Daily Archives: December 21, 2012

Possible Christmas Snow Storm – Friday Evening Update

Highlights

  • Confidence in a Cold Air Outbreak in Texas by Christmas Afternoon is high
  • Confidence in light snow occurring in Texas is moderate.
  • Confidence WHERE in Texas snow will fall is low.
  • Confidence in accumulating snowfall in Texas is low.
  • We will begin to get better weather model consistency on Saturday, at least that is the hope.

Discussion

We’ve been talking about it since last weekend, but even now we still have a great deal of uncertainty about the possible Christmas winter storm. We are confident that there will be another cold air outbreak that will make temperatures quite chilly by Christmas evening (sooner in northern parts of Texas) with some rain possible on Christmas Eve along and east of Interstate 35. By Christmas morning, cooler air will be filtering into Texas. That will allow rain to change over to snow in portions of the Texas Panhandle, Northwest Texas, North Texas, and Northeast Texas. Exactly where the rain/snow line sets up is debatable, but certainly not exactly predictable. Remember, we’re still over 84 hours out from this potential event. This storm has not yet made it into the United States Upper Air Network nor has the system entered the range of our high-resolution weather models. Everything you see below will change at least once over the weekend, so be sure to check back for updates. I plan to release my own personal thoughts on the Christmas Event once we get into Saturday and we receive more data about the system. For now, I’m going to post the forecast graphics from several of the National Weather Service offices in the affected regions.

Graphics

NWS Amarillo

NWS Amarillo

NWS Norman/Oklahoma City

NWS Norman/Oklahoma City

NWS Fort Worth

NWS Fort Worth

NWS Tulsa

NWS Tulsa

NWS Shreveport

NWS Shreveport

Special Statement for Christmas Storm in North Texas

I’ll have a complete forecast update later tonight explaining the latest trends with the weather model data and why it may mean more snow for North Texas. The National Weather Service in Fort Worth just issued their first Winter Weather Outlook for the possible Christmas Storm. Here is their statement…

 

Winter weather outlook for December 25th…

A strong storm system is expected to move over North Texas on
Christmas day. While this system has been forecast to affect the
region for several days… confidence is increasing that wintry
precipitation associated with this system will fall over portions
of North Texas on Christmas day. Based on the latest forecast
models… the track of this system will move over the Interstate 20
corridor. Assuming this track verifies… this would support a
change over from rain to snow with some snow accumulations for
locations generally along and north of the Interstate 20 corridor
during the afternoon and evening hours on Christmas day. Just
south of the Interstate 20 corridor… a mix of rain and snow will
be possible with little to no snow accumulations possible.

At this time snowfall is expected to be brief… and may only
persist for a few hours. Currently our forecast calls for less
than 1 inch of snowfall for locations along and north of the
Interstate 20 corridor. As the change over to snow
occurs… northwest winds are expected to increase in intensity to
20 mph gusting to 30 mph. Any snow accumulation will blow around
in these winds… which may reduce visibility if traveling after
sunset on Christmas day.

As of this afternoon… this upper level storm system remains over
the open waters of the North Pacific Ocean. There remains a great
deal of uncertainty regarding the exact track and any associated
wintry precipitation falling over North Texas on Christmas day at
this time.

A more northerly track to this system will likely result in little
to no wintry precipitation over North Texas on Christmas day.

A more southerly track to this system could result in a change in
the areal coverage and possible increase in snow accumulations
across North Texas on Christmas day.

Bottom line… wintry precipitation is looking more likely along
and north of the Interstate 20 corridor for the afternoon and
evening hours of Christmas day. Check back for the latest forecast
updates in the coming days as these details are very likely to
change in future forecasts.

Cavanaugh

Christmas Snow for North Texas?

It’s still a non-zero chance…which means there IS still a chance for snow to fall in the northern parts of the state for Christmas.  Nothing is certain yet, but it’s not been completely ruled out at this time.

I wanted to share with you a wonderful educational discussion posted by the National Weather Service office in Ft. Worth yesterday evening.  In it, they break down the processes that would need to be in place with this next system for snow to develop.  It’s very interesting and not extremely nerdy.  I did include some definitions in parenthesis where appropriate.  It’s worth a read!

SINCE IT IS A QUIET NIGHT…ITS WEATHER CLASSROOM AND LAB TIME FOR THOSE INTERESTED… THE CHRISTMAS NIGHT FORECAST IS ON EVERYONE/S MIND. WE HAVE BEEN WATCHING THIS SYSTEM IN THE EXTENDED MODELS FOR SEVERAL DAYS…AND CONTINUE TO RIGHTLY ASSESS THAT A POTENTIAL FOR A WINTER EVENT EXISTS. UNFORTUNATELY, MODEL GUIDANCE DOES NOT HAVE THE SKILL IN THAT TIME RANGE TO ACCURATELY PREDICT THE INTENSITY AND LATITUDE THE UPPER LEVEL SYSTEM WILL TAKE BECAUSE OF THE UNCERTAINTY ON EXACTLY WHERE A COLD FRONT WILL BE ON TUESDAY.  AS WE HAVE SAID IN THE LAST 2 AFDS (area forecast discussions)…THIS COLD FRONT IS THE KEY TO THE FORECAST. BUT TO UNDERSTAND WHY…IT IS HELPFUL TO DISCUSS BAROCLINICITY AND ITS AFFECTS ON THE MOVEMENT AND STRENGTH OF WEATHER SYSTEMS.

