Ramblings of a Graduate Student

A very active weather pattern is shaping up for the next 10 days or so beginning with a shortwave tough (upper-low) moving through the southern United States.  I debated whether to talk about this upper-low as it currently is moving through the south central US or discuss what will happen (partly as a result of this upper-low) along the east coast starting tomorrow into the weekend (another major snow storm for the mid-Atlantic states!).  I figured there would be a lot to talk about tomorrow with respect to the mid-Atlantic snow so I decided to talk about the southern plains tonight.

Below is a radar image taken tonight from the National Weather Service radar in southwest Oklahoma.  At the time this image was taken, southwest Oklahoma was experiencing light to occasionally moderate intensity rain – at the surface.  Why do I make this distinction?  Because above 6000 feet above the ground it was actually snowing!

The radar image above has what is known to meteorologists as a “bright band” signature.  The bright band is the dark green and yellow pixels that appear to make a circle around the radar (the cyan dot with KFDR label).   What is happening is that high in the cloud, the precipitation starts out as snowflakes in the cold air aloft.  As the snow falls and gets closer to the surface it encounters air that is above 0C (32F) that causes the snowflake to begin to melt.  This partially melted snowflake shows up on a radar image much more easily than a snowflake or a raindrop itself would show up.  This causes the level with the most partially melted snowflakes to show up as a bright band or bright circle around the radar site.  (Without getting in the math and physics of it all, everywhere along the the dark green and yellow circle is essentially at the same height.   It has to due with the curvature of the earth’s surface.)

Because the image below has a fairly easily identifiable bright band, I can say with a lot of confidence that it is snowing higher up in the clouds.  Now, the million dollar question, will any of those snowflakes make it to the ground?

(Note:  Clicking on the above picture may take awhile to load the image.  It is quite large.)

In the image above, areas circled in yellow have clear skies which allows for the satellite’s sensors to “see” the white snow on the ground.  Because the satellite can’t distinguish between snow and clouds, the snow on the ground gets displayed on the resulting image.  One way a meteorologist can distinguish snow from clouds on a visible satellite image is by looking at the details.  Unless a lake or river is completely frozen (which doesn’t happen very often in the south), the lake and river won’t have snow on it.  Thus a visible satellite image would show a dark spot in the middle of the bright white snow cover.  If you look in the areas circled in yellow, you can easily see how splotchy the white looks and the occasional “crease” that runs through the white.  The splotches and creases are the lakes and rivers standing out against the white backdrop of the snow.

Contrast these areas with the areas over central Oklahoma, eastern Kansas, and Nebraska.  Notice how smooth the white looks; these are clouds.  Lakes and rivers are on the surface, beneath the clouds, so there are no splotches and creases to stand out.  Those of you living in Oklahoma, particularly the central part of the state, may be wondering why it has been so foggy lately.  After all, weren’t we supposed to warm up and see some sun by now?  Ironically, the mechanism by which we were supposed to warm up has produced a lot of low clouds and fog which has hampered our warm up.

A shortwave tough (upper-low) has been slowly moving across the northern United States.  As this shortwave trough was moving to the north of Oklahoma, it was trying to draw northward into it the relatively warm, moist air from Texas.  In order for the warm, moist air to reach the shortwave trough, it has to move over the cold ground (thanks to the fresh snow cover) in Oklahoma. As it does this, the warm, moist air is cooled by the ground until the air because saturated, and a cloud forms.  We call this cloud that forms on the ground “fog”.  So, in other words, because warm, moist air is trying to move through Oklahoma, fog continues to be created.  The thicker the fog becomes, the less sunlight is able to reach and warm the ground.

For the past few days, the models were accurately predicting a warm up because of this warm, moist air moving through our area.  Unfortunately, the models underestimated the amount of snow Oklahoma received last Friday and therefore handled the ground temperatures rather poorly.  Because of this, the models weren’t predicting the fog that developed.  Because the models weren’t predicting the fog, the models have been forecasting higher temperatures than we’ve been having.  As long as we continue to have warm, moist air moving through Oklahoma, and there is snow on the ground, we will continue to have fog.  As such, we will continue to to be colder than the models predict.  The shortwave trough responsible for drawing the warm, moist air northward should move off to the east by tomorrow and this should lessen the amount of warm, moist air being drawn northward.  If this happens, we might see some sun tomorrow afternoon.  Unfortunately, a new shortwave trough will begin approaching the area late tomorrow.  As this shortwave trough approaches, the warm, moist will once again resume moving northward through Oklahoma – and the cloud/fog should return.  Thus, tomorrow afternoon will be our best chance for sun over the next few days…or until a majority of the snow melts.

