NWS Area Forecast Discussion

FXUS64 KFWD 180557

Area Forecast Discussion
National Weather Service Fort Worth TX
1157 PM CST Sun Dec 17 2017

Difficult aviation forecast with respect to low clouds and fog
potential tonight. Fog has become dense in some areas, but since
low clouds have already settled in over the area, visibilities are
most impacted at stations with temperatures in the mid 40s which
are lower than the average mid 40s dewpoints in the region. Thus
fog is more advective in nature, and the heat of the Metroplex is
likely to keep DFW and DAL visibility higher than GKY which
cleared out earlier this evening and is in the low 40s. If stratus
were to build down to the surface, these visibilities may
diminish areawide. Monitoring temperatures will likely be the key
to what occurs.

Regardless of the difficult visibility forecast it is a given
that CIGs will be in the LIFR to IFR category through the night
and into the morning. Believe the RAP and NAM have the right idea
with the inversion remaining strong through the day tomorrow so
will show IFR CIGs prevailing through mid afternoon with perhaps a
brief period of MVFR by late afternoon and evening. By Monday
evening, stronger south winds near the surface will bring
warm/moist advection to the area and CIGS/VSBY will fall again to
LIFR or possibly VLIFR.

Very light southeast winds will become southerly tomorrow
afternoon at speeds of 5-10kt. Still have opted not to mention any
showers in the TAF through this set of TAFs, but rain/storms may
be needed by Tuesday morning.



.UPDATE... /Issued 800 PM CST Sun Dec 17 2017/
The low clouds continue to be the main influence on the sensible
weather forecast overnight. Where these clouds cleared during the
day temperatures fell quickly after sunset and some fog has
started to develop. Meanwhile where the clouds remained,
temperatures likely will not fall much tonight. Low clouds should
fill back in over the entire area tonight. Have updated the sky
grids and forecast lows to account for these trends. Patchy dense
fog looks most likely to impact the region south of I-20 for the
next few hours. As low clouds fill back in this fog may become
less dense so will monitor trends before deciding on any dense fog
advisories.  TR.92


.SHORT TERM... /Issued 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.



.LONG TERM... /Issued 303 PM CST Sun Dec 17 2017/
/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



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


Dense Fog Advisory until 9 AM CST Monday for TXZ091-100>102-



NWS FWD Office Area Forecast Discussion