Area Forecast Discussion
Issued by NWS Dallas/Fort Worth, TX

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805
FXUS64 KFWD 142141
AFDFWD

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

.SHORT TERM...
/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
mid-December.

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.

25

&&

.LONG TERM...
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 difinitive forecast
guidance. At this time, model spread remains MUCH too high to
"sound the alarm" for a winter event.

TR.92

&&


.AVIATION... /Issued 1223 PM CST Thu Dec 14 2017/
/18Z TAFs/

VFR conditions will prevail with north flow.

North winds will prevail in the wake of last night`s cold front.
Gusts will occasionally top 20kts this afternoon. Wind speeds will
gradually diminish around sunset and remain around 10kts or less
through Friday. A surface ridge axis will finally move through the
region late in the day with southerly winds returning Friday
night.

As an upper trough approaches tonight, lift will steadily increase
in the mid-levels. Any precipitation will be confined to the core
of the subtropical plume across South Texas, but mid clouds should
be on the increase, particularly across Central Texas. Waco will
remain VFR, but a broken deck above FL100 may be introduced with
subsequent TAFs.

25

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    37  57  38  59  45 /   0   0   0  10  60
Waco                37  58  34  56  43 /   0   0   0  30  70
Paris               34  54  34  57  43 /   0   0   0  10  80
Denton              32  56  31  59  43 /   0   0   0  10  50
McKinney            33  55  32  57  44 /   0   0   0  10  70
Dallas              38  57  41  60  45 /   0   0   0  10  60
Terrell             34  58  34  57  44 /   0   0   0  20  70
Corsicana           36  57  37  57  45 /   0   0   0  30  70
Temple              39  58  35  54  44 /   5   0   0  40  60
Mineral Wells       31  56  32  59  43 /   0   0   0  10  40

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$



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