Ramblings of a Graduate Student

As I mentioned last night, a substantial heatwave is forecast to grip a large portion of the country beginning this weekend and lasting into next week. Local National Weather Service (NWS) offices across the southern United States have already issued Heat Advisories or Excessive Heat Warnings in preparation. If you live across the southern half of the United States, please take this threat seriously. On average, more people are injured or killed by heatwaves every year than any other weather-related disaster. Please read the Center for Disease Control (CDC)’s webpage on Extreme Heat.

One last thing. Notice the “desert” southwest is blanketed with various forms of flood watches and warnings. This is fairly typical this time of year as the North American monsoon is in full swing. This is when a large portion of Arizona and New Mexico receive most of their yearly rainfall.

Last night I mentioned a possible high risk, high reward scenario for storm chasers on “the day before the day before”, also known as Wednesday.  Well, it turns out that the Storm Prediction Center is keeping an eye on the potential as well.  Above is this afternoon’s “Convective Outlook” for tomorrow (Wednesday).  Notice the words “See Text” over Oklahoma?  This means that while the threat of severe thunderstorms does not appear to be sufficient for a “Slight Risk” (there are spatial and frequency requirements for severe thunderstorms to justify a Slight Risk), the threat is certainly non-zero and the reader should read the outlook for more information.  The official text is below.

   DAY 2 CONVECTIVE OUTLOOK
   NWS STORM PREDICTION CENTER NORMAN OK
   1213 PM CDT TUE APR 20 2010

   VALID 211200Z - 221200Z

   ...NO SVR TSTM AREAS FORECAST...

   FAIRLY IMPRESSIVE UPPER TROUGH WILL MOVE ONSHORE ALONG THE CA COAST
   BEFORE MOVING INTO THE GREAT BASIN LATE WEDNESDAY.  DOWNSTREAM OF
   THIS FEATURE...UPPER RIDGING WILL GRADUALLY SHIFT EAST INTO THE
   CNTRL/SRN PLAINS AS HEIGHT FALLS SPREAD ACROSS THE SRN ROCKIES.  AT
   THE SURFACE...WEAK CYCLOGENESIS IN THE LEE OF THE SRN ROCKIES SHOULD
   DAMPEN OUT OVER THE TX PANHANDLE EARLY IN THE PERIOD AS LARGE SCALE
   FORCING BEGINS TO FOCUS ACROSS THE SRN ROCKIES IN ADVANCE OF UPPER
   TROUGH.  RESULTANT LEE TROUGH/DRY LINE SHOULD RETREAT INTO EXTREME
   ERN NM BY PEAK HEATING WITH AN E-W SURFACE TROUGH OR WARM FRONT-LIKE
   BOUNDARY EXPECTED TO EXTEND ALONG THE RED RIVER.  THE RED RIVER
   BOUNDARY WILL LIKELY SERVE AS THE DEMARCATION FOR EARLY-MID DAY
   CONVECTION DRIVEN IN LARGE PART BY WARM ADVECTION.  THIS
   ACTIVITY...POSSIBLY AN MCS-LIKE CLUSTER...SHOULD SPREAD ACROSS OK
   BEFORE WEAKENING OVER AR AS VEERED LLJ DECREASES AND BEGINS TO
   RESPOND TO WRN U.S. TROUGH OVER THE SRN HIGH PLAINS.  STRONG HEATING
   WILL LIKELY BE FOCUSED OVER THE PLAINS OF ERN NM/WEST TX SOUTH OF
   THE WARM FRONT AND FORECAST SOUNDINGS SUGGEST SFC-BASED PARCELS
   COULD REACH THEIR CONVECTIVE TEMPERATURES ACROSS UPSLOPE REGIONS OF
   THE HIGH PLAINS BY LATE AFTERNOON.  EVEN SO...DEEP LAYER SHEAR IS
   NOT EXPECTED TO BE PARTICULARLY STRONG...BUT VEERING PROFILES WITH
   HEIGHT DO FAVOR ROTATING UPDRAFTS...ESPECIALLY WHERE LOW LEVEL FLOW
   IS DECIDEDLY ELY ALONG/NORTH OF WARM FRONT.  AT THIS TIME WILL
   MAINTAIN 5% PROBABILITIES FOR PRIMARILY LARGE HAIL WITH ANY SLOW
   MOVING SUPERCELLS.  A SLIGHT RISK MAY NEED TO BE ADDED TO PORTIONS
   OF THE SRN HIGH PLAINS IN LATER OUTLOOKS IF IT APPEARS DIURNAL
   HEATING WILL CONTRIBUTE TO SUBSTANTIAL CONVECTIVE
   DEVELOPMENT...PRIMARILY ACROSS THE TX SOUTH PLAINS.

   ..DARROW.. 04/20/2010

The “See Text” area is outlined by the 5% risk level contoured above.  Notice how it is shaped very similar to the areas I discussed yesterday.

Lastly, the local National Weather Service Forecast Office in Norman, Oklahoma is also watching the same scenario very closely.  The image above was taken from their enhanced weather and graphicast webpage.

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).)