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

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FXUS64 KFWD 210600 AAB
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1200 AM CST Wed Feb 21 2018


.AVIATION...
/06z TAFs/

The next wave of ascent appears to be moving off the high terrain
of Coahuila near Del Rio, Texas late this evening. Broad warm
advection continues ahead of this feature which is resulting in
continued showery activity occasionally impacting area terminals.
While most activity has been slowly weakening as it`s been
approaching the Metroplex TAF sites, just enough elevated
instability remains for a few embedded storms for a few more hours
before this instability aloft decreases (briefly). Recent hi-res
guidance is in fairly good agreement that this aformentioned
increased ascent will result in the development of another rather
widespread area of showers late tonight which will impact the
Metroplex terminals into the mid-morning hours. The latest
indications are for this next batch of precipitation to gradually
clear the Metroplex towards 15z. This is important because
surface temperatures by this time are forecast to approach and
fall below the freezing mark, especially at KAFW and KFTW. Since
sufficient evidence exists to suggest that a gradual drying trend
may take place, have left out the mention of -FZRA in this TAF
issuance. However, should this precipitation hang on longer than
currently anticipated, impacts from some light freezing rain will
be possible, potentially even at DFW perhaps during the 14-17z
time frame.

After a brief lull in activity, another disturbance is forecast to
ripple overhead along with increasing elevated instability once
again with SHRA and VCTS Wednesday afternoon. As this batch clears
the Metroplex sites, low-level saturation and lingering weak lift
down low may support drizzle and mist through the overnight hours.
At this time, temperatures Wednesday night are forecast to be just
above freezing.

At Waco, overall similar trends are anticipated with the same
slug of moisture and lift resulting in the development of
additional showers--perhaps with more in the way of embedded
lightning--overnight. Here, temperatures aren`t expected to be as
much of an issue with the freezing line staying off to the north
and west.

Overall, IFR cigs are anticipated through the forecast period
right around FL008-010, but periodic fluctuations to low-MVFR will
be possible.

Carlaw

&&


.MESOSCALE UPDATE... /Issued 634 PM CST Tue Feb 20 2018/

A strong cold front continues to make southward progress and is
located roughly from Comanche to Paris as of 6pm. Initially, some
drying should occur farther behind the front while showers and
thunderstorms continue immediately along and behind it. Areas
from Graham to Bowie have seen a marked decrease in rain coverage
in the last hour and this should generally be the trend for
locations more than ~50 miles behind the front. Temperatures in
these locations, however, have fallen to near freezing as shallow
arctic air continues to funnel down the southern Plains. At this
point, the current forecast temperatures and Advisory
configuration for potential freeing rain overnight still are on
track. We`ll continue to assess temperature trends in guidance
through the next several hours, but at this time, there is no
indication the Advisory needs to be expanded southeastward. After
about midnight, a stronger wave of ascent should pivot through
the upper trough to our west. Isentropic lift within the 650-750mb
layer along with steepening lapse rates of as much as 6-7 C/km
should support some convective rain showers where temperatures
will have fallen to near or perhaps just below freezing. With
100-300 J/kg of MUCAPE available in this area, can`t rule out some
thunder occurring as well. If convective freezing rain occurs
with temperatures in the upper 20s, bridges and overpasses will
quickly deteriorate overnight and into Wednesday morning.

To the east and south of the current Winter Weather Advisory,
temperatures are expected to remain just above freezing overnight.
The cold front will have cleared our southeastern counties by
midnight or shortly after. However, the threat for heavy rain and
some flooding will continue in these areas as activity
intensifies with the aforementioned wave of ascent. Much of the
forecast area has already received 1-2" of rain with isolated
spots closer to 3-4". As continued waves of rainfall arrive into
Wednesday, the threshold for flooding will be decreased due to
soils becoming saturated. Additional river flooding and isolated
instances of flash flooding will be the primary concern for these
areas.

