FXUS64 KFWD 121756

Area Forecast Discussion
National Weather Service Fort Worth TX
1156 AM CST Wed Dec 12 2018

Thick mid and high level cloud cover continues to stream
overhead, which has limited insolation and kept temperatures
mainly in the 50s thus far today. The exception has been across
the southeast where warm air advection has allowed temperatures to
climb to near 60, so the forecast will be left alone in that
region. Elsewhere, high temperatures have been lowered a few
degrees based on the latest trends and the most recent guidance.

Otherwise, we continue to assess the potential for winter weather
for late Thursday into early Friday across the western half of the
forecast area. It still appears that the potential for
accumulating snow will be primarily limited to the higher
elevations across the west and southwest counties, and we will
have updated information available with the afternoon forecast



Southerly low level flow will continue today and tonight ahead of
a pair of shortwave troughs approaching from the west and
northwest, as well as a cold front approaching from the north.
The concern for the short-term has been assessing the potential
for MVFR ceilings as low level moisture increases. Low clouds have
in general been on the increase over the past few hours, so there
will at least be a TEMPO group for MVFR in the near term for all
airports. Guidance has been inconsistent regarding the prevalence
of low cigs, with the majority showing VFR conditions resuming
this afternoon. However, based on trends and some of the more
recent guidance, a prevailing MVFR group may eventually be needed
for the afternoon and evening time frame.

The cold front is set to push through the region on Thursday,
reaching both the DFW Metroplex and the Waco area by early
afternoon. A shift to northwest winds will accompany the front,
with speeds increasing through the afternoon. The 22KT in the
extended portion of the DFW TAF may need to include some stronger
gusts in future forecast for the Thursday night / Friday period.

At the upper levels, strong lift will occur as the upstream
systems merge, wrap-up and transition into a potent cut-off low
over North and Central Texas. A few thunderstorms cannot be ruled
out, though the better instability and stronger ascent will be
just east of the area. In addition, a low-end chance of a rain-
snow mix will exist (with little or no accumulation) late Thursday
into early Friday, but this will be addressed in later forecasts.



.SHORT TERM... /Issued 455 AM CST Wed Dec 12 2018/
/Today and Tonight/

Our impending winter storm is currently bringing snow to the North
Cascades of Washington State and British Columbia. However,
tonight's rainmaker is the upper low moving equatorward along the
Pacific coast of Baja. As the upstream jet streak associated with
the Pacific Northwest storm surges into the Intermountain West
today, the Baja low will begin to track east across the high
terrain of Mexico. With the feature maintaining its latitude, it
will be forced to open up, eventually spreading mid-level lift
across Texas this evening. Its downstream conveyor is already
streaming high clouds across the Lone Star State, and these clouds
will continue to clutter the skies today. Temperatures are 10 to
20 degrees warmer than this time yesterday, and despite limited
sunshine today, the head start to the day and additional low-
level warm advection should assure highs exceed Tuesday's by a few
degrees. When forcing for ascent arrives this evening, elevated
showers will approach from the west, becoming more numerous as
they ingest the richer moisture across the eastern half of the
state. Mid-level lapse rates will be marginal for thunder, but a
few embedded cells may be intense enough to produce lightning.
These updrafts will intensify across East Texas, but the potential
for strong/severe storms will be greater east of our area as the
shearing system moves into the Ark-La-Tex Thursday morning.



.LONG TERM... /Issued 455 AM CST Wed Dec 12 2018/
/Thursday Through Next Week/

Not that it's any secret, but big changes await Thursday into
early Friday morning regarding sensible weather across North and
Central Texas. A little potpourri of everything will occur from
start to finish on Thursday, with the strongest, non-convective
winds we've seen in quite a long time later in the day and through
early Friday morning.

In addition to the intense north and northwest winds arriving
later in the day, other impacts and weather elements we'll be
dealing will be a window later Thursday morning to Thursday
evening for for a strong or even marginal severe storm or two
across our far east and northeast counties. South to southwesterly
effective bulk shear values will not be in question, with values
40-50 knots, as wind fields from 850mb upward to jet level
continue to intensify through the day. Luckily, 0-1km bulk shear
will be only around 10 knots, though some backing could occur in
advance of the surface low and surface dryline tracking southeast
across the heart of the CWA. The one detriment for the potential
for severe storms will be moisture quality, with the Gulf of
Mexico having been well-scoured by recent arctic surface cold
fronts. Surface dew point temperatures are only forecast to be in
the 50s across these areas, so surface-based instability will be
fairly marginal, but mid level lapse rates will be steepening
quite a bit with the approach of the strong mid-upper level low
pressure system. Models are now finally coming into some sort of
agreement with a further south track, somewhere south of the I-20
corridor and into Central Texas. This will likely limit the dry-
slotting somewhat further south of our CWA. Most convection
"should" be elevated in nature, though obviously the strong winds
aloft and shear component could lead to mesocyclone development in
the mid levels with hail and add to the already gradient-driven,
increasing gusty winds at the surface. At this time, due to the
low level moisture/CAPE, any tornado threat should remain south
and east of area through the day.

