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

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FXUS64 KFWD 172103

Area Forecast Discussion
National Weather Service Fort Worth TX
303 PM CST Sun Dec 17 2017

/Tonight through Monday Evening/

Cool and damp conditions are anticipated over the next 24 to 30
hours for parts of North and Central TX. There will also be the
potential for another round of overnight fog across a good portion
of the area. Temperatures will remain near seasonal values.

Low clouds remain entrenched across the eastern two-thirds of
North and Central TX. Another row of counties or two could
experience some partial clearing over the next couple of hours,
but that should be about it. With low level moisture slowly
sloshing back towards the west and dewpoint depressions falling to
near zero, the stage should be set for another round of low
stratus and fog. Outside of areas across the west, temperatures
tonight shouldn`t change too much due to the moist airmass and
cloud cover. With regards to the fog potential, it`s probable that
some fog could become dense. Exact locations, however, preclude
mention in the forecast at this point in time and for now, I`ll
have to defer to later shifts to fine tune these details.

Rain chances will start to ramp up around daybreak on Monday as
low level isentropic ascent increases. At this time, the strongest
lift looks to be across Central TX where I`ll continue a mention
of a few showers in the morning. Lift should continue to
overspread southeastern and eastern zones through the morning and
into the afternoon hours. Forecast soundings do indicate some
elevated instability which should support at least an isolated
thunder threat. At this time, I don`t anticipate much in the way
of severe weather as instability will be on the weaker end of the
spectrum. Can`t completely discount that some of the more robust
convection may contain some very small hail given some wind shear,
but this potential is conditional given that the vigor of most
storms should be on the low side. Widespread cloud cover should
keep temperatures in check across the area on Monday and most
areas will likely remain below 60 degrees for afternoon highs.



/Monday Night Through Saturday/

The vigorous shortwave disturbance over Northern Mexico and The
West Texas Big Bend Country will lift east slowly over West Texas
late Monday night. A 20-30 knot southwesterly LLJ will combine
with increasing pressure advection on the 305K-310K isentropic
surfaces to induce strong warm advection by Tuesday morning from
the Hill Country, across Central Texas and into East Texas. Light
surface winds and rich moisture below the boundary layer may lead
to at least patchy fog, possibly dense once again Tuesday
morning. This dynamic shortwave will take on a negative tilt
across the state Tuesday afternoon and Tuesday evening and create
widespread showers with scattered storms. Much, if not all of this
activity will be elevated above an inversion anchored between
900mb and 850mb. As noted yesterday, skinny MUCAPE averaging at or
just above 500 J/kg will combine southwesterly effective bulk
shear near 50 knots and lapse rates of 6.5-7 deg C for a few
strong elevated storms Tuesday afternoon, then in the evening east
of I-35/35E with some smaller hail, frequent lightning, and heavy
downpours. Elevated instability will be greatest across the
southeast half of the area where I cannot rule out possibly a
brief, marginal severe storm where the main threat would be hail
possibly up to quarter size, especially our southeast counties.
Otherwise, atmospheric moisture content Tuesday will be sufficient
for decent rain totals between 1 and 2.5" along and south of I-20
and especially east of I-35 across Central Texas. With ground
temperatures having cooled off lately, some run off and minor
flooding is possibe southeast of the D/FW Metroplex.

With deepening energy across the Great Basin and Western U.S.
Tuesday night and early Wednesday, look for this system to dampen
and kick out of the area quickly Wednesday morning. Light and
variable weak surface wind speeds will quickly increase from the
south by Wednesday afternoon with sunshine returning to
the western half of North and Central Texas. The increased
insolation and southernly winds will bring a warm up back well
into the 60s and possibly lower 70s out west where less rainfall
is expected with the early week system. The wet soils and
lingering cloud cover in the east should keep things from warming
up to quickly. We begin to see the Western U.S. trough lifting
east toward the 4-corners region of the Rockies by Thursday which
increases lee-cyclogenesis and associated low level advection
ahead of a shallow, but strong cold front marching south through
the Central and Northern Plains into the Midwest and Mississippi
River Valley. This will be our warmest day for the week, as veered
low level flow above the surface and 850mb temperatures pushing
14-15 deg/C combine for temperatures in the low-mid 70s most
areas. If not for the moist soils from previous rainfall, a few
western areas would likely see highs near 80, but that`s likely
not happening this time around.

