Severe Thunderstorm Warning 8, NWS Fort Worth, TX

Area Forecast Discussion
National Weather Service Fort Worth TX
547 AM CST Sat Feb 24 2018

.AVIATION...
The primary aviation weather concerns this morning will be in the
first 6 hours of the TAF cycle with periods of rain and
thunderstorms and LIFR ceilings and visibilities at all sites.
Elevated thunderstorms were ongoing across the western Metroplex
TAF sites as of 12Z TAF issuance. These storms will reach DAL and
Waco between 12Z and 13Z. A more organized line of storms
(currently developing west of a Bowie to Eastland line)will move
quickly across all TAF sites a bit later this morning. These
storms will have a potential to produce some large hail and gusty
winds. The only good news is that the storms will be fast moving
and should clear all TAF sites by midday with rapidly improving
ceilings and visibilities during the afternoon. The only issue
this afternoon at the terminals will be a strong and gusty west
wind between 13 and 17 knots which may cause some crosswind issues
on north/south runways. Wind speeds will decrease around sunset
and winds will become more northerly.

VFR conditions are expected this evening and overnight with winds
generally less than 10 knots. There will be a potential for some
fog overnight due to clearing skies, light winds and a moist
ground, but wind speeds should stay up enough to prevent dense fog
from developing.

79

&&

.SHORT TERM... /Issued 328 AM CST Sat Feb 24 2018/
/Today and Tonight/

A surface warm front was analyzed snaking its way from near a
Paris to Sulphur Springs to Mexia to Temple line early this
morning, delineated more-or-less by the 60 degree isodrosotherm.
To the north and west of this feature, winds remain light and out
of the northwest with a completely saturated airmass in the lowest
two thousand feet or so. This is resulting in a combination of
widespread fog and drizzle across most of North and Central Texas.
Based on current observations and short-term model guidance, we
have elected to expand the Dense Fog Advisory a bit farther to the
north and west. We may need to add the remaining three
northwestern counties (Young, Jack, and Montague) at some point
depending on observation trends.

Radar mosaic imagery is already showing the development of another
wave of showers and embedded thunderstorms just to our south and
west. This activity seems to be associated with strong warm
advection in the 850-700 mb layer ahead of an intense vort max
barreling east of the Four Corners region at this hour. The TTWRF
and most recent 06z 3-km NAM seem to be handling this ongoing
activity the best, and show a continued expansion of thunderstorm
coverage across roughly the western half of the CWA through
sunrise. Recent data from the SPC mesoanalysis shows the MUCAPE
values are increasing past 1000 J/kg at this hour (above the
frontal inversion) aided by very steep mid-level lapse rates
which are approaching 7.5 to 8.5 C/km in spots. In addition,
effective deep layer shear values are increasing as a 130+ kt jet
streak is starting to impinge on our region. These values are
increasing past 55-60 kts across the Concho Valley. This parameter
space will easily support elevated supercells this morning,
developing perhaps as early as 5-6 AM as this activity continues
to intensify to our south and west. Large hail, perhaps to the
size of golfballs, will be the main threat with this initial
activity considering the steep mid-level lapse rates and very
strong shear. I can't rule out some stronger winds working their
way to the surface this morning, but the temperature inversion
remains quite robust, and it will take a good deal of momentum
transfer to realize strong-severe wind gusts here.

We will need to keep an eye on our south and eastern counties by
mid-morning and onwards as this initial wave of thunderstorms
starts to move towards the warm front. Storms may attempt to
become surface-based at this point roughly south and east of a
Sulphur Springs to Terrell to Temple line. As this occurs, the
threat for damaging wind gusts and even a few tornadoes will ramp
up. One mitigating factor here will be the time of day, as this
morning wave will not give much, if any, time for robust
destabilization out east. Still, an extremely impressive low-level
wind field continues to drive mid and upper 60s dewpoints into
our southeastern counties at this hour, and long, clockwise-curving
hodographs with 0-1 km shear values pushing 35-40 kts are causes
for concern.

An additional area of showers and storms may actually develop
behind this initial warm-advection-induced wave towards the mid-
late morning hours across our NW as mid-level heights really
begin to fall in advance of the incoming shortwave. How much the
atmosphere is "worked-over" from the morning activity is in
question, but this secondary activity will pose an additional
large hail risk near and west of the I-35 corridor into the early
afternoon. This wave of convection should tend to become more
linear with time and spread east of I-35/35E after about 12-1 PM
or so. As this activity presses east during the afternoon and
early-evening, the severe threat will depend on how much the
atmosphere has been contaminated by the morning wave. Given the
thermodynamic-shear space, all modes of severe weather will remain
possible, however.

