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

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544
FXUS64 KFWD 201813
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
1213 PM CST Tue Feb 20 2018


.AVIATION...
/18Z TAFS/

Widespread showers and a few thunderstorms are in progress across
North Texas ahead of an approaching strong cold front. The cold
front is expected to move through this evening in the 5-7 pm
timeframe in the Metroplex and around 6-8 pm at Waco. Ahead of the
front, persistent MVFR cigs around 2000 ft will prevail with
scattered showers and a few embedded thunderstorms. We`ll carry a
TEMPO for TSRA this afternoon from 20-23Z when surface based
instability may be maximized. The current forecast will show the
front coming through the Metroplex around 23Z with an abrupt wind
shift to the north. There will continue to be a chance for
thunderstorms during this time, although showers should become the
prevailing precip type as the cold air gets a little deeper. A
period of IFR cigs is likely to occur post frontal. There may be a
brief lull in the precipitation late tonight prior to another wave
of ascent spreading across the region Wednesday. Additional areas
of showers and a few thunderstorms will be possible through the
day.

There is some chance for freezing rain or sleet mainly to the
northwest of the Metroplex as temperatures drop late tonight and
especially on Wednesday. For now, we`ll keep the mention of any
FZRA out of the current forecast, but will continue to monitor
through the evening.

Dunn

&&

.UPDATE... /Issued 1105 AM CST Tue Feb 20 2018/
Showers and thunderstorms continue to train from southwest to
northeast, with the focus so far this morning being along and
west of the Interstate 35 corridor. Additional development is
expected as a cold front pushes in from the northwest. With the
main upper trough remaining off to the west, we will continue to
see good lift and widespread precipitation over the next 24 to 36
hours. Some of this activity can already be seen redeveloping off
to the west where the strongest ascent is currently located. The
main flood threat will eventually shift farther east along the Red
River as the front moves in, but will continue to extend back to
the southwest along the boundary in a region of strong forcing.
Based on RADAR trends this morning and new guidance, the Flood
Watch has been expanded farther west to include the Gainesville,
Stephenville and Comanche areas.

In addition, some of the new hi-res guidance is coming in colder
and indicating sub-freezing temperatures across the west and
northwest counties late tonight and through the day Wednesday.
More details will be ironed out with the afternoon package, but
feel it was prudent to at least include some light freezing rain
to the forecast with this update, beginning late tonight or
Wednesday morning for areas north and west of the Metroplex.

30

&&

.SHORT TERM... /Issued 352 AM CST Tue Feb 20 2018/
/Today and Tonight/

A rather complex forecast will evolve today and tonight as a
drawn-out active/unsettled weather pattern begins across the
region. In the immediate term, the first wave of ascent is
arriving early this morning out of the Concho Valley, and this
has helped to initiate a cluster of showers and thunderstorms
which will continue to rapidly push north and eastward within
the fast southwesterly flow aloft. Effective deep layer shear
values are running on the order of 40-50 kts, and this has been
supporting persistent mid-level rotation in these storms with high
velocities showing up aloft. Thus far, this enhanced momentum
seems to be having a difficult time making it down to the surface
and high-res guidance insists on a gradual weakening trend, but
this activity may still pose a strong and gusty wind and small
hail threat as it races towards the Metroplex through the 4-6 AM
time frame. Anticipate that additional convection will fire in
northeast to southwest oriented bands through the morning hours,
aligned roughly along and just west of the I-35 corridor where the
highest PoPs have been painted. Some locally heavy rainfall and
associated isolated hydrologic issues are possible with this
initial activity along the I-35 corridor, but most of the shower
and thunderstorm activity should move along at a decent enough
clip to preclude any substantial issues.

During the afternoon hours, the sharp arctic front, which is
currently draped just north of I-40/44 in Oklahoma (and actually
this front is slowly easing back north and west as a warm front at
this hour!) will begin to make its notable southward push. The
faster NAM, 3km NAM, and RAP output was generally followed for
the front timing given the much colder (and denser) airmass
analyzed across far northwest Oklahoma and southern Kansas than
earlier guidance had indicated. With this faster guidance, the
initial north wind shift is forecast to arrive across our
northwest counties around 4 PM, into the Metroplex around 6-8 PM,
and then through our southeastern counties by daybreak Wednesday.
Additional showers and storms should blossom ahead of the surging
cold front late this morning and through the afternoon, and storm
training may begin at this point as upwind corfidi vector
velocities decrease to under 5-10 kts along with boundary-parallel
cloud-bearing flow. PWAT values are forecast to be in the 1.6 to
1.8 inch range, which are easily records for the date, and would
even be near-record values by April and May standards. The
probability-matched mean QPF products from the HREF indicate the
axis of heaviest rainfall into the early evening hours stretching
from near I-35/35W to a Paris to Hillsboro line which seems
reasonable and this appears to be well represented by the current
Flood Watch.

