Ramblings of a Graduate Student

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.

The upcoming winter storm (identified on yesterday’s post) is certainly what everyone is talking about in Norman.  In fact, at today’s Hazardous Weather Testbed (HWT) Map Discussion there were over 40 people in attendance – all focused on what would unfold later this week.  Tonight I thought I would take a look at what the Hydrometeorological Prediction Center (HPC) is currently forecasting.

Above is the current 5-day HPC precipitation forecast.  Almost all of the precipitation across the southern plains is associated with the shortwave trough forecast to affect the area Wednesday night into Friday morning, aka the potential Winter Storm.  Ultimately, the big question is what will the precipitation type of that amount be.  As a rule of thumb (and a starting point), 1″ of liquid is approximately 10″ of snow – however, a lot of things can affect that ratio.

Using the 10:1 ratio above, if all of the precipitation were to fall as snow, a good portion of the southern plains would be buried under almost 2 feet of snow.  The good news for Oklahomans and Arkansans is that all of this precipitation is not forecast to fall as snow.  So what then are meteorologists currently forecasting?  The next few images should help break that down as best as meteorologists can currently forecast.  It should be pointed out that the forecasts below are valid from 00Z Thursday (6PM CST Wednesday) until 00Z Friday (6PM CST Thursday), whereas the precipitation forecast above is valid through 00Z Sunday (6PM CST Saturday).  Thus, additional precipitation will fall after 00Z Friday (6PM CST Thursday) and most of that will fall in the form of some wintry precipitation.

Above is the probability of an area receiving at least 4″ of snow.  As you can see, HPC forecasters have greater than 70% confidence that a large portion of northwest Oklahoma, northern Texas Panhandle, and southern Kansas will receive at least 4″ of snow.

Above is the probability of an area receiving at least 8″ of snow.  Again, HPC forecasts have fairly high confidence that a good potion of the northern Texas Panhandle and northwest Oklahoma will receive over 8″ of snow.

Above is the probability of an area receiving at least 12″ of snow.   It isn’t too often you see confidence high enough to warrant a 70% area for at least a foot of snow this far into the future for a southern plains winter storm.  However, a large swath of 40% probabilities is nothing to sneeze at!  Remember, more snow will fall in parts of that outlooked area after the end of that time period!

And last, but not least, what about areas to the south and east of the heavy snow?  What does all that precipitation fall as?  Unfortunately, it appears that there is enough confidence to warrent a 40% probability that some areas will receive at least 0.25 inches of freezing rain.  This is the criterion for what is known as an Ice Storm.  Couple this much freezing rain with the expected strong winds, some places will most likely see power outages.

Editor’s note:  While I was writing this data from the latest (00Z, 6PM CST) run of the North American Model (NAM) began to come in.  This run is considerably slower and stronger than previous runs.  It holds off most of the heaviest winter precipitation until after 00Z Thursday (6PM Wednesday).  In fact, between 1.5 and 2 inches of liquid equivalent is forecast in the cold sector of the storm Thursday night.  The model forecasts this to fall as heavy, wet snow along the I44 corridor in Oklahoma.  This would easily result in over 12″ of snow in this portion of Oklahoma – again – if this model run turns out to be true.  A lot can (and will) change over the next few days.  Don’t get caught up in the details of a single run, just know that there is the potential for a major winter storm late this week!