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

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FXUS64 KFWD 120255

Area Forecast Discussion
National Weather Service Fort Worth TX
955 PM CDT Fri Aug 11 2017

A frontal boundary remains draped near the I-20 corridor this
evening. Isolated to scattered showers and thunderstorms continue
across the area from Fort Worth to Athens. This activity should
slowly weaken through the early morning hours. Have left 20-30
percent PoPs from Fort Worth to Athens line through 1 AM. No other
changes were made to the previous forecast.



.AVIATION... /Issued 638 PM CDT Fri Aug 11 2017/
Main concern through the forecast period will be potential for
thunderstorms affecting Dallas/Fort Worth Metroplex TAF sites.
This evening...a few showers have developed across the Metroplex
near a weak cold front. No lightning strikes have been observed so
far and this activity should weaken over the next couple of hours
with the loss of daytime heating. Went with VCSH at KAFW, KFTW,
and KGKY to cover the spotty shower activity through the evening,
though a couple rumbles of thunder cannot be totally ruled out.

Winds should turn out of the SE by late Saturday morning. While
the TAFs for Metroplex sites explicitly mention winds of 7
knots...winds may increase somewhat during the afternoon hours in
response to an approaching cold front. Thunderstorms should begin
to develop across North Texas by 2000Z. While the greatest
coverage is expected to be near the Red River...chances will be
high enough across the Metroplex to warrant a VCTS mention during
the late afternoon hours with a second round of thunderstorms
potentially impacting Metroplex sites after 0300Z Sunday. Given
the uncertainty beyond 0000Z in the evolution of
thunderstorms...opted to go with VCTS for KDFW at 0300Z.

For significant issues are expected during the next 24
hours with VFR conditions forecast to prevail...and thunderstorm
activity expected to remain well to the north.



.UPDATE... /Issued 609 PM CDT Fri Aug 11 2017/
A cold front is quite apparent on radar late this afternoon. The
frontal boundary extended along a line from Benjamin (Knox County)
to Newcastle to Fort Worth to Terrell to Grand Saline. Scattered
showers and thunderstorms have developed along the boundary. Cloud
to ground lightning, gusty winds and brief heavy rain will
accompany some of the storms. This activity will dissipate after

We took a second look at the Heat Advisory and even though it may
be borderline in some areas, depending on cloud cover and when the
convection initiates Saturday afternoon, we decided to extend
it another day and also include areas farther east through Canton
and Palestine.



.SHORT TERM... /Issued 339 PM CDT Fri Aug 11 2017/
Primary concern through the next ~6 hours is rain/thunderstorm
potential through this evening. Rapidly developing cumulus is
located across North TX along an axis of surface convergence. This
trough axis precedes a broad frontal zone positioned through far
southern OK. Another area of low level convergence exists across
Central/East TX associated with this mornings outflow boundary
that surged through the forecast area. Both of these convergent
areas could serve as a focus for convective initiation through
the early evening hours. Expect most activity to primarily consist
of showers, but a few isolated lightning strikes are possible as
well. Any activity that may develop should begin to dissipate with
loss of heating. With limited coverage expected, a 20% PoP will
suffice through this time period.

Aside from thunderstorm potential, the other concern today has
been the heat index values within an ongoing Heat Advisory. Calm
winds and clearing skies within the Advisory area have allowed
temperatures to climb into the upper 90s to near 100 degrees with
heat index values at or just above 105. The Heat Advisory will be
allowed to expire at 6pm however, as heat index readings above 105
will be more sparse on Saturday with increased clouds and
convective coverage during peak heating.

Later this evening, the frontal zone is expected to pivot and
become oriented NW to SE through North TX. Where this front is
positioned by Saturday morning will determine where the highest
potential for thunderstorms will be later Saturday afternoon and
evening, as discussed below.



.LONG TERM... /Issued 339 PM CDT Fri Aug 11 2017/
/Saturday through Friday/

A flat upper ridge will be in place across the Southern Plains at
the start of Saturday with primarily zonal flow across North
Texas and Oklahoma. A well defined shortwave trough will ride
around the northern periphery of the ridge and approach the Red
River by early morning. At the surface, a quasi stationary front
will be located very near or just south of the Red River. Moisture
pooling along the frontal boundary and strong heating through the
day will result in moderate to strong instability developing
generally from the I-20 corridor northward. As the shortwave
trough gets closer and large scale forcing for ascent overspreads
the Red River region by mid afternoon, scattered thunderstorms are
expected to develop along the front and will increase in coverage
through the evening hours. Forecast soundings from the Gainesville
area over toward Bowie are actually rather impressive for this
time of year with 35-40 kt of westerly flow at 500 mb and 15-25
kt of flow from 850-700 mb. This yields a fairly decent 0-6 km
shear profile with moderate to strong instability in the vicinity
of the frontal boundary. With thunderstorms expected to develop
during peak heating, at least some organized clusters of
thunderstorms are expected with shear and instability sufficient
for rotating updrafts and an accompanying damaging wind and hail
threat. In the immediate vicinity of the frontal boundary, any
discrete updrafts would also have some increased low level
vorticity ingestion potential and an attendant tornado threat.

As we get into the late evening hours, thunderstorms should
increase in coverage along and north of the frontal boundary.
Where this boundary sets up is a little uncertain and may be
impacted by convection earlier in the afternoon. We`ll generally
have high PoPs along and north of I-20 through the overnight hours
Saturday night into Sunday. The potential for heavy rainfall and
some isolated flash flooding will increase during this time as low
level flow becomes more veered and we could see some backbuilding
of echoes. Activity should diminish across the region during the
day Sunday. By Sunday night, with the boundary still in the
general vicinity of the Red River, additional showers and
thunderstorms are expected to develop. We will have to monitor
this area for heavy rainfall potential into Monday morning. Rain
chances will diminish overall across the region Monday night into

Concerning Flash Flood potential... At this time we`ll hold off
on issuing a Flood Watch for any part of our area but we will have
to monitor convective potential north of I-20 tomorrow afternoon
and evening. If it becomes more apparent that the synoptic frontal
boundary sets up farther south into our area, then we may need a
flood watch across parts of our area.

Concerning Heat Advisory extension...We`ll let the current heat
advisory expire this evening. Heat indices will still remain in
the 100-105 degree range tomorrow across most of the area.



Dallas-Ft. Worth    81  98  77  94  79 /  20  40  60  30  30
Waco                77  99  78  99  78 /  10   5  10   5  10
Paris               74  91  73  86  74 /  10  40  80  70  50
Denton              78  96  76  93  76 /  10  40  70  40  40
McKinney            77  96  75  91  76 /  10  50  80  50  40
Dallas              81  98  78  94  80 /  10  40  60  30  30
Terrell             78  97  77  92  77 /  20  40  60  30  20
Corsicana           78  97  77  95  77 /  20  10  20  10  20
Temple              76  99  76  99  76 /  10   5   5   5   5
Mineral Wells       76  95  76  96  76 /  10  20  50  20  30


Heat Advisory until 6 PM CDT Saturday for TXZ118>122-133>135-



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