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

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FXUS64 KFWD 200409

Area Forecast Discussion
National Weather Service Fort Worth TX
1109 PM CDT Fri May 19 2017

The tornado threat has shifted north of the Red River where
numerous ongoing storms have occasionally produced tornadoes.
There is a break in the activity from the Red River down to about
I-20, then a loosely organized linear MCS is trying to get going
from south of Mineral Wells to near Lampasas. The current feeling
is that we may see showers and storms increase in coverage again
overnight as a front makes its way southeast into the area and
stronger lift from the Colorado upper low spreads farther east.
The severe threat should see a general decrease as we lose diurnal
instability, though a few storms could still pose a hail and
damaging wind threat. In fact the storms moving in on the
Metroplex from the west are showing signs of gusting out and and
possibly generating some damaging straight line winds, so please
keep an eye on any warnings issued for your area. For now there
are no watches in effect due to the isolated nature of severe
convection, though an occasional warning will remain possible for
any storms which intensify. Showers and storms will linger across
the southern counties Saturday morning as the front eases slowly
south of I-20, with locally heavy rain becoming a greater threat.



/ISSUED 705 PM CDT Fri May 19 2017/
Thunderstorms continue to erupt in two general areas both
southwest and northwest of North Texas TAF sites. The active
weather is associated with an upper low over Colorado, a shortwave
rotating around the southeast flank of the low and an approaching
cold front.

The northeasterly storm motion in the ongoing activity means that
the scattered strong to severe convection approaching Glen Rose
will begin to affect the Ft Worth area airports around 01Z or so,
then DFW and DAL shortly thereafter. For the 01Z to 03Z
timeframe, thunderstorm coverage in the Metroplex may remain low
enough to preclude a tempo or prevailing group, so will keep VCTS
going to account for the scattered activity. Forcing from the
advancing shortwave will increase as we proceed later into the
evening and prevailing TS beginning 03Z seems warranted. Storm
coverage will likely increase as the cold front moves in from the
northwest, and a tempo group from 06-09Z will be for the latest
thinking of when the front and associated line of storms move
through the DFW area.

For KACT, it appears that storms should remain west and northwest
for the time being, but should increase in the area around 06Z as
lift increases. A tempo group has been added from 09-12Z when the
front moves into the area.



Did a quick update to account for backed flow and stronger
surface winds than previously forecast. Many locations are
gusting over 30 MPH out of the southeast, which if nothing else
will at least keep the severe thunderstorm and tornado threat
going within tornado watch 249 (which remains in effect until 11
PM). The latest indications are that the scattered convection just
west of the region will eventually approach the I-35 corridor 8
to 10 PM. Another round of storms will be possible after midnight
as the strongest forcing associated with a shortwave disturbance
arrives and a slow moving cold front moves in from the northwest.
The severe threat should continue overnight as the low level jet
develops, but the threat will likely be lower due to the loss of
diurnal instability. A locally heavy rain threat can also be
expected overnight.



.PREV DISCUSSION... /ISSUED 225 PM CDT Fri May 19 2017/
Our slow-moving upper low continues to shift east across CO this
afternoon, with disturbances active within strong southwest flow
aloft over the western half of the state of TX. Shortwave energy
will continue to round the base of the upper low across the state
and northeast across OK through the weekend. At the surface, a
surface low was located between Midland and Lubbock, with a surface
cold front progressing southward across the OK/TX Panhandles. This
feature will be an additional player in convective developing late
tonight and lingering across Central TX later on Saturday and
possibly into Saturday night.

A corridor of enhanced isentropic lift and jet forcing aloft
remains centered from West-Central TX, northeast across the South
Plains and into southern and eastern OK this afternoon. Per latest
high res model guidance (3km NAMNEST the best on initialization
w/the HRRRx a close second), we expect our main threat through
early evening to remain west of I-35 and particularly north of
I-20. Model sounding forecasts across this area indicate plenty of
available instability(including mid level lapse rates 7-8 deg
C/km) and wind shear for severe storms containing very large hail
at times, along with damaging winds. Initial curved hodographs in
the lowest 5-10 kft do indicate a window of tornadic potential
through at least mid evening, especially with any discrete
activity, along with ample, sub-cloud instability to draw
horizontal vorticity surface upward into any rotating updrafts.

As the cold front enters our northwest CWA mid-late evening,
early indications are that discrete severe storms could congeal
into a slow, east-southeast moving QLCS or broken squall line
through the overnight hours, before weakening around daybreak
Saturday morning. The slow-movement of the cold front, along with
potential training of convection, heavier rain rates within an
area of 1.5 or greater PWATs will likely result in between 1-3
inches of rainfall along and north of I-20. We can`t rule out
isolated higher amounts. Since we`re so close to the event and
with some uncertainty on rainfall amounts, a Flash Flood Watch
will not be hoisted. That said, localized flash flooding is
certainly possible tonight and early Saturday, especially urban
areas around the DFW Metroplex, northeast through Sherman/Denison.

The surface cold front, assisted by storm outflow and the gradual
eastward progression of the upper low across the Central Plains
will continue to progress the front through Central TX during the
day Saturday into Saturday night. Better rain and convective
chances will accompany the cold front southward into East and
Central TX during this time. A few strong to severe storms are
possible where any instability can establish itself, though
coverage of severe weather should be much less than recent days.
In addition, better ascent and wind shear will be north of the
region, lessening the large-scale ascent across the area.
Depending on how fast the cold front moves southeast, areas of
heavy rain and flooding potential are uncertain. If the front is
slower, then more potential for heavier rains will exist. If the
front is more progressive, then flooding will be less likely
across Central Texas Saturday and Saturday night.

Drier low level air should continue to filter into the region
from the north the latter half of the weekend, as flow aloft
dampens into a westerly regime in behind the departing upper level
trough. Periodic energy moving east across Central TX may continue
some off and on showers and a few thunderstorms, but any flash
flood and severe weather threat will have ended. Weak upper
troughing and surface high pressure across the area will keep
sensible weather relatively quiet and cooler Sunday into Monday,
before the surface high shifts east.

Gradual low level moisture return will occur Monday night, ahead
of another shortwave trough aloft and associated cold front
arriving on Tuesday. Tuesday should be our next good, but brief
shot for showers and storms, though right now the thermodynamic
environment is too unknown and currently not looking overly
unstable for any severe weather. A cooler and brisk day Wednesday.
Shortwave ridging aloft and returning southerly surface winds the
end of the week will bring more seasonal late May warmth back to
the area. Rain chances look close to nil as well.



Dallas-Ft. Worth    68  81  59  82  62 /  80  50  30  10   5
Waco                70  83  61  82  63 /  60  70  60  20  20
Paris               68  79  56  80  58 /  70  60  30   5   0
Denton              65  78  55  82  58 /  80  40  20   5   5
McKinney            68  80  57  82  59 /  80  50  30   5   5
Dallas              70  82  61  83  63 /  80  50  40  10  10
Terrell             70  82  60  82  61 /  70  60  50  10  10
Corsicana           70  83  61  81  63 /  60  70  60  20  10
Temple              70  84  61  81  63 /  50  60  60  30  20
Mineral Wells       64  77  55  82  58 /  80  30  20   5  10



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