Ramblings of a Graduate Student

This is the last image, of the last model runs, for the last day of the 2010 Hazardous Weather Testbed (HWT) Experimental Forecast Program (EFP). The image above depicts the models’ forecast reflectivity. In other words, this is what the model (or in the case, 4 different models) are forecasting the radar to look like.

The 2010 HWT EFP was the second of two major projects I was involved in during the last 6 weeks. (VORTEX 2 was the other one.) Below is a brief explanation of what the 2010 HWT EFP was all about this year

The NOAA HWT Spring Experiment is a yearly experiment that investigates the use of convection-allowing model forecasts as guidance for the prediction of severe convective weather. A variety of model output is examined and evaluated daily during the experiment and experimental severe weather forecasts are created and verified. The variety of model output allows us to explore different types of guidance, including products derived from both ensembles and deterministic forecasts.

The 2010 Spring Experiment will be held from May 17th through June 18th in the HWT facility at the National Weather Center in Norman. The Experiment is scheduled to run Monday through Friday from 730am to 4pm. In addition to the traditional focus on severe convection, the 2010 Experiment will also explore use of convection-allowing models to address thunderstorm aviation impacts and for convective heavy rain forecasting. More information about this year’s Experiment can be found below in the 2010 Spring Experiment Operations Plan.

You can check out the 2010 HWT EFP webpage by clicking here. At this link you can check out all the experimental forecasts, look at the experimental model runs, and a whole lot more.

For the past few weeks I’ve alluded to the fact I will have an extremely active spring.  I’m involved in both VORTEX II (although my role has continued to evolve over the last few days) and the Experimental Forecast Program (a joint experiment hosted by the National Severe Storms Laboratory [NSSL] and the Storm Prediction Center [SPC] and held in the Hazardous Weather Testbed [HWT; I’m the one in the blue shirt in the image on the upper right!)  This year’s experiment will have collaborators from the Aviation Weather Center (AWC) in Kansas City, MO and the Hydrometeorological Prediction Center (HPC) in the Washington D.C. / Silver Springs, MD area.

In preparation of this year’s experiment I was on a teleconference this morning with HPC and SPC so tonight I thought it appropriate to display a graphic produced by HPC.  Above is the forecast precipitation totals over the next 5 days.  As you can see, heavy rain is forecast for places that were hit hard by last weekend’s tornado outbreak.  This does not bode well for cleanup efforts.  This heavy precipitation is the result of another slow moving trough (#4 in last night’s post) that will move through the southeast late in the week into the weekend.

As some of you know, I’m heavily involved in the VORTEX II project, which this year begins on 1 May and runs until 15 June.  Additionally, I’m heavily involved in the National Severe Storms Laboratory (NSSL) and Storm Prediction Center (SPC) Experimental Forecast Program (EFP) which begins 17 May and runs through 18 June.  As you can imagine, I’m up to my eyeballs with respect to all the things that still have to get finished before the start of these projects.  Since the weather has been rather tranquil of late, I’ve tried to get a lot of work done for VORTEX II and the NSSL/SPC EFP.  This way, when the weather becomes active again, I can devote some attention to the blog.  (Yes, this is why the blog posts have been rather short of late.)

The image above is screenshot of one of the projects I’m working on for VORTEX II.  It takes some of the data feeds we have here at the NSSL and SPC (particularly the mesoanalysis data) and converts into an overlay for the Gibson Ridge radar viewer(s).  The image above depicts the amount of Convective Available Potential Energy (CAPE).  A simple way to understand CAPE is that the greater the amount of CAPE, the more energy thunderstorms have available to them.  Thus, they can become stronger. However, a large CAPE value does not mean a thunderstorm will develop (just look at the image above; the storms aren’t located in the maximum CAPE values!).  That’s an entirely different question altogether!  This tool will be one of many used by VORTEX II personnel in trying to identify where the strongest thunderstorms will develop, and differentiate between those that are more likely to produce tornadoes and those that are not.

