Severe Thunderstorm Warning 218, NWS Fort Worth, TX

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
630 PM CST Wed Dec 26 2018

.AVIATION...
/00Z TAFs/

The widespread thunderstorms that dominated the mid to late
afternoon brought one final round to the Metroplex around 00Z
(6 pm CST). Although some showers are still upstream, the forcing
responsible for the deeper convection is passing north of the
I-20 corridor, and a brief lull will precede a line of storms
approaching from the west. The AWW will be allowed to expire on
time (0030Z or 6:30 pm CST), but another will likely be needed
after 01Z (7 pm CST).

Strong southerly flow is advecting MUCAPE values in excess of 2000
J/kg that will soon interact with the advancing linear complex.
In addition, an axis of enhanced mid-level lapse rates closer to
the I-35 corridor will also aide in the line's intensification and
a corresponding uptick in lightning activity. This squall line
should continue peeling up the buoyant surface layer, and the
downward transport of momentum from a powerful low-level jet will
maintain the potential for damaging winds. Although outside the
current tornado watch, embedded tornadoes will also remain a
threat as the line reaches the I-35 corridor.

The line is expected to enter Tarrant County after 01Z (7 pm CST)
and move rapidly eastward across the Metroplex TAF sites. A
similar, but slightly delayed, time line is anticipated for Waco.
The squall will feature westerly winds, but backing will occur
within the trailing precipitation. A rapid clearing after midnight
will maintain VFR conditions with southwesterly flow into the
daylight hours Thursday.

The surface front associated with this storm system will arrive
Thursday afternoon, shifting the winds to the west then northwest.
Although it may mimic a Pacific front, the postfrontal air will be
cP in origin and not starved of moisture. As a result, stratocu
may briefly clutter the sky after FROPA, but the deck should be
primarily scattered.

25

&&

.SHORT TERM... /Issued 135 PM CST Wed Dec 26 2018/
/Through Tonight/

Scattered thunderstorms are increasing in coverage this afternoon
along and west of the I-35 corridor, in response to a
strengthening low-level jet and warm-air advection regime
centered near the Big Country. General thinking is most of this
convection should remain rooted above a surface stable layer
for another hour or two. Still, will continue to monitor these
cells for any more notable surface-based characteristics, which
would suggest a quicker increase in the threat. Regardless, as
forcing for ascent with the primary shortwave trough (very evident
in satellite imagery over the Permian Basin) approaches North and
Central Texas, a line of storms is then expected to intensify as
it moves into our western counties around 6-8pm.

Ahead of this line, the strengthening low-level jet and
south/southeasterly surface flow will draw dew points in the lower
to possibly mid 60s northwestward through Central Texas towards
the Big Country. This should bring sufficient low-level and
mixed-layer CAPE (e.g., upwards of 1000-1500 J/kg of MLCAPE)
to primarily areas south of I-20 and west of I-35 to support the
development of surface based updrafts through the evening hours.
In turn, a threat of a few damaging wind gusts, marginally severe
hail, and a few tornadoes is expected with this line, especially
given rather stout low-level flow. Farther north (generally I-20
and points northward), decreasing surface theta-e should limit the
severe threat more. Still, a few isolated damaging gusts will be
possible and a couple of brief tornadoes cannot be ruled out
either during the late evening hours.

Storms should cross the I-35 corridor through about 8pm to
midnight, before reaching East Texas around 4am. The severe threat
should then come to an abrupt end with the departure of the line
to the east.

Picca

&&

.LONG TERM... /Issued 135 PM CST Wed Dec 26 2018/
/Thursday through Wednesday/
With the passage of the upper trough overnight, drier westerly
flow will prevail behind the Pacific front Thursday. Temperatures
will be near or even a little warmer than today compliments of
the downslope flow and sunny skies. Winds by the evening hours
will become more northwesterly as a surface high builds down in
through the Plains. This will begin a modest but persistent cold
advection regime over the region. Temperatures will gradually fall
and it will bring a few days of chilly weather. Friday's highs
will be in the mid 40s in the northwest zones to the mid 50s
southeast, and lows Friday night will range from the upper 20s
northwest to the upper 30s southeast.

