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FXUS64 KFWD 150508

Area Forecast Discussion
National Weather Service Fort Worth TX
1108 PM CST Thu Dec 14 2017

VFR conditions will prevail through the TAF period. North winds
around 10 kts or less will continue overnight and through most of
Friday. However, a return to light south winds will occur Friday 
evening, likely around 03z. Otherwise, a plume of subtropical high
cirrus will continue to stream over the region within a strong
upper jet streak.



.SHORT TERM... /Issued 341 PM CST Thu Dec 14 2017/
/Through tonight/
North winds have pushed deep into South Texas, but even with the
breezy conditions behind last night's front, the cold advection 
has been weak. The sunshine and dry air has helped daytime 
temperatures to effectively moderate, but even across Central 
Texas where cirrus has blotted out the sun, afternoon temperatures
are similar to those from 24 hours earlier. While temperatures 
are certainly cooler than yesterday's across the bulk of North
Texas, many locations reached the 60s, which is above normal for 

The plume of cirrus will remain primarily across South Texas, but
as an upper trough approaches from the northwest tonight, mid and
high clouds will increase. The dense clouds across Central Texas
will keep temperatures in the upper 30s and lower 40s tonight. But
across the northern half of the area, the clouds are unlikely to
inhibit radiational cooling, and Friday morning low temperatures
will generally be in the lower to mid 30s. Some areas across the
north and west will dip below freezing.



.LONG TERM... /Issued 341 PM CST Thu Dec 14 2017/
Friday will be a pleasant day with some sunshine through the high
clouds and seasonable temperatures. While humidity will once 
again drop below 30% area wide, wind speeds will be fairly light 
and limit the wildfire danger. That's not to say that fires won't 
start easily, but their rate of spread should be slow enough for 
fire fighters to contain.

The upper level low near Baja today will head eastward toward the
region on Saturday as another upper wave drops down the Pacific 
coastline. Ahead of the upper level low, southerly winds will 
increase and eventually bring Gulf moisture northward. Saturday 
will start out clear and chilly, but low and mid clouds will 
overspread from south to north during the morning hours with winds
increasing to near 15 mph. Temperatures will stall in the mid 50s
over the southern zones to near 60 over the north for highs. With
winds increasing and humidity once again dropping below 30%, it 
raises the concern for wildfires, particularly across the NW zones.
At this time it appears that the arrival of cloud cover and fairly
cool temperatures will keep conditions from getting critical. We 
will not be issuing any fire weather watches. 

Rain should begin to develop over Central Texas by Saturday
afternoon and spread northward into North Texas during the evening
hours as strong isentropic lift organizes. While the coverage of
this rain will be high, especially east of I-35, the amounts look
to be on the light side with an average of less than 0.10" of an 
inch in the western zones ranging to 0.75" in the far southeastern
zones where the better moisture/lift/instability will be located.
The predominant character of the precip will be light showers, but
a few isolated thunderstorms with moderate rainfall may be possible.
All of the rain should quickly end early Sunday morning with
clearing skies and temps warming into the 60s during the day.

The second upper level system dropping through the West Coast on
Saturday will move across our region in a weakened state late
Monday. Both moisture and lift look much weaker with this system,
but will carry a low chance of light showers over the eastern 
zones Monday and Monday night. Zonal upper level flow will prevail
across North and Central Texas Tuesday through Thursday with high
temperatures ranging from the 60s to lower 70s and low
temperatures in the mid 30s to mid 40s. 

The big weather story is the forecast leading up to Christmas.
For several days we've been seeing indications of a long-wave
pattern shift across North America with virtually all of the
guidance showing a flip from an East Coast trough to either a 
West Coast or Plains trough. The latter solutions certainly spell 
colder temperatures for our region. Model guidance is clustered 
around a strong frontal passage in about 1 week (Thursday night).
Upper level ridging across the Canadian Pacific coastline will 
induce strong anti-cyclogenesis and tap into a pool of arctic air 
and send it southward into the northern Plains. Just how much of 
this Arctic air makes it into our region is uncertain and highly 
dependent upon the exact configuration of upper level pattern and 
how it evolves. 

Over the past several days we've seen wild run to run variations 
of the upper level pattern so it is much too early to say with 
confidence just how cold it will get or whether there is a threat
of wintry precipitation. A stronger and slower evolving trough 
like what the ECMWF has been showing for several runs means a more
shallow and modified cold airmass will succeed the front for the 
22nd and 23rd. While a solution like the GFS which quickly digs 
the upper trough into the Desert SW would bring in a very shallow
but arctic airmass right behind the front. Both the GFS and ECMWF
forecasts raise our concern for wintry precipitation. The GFS 
continues to indicate a risk of freezing rain on the 23rd and the 
ECMWF is now indicating mixed precip on the 24th and 25th. 
Obviously those forecasts are 8-10 days out and we would be
shocked to see a perfect forecast from either model. However the
verifying forecast could be in that ballpark, or it could look 
like the Canadian and most of the ensembles means which are much 
faster and farther east with the upper level trough. This would 
bring a deep and very cold airmass into the region with clear 
skies and no chance of precip.

In summary, the key points: 
1) Confidence in colder weather arriving in about 1 week is high.

2) There is tremendous uncertainty regarding the post-frontal 
precipitation potential.

3) As always, more confidence will come with subsequent 
forecast data. The sooner all computer models settle on a 
solution the sooner we can provide more definitive forecast 
guidance. At this time, model spread remains MUCH too high to 
"sound the alarm" for a winter event.



Dallas-Ft. Worth    36  57  38  59  45 /   0   0   0  10  60 
Waco                37  58  34  56  43 /   0   0   0  30  70 
Paris               33  54  34  57  43 /   0   0   0  10  80 
Denton              30  56  31  59  43 /   0   0   0  10  50 
McKinney            32  55  32  57  44 /   0   0   0  10  70 
Dallas              38  57  41  60  45 /   0   0   0  10  60 
Terrell             33  58  34  57  44 /   0   0   0  20  70 
Corsicana           36  57  37  57  45 /   0   0   0  30  70 
Temple              40  58  35  54  44 /   5   0   0  40  60 
Mineral Wells       29  56  32  59  43 /   0   0   0  10  40 



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