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

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FXUS64 KFWD 221744

Area Forecast Discussion
National Weather Service Fort Worth TX
1244 PM CDT Sat Jul 22 2017

Subsidence provided by a broad ridge aloft should maintain the
quiet weather pattern for another day. An isolated storm will be
possible late this afternoon near the western DFW TRACON boundary
where the hottest surface temperatures are forecast, but it
appears any of this activity would remain west of North/Central
TX TAF sites. Otherwise, scattered convection can be expected well
southeast of the area along the TX & LA coastal areas.

A swath of Gulf moisture will push in from the east overnight and
Sunday around the north side of the Gulf Coast upper low, with
PWATs increasing from 1.5 to near 2 inches by this time tomorrow.
This should aid in scattered thunderstorm development during peak
heating hours Sunday afternoon as diurnal instability increases.
In addition, a weak upper level disturbance is progged to drop
southeast through the Southern Plains, perhaps weakening the ridge
overhead. Though not a definite, confidence at this time is high
enough to include VCTS in the extended DFW TAF. Otherwise,
conditions will remain VFR outside of any convection.



.SHORT TERM... /Issued 343 AM CDT Sat Jul 22 2017/

/Today and Tonight/

The upper ridge that has been dominating the weather across North
and Central Texas has continued to shift eastward. A weak upper
level low can be seen on water vapor satellite imagery over the
South Plains of West Texas but this feature is expected to remain
to our west. Meanwhile, a broad upper level low can be seen on
water vapor satellite imagery over southeastern Louisiana. This
system will continue to track west along the southern periphery of
the aforementioned upper level ridge. We will start to see an
increase into precipitable water values across the eastern zones
this afternoon. There will be a low chance of showers and
thunderstorms southeast of a Temple to Athens line by late
afternoon. Highs today will range from the mid 90s across the east
to around 103 degrees across the northwest. Heat index values
will range from 101 to 108 degrees. A Heat Advisory will remain in
effect through 8 PM for areas along and east of a Bowie to
Killeen line. Winds will be southerly around 10 mph. Skies will be
clear to partly cloudy tonight with lows in the mid to upper 70s
except for the lower 80s in parts of the Metroplex.



.LONG TERM... /Issued 343 AM CDT Sat Jul 22 2017/

/Sunday through Friday/

The main headlines through the initial portion of the long term
forecast period will be chances for rain and thunderstorms with
continued heat and high humidity. Thereafter, heat will be the
main story.

Sunday offers intriguing PoP, Max T and Apparent T forecasts.
Previous forecasts noted some weak isentropic upglide along the
310K theta surface up across northwestern zones on Sunday morning.
While the flow is weak along this isentropic surface, forecast
soundings do indicate near saturation of a 40-45mb layer centered
around 700mb. In addition, the 00 UTC NAM NEST and 00 UTC NCAR
Ensemble indicate the potential for what would likely be some weak
elevated convection. With these signals, I`ve continued and
expanded the mention of isolated storms near and northwest of a
Comanche to Mineral Wells to Gainesville line. The weak wind shear
should preclude any type of severe weather risk with this morning
activity, though I wouldn`t completely rule out some strong
downburst winds. There may also be a less than 20% chance for
showers and storms for areas near and southeast of a Temple to
Canton line on Sunday morning as well as a weak lobe of vorticity
rotates in from the east.

Sunday afternoon should be another hot and humid day across a
good portion of North and Central TX. While temperatures may not
peak quite similar to what we`ve experienced over the past several
days (courtesy of additional moisture, resultant cloud cover and
possible rainfall), it`s likely that max heat index values in the
105 to 107 degree range. While this is forecast at this time,
there`s uncertainty regarding just how high these values will get.
As a result, we will hold off on extending the Heat Advisory into
Sunday at this time. If the coverage of showers and storms is
less than currently forecast, a Heat Advisory will likely be
required for parts of the forecast area. With regards to
precipitation potential on Sunday afternoon, confidence remains
about the same that isolated to scattered convection will develop
through the day. The most favored area for a greater concentration
of storms appears to be for locales east of I-35, though we will
have to keep close monitor across areas back to the northwest as
forcing in the form of a broad upper level deformation zone may
assist with convective development. Deep layer wind shear
necessary for organized storm modes still appears very low at this
time. However, should high temperatures climb into the mid to
upper 90s as forecast, there will be an appreciable risk for
isolated to scattered severe downbursts with the more vigorous
convection across just about all of North and Central TX. The slow
moving nature of likely heavy rain producing storms will also
result in an isolated flood risk as well.

The coverage of Sunday afternoon convection should wane towards
sunset, but there continues to be a persistent signal in model
guidance of a weakening complex sliding southward into North TX
through the evening and nighttime hours. The low level wind field
appears generally on the weaker side, so for now, I`ll maintain
just a 20 PoP for the northern half of the forecast area.

With the mid/upper low continuing to keep the H5 ridge at bay,
I`ll also continue with low rain chances through the day on
Monday. Similar to Sunday, there will be a risk for strong to
severe downbursts with the more intense convection. Rain chances
will linger into Tuesday, mainly across northern and northeastern
zones where a weak complex may be aided by subtle forcing from the
tail end of a broad trough. The threat for downbursts will still
exist, but the overall intensity this far out is unknown---stay
tuned for details in later forecasts. Otherwise, the temperatures
will start to moderate on Monday and Tuesday as the coverage of
rain/clouds should be less. One interesting note is that GFS
forecast soundings appear to mix very deeply on Monday and
Tuesday. Low level flow also appears veered and if this occurs,
portions of Central TX could be the beneficiaries of some weak
compressional warming (and perhaps see temperatures closer to 103
F). For now, we will keep an eye on this potential as it appears
low right now.

For Wednesday and beyond, the oppressive and dangerous heat will
return! ECMWF and GFS are in good agreement that the H5 ridge will
build back towards the region. While this week has been hot, next
week has the potential to be even hotter. It`s likely that
additional Heat Advisories will be necessary and some guidance
even suggests that Excessive Heat Watches/Warnings may be in order
for some. For now, will go with a slightly "cooler" forecast as
there`s some uncertainty with regards to the exact placement of
the ridge. Regardless, it`ll be hot for consecutive days with
little in the way of relief during the overnight hours as low
temperatures will only fall to near 80, especially in urban areas.



Dallas-Ft. Worth   100  80  99  80  97 /  10   5  20  20  20
Waco               100  77  99  77  99 /  10   5  20  20  10
Paris               96  75  96  75  94 /  10   5  30  20  20
Denton              99  78  98  77  97 /   5   5  20  20  20
McKinney            97  77  96  77  95 /  10   5  20  20  20
Dallas              99  81  98  80  98 /  10   5  20  20  20
Terrell             97  76  96  75  96 /  10   5  30  20  20
Corsicana           97  76  97  76  97 /  10   5  30  20  10
Temple             100  76  99  76 101 /  10  10  20  10  10
Mineral Wells      100  75  99  75  97 /   5   5  20  20  10


Heat Advisory until 8 PM CDT this evening for TXZ091>095-102>107-



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