By Saturday clouds and rain chances will increase as isentropic
lift sets up over the area in response to energy translating
through the Panhandles within a stationary positive tilt upper
level trough axis. Moisture will be a bit limited with this first
round of lift so the best rain chances, while still low, will be
across the southeastern half of the area. Another shortwave will
track through Texas in southwesterly flow aloft late Sunday and
we'll see more numerous showers, especially Sunday night. Again
the best chances of rain will be over the southeast half of the
CWA where moisture is greatest and PoPs will range from likely
there to slight chance in the northwest. Daytime temperatures will
be held down by the clouds and sporadic rainfall with 30s and 40s
for highs this weekend. Likewise the cloud cover will keep low
temperatures from cooling appreciably, so believe the threat for
any freezing rain is too low to mention. While one might normally
be concerned that we could see wet bulb cooling to force temps
below freezing, by this time wind speeds are nearly calm and
therefore it would be a situation where evaporative cooling is
minimal and warm raindrops are effectively transporting heat from
the cloud layer to the ground.

This round of rain will end during the day Monday from west to
east with the passage of the shortwave trough. Clearing skies and
drier weather will prevail through the end of the week.
Temperatures will continue to be near or slightly below normal.
New Years Eve and Day festivities look seasonably chilly, but
pleasant.

TR.92

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    53  64  37  51  34 /  90   0   0   0   0
Waco                51  67  39  55  36 /  90   0   0   0   0
Paris               53  66  36  50  33 /  90  10   0   0   5
Denton              50  63  34  48  32 /  90   0   0   0   0
McKinney            52  65  35  49  33 /  90   0   0   0   0
Dallas              53  66  37  51  35 /  90   0   0   0   0
Terrell             54  66  37  52  35 /  90   0   0   0   0
Corsicana           54  66  38  54  36 /  90   5   0   0   5
Temple              52  68  39  56  37 /  90   0   0   0   0
Mineral Wells       48  61  32  48  29 /  90   0   0   0   0

&&

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

25/05

Hazardous Weather Outlook
National Weather Service Fort Worth TX
217 PM CST Wed Dec 26 2018

TXZ091>095-100>107-115>123-129>135-141>148-156>162-174-175-271245-
Montague-Cooke-Grayson-Fannin-Lamar-Young-Jack-Wise-Denton-Collin-
Hunt-Delta-Hopkins-Stephens-Palo Pinto-Parker-Tarrant-Dallas-
Rockwall-Kaufman-Van Zandt-Rains-Eastland-Erath-Hood-Somervell-
Johnson-Ellis-Henderson-Comanche-Mills-Hamilton-Bosque-Hill-Navarro-
Freestone-Anderson-Lampasas-Coryell-Bell-McLennan-Falls-Limestone-
Leon-Milam-Robertson-
217 PM CST Wed Dec 26 2018

This Hazardous Weather Outlook is for North and Central Texas.

.DAY ONE...This Afternoon and Tonight.

Scattered to numerous thunderstorms are expected to further develop
west of the I-35 corridor this afternoon. The primary threat will be
locally heavy rain and generally small hail. Isolated marginally
severe hail cannot be ruled out, though.

The severe threat will increase early this evening as storms develop
across western North Texas and Central North Texas through 6-8pm.
There will be a threat of damaging winds, marginally severe hail, and
a few tornadoes, especially south of I-20 and west of I-35.

Strong to severe storms should push across the I-35 corridor during
the late evening hours, and then move into East Texas after midnight.
There will be a continued threat of severe weather, with the highest
potential remaining south of I-20. Localized flash flooding will be
possible as well.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.
A few storms may linger along and east of a line from Hearne to
Canton early Thursday morning, with lightning and localized flooding
being the primary threat.

.SPOTTER INFORMATION STATEMENT...
Spotter activation is likely south of I-20 and west of I-35 this
afternoon, and area-wide this evening and overnight.

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