ESSENTIALLY THESE STORM SYSTEMS ARE BLOBS OF UPPER LEVEL ENERGY…OR VORTICITY…OFTEN CALLED SHORTWAVES OR DISTURBANCES. IN AN ATMOSPHERE WHERE THERE IS VERY LITTLE TEMPERATURE DIFFERENCE FROM LOCATION TO LOCATION…THE MOTION OF THESE DISTURBANCES IS BASICALLY DICTATED BY THE FLOW THEY ARE A PART OF. THEY BEHAVE MUCH LIKE A LEAF WOULD IF IT WERE FLOATING DOWN A RIVER. HOWEVER WHEN HORIZONTAL TEMPERATURE GRADIENTS EXIST…SUCH AS WHEN A COLD FRONT IS NEARBY…THIS SIMPLE RULE GOES OUT THE WINDOW. THIS IS CALLED A BAROCLINIC ATMOSPHERE. A DISTURBANCE THAT MOVES INTO A BAROCLINIC ZONE WILL INTENSIFY. THIS IS BECAUSE LOW LEVEL TEMPERATURE ADVECTION (Transfer of heat by a fluid such as water vapor) BEGINS TO INTERFACE WITH THE UPPER LEVEL SYSTEM. IN SHORT…THERMAL ADVECTION IN THE LOW LEVELS CAUSE HEIGHT CHANGES (Height =  the height in meters above sea level of a specific air pressure level such as 500 millibars of air pressure) IN THE UPPER LEVELS…WHICH IN TURN STRENGTHEN THE UPPER LEVEL VORTICITY CENTER…WHICH IN TURN STRENGTHENS THE LOW LEVEL THERMAL ADVECTION…AND ETC. THE WHOLE THING BECOMES A FEEDBACK LOOP. AN UPPER DISTURBANCE UNDERGOING BAROCLINIC INTENSIFICATION WILL TRANSFORM INTO A CYCLONE AND WILL NO LONGER MOVE WITH THE MEAN FLOW…BUT WILL CURVE TOWARD THE POLE. IN THE PROCESS OF TURNING TOWARD THE POLE THE CYCLONE OFTEN OCCLUDES…MEANING THEY LOSE THEIR ACCESS TO THE TEMPERATURE GRADIENTS AND THEY WEAKEN.

WE HAVE ALREADY SEEN MANY MODEL RUNS OF A BIG WINTER CYCLONE DEVELOPING OVER THE SOUTHERN PLAINS AND CURVING OFF TO THE NORTH. IN THE LATEST MODELS WE NO LONGER SEE THAT SOLUTION AND SEE A WEAK TROUGH MOVING OVERHEAD. THE REASON WHY THE MODELS HAVE BEEN ALTERNATING BETWEEN A POWERFUL WINTER CYCLONE IN THE PLAINS AND A WEAK DISTURBANCE HAS BEEN DUE TO THE LOCATION OF THE BAROCLINIC ZONE /OR COLD FRONT/ WHEN THE DISTURBANCE REACHES THE SOUTHERN PLAINS. THE TREND IN MOST OF THE MODELS HAS BEEN TO MOVE THE FRONT/BAROCLINIC ZONE INTO SOUTH TEXAS. AS A RESULT THE UPPER LEVEL DISTURBANCE TRAVELS OVER TEXAS WITHOUT STRENGTHENING BECAUSE THE TEMPERATURE GRADIENTS ARE WEAK SO FAR BEHIND THE FRONT. THE SYSTEM THUS DOES NOT INTENSIFY UNTIL IT REACHES THE NORTHERN GULF COAST WHERE A BAROCLINIC ZONE EXISTS. THIS SCENARIO…WHICH WE ARE CURRENTLY FORECASTING…IS ONE THAT WOULD ONLY PRODUCE LIGHT AMOUNTS OF WINTER PRECIP. THIS IS BECAUSE THE SYSTEM WILL BE WEAK AND HAS NO ACCESS TO THE THERMAL GRADIENTS IT NEEDS TO INTENSIFY. AN INTENSIFYING LOW WOULD BE ABLE AUGMENT THE WIND FIELDS AND THUS TAP INTO THE GULF FOR MOISTURE.

IN ORDER TO GET A SIGNIFICANT SNOW STORM HERE…WE WOULD NEED THE FRONT TO BE FARTHER NORTH THAN CURRENTLY FORECAST…BUT NOT SO FAR NORTH THAT THE SYSTEM BEGINS ITS POLEWARD TURN AND STAYS NORTH OF THE REGION ALTOGETHER. WE NEED A GOLDILOCKS POSITION OF THIS FRONT.

AGAIN…KNOWING THE PRECISE LOCATION OF A FRONT BEYOND 5 DAYS IS THE DIFFICULTY WITH THIS FORECAST. THANKFULLY THE LEAD SHORTWAVE RESPONSIBLE FOR THE LOCATION OF THIS FRONT WILL BE ONSHORE IN CALIFORNIA SATURDAY AND WILL BE SAMPLED BY THE BALLOON NETWORK. IN ADDITION…WE WILL BE IN RANGE OF THE NAM (North American Model) BY TOMORROW FOR HAVING ANOTHER OPINION ON THIS FRONT/S STRENGTH AND LOCATION MONDAY NIGHT AND TUESDAY…EVEN IF WE DONT GET AN EXPLICIT DEC 25TH FORECAST FROM THE NAM UNTIL SATURDAY.

I’ve included some graphics below from the Ft. Worth, Amarillo and Lubbock weather offices that were produced after their own individual analysis of this potential system.  All this to say…don’t give up hope for some snow just yet!

Christmas snow

NWS Amarillo snow

NWS Lubbock snow

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