The NWS is forecasting a high of 45 in Norman, OK tomorrow, along with some sunshine.  If we don’t see the sunshine, I’m willing to bet we will remain in the 30s.  By this time tomorrow night, we’ll know what happened…

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.

Two days ago I posted an image of the 5-day precipitation forecast from the National Weather Service’s (NWS) Hydrometeorological Prediction Center (HPC).  Tonight’s forecast is a little more sobering.  The San Gabriel mountains to the east and northeast of Los Angeles are forecast to receive over 11 inches of rain during the next120 hours.  This much rain in such a short period of time will result in widespread flooding and a high potential for mudslides in and around the greater Los Angeles area.  Everyone in southern California should be on high alert over the next week or so.

(Image courtesy from the NWS HPC.)

The previous couple of posts have alluded to a shift in the United States weather pattern from extreme cold in the east to extreme wet in the south.  Below is a 5-day forecast of rainfall amounts from the National Weather Service (NWS).  The heavy rain maker from last night’s post is continuing to move slowly east across the Gulf of Mexico.  As it moves back onshore in the southeast US, heavy rain will once again be possible, and is highlighted by the widespread 3-5 inch predicted rainfall amounts

The next storm system in the active southern stream is currently off the west coast about to batter California.  Widespread 3-5 inch rainfall amounts are possible in the lower elevations with higher elevations seeing upwards of 8-9 inches of liquid equivalent (in other words, if the precipitation falls as snow, the amount of water left over after melting the fallen snow).

As a good friend of mine recently stated, “the West Coast may have natural water parks by the end of next week.”

(Image courtesy from the NWS Hydrometeorological Prediction Center (HPC).)

Well, as the majority of the blog posts so far have indicated, most of the country is in the midst of an extended cold snap.  Unfortunately, it appears that the cold snap is about to get colder.  Today’s weather graphic is once again taken from the National Weather Service (NWS).  It is an image containing all valid NWS watches, warnings, and advisories that are currently in effect.   A good portion of the eastern two-thirds of the country has some sort of winter weather product in effect.  These range from a very small Blizzard Warning in eastern Montana (bright red) to a winter storm warning for much of western Iowa to a Hard Freeze Warning for a good portion of the southeast to a Freeze Warning throughout much of the Florida peninsula.

The good news is that the NWP models meteorologists use to help predict the weather are suggesting that Old Man Winter should lessen his icy grip on the US sometime next week.  We certainly won’t see summer like weather, but places that aren’t used to spending days below freezing should return to more normal winter temperatures.  Remember, a lot can happen between now and then, and the forecasts may change, but it should offer hope to those who are tired of the cold.

(Image courtesy of the National Weather Service.)

I hope you enjoyed the deviation from the cold air with yesterday’s post on severe thunderstorms and tornadoes.  Today, we’re back to highlighting the impending cold air outbreak.  Below is a graphic created from the Global Forecast System (GFS) numerical weather prediction model (NWP).  It is taken from the 12UTC (6 AM CST) run¹ from 4 January 2010 and is valid² at 12 UTC (6 AM CST) Friday, 8 January 2010.  In other words, this is a good approximation as to what the NWP model is predicting for overnight low temperatures Thursday night/Friday morning (for locations in central time zone).  The temperatures contoured are predicted 10 meters above ground level temperatures, which means locations where the winds are calm and the sky is clear, the surface temperature will most likely be even colder.

A couple of things to note about this forecast.

1. The temperatures are in degrees Celsius.  For those who don’t like math³, -30C = -22F ; -20C = -4F ;
   -10C = 14F ; 0C = 32F ; 10C = 50F ; 20C = 68F ; 30F = 86F.
2. The green color fills are the liquid equivalents for any precipitation that has fallen during the previous 6 hours⁴.
3. Almost everywhere is below 0C (32F).  The only places where this isn’t the case is the west coast, extreme southern Texas, and the peninsula of Florida (which will most likely experience below freezing temperatures the following night).

(Image Courtesy of the NWS National Centers for Environmental Prediction Central Operations)