As some have noted from the previous overnight forecast, a few
things have changed with regarding of our end of the week into the
holiday weekend forecast. Some will be happy for travel reasons,
some will be sad as maybe what they hoped for doesn`t appear to
the be the scenario unfolding. One element that is a certainty
will be a very shallow arctic frontal push occurring underneath
strengthening southwest flow aloft in advance of this next deep
upper trough. When I say shallow, I mean initially not even as
deep as even the boundary layer, which is causing some havoc with
models and the interpretation of the density of this airmass and
its associated temperatures. Look for this shallow cold front to
outrun what will eventually be an open and progressive shortwave
lifting east across the Central and Southern Plains by Friday.
With this scenario playing out, better moisture and even some
instability will be mainly relegated to areas along and east of
I-35 in the ambient warm sector where even a few surface-based
strong storms possible across our far southeast counties early in
the day. Otherwise, a bulk of the lift arrives after the deeper
and higher quality moisture exits east-southeast of the area. So
right now any precipitation we do get Friday and Friday evening
will be in the form of showers and isolated storms early, morphing
into some modest chances for a cold rain across the east by
Friday evening and after the shallow cold front has well passed
the area.

As for temperatures, as noted above, I`m feeling the national
blend of models is too warm with the assumption the warm advection
above the frontal inversion is actually mixing through the very
shallow dense arctic airmass near the surface. I did warm up highs
both Friday and Saturday some, but not to the magnitude of the
national blend with values mostly in the 40s, after a chilly
start to the day Saturday in the 20s northwest, to the 30s
southeast. We will be watching another strong shortwave diving
southeast across the Central Rockies next Saturday with the base
of a broad, latitudinal trough across much of the U.S. The cold
air near the surface deepens late Saturday and through Christmas
Eve, but is not expected to be excessively cold. The colder air
will actually arrive shortly after Christmas day. With the broad,
large scale trough continuing to deepen across the Central U.S.
through Christmas and after (as noted by 500mb thickness below
540DM), we`ll need to watch for signs of mid level frontogenesis
that could result in some light snow, but as of right now, it is
too far out to determine if this process will actually occur, not
to mention moisture availability and instability aloft. We`ll
continue to monitor as the transition longwave pattern across the
U.S. becomes better sampled by the upper-air network of Southern
Canada and the U.S. One thing is for certain is that some of the
coldest air looks on tap for our area between Christmas and New



.AVIATION... /Issued 1210 PM CST Sun Dec 17 2017/
18 UTC TAF Cycle

Concerns---Ceiling and visibility trends. Potential for VCSH on

Poor flying conditions are expected over the next 24 to 30 hours
due to low ceilings and visibility. Widespread IFR stratus is
expected to persist into the early to mid-afternoon hours today at
the Metroplex before cigs lift to MVFR. IFR cigs and vis should
both return this evening. With a low temperature-dewpoint
temperature spread, vis down to 2SM is expected with some BR/DZ.
Cigs could become lower this evening and an introduction of FG may
be warranted, but at this time, confidence isn`t high enough to
add this to the TAF. MVFR cigs may return to the Metroplex TAF
sites as increasing low level flow helps to lift cloud bases on
Monday morning. In the D/FW extended, I`ve added a mention of of
VCSH as low level lift increases. There may be a threat for
thunder, but this potential is too low to include this far out in
the TAF.

IFR cigs will likely persist at Waco through the entire 18 UTC
TAF cycle with IFR vis starting tonight. Showers appear to be a
good bet Monday morning and I`ve inserted a VCSH around 15 UTC at



Dallas-Ft. Worth    48  57  53  62  51 /   0  30  30 100  60
Waco                47  56  56  63  47 /   0  50  60 100  20
Paris               44  55  54  61  50 /   0  10  40  90  90
Denton              44  54  52  62  47 /   0  20  30  80  60
McKinney            44  55  53  60  49 /   0  20  30  90  70
Dallas              47  57  54  61  51 /   0  30  40 100  70
Terrell             46  58  54  63  51 /   0  40  50 100  70
Corsicana           49  57  56  62  51 /   0  60  60 100  60
Temple              48  56  56  63  47 /   0  60  70 100  20
Mineral Wells       45  56  50  61  44 /   0   5  20  70  30




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