Ah, the night before a forecast big storm. Meteorologists keep checking the data non-stop hoping the latest piece of information is available. When the numerical weather prediction models’ forecast transitions away from the forecaster’s desired forecast, a lot of “wishcasting” takes hold. Meteorologists will often tend to find any piece of information to sustain hope that what they want will ultimately develop. This internal struggle can be a challenge for a meteorologist. I know I struggle with it from time to time.

As residents throughout the southern plains head to bed this evening, most are still unsure what tomorrow’s weather will hold.  This is because forecasters are still struggling to understand the evolution of the winter storm.  It is apparent that somewhere will see heavy snow, somewhere will see sleet and freezing rain, and somewhere will only see rain.  The problem is pinpointing those exact locations is difficult even 24 hours in the future.

A couple of things struck me tonight with respect to the shortwave trough (upper-low) responsible for the difficult forecast…

Above is a satellite image depicting the moisture content of the middle-to-upper atmosphere (brighter colors means more moisture aloft).  This is important because the most efficient way to get moisture aloft is for there to be rising motion in the atmosphere.  Thus, wherever there is the brighter colors, we can infer rising motion.  Rising motion is important because it is a key (but not a sufficient) ingredient  in the development of precipitation.  Thus, with all that rising motion in the southwest United States, one would expect a lot of precipitation in the southern plains as the rising motion moves into the area.  This agrees quite well with the precipitation forecasts from the Hydrometeorological Prediction Center (HPC) shown over the last few days.

Below I’ve identified the center of the upper-low (red L) and what I suspect will be the eventual forecast path based on a combination of observational and model trends.  This is considerably farther north and west than what was forecast several days ago.  As I mentioned last night, a farther north/west track will result in more places being affected by the dry slot – including Oklahoma City and points south and east.  However, before the dry slot affects central and eastern Oklahoma, these places will spend an extended period of time in the warm conveyor belt which will bring warm, moist air.  This warm, moist air will result in heavy precipitation and a warming of the temperature aloft (and possibly at the surface).  This warming aloft will prevent snow crystals from forming in central Oklahoma which leaves sleet, freezing rain, or a cold rain as the resulting precipitation types.

As you can imagine, I’ve been pretty busy the past few days handling all the requests for information and discussion regarding tomorrow’s (possible) winter storm.  I didn’t have time to annotate any more graphics tonight, but I will leave you with something better.  Below is a recording of today’s HWT Map Discussion.  It may be a little too technical, but it can give a better glimpse as to what a forecaster must look at in situations like these.  Please feel free to ask questions and provide feedback regarding anything in the last few days worth of blogs and / or the video below!

27 January 2010 NOAA HWT Map Discussion from Patrick Marsh on Vimeo.

I have had a difficult time trying to write tonight’s blog. Normally I have this problem because there is nothing really “exciting” to write about. Tonight the problem is I don’t know what to write without creating a dissertation! As I hinted at last night, almost every meteorologist in the southern plains is focused on the exact evolution of the southern plains winter storm. There were close to 70 National Oceanic and Atmospheric Administration (NOAA) and University of Oklahoma (OU) meteorologists in attendance at today’s Hazardous Weather Testbed (HWT) map discussion. I’m sure tomorrow we’ll have even more.

The official precipitation forecast from the National Weather Service’s (NWS) Hydrometeorological Prediction Center (HPC) continues to have over 2.5-3″ of liquid equivalent (the amount of rain or water from melted ice/snow) falling across portions of Oklahoma, western Arkansas, and northern Texas.  Generally speaking, almost all of today’s numerical weather prediction (NWP) models are in excellent agreement with this HPC forecast.  Where the models differ – between each other and also with different simulations of the same model – is in the form that this precipitation will fall.  Hopefully by the end of this post someone will have an idea as to what will happen…

The culprit for the soon-to-be winter storm is easily identified in current satellite imagery.  In the image above, the shortwave trough (upper low) that is the batch of white clouds off the southwest coast of California.  Over the next 24 hours the upper low should track south-east into northern Mexico.

Sometime during the 24-36 hour time-frame the upper-low should begin to turn more toward the east and then northeast.  When this turn to the northeast happens is crucial for determining precipitation types and duration across portions of the Texas Panhandle, Oklahoma, and western Arkansas.  One reason why where the turn occurs is important, is that it will have a direct impact in where several important features of the cyclone will become established.

