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

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FXUS64 KFWD 192230

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

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.




/ISSUED 103 PM CDT Fri May 19 2017/
/18Z TAFS/
A considerable amount of uncertainty exists with this TAF
package, primarily regarding the timing of individual thunderstorm
sequences this evening/overnight. A powerful upper low over
Colorado will continue to move eastward through tonight,
promoting strong forcing for ascent over N Texas. In conjunction
with this upper low, a pronounced cold front will also move
southward across the region overnight. In addition to these
forcing features, smaller scale surface boundaries exist over N TX
at midday, in association with previous morning convection. The
airmass also remains quite moist and unstable, fed by strong low
level southerly flow. UPSHOT: Plenty of forcing AND "juice" for
convection exists over the next 18 hours, and all TAF sites will
experience thunder at one or more points in time.

Don`t have great confidence in the current short-term model
output, other than the fact that they`re all advertising multiple
modes of convection through the next 18 hours: isolated discrete
cells, MCS modes, and linear modes. Expect convection at the DFW-
area TAF sites and KACT to be quite limited this afternoon, with
just scattered showers present. The first round of organized
convection, in the form of thunderstorm clusters, is likely to
impact the DFW sites in the 01-04z timeframe, and a bit later at
KACT. Expect there`ll be residual scattered SHRA/TSRA activity in
the area from 04z until at least 07-08z, at which time another
round of convection - likely associated with the cold front itself
- should start to impact the DFW sites. The timing for this second
  round is particularly sketchy, and could vary as much as 1-3
  hours either side. Do think that the majority of TSRA activity
  will be mainly E/SE of DFW by 12z/Saturday, though if the front
  slows down, this forecast cessation would have to be delayed a
  few hours.

MVFR cig conditions should be the rule region-wide after 04z
tonight, potentially lifting to VFR after 18z Sat. We may see
temporary windshifts to the NW anytime after 01z this evening in
conjunction with the first wave of convection, becoming persistent
after about 04z. NWly flow will be rule through Saturday.



.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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