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

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FXUS64 KFWD 231234
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
634 AM CST Fri Feb 23 2018


.AVIATION...
An extended period of IFR ceilings/visibilities is expected at
all TAF sites through sunrise Saturday as moisture remains both
above and below the frontal inversion.

Numerous showers and thunderstorms will impact all TAF sites
today as large scale lift interacts with increasing moisture. The
best storms chances will be during the morning hours as an upper
level disturbance moves north out of the Texas Hill Country.
There should be a decrease in activity this afternoon/evening
before the next disturbance arrives after sunrise Saturday.

a light north to northeast wind this morning will slowly veer to
the east this afternoon and southeast overnight/Saturday morning.
Wind speeds will remain below 12 knots through 12Z Saturday.

&&


.DISCUSSION... /Issued 350 AM CST Fri Feb 23 2018/
While we`ve managed to send the persistent sub-freezing airmass
back north of the Red River (save for Graham which has been
reporting 32-33), the highly active and unsettled weather pattern
will persist today and into Saturday. At the surface, winds remain
out of the north or northeast, but stout southerly flow is
increasing about 1500 feet off the surface. The induced upglide as
warm and moist air is forced to ascend above the shallow lingering
frontal inversion is resulting in two NE-SW oriented bands of
showers and embedded thunderstorms across parts of the region
early this morning. In general, precipitation rates have been
averaging under an inch per hour, but the presence of nearly
saturated grounds--as evidenced by near-surface relative soil
moisture readings in the 70-90%--means that much of what is
falling is quickly being converted to runoff.

As the aformentioned 35-40 kt southerly winds persist above the
surface, renewed moisture will be pumped northward across the
region, and PWATs will head above 1.25" across much of North and
Central Texas by later this morning. The next batch of showers and
thunderstorms is expected to develop over the next few hours
across the Concho Valley as strong warm advection in the 925-850
mb layer gets going. Given how robust this forcing is forecast, am
a bit perplexed by recent runs of the HRRR which show fairly
limited/brief precipitation chances this morning. The 00z run of
the 3 km NAM seemed more reasonable, and this solution also agrees
with the NSSL WRF and the parameterized precipitation output from
the ECMWF. As a result, PoPs were raised dramatically across
roughly the northern 2/3rds of the forecast area through the
morning and into the afternoon hours today as showers and storms
expand in coverage. With the increase in atmospheric moisture
content and tall, skinny (elevated) CAPE profiles, any embedded
storms will be capable of producing very heavy rainfall. With this
in mind, and given the antecedent soil conditions in place, we
have expanded the flood watch westward by two rows of counties to
Montague and down to Erath. A combination of flash flooding and
subsequent river flooding is anticipated.

There is also a threat for some of the over-achieving storms to
produce some nickel to perhaps some slightly larger hail. The
mitigating factors today for a more appreciable hail threat will
be the rather anemic CAPE profiles and effective bulk shear values
generally under 35 kts thanks to the effective inflow layer for
any storms rooted in the 900-800 mb layer.

Finally, a large contrast in temperatures will exist today with
our far NW counties unlikely to get much past 40 degrees, while
lower 70s will be possible across the far southeast as a surface
warm front approaches. We will also be monitoring the potential
for some dense fog this morning near the Red River and then west
of I-35.

Later this afternoon and evening, much of this activity should
push north of the Red River. Copious low-level moisture will
remain, however, and prospects look decent for the development of
widespread mist or drizzle tonight (in addition to any lingering
showers or isolated storms).

By early Saturday morning, rapid lee cyclogenesis will be taking
place to our north and west as a potent shortwave ejects out of
New Mexico/Colorado and into the Central Plains. Cooling mid-level
temperatures will result in gradually increasing instability
across parts of the region as the left exit region of a 130+ kt
jet streak screams overhead. The resulting increasing kinematic
profile will yield 45-55 kts of effective deep layer shear by mid-
morning Saturday with impressive low-level shear values as well
(perhaps pushing 35 kts in the 0-1 km level). This certainly
portends the potential for strong to severe storms, but the main
question is where surface-based instability will reside. At this
time, the multi-model consensus is for this to be south and east
of a Paris to Terrell to Temple line. A line of strongly forced
convection is forecast to develop perhaps as early as 7-8 AM
across our west and march eastward into the late-morning and
early-afternoon hours. As this line moves east of the
aformentioned line, it may become surface-based. Given the shear
and thermodynamic parameter space, a damaging wind threat may
materialize in addition to the potential for a few embedded
tornadoes with the main tornado threat likely east of a Sulphur
Springs to Corsicana to Athens line. With such strong shear
forecast, I can`t rule out the potential for a significant
tornado, but much of this will hedge on just how readily things
destabilize out east.

If this weren`t enough, heavy rain will also be a threat with this
line of storms, and flooding/flash flooding issues will likely
only be exacerbated, especially across our north and eastern
counties. If this line looks like it will slow down some, the
flood watch will need to be extended into Saturday evening out
east.

The next chance for showers/storms will develop Tuesday night and
into Thursday with the approach of the next upper-level
disturbance.

Carlaw

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    50  47  67  38  69 / 100  60  90   5   5
Waco                57  56  70  39  65 /  80  30  80  10  10
Paris               53  53  65  38  63 / 100  60  90  10   5
Denton              45  45  66  34  67 / 100  60  90   5   5
McKinney            50  48  65  36  65 / 100  60  90   5   5
Dallas              52  49  67  39  67 / 100  60  90   5   5
Terrell             58  55  68  38  65 / 100  50  90  20  10
Corsicana           61  57  71  40  66 /  90  30  90  20  10
Temple              61  58  71  40  66 /  60  30  80  10  20
Mineral Wells       46  44  65  31  69 / 100  60  60   0   5

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Saturday morning for TXZ091>095-101>107-
116>123-130>134.

&&

$$

79/90


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