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

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FXUS64 KFWD 220218 AAB

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
918 PM CDT Thu Sep 21 2017

Main update to the forecast was to remove PoPs for the remainder
of the evening and add in a small mention of patchy fog across far
southeastern zones.

Diurnal thunderstorm activity has diminished quickly with the
loss of daytime heating this evening. With no real forcing for
ascent present, additional convection is unlikely. As a result,
I`ve removed the mention of any rain/thunder from the weather
grids for tonight. Low level moisture is expected to surge
northward tonight and into Friday morning in response to a
strengthening low level jet, mainly near and west of I-35.
Moisture advection east of I-35 will occur, but the overall
magnitude of winds at the surface and aloft will be weaker. With
moistening occurring in the near surface layer and weaker winds
this environment may be conducive for some patchy fog, mainly
across low-lying/sheltered areas and near bodies of water. Patchy
fog development may also be favored near areas that saw rain this
afternoon (mainly across southeastern zones). With that in mind,
I`ve added a small mention of patchy fog for far southeastern
zones. Widespread dense fog appears unlikely.

The remainder of the forecast is in good shape and updated
products have already been sent.



.AVIATION... /Issued 632 PM CDT Thu Sep 21 2017/
A few showers and thunderstorms will continue to approach the
Metroplex Terminals from the southeast along a northwestward
moving outflow boundary. This activity should weaken before
reaching any of the Metro Terminals, but will have some minor
impacts at the Cedar Creek corner post. Even though there should
be no direct impacts, have decided to add showers in the vicinity
at DAL and GKY through 01Z. Once the sun sets, all activity should
dissipate with VFR conditions expected through the evening.

A 25 knot low level jet will develop overnight and bring a surge
of moisture northward, resulting in stratus. MVFR ceilings should
reach Waco around 09Z and shortly after at the Metro Terminals.
Ceilings will begin to lift after sunrise and scatter out before
midday. Although some minor visibility restrictions are possible
across south Texas and the Hill Country around sunrise, wind
speeds should be too strong for any major visibility reductions.

A south wind will continue in the 10 to 14 knot range along with
some higher gusts.



.SHORT TERM... /Issued 226 PM CDT Thu Sep 21 2017/
/Rest of today and tonight/

Temperatures early this afternoon have warmed into the mid 90s in
most locations which, combined with dewpoints in the low 70s is
resulting in heat index values near the century mark. Latest
visible satellite imagery reveals a fairly flat cumulus field
streaming overhead, and this is probably due in part to relatively
warm and dry air present just under 500 mb based on recent ACARS
soundings. A more active and roiling cumulus field is apparent
just south and east of our region where showers and thunderstorms
are ongoing inland of the Upper Texas Coast. With moisture in
better supply here, it seems reasonable to expect additional
isolated to scattered thunderstorm development through the rest of
the afternoon along the northward surging outflow. Paltry mid and
upper-level flow mean individual cells will pulse up and rain
themselves out, but will be capable of producing some strong
downburst wind gusts as their cores collapse. Activity will
diminish this evening with the loss of heating.

Low cloud cover will surge northward out of the Texas Hill Country
once again tonight, and temperatures overnight will once again be
pretty mild--only falling into the lower to upper 70s. We`ll need
to keep an eye on our south and eastern counties late tonight and
into Thursday morning as there is some potential for fog
development. Winds atop the cooling and moistening boundary layer
do not appear overly strong, and mixing ratios increasing with
height portend some potential for downward-direction moisture
fluxes. The 12z NSSL WRF and SREF probabilities highlight locales
from a Sulphur Springs to Terrell to Cameron line as potential
candidates for fog formation. Confidence in low visibilities
(under 1 mile), however, is not high enough to warrant the
introduction of fog into the forecast at this time, but the
evening shift will monitor this potential as additional high-
resolution guidance becomes available.



.LONG TERM... /Issued 226 PM CDT Thu Sep 21 2017/
Friday through the weekend will be defined primarily by a
deepening upper trough across the western CONUS, an elongated
ridge from Texas to the Great Lakes and a weak inverted trough
across the Southeast. The upper trough to the west will slowly
shift eastward late this weekend and next week, while the
inverted trough to the east works its way westward around the
base of the ridge. The inverted trough will be responsible for
diurnal convection across the Southeast States this weekend. As we
head into next week, however, the system will become absorbed
into the West Coast system (the leading edge of which will have
worked its way into the Plains) and become a non-factor.

All eyes will be on the large upper level trough as it trudges
eastward towards the middle part of the country. Forcing for
ascent will remain west of the forecast area this weekend, but
will begin to affect the western counties of North Texas early in
the workweek. Chance POPs are in place west of I-35/35 W late
Monday and Tuesday. The persistent slow but steady eastward
progression of the trough will spread strong ascent slowly
eastward, causing an increase in showers and storms across the
central and eastern portions of the forecast area late Tuesday
and Wednesday.

The upper level system will be accompanied by a cold front, which
could locally enhance rain rates as it provides additional focus
for development. The front is currently progged to push steadily
southeastward across the region late Tuesday and Wednesday,
eventually clearing the southeast counties sometime Thursday.

Though it`s still early on in the forecast, the current thinking
is that instability and shear will be sufficient for a few strong
storms, but the severe threat appears pretty low. The bigger
concern may turn to locally heavy rainfall and the potential for
flooding. Tropical moisture will surge northward late Tuesday,
with fairly widespread 2" PWATs by Wednesday. Additional moisture
pooling along the surface boundary could create a narrow swath of
2.25" PWATs along the front as it pushes through Wednesday into
Thursday. Fortunately, the region has dried out significantly due
to the lack of any appreciable rain this month. That said, the
possibility of heavy rain will be something to keep an eye on as
we head for the middle of next week.

The upper level trough and cold front will progress east of the
forecast area late next week, which should hopefully bring more
comfortable early Fall weather by next weekend.



Dallas-Ft. Worth    77  93  75  94  74 /   5   5   0   5   5
Waco                75  93  72  94  71 /   5   5   0   5   5
Paris               72  91  71  92  71 /   5   5   0   5   5
Denton              75  91  73  93  72 /   5   5   0   5   5
McKinney            75  91  72  92  72 /   5   5   0   5   5
Dallas              77  93  76  94  75 /   5   5   0   5   5
Terrell             74  92  71  94  72 /   5   5   0   5   5
Corsicana           74  92  71  94  72 /   5   5   0   5   5
Temple              74  91  70  93  70 /   5  10   0   5   5
Mineral Wells       74  92  70  93  70 /   5   5   0   5   5




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