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Area forecast discussion
National Weather Service Fort Worth Texas
1245 PM CDT sun Jul 23 2017



Aviation...
the upper ridge currently anchored out west is being weakened on
its eastern flank by an upper low over southern Louisiana, and a
shortwave trough dropping southeast through the Southern Plains.
This vulnerability will last through Monday, leading to
multiple opportunities for convection over the next 24 to 36
hours.

The 1.73 inch precipitable water value on the morning sounding indicate that
the atmosphere has moistened considerably over the past 24 hours.
With isolated storms still ongoing around the region, the feeling
is that coverage will increase considerably after a few more hours
of surface heating and temperatures approach upper 90s. Due to
the expected scattered nature of the convection, thunderstorms in the vicinity will have to
suffice in area tafs for the time being. If it becomes apparent
that storms will affect a particular Airport, an amendment for
tempo ts will be done asap.

Activity should decrease in coverage this evening as surface
heating wanes. There is some indication that a loosely organized
mesoscale convective system will develop over the Southern Plains in association with the
shortwave trough, and approach from the north during the
overnight hours. Storms would likely weaken as they approached, so
have stuck with vcsh tor the Monday morning hours. Thunderstorms in the vicinity has been
added to the extended dfw taf for anticipation of another round of
scattered convection Monday afternoon.

30

&&



Update... /issued 1032 am CDT sun Jul 23 2017/
scattered convection has been persistent this morning generally
south of I-20 and west of I-35. We expect showers and storms to
become more numerous through the afternoon as moisture slowly
increases, temperatures warm into the 90s, and moderate
instability remains in place. The primary threat with any storms
will be from downburst winds, lightning and locally heavy
rainfall.

We will send out a quick update to raise pops up a bit through
mid afternoon and adjust hourly temp/dewpoint and cloud grids
based on current trends.

79

&&

Short term... /issued 358 am CDT sun Jul 23 2017/

/today and tonight/

The overnight isolated to scattered showers and thunderstorms
have remained mainly west of the forecast area with only a few
showers brushing western Young and Stephens counties. There will
be a low chance of showers and thunderstorms this morning west of
Bowie to Killeen line in an area of deeper moisture and
instability. As an upper level low over the northwest Gulf of
Mexico off the coast of southwestern Louisiana drifts westward,
precipitable water values near 2 inches will spread northwestward
across central Texas and the southeastern part of North Texas. We
should see scattered showers and thunderstorms across the forecast
area this afternoon, which will bring some brief relief from the
heat for those that see rain. Of course, with afternoon
temperatures reaching the mid 90s to around 100 degrees, there
will be the potential for damaging downburst winds with the
strongest storms. Dcape (downdraft convective available potential
energy) values will be 1000 j/kg or more across much of the
forecast area.

The showers and thunderstorms are expected to decrease in areal
coverage toward sunset but low chances will remain into the
overnight hours. Lows will be mostly in the 70s, except around 80
in the warmer areas of the metroplex.

58

&&

Long term... /issued 358 am CDT sun Jul 23 2017/

/Monday through Friday/

Hi-res guidance and to some degree coarser resolution guidance
both indicate the potential for additional generation of
convection along likely thunderstorm outflow on Monday morning.
Coarser resolution guidance is split on the overall magnitude of
low level wind fields during the morning hours and this results
in a low confidence pop forecast. For now will maintain slight
chances for showers and storms across much of north and central
Texas.

There's been a persistent signal in model vorticity fields over
the past few days that an mesoscale convective vortex will develop in the wake of the
morning convection near the Red River. With the ridge in a
weakened state, daytime heating combined with plentiful low level
moisture and lift from this potential mesoscale convective vortex should yield another
afternoon of scattered showers and thunderstorms. If morning
activity persists longer than currently thought, it's possible
that the afternoon convection may not be quite as
robust/widespread as currently thought. Nevertheless, the best
chances for showers and storms appear to be along the Red River
per hrrrx and NAM nest output. As a result, i've nudged pops up to
around 40% for Monday afternoon across this region. If an mesoscale convective vortex
does materialize, it's possible that these pops are not high
enough. Confidence diminishes with southern extent across the cwa,
so i've tapered pops downward across central Texas. Similar to
Sunday, large amounts of dcape, inverted-v profiles and afternoon
high temperatures in the mid 90s all support a damaging downburst
risk. Pwats will also near 2" and with slow storm motions, there
will be an isolated flood risk.

Rain chances should diminish through the nighttime hours, with
just low rain chances across far northeastern zones on Tuesday
morning, perhaps continuing into the early afternoon. Late Tuesday
afternoon should see less in the way of storms and with
additional sunshine, the return of the oppressive heat and
humidity will begin. Across central TX, it's possible that heat
indices will climb into the 105-107 degree range on Tuesday.

The oppressive heat will be in full force on Wednesday and
Thursday as the 500 mb ridge shifts towards the east. Depending on how
much rain falls, there could be an added source for low level
moisture via evapotranspiration, resulting in even more oppressive
conditions. Temperatures will soar into the 96 to 102 degree range
across the region with heat indices of up to 109 degrees possible.
These values are on the cusp of excessive heat watch/warning
criteria, but for now will abstain from these headlines at this
time until we gain greater confidence in temperature and dewpoint
forecasts. Regardless of advisory/watch/warning, it'll be hot and
individuals need to continue to review and exercise heat safety
tips located on our website.

The heat will continue into Friday, but there could be some minor
relief in the extended range forecast as the European model (ecmwf) and GFS are in
very good agreement with regards to the upper air pattern. Both
models build a very large ridge across the intermountain west
with a large and deep upper trough to the east. This would place
north and central Texas beneath northerly flow aloft which may allow
for a late season frontal boundary to protrude southward. Before I
induce a ton of excitement, it should be noted that the front
would likely only drop temperatures by a degree or two with
slightly lower dewpoint temperatures---if the front makes it this
far south. The real cool down would be from showers and
thunderstorms along this boundary which at this point in time is
difficult to pinpoint. There are some hints that showers and
storms will be possible across far northwestern zones as early as
Friday afternoon with continuing rain chances into the day on
Saturday. For now, will leave broad brush pops for this time
period as we will have the ability to better refine the forecast
as next weekend nears.

24-Bain

&&

Preliminary point temps/pops...
Dallas-ft. Worth 100 80 98 80 100 / 30 20 40 20 10
Waco 100 77 99 77 100 / 30 20 20 10 10
Paris 96 75 93 75 96 / 30 30 40 20 10
Denton 99 77 95 77 99 / 30 20 40 20 10
McKinney 97 76 96 77 98 / 30 20 40 20 10
Dallas 99 80 98 80 100 / 30 20 40 20 10
Terrell 96 76 95 77 98 / 30 20 40 20 10
Corsicana 96 77 96 77 98 / 30 20 30 10 10
Temple 100 76 99 76 100 / 30 20 10 5 10
Mineral Wells 99 75 98 75 99 / 30 20 30 10 10

&&

Forward watches/warnings/advisories...
heat advisory until 7 PM CDT this evening for txz091>095-102>107-
117>123-131>135-144>148-157>162-174-175.

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