-Stalley

&&

.SHORT TERM... /Issued 251 PM CST Tue Feb 20 2018/
/Tonight/

Multiple shortwaves continue to work their way around a parent
trough located across the Rockies, generating enough lift to
produce numerous showers and storms across the North and Central
Texas. The axis of heavier precipitation has thus far remained
over the western two-thirds of the forecast area. Convection is
currently beginning to become focused along a cold front currently
making its way into the northwestern zones. The axis of heavier
precipitation will shift to the southeast tonight as the front
pushes southeast through the forecast area. A few strong to
marginally severe storms will remain possible due to good shear
and modest instability, but the focus will shift to heavy rain and
possible flooding over the next 24 hours.

Another issue that we have been monitoring over the past few days
is the amount of cold air present behind the front. Judging by
upstream observations, it is one again appearing that the NAM has
done a better job depicting the cold air within the shallow arctic
airmass. The latest temperature forecast has leaned toward the
colder NAM, which indicates sub-freezing temperatures across the
northwest counties overnight and through most of the day
tomorrow. The threat for a switch-over to freezing rain is
looking more likely overnight and Wednesday for areas to the west
and northwest of the Dallas-Fort Worth Metroplex. A Winter Weather
Advisory has been issued for areas along and northwest of a line
from Gainesville to Weatherford to Eastland, where freezing rain
accumulations between a hundredth and a tenth of an inch will be
possible late tonight through Wednesday.


&&

.LONG TERM... /Issued 251 PM CST Tue Feb 20 2018/
/Wednesday through early next week/

Travel west on I-20, northwest on Highway 287 and north on
Interstate 35 could become difficult at times by midday Wednesday.
The advisory is set to expire at 6 PM Wednesday, but may need to
be extended if future trends suggest that cold air and precipitation
will linger beyond that time. In addition, if later guidance
trends colder farther south, we may see some minor impacts creep
into the Metroplex during the morning hours Wednesday. Will leave
it to the evening and overnight shifts to monitor temperatures
and adjust the advisory area if it becomes necessary.

Meanwhile, the cold front will continue sagging southeast,
reaching the southeastern counties by Wednesday afternoon. The
focus for the heaviest precipitation will likewise shift east of
I-35 and south of I-30. It may become necessary to remove western
counties from the Flash Flood Watch as the heavier precipitation
moves east. However, due to the presence of strong ascent
associated with the Rockies trough, moderate rain from elevated
convection will remain possible, and at this time the Watch is
set to continue for all counties along and east of a line from
Gainesville to Stephenville to Goldthwaite.

The Flood Watch is set to expire at midnight Wednesday night
after one of the stronger shortwaves lifts northeast of the
region. The cold front will have briefly become stationary over
southeast Texas, but should begin to lift northwest as a stronger
shortwave swings down through the Desert Southwest and a lee-side
surface cyclone develops. So after a lull in precipitation
Wednesday night and Thursday, showers and storms will ramp up
again Thursday night and Friday as strong lift arrives from the
west. The surface boundary will again provide added focus for
development, and some locally heavy rainfall may again be a
possibility. We will likely need to look into the possibility of
another Flood Watch sometime between late Thursday and late
Saturday.

The main trough axis is progged to finally move east across the
Plains over the weekend. Precipitation will come to an end from
west to east late Saturday or Sunday. A few days of dry and
seasonable weather is likely Sunday and Monday, followed by
another system approaching around the middle of next week.


30

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    35  36  36  51  47 / 100  90  50  70  80
Waco                42  42  39  52  48 /  80  90  40  60  70
Paris               39  42  42  55  50 / 100  90  70  70  90
Denton              34  35  35  50  44 / 100  80  50  70  80
McKinney            36  36  36  51  47 / 100  90  50  70  80
Dallas              37  37  37  52  48 / 100  90  50  70  80
Terrell             40  40  40  55  49 / 100  90  60  60  80
Corsicana           41  42  41  55  51 / 100 100  60  60  80
Temple              42  44  39  53  49 /  90  90  40  60  70
Mineral Wells       31  33  32  47  40 / 100  80  40  70  60

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Wednesday evening for TXZ092>095-102>107-
116>123-130>135-143>148-157-159>161.

Winter Weather Advisory until 6 PM CST Wednesday for TXZ091>093-
100>103-115>117-129.

&&

$$


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