Above is the 700mb (~10,000 feet above ground level) chart from the 00Z (6PM CST) North American Model (NAM).  The 700mb chart is important for winter weather forecasting because a lot of important features are easily identified on it.  Below, I’ve annotated the same chart as displayed above.  The red arrows represent initially warm, moist air (moving in the direction of the arrows), whereas the blue arrow represents initially cold,dry air (also moving in the direction of the arrow).  The red arrow is often referred to as the “Warm Conveyor Belt” and the blue arrow is often referred to as the “Dry Slot”.

In the warm conveyor belt, warm moist air from near the surface flows northward into the developing cyclone.  As it flows northward, it tends to encounter colder, drier air at the surface.  Warm, moist air is less dense then cold, dry air so the warm, moist air flows up and over the cold air.  So, in addition to the warm, moist air flowing northward, it is also flowing upward (from the ground).  As warm, moist air reaches higher altitudes, it encounters lower atmospheric pressure and begins to expand.  This expansion of the warm, moist air causes the temperature of the warm, moist air to cool, condensate, and eventually precipitate.

Depending on the strength of the developing cyclone, the warm conveyor belt will either continue to develop ahead of the (weak) cyclone or a portion of the warm conveyor belt will get wrapped around the backside of the cyclone…typically near and north of the 700mb low.  When the warm conveyor belt wraps around the cyclone, the cyclone takes on the typical “comma” shape often seen on satellite imagery and in text books.  This conveyor of warm, moist air aids in the development of clouds and precipitation on the backside of the low.  Precipitation resulting from this process is often referred to as “wrap-around” precipitation because it has wrapped all the way around the low.  Because this is occurring on the backside of the low, the surface temperatures are often falling as the surface cold front has most likely passed through.  Thus, in winter, snow is often found in the “wrap-around” precipitation.

Equally important in the life-cycle of a cyclone is the “Dry Slot”.  Unlike the warm conveyor belt that starts near the surface, the dry slot originates in the upper portion of the troposphere.  Here, cold, dry air begins to get entrained into the mid-level cyclone.  As mentioned above, cold, dry air is more dense than warm, moist air – which is typically found near the surface.  Thus, the cold, dry air aloft attempts to sink toward the surface.  As it does this, the cold, dry air encounters air at a high pressure and is forced to compress.  This sinking and compressing results in a relative warming and substantial drying.  This drying out of the atmosphere tends to supress precipitation development which also aids in the development of the comma shape often seen.  The dry slot is often found just south of the track of the 700mb low.

So what do the warm conveyor belt and dry slot have to do with when the storm turns north?  Well, the earlier the upper-low makes the northward turn, the farther north the 700mb low will track and a good portion of southern Oklahoma into western Arkansas will be “dry slotted” at the same time the temperature become cold enough to support snow.  Places that are dry slotted, in turn, would receive considerably less snow than places that remain in the warm conveyor belt.

So, based on the images above, what does the NAM forecast?

Southern Oklahoma and western Arkansas dry slot…

With this said, the warm conveyor belt is producing so much precipitation ahead of the dry slot that Oklahoma and Arkansas should still see over 2.5-3″ of liquid equivalent…before the dry slot overtakes them.  For Oklahoma, surface temperature would support much of this falling as freezing rain and sleet (to the tune of over 0.5″ of ice and 2-4″ of sleet!) while west-central Arkansas would see mainly rain (with a brief opportunity for some ice toward the end of the precipitation).  As for the places remaining in the warm conveyor belt?  This run of the NAM predicts over a foot of snow in a wide area from Tulsa westward to Edmond, Enid, Woodward, etc.

Remember, this is just one run of one computer model.  The forecasts continue to change as we learn more about the approaching short-wave trough (upper low).  The above scenario should be taken as 1 possibility out of many others.  In fact, another model run at the same time as the NAM predicts much more sleet across the aforementioned area, and does have as pronounced of a dry slot.

Only time will tell with this storm.  Please refer to your local National Weather Service office for more details for your specific area.