The other threat to contend with today will be the potential for
some strong to perhaps severe storms in the warm sector this
afternoon and evening. Forecast soundings ahead of the front show
a thermodynamic/shear space which could support storms with a
damaging wind or even an isolated tornado threat where surface-
based instability resides (south and east of a Sulphur Springs to
Hamilton line). Given the surging nature of the front, it`s
possible any initially surface-based convection gets undercut, but
the presence of very strong 0-1 km shear is a concern, especially
if the front hangs up a bit more than forecast.

Finally, an additional wave of warm advection/isentropic ascent
is forecast to materialize late tonight across the Concho Valley.
This should lead to the development of additional showers and
perhaps some isolated storms above the shallow frontal inversion
across our western counties. Temperatures here will be falling
through the 30s, and am growing a bit more concerned about some
freezing rain potential developing towards daybreak on Wednesday
north and west of an Eastland to Gainesville line. Each subsequent
NAM/SREF run is coming in colder, and forecast soundings are
showing steepening 850-600 mb lapse rates facilitating increasing
MUCAPEs late tonight supporting the potential for elevated
convection. With the exceptionally warm air just off the surface
(+12 to +14 C at 900 mb), any heavier convection may drag this
warmer air to the surface, reducing potential impacts from any
freezing rain, but this threat will continue to be scrutinized
very closely today.

Carlaw

&&

.LONG TERM... /Issued 352 AM CST Tue Feb 20 2018/
/Wednesday through Monday/

The cold front will have moved through the southeastern parts of
the forecast area as of 6 am Wednesday. Lift over the frontal
boundary will be on-going. This along with the upper level trough
remaining across the western United States, will result in
continued chances of rain. The heaviest rain should shift to the
east of the forecast area by Wednesday afternoon but some light to
occasionally moderate rain will continue area wide Wednesday
afternoon. With the cold advection behind the front, cloud cover
and precipitation, temperatures will not rise much, if at all,
during the day. After early morning lows in the lower 30s
northwest to the upper 50s southeast, daytime highs will only be
in the mid 30s northwest to the lower 60s southeast.

Rain chances should decrease some Wednesday night, especially
across the west as we await the next shortwave. However, some
chances will remain and as temperatures will be near or slightly
below freezing across the northwest, have included a mention of
rain of freezing rain for the overnight Wednesday night into the
early morning Thursday period for locations along and northwest of
an Eastland to Bowie line. Fortunately temperatures are expected
to be 30-32 degrees and given how warm we have been, no
significant icing on roads is expected at this time. Rain chances
will increase Thursday afternoon and Thursday night. Lows
Wednesday night will range from around 30 northwest to the mid 40s
southeast. Highs Thursday will range from the lower 40s northwest
to the mid 60s southeast.

Chances of showers and isolated thunderstorms will continue
Thursday night through Saturday. As the upper level trough over
the Southwestern United States moves eastward across the Rockies,
a lee side low will develop. In response to the developing low,
surface winds across the forecast area will come around the south
Friday. Overnight lows Thursday night will range from the upper
30s northwest to the upper 50s southeast and highs Friday will
range from the upper 50s northwest to around 70 degrees southeast.
The upper level trough will finally move east out of the Rockies
and across the Plains Saturday. As this system moves east, rain
chances should end across all but the extreme eastern zones by
Saturday night.

58

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    37  42  38  51  47 /  90  90  50  40  80
Waco                42  49  40  54  48 /  90 100  50  50  70
Paris               40  46  43  55  49 /  90 100  70  40  80
Denton              35  39  36  49  44 /  70  90  50  40  70
McKinney            36  43  39  51  47 /  90  90  50  40  80
Dallas              37  44  40  52  48 /  90  90  50  40  80
Terrell             39  47  42  55  50 /  90 100  60  40  80
Corsicana           42  51  44  56  51 /  90 100  70  40  80
Temple              44  51  41  54  50 /  90 100  50  50  70
Mineral Wells       34  38  33  46  41 /  50  90  40  60  60

&&

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

&&

$$



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