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

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FXUS64 KFWD 180909

Area Forecast Discussion
National Weather Service Fort Worth TX
409 AM CDT Wed Oct 18 2017

A dry autumn air mass remain in place across North and Central
Texas. South winds made a tepid return on Tuesday, but dew points
have remained relatively steady in the low to mid 40s. It was
another night with clear skies and low humidity, but warm
advection of 20-25kts just above the nocturnal boundary layer has
disrupted the radiational cooling process. As a result, many
locations are several degrees above readings 24 hours earlier.
But with normal lows generally in the 50s this time of year, much
of the region will still be below normal at daybreak this morning.

With abundant sunshine again today, the thin layer of cool air
will quickly mix out. Temperatures will be in the low to mid 70s
by noon. The ridge axis will finally shift east of the region
today, allowing lee troughing to control the wind field across
North and Central Texas. South winds may exceed 10mph at times
today, but with the 60F isodrosotherm still well offshore, the
humidity will remain pleasant today as highs top 80F across much
of the region.

Shortwave ridging aloft will transit our longitude late in the
day. As the upper-level winds back, cirrus will invade from the
west. Despite decreasing surface winds this evening, rich Gulf
moisture will enter Central Texas. A stratus deck may arrive
before the surface layer is able to cool to saturation, but some
patchy fog will still be possible across our southern zones
Thursday morning, particularly where the surface winds subside.



/Thursday Thru Tuesday/

An amplifying shortwave disturbance aloft across the Great Basin
of Utah/Arizona/Nevada will slowly move east over the Four
Corners region of the Southern Rockies on Thursday. Meanwhile, a
shortwave upper ridge over our region will shift east toward the
Lower Mississippi Valley and Central Gulf States with deepening
SSW flow aloft across the Texas. Continued southerly flow in the
low levels will continue to bring modified moisture from the Gulf
of Mexico, before more rich, yet shallow surface dew pts in the
lower to mid 60s arrives northward across areas west of I-35 and
35W progressing into Thursday Night and Friday. An initial subtle
shortwave disturbance will lift across the state during the day
Thursday to help enhance low level warm advection and modify the
strong EML (capping inversion) in place, with another much
stronger shortwave arriving Thursday night into Friday. I still
have my doubts on how quickly our EML and environment will modify
during the end of this week, but with low level moisture quality
on the increase with PWAT values across the west increasing to
near 1.5" and steepening lapse rates, felt low convective chances
of the general variety were warranted. Main window for spotty
showers and storms appears to be during the overnight hours
through the morning hours Friday, with this disturbance moving
away with activity ending across across eastern counties before
midnight Friday. Any outdoor activities Friday evening should be
OK, but areas east of I-35/35E could possibly be impacted early

Most of Friday night and Saturday morning should be quiet as we
remain in between disturbances aloft, though low level warm
advection will be in full swing with a full surge of rich Gulf
moisture northward, especially across areas east of I-35. I`m
still a little leery about whether the richer moisture across the
Bay of Campeche and Central Gulf of Mexico can eventually break
free. For now, I`ll buy off that this occurs considering the
strength of our southerly low level flow in advance of our main,
very strong southern stream mid level disturbance lifting east
over the southern High Plains late Saturday. Saturday will be our
warmest and most muggy day per the aforementioned processes, with
highs in the lower to middle 80s many areas. Warm advection-
driven isolated to scattered showers and storms appear possible
across our eastern counties, but a persistent, though weakening,
EML should limit the intensity through early Saturday evening.

As the main latitudinal upper support lifts east over the Plains,
strong height falls will progress across the area later Saturday
night. At the surface, Low pressure over northeastern Colorado
will lift quickly northeast toward eastern Nebraska and Iowa by
Sunday morning, allowing a strong cold front to surge rapidly
south-southeast and into at least the western half of the CWA by
daybreak Sunday morning. This is where things become complicated
with this system, as medium range models and even their ensemble
members are having trouble deciphering the deep Central U.S.
longwave trough and energy embedded within it. The ECMWF remains
consistent with its previous runs with a more progressive
solution, while the GFE/GEFS and Canadian models project deeper
southern stream energy and thus a more cut-off and slower
solution, which really makes timing of the cold front and
beginning/ending of convection very challenging.

In addition to those challenges, deciphering environmental
evolution Saturday night into Sunday remains difficult. With
these models and their ensembles continuing to struggle and waffle
on run-to-run consistency, my confidence remains marginal on
timing and potential for strong to severe storms. Taking the
moisture, steepening lapse rates with the axis of the trough,
lift, and effective bulk shear forecast, I cannot argue that a few
strong to severe storms with damaging winds and some marginally
severe hail late Saturday night and early Sunday morning. If the
time frame had been more during the day, I`d be more convinced on
a more widespread severe weather threat. That said, interests
across North and Central Texas should be ready for these hazards
Saturday night and early Sunday.

For now, will take an average of these solutions, which will not
be too different from previous forecasts. Highest convective
chances with the arrival of the surface cold front and main
dynamics occurs Saturday night and Sunday morning, especially
across the north. I have lingered high convective chances with the
progression of the cold front across our southeast counties later
Sunday, while ending across the western half of the CWA later in
the day. Lingering slight chances will continue Sunday evening
across eastern zones with trailing mid level energy. Otherwise,
brisk and much cooler conditions will occur in the wake of the
cold front by Sunday evening and continue into Monday. With the
model discrepancies heading into early next week regarding the
cutting off of mid-level energy across East Texas/western
Louisiana, it is possible low cold core showers may need to be
maintained, but will leave the forecast dry for now Monday and
Tuesday. I will continue to introduce highs in the 70s with modest
low level north flow and clouds dispersing eastward. However, if
the cut-off solution ends up verifying early next week, then cloud
cover will likely linger with lower high temperatures in the 60s
across areas along and east of I-35. Hopefully, medium range
models will get a better handle on this mid-level trough and
associated energy and environment later this week.



.AVIATION... /Issued 1142 PM CDT Tue Oct 17 2017/
VFR conditions will prevail through 06z Thursday as high pressure
aloft continues to dominate the weather across the region. South
to southeast winds around 6 knots will become southerly around 10
knots by 15z Wednesday. Some fog may develop across Central Texas
after 06z Thursday but that is beyond the forecast period of the
Waco TAF.



Dallas-Ft. Worth    81  58  83  64  82 /   0   0   5   5  20
Waco                82  57  84  62  82 /   0   0  10  10  30
Paris               78  52  79  57  80 /   0   0   5   5  20
Denton              81  56  81  62  81 /   0   0   5  10  20
McKinney            79  55  81  60  80 /   0   0   5   5  20
Dallas              82  58  83  64  82 /   0   0   5   5  20
Terrell             82  54  82  60  83 /   0   0   5   5  20
Corsicana           81  56  82  62  81 /   0   0   5   5  30
Temple              82  57  83  63  82 /   0   0  10  20  40
Mineral Wells       83  54  82  62  81 /   0   0   5  20  30




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