We have set the flood watch to expire at noon today for the two
columns of counties west of I-35 as all the heavy precipitation
should be east by then, with the watch continuing through 4 PM to
the east. While this activity today will move at a good clip,
very heavy rainfall rates will be possible, leading to additional
instances of flooding.

Breezy westerly winds will materialize this afternoon as
convection clears the I-35 corridor, driving much drier air into
the region. In fact, many of us from the Metroplex and points west
will actually see the sun for the first time in many days!
Relative humidity values will fall into the 20-30 percent range
out west. This combination would normally result in an elevated
fire weather threat, but with the recent rainfall, the fire
weather potential appears low this afternoon.

As skies clear tonight, some radiation fog is possible given the
recent rains, but the signal was not strong enough to warrant an
addition of fog to the overnight forecast.

Carlaw

&&

.LONG TERM... /Issued 328 AM CST Sat Feb 24 2018/
/Sunday through Friday/

A few showers may redevelop across the southern portions of the
forecast area on Sunday as a shortwave moves east out of the
Central Rockies and lifts moisture over the stalled cold front
located near the the Texas coast. Precip chances will end Sunday
evening as the shortwave moves to the east.

Brief ridging aloft and dry air in the low levels of the atmosphere
will result in a mostly sunny and mild day on Monday with highs
in the 60s and lower 70s.

The ridge axis will move east of the region Monday night while
another low pressure system strengthens in the west. As a result,
low level moisture and large scale upward vertical motion will
both increase Tuesday through Wednesday, resulting in a chance of
showers and thunderstorms. The upper trough should move east of
the state Wednesday night and allow a cold front to push into the
region through Thursday, bringing drier and cooler weather
Thursday and Friday. There are some differences between the
medium range models regarding the speed of the upper trough and
cold front, so for now we will keep only chance PoPs in the
forecast.

Temperatures Tuesday through Friday will be near or above seasonal
normals with highs mainly in the 60s and lows generally in the
40s and lower 50s.

79

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    67  40  67  41  66 / 100   0   0   5   0
Waco                73  42  65  38  69 /  90   5   5   5   5
Paris               68  40  63  41  67 / 100   5   5   5   0
Denton              64  38  66  35  66 / 100   0   0   5   0
McKinney            65  37  65  38  65 / 100   0   0   5   5
Dallas              67  41  66  41  66 / 100   0   0   5   5
Terrell             69  41  65  41  67 / 100   5   5   5   5
Corsicana           72  45  66  42  67 /  90   5  10   5   5
Temple              73  43  65  40  70 /  80   5  20   5   5
Mineral Wells       67  36  68  33  69 /  90   0   0   0   0

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 4 PM CST this afternoon for TXZ093>095-103>107-
118>123-133-134.

Dense Fog Advisory until 9 AM CST this morning for TXZ092>095-
102>107-115>123-129>135-141>147-156>161.

Flood Watch until noon CST today for TXZ091-092-101-102-116-117-
130>132.

&&

79/90

Hazardous Weather Outlook
National Weather Service Fort Worth TX
408 AM CST Sat Feb 24 2018

TXZ091>095-100>107-115>123-129>135-141>148-156>162-174-175-251015-
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-
408 AM CST Sat Feb 24 2018

This Hazardous Weather Outlook is for North and Central Texas.

.DAY ONE...Today and Tonight.
Dense fog will be possible across most of North and Central Texas
through 9 AM. Visibility will be reduced to less than 1/2 mile in
many locations.

Thunderstorms are expected across the entire region today. Locations
along and west of the I-35 corridor will have the best chance of
seeing some severe storms through mid morning with large hail being
the primary hazard. The severe weather threat will continue through
the afternoon, with the most favored area shifting east of the I-35
corridor. Large hail, damaging winds and a few tornadoes will all be
possible. In addition, heavy rain will aggravate any ongoing
flooding. All thunderstorms will move southeast of the region this
evening.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.
Thunderstorm chances will return to all of North and Central Texas
Tuesday through Wednesday night. A few strong storms will be
possible.

.SPOTTER INFORMATION STATEMENT...
Spotter activation is possible today. The most likely time for
activation west of the I-35 corridor will be before noon with the
activation possible elsewhere generally from late morning through
the afternoon.

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