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

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FXUS64 KFWD 151731

Area Forecast Discussion
National Weather Service Fort Worth TX
1131 AM CST Fri Dec 15 2017

While flying weather is generally good today...we have received several
PIREPs of moderate to severe turbulence between FL170 and FL250.
This turbulence is associated with a potent jet stream which is
currently passing through northern portions of our forecast area.
This jet should move out of our area by evening...allowing for
this turbulence to diminish.

VFR is forecast to prevail through the TAF period. N`ly flow
conditions are in place across the region...with a transition to
S`ly flow expected late this evening as a trough of low pressure
develops across the Southern High Plains. Cloud cover will begin
to increase after 16/1500Z...but ceilings should remain VFR
through the TAF period. Winds will increase on Saturday as
well...though this is not mentioned explicitly in the TAFs as
winds should remain S`ly.

Just beyond the TAF period...expect chances for SHRA to increase
Saturday afternoon across Central Texas and spread into North
Texas by evening. As the rain overspreads the area...expect
conditions to deteriorate to MVFR...and possibly IFR for a time.
This rain should exit the area Sunday morning with conditions
improving throughout the day.



.SHORT TERM... /Issued 340 AM CST Fri Dec 15 2017/
/Today and Tonight/

Surface high pressure will settle over the state today and result
in dry and relatively calm conditions with a northwest wind in
the 5 to 10 mph range. The lighter winds today will keep the fire
threat relatively low even though fine fuels will remain abundant
and afternoon relative humidities will bottom out in the 20s and
lower 30s.

There will be a continuous feed of upper level moisture across
mainly the southern zones today as an upper low currently near
Baja lifts slowly northeast. These clouds will hinder daytime
warming a bit, but temperatures should still reach near seasonal
normals with highs generally in the middle and upper 50s.

High clouds will increase across all of North and Central Texas
tonight as the upper low inches closer. The high clouds should
slow cooling a bit, but a lack of low level moisture return will
still allow temperatures to fall into the 30s overnight.



.LONG TERM... /Issued 340 AM CST Fri Dec 15 2017/
/Saturday and Beyond/

The evolution of Saturday`s weather will be influenced primarily
by a small, but rather potent vort max--the remnant shear-axis
left over from a closed upper-low currently pinwheeling about
over the Gulf of California and Sinaloa, Mexico. By late Saturday
morning, this feature will be situated from the Texas Panhandle
and back into the Trans Pecos region of southwest Texas. Deep
southwesterly flow aloft ahead of this system will continue to
pump a surfeit of mid- and high-level moisture across the region,
and this will result in a steadily thickening and lowering veil
of dense cloud cover across North and Central Texas. The mass
response in the lower-levels to the approaching wave will also
help induce some gusty southerly winds, although things look to
remain safely below wind advisory criteria.

While thickening cloud cover will certainly curtail the degree of
insolation on Saturday, stout warm advection induced by the 15-20
mph southerly winds should still allow temperatures to rise into
the lower or possibly mid 60s across the northern half of the
region, with 50s more prevalent across the south. Since it
continues to look like dewpoints will not really begin to surge
northward until Saturday evening, afternoon relative humidity
values from I-20 and north may fall into the 25-35 percent range.
This dry air, coupled with the gusty winds may foster an elevated
or near-critical fire weather threat across this portion of the
region. While highs will be above-normal, we don`t feel a fire
weather watch is necessary at this time with temperatures just a
handful of degrees above climatological values.

By Saturday afternoon, top-down moistening and subsequent
increasing warm advection/isentropic ascent will result in the
development of the first appreciable chances at measurable
precipitation since the beginning of the month across our southern
counties. Forcing for ascent will continue to increase through the
evening hours, and this will spread precip chances across all of
our forecast area overnight. It seems quite likely that locales
east of I-35 will get in on a much-needed wetting rainfall before
this quick-moving system departs the region Sunday morning. While
most of the precipitation is expected to fall as light to moderate
rain showers, enough elevated instability will be present to
foster at least some isolated embedded thunderstorms. The most
vigorous and focused ascent will overspread the southeastern third
of the CWA late Saturday evening-overnight, where intense
frontogenesis may allow a band of moderate to heavy rain to
develop. While overall precipitation amounts of a half an inch or
less are expected, some lucky locations across our southeast could
pick up over an inch of rain by daybreak Sunday.

Drier and subsident air will quickly descend upon the region
Sunday morning which will bring a swift end to precipitation
chances from west-to-east. Another embedded vort lobe may bring
another quick shot at some showers to our southeastern counties
Monday into Tuesday, but moisture looks to be in shorter supply.

Temperatures through Thursday should manage to moderate into the
mid 60s to 70 degrees. The next feature of interest will be a
strong cold front which is set to plow through the region Thursday
evening and overnight. Interestingly, all of the available global
guidance, from the ECMWF to CMC to the GFS show this initial surge
of strong winds and colder air arriving all within about 6 hours
of one another. Some light precipitation appears possible across
our southeast as this front encounters better moisture, but this
will remain of the liquid variety at ground level with
temperatures well above freezing.

While a return to much colder conditions is appearing more likely
towards the end of next week, that`s pretty much where the higher
confidence stops. Agreement among the global guidance regarding
the pertinent positioning and strength of upper-level features
remains extremely volatile (from run-to-run). While several
previous runs of the GFS depicted a deep closed low over Baja
California, this evening`s run has no such feature at all,
replaced instead by robust and amplified ridging along the West
Coast. The ECMWF, on the other hand, has decided that trading
places with the GFS was in store, and is now the only piece of
global guidance to depict a deep cutoff Baja low.

None of this model volatility is all that unexpected considering
the nascent shortwave is still perhaps somewhere over the
Kamchatka Peninsula at this hour, and goes to show that relying
on individual model solutions beyond 7 days in these situations
is extremely unwise. While it`s certainly conceivable that some
wintry precipitation could materialize in the post-frontal airmass
next weekend, it remains nothing more than pure speculation at
this point.



Dallas-Ft. Worth    57  38  62  45  64 /   0   0  20  60   5
Waco                58  34  55  43  66 /   5   0  60  70   5
Paris               54  33  58  43  60 /   0   0  10  80  10
Denton              55  33  62  44  63 /   0   0  20  60   5
McKinney            55  33  60  44  62 /   0   0  20  70  10
Dallas              57  39  61  45  64 /   0   0  20  70   5
Terrell             57  34  58  43  64 /   0   0  20  90  10
Corsicana           58  36  55  44  64 /   5   0  40  90  10
Temple              58  35  54  44  67 /   5   0  60  70   5
Mineral Wells       57  32  62  42  67 /   0   0  20  50   0




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