favorable jet... some obviously already mentioned

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Sometimes i think the things that inhibit severe can be better to know; dry air, cap, stable marine LL BL, too much shear, lack of mid level lapse rates, no trigger/lift, no cold pool aloft, bad timing, no curved hodo signature, no favorable jet... some obviously already mentioned

Not sure "better" is accurate. Alot of these are just the opposite of features you assess for severe risk...so if you learn what causes severe wx, knowing these is a no-brainer.

We are taught to assess severe parameters and look for severe where they overlap, like on a severe composite map. Then once the threat area is outlined, you assess the features that would hinder it. At least, that's what I'd do. :)

For example, if you look at severe parameters and find an area where they all overlap, there likely won't be a stable marine layer/lack of ML lapse rates/lack of shear/lack of trigger/no favorable jet/lack of hodo sig/etc there in the first place. You initially found an environment where the opposite of all of these are present. Make sense?

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I found this link (http://www.wunderground.com/radar/help.asp) which nicely explains how radars work, but I do have a noob question:

I sometimes see people talking about level 3 or level 2 radars, which I assume refers to NEXRAD's data, but what is the difference between those "levels"? From what I've seen, people talk about level 3 data as if it's inferior, but why?

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I found this link (http://www.wundergro.../radar/help.asp) which nicely explains how radars work, but I do have a noob question:

I sometimes see people talking about level 3 or level 2 radars, which I assume refers to NEXRAD's data, but what is the difference between those "levels"? From what I've seen, people talk about level 3 data as if it's inferior, but why?

http://www.grlevelx.com/ this is what is being referred to by the "levels" of radar...

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http://www.grlevelx.com/ this is what is being referred to by the "levels" of radar...

In many data sets, particularly for observations, the "level" usually is in reference to the amount of processing that has been done to the raw measurement. For NEXRAD, you can find descriptions for

Level2 Here: http://www.roc.noaa.gov/WSR88D/Level_II/Level2Info.aspx

and

Level3 Here: http://www.roc.noaa.gov/WSR88D/Level_III/Level3Info.aspx

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Mesoscale book that I would highly recommend. Served me well at Millersville and has a fair amount of pictures/diagrams if you are a more visual learner. It doesn't really make a hard effort to avoid the mathematics, but it also tries to break things down conceptually so if you have the interest you will likely enjoy it.

http://www.amazon.co...r/dp/0470742135

BTW, this a GREAT thread!

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This is probably a good thread to post all our favorite weather links. I'm terrible about organizing my own. I almost feel like paying someone to send me an organized bookmark folder with all the good ones.

Here's a few I really like:

Making composites for analog years:

http://www.esrl.noaa.gov/psd/cgi-bin/data/getpage.pl

Major teleconnection forecasts and historical data:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/teleconnections.shtml

One Stop Enso:

Nino: http://www.elnino.noaa.gov/index.html

Nina: http://www.elnino.noaa.gov/lanina.html

Historical Data: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml

MJO ensemble forecast (GFS):

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/foregfs.shtml

Aweome climate data site (especially for historical location snowfall):

http://climate.usurf.usu.edu/products/data.php

Favorite MD radar (great resolution with temp overlay):

http://www.marylandwx.com/radar/lwx/flashklwxstatebr.php

Favorite Model Links:

GFS/NAM/GEFS/RUC/SREF: http://mag.ncep.noaa.gov/NCOMAGWEB/appcontroller?prevpage=index&MainPage=index&cat=MODEL+GUIDANCE&page=MODEL+GUIDANCE

Euro: http://www.wunderground.com/wundermap/?zoom=4&rad=0&wxsn=0&svr=0&cams=0&sat=0&riv=0&mm=1&mm.mdl=GFS&mm.type=SURPRE&mm.hour=0&mm.opa=100&mm.clk=0&hur=0&fire=0&tor=0&ndfd=0&pix=0&dir=0&ads=0&tfk=0&fodors=0&ski=0&ls=0&rad2=0

There are 100's of other great links out there. Please share your faves. I need to get my stuff organized and I'm always looking for the best links out there. I'm sure there are many other board members who would like to add to their list as well.

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Mesoscale book that I would highly recommend. Served me well at Millersville and has a fair amount of pictures/diagrams if you are a more visual learner. It doesn't really make a hard effort to avoid the mathematics, but it also tries to break things down conceptually so if you have the interest you will likely enjoy it.

http://www.amazon.co...r/dp/0470742135

BTW, this a GREAT thread!

Seconded suggestion for this book! Great, comprehensive resource for all things mesoscale. :)

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