Now, for the late Thursday afternoon into Friday morning time
period everyone is waiting for. With the further southward track
of the mid level system, the potential for seeing the "backward J"
PVA signature and intense tropopause fold (down to between
300-500mb) later in the day across areas west of I-35 will combine
with intensifying 925mb to 700mb frontogenesis in the northwest
"TROWAL" region of the system for the potential of a heavy and wet
snow either mixing in with a cold rain early on along and west of
U.S. 281 from Jacksboro to Evant and points west. The potential
for the oft-used term "dynamic cooling" will be high and allow the
column just above the surface to cool into a more favorable,
sub-0 degrees C dendritic zone due to the intense ascent. The
issue will be how it correlates with more subtle low level cold
advection, as the airmass upstream is not expected to be overly
cold. That said, the effect of wet-bulb cooling cannot be
understated here. Per detailed collaboration with our western
neighbors at WFOs EWX, OUN, and SJT, it was agreed upon to keep it
a rain/snow mix, though with enough cooling to input snowfall
amounts of 2-5 inches despite surface temperatures being above
freezing. We don't know how much surface temperatures will be
affected and how much impact will actually occur on road and
highway surfaces, but this much is certain: even low snow-water
ratios with a heavy/wet snowfall will at minimum cause slushiness
and impacts with any bursts. Typical problem areas such as Ranger
Hill on I-20 will especially feel the effects. A Winter Storm
Watch was agreed upon among our office and WFOs OUN/SJT with the
mix of a heavy wet snow, slushiness, and expected very gusty and
strong north-northwest winds from later Thursday afternoon through
at least daybreak Friday morning, as the cold core upper low
transits southeast across the region.

Some are probably wondering -- what about the high winds in
combination with the heavy snowfall? Well, with the wetness of
the snow with temperatures above freezing, yes it will be a
problem, but shouldn't blow around as much as one would think.
Thus, a Winter Storm Watch for heavy snow was preferred over a
Blizzard Warning. Regardless, it won't be pleasant late Thursday
into early Friday. As for the intense winds, they should remain
just below High Wind criteria (sustained 40 mph with gusts to 58
mph), and for simplicity sake early this morning, no High Wind
Watch was issued, but will need to be looked at very closely later
today for any additional highlights. As for temperatures Thursday
and Thursday night, with so many variables in the game (effects
of wet-bulb cooling, strength of low-level thermal advection, and
precipitation rates), it was agreed upon to go with cooler than
guidance blends in the west, but not as cold as it could possibly
get if the aforementioned processes are more intense than
expected. We'll just have to wait to see what really plays out
here. The system should continue to lift east away from the area
during the day Friday, with brisk and chilly conditions continuing
in its wake. Have some lingering light rain or rain-snow mix
across especially our east-northeast counties, including the DFW
and Sherman/Denison areas. At this time, temperatures are expected
to warm enough at the surface rise with partial west-east clearing
through the day that at this time, major impacts on roads and
travel are not expected, but do bear watching, especially Friday
morning for the usual metro commutes. The kiddos may get to see a
few big heavy flakes fall and possibly briefly accumulate on
grassy and elevated surfaces with a "dusting" in general terms,
but it wouldn't last long. Highs will slowly warm into the 40s,
though obviously highs will be dependent on any snow on the
ground, especially our far western counties and the timing of any

Shortwave ridging is expected to evolve aloft across the Southern
Plains and North Central Texas this coming weekend. Moderating
temperatures with highs in the 50s (to possibly lower 60s in the
southwest counties) are expected with the passage of a broad
surface ridge on Sunday. As already noted, temperatures out west
will depend on any residual snow on the ground from our potent
late week system. Otherwise, dry conditions are expected early
next week (Monday and Tuesday) as we await yet another storm
system developing across the Desert Southwest and Intermountain
West for later next week. For now, have removed low PoPs Tuesday
and will introduce very low convective chances on Wednesday with
low level warm advection ensuing with mid level ridging
dampening. As one would expect, medium range models and their
ensemble solutions are all over the place, so no bold predictions
for what to expect late next week.



Dallas-Ft. Worth 61 55 61 39 49 / 0 30 60 60 40
Waco 63 54 61 38 49 / 5 40 30 40 20
Paris 58 54 62 41 48 / 0 70 70 70 60
Denton 60 53 59 38 48 / 0 30 60 70 40
McKinney 60 56 61 39 48 / 0 40 60 70 50
Dallas 62 56 62 39 49 / 0 40 60 60 40
Terrell 61 57 63 39 50 / 0 50 50 60 40
Corsicana 64 56 62 39 48 / 0 50 40 50 30
Temple 64 52 61 38 50 / 5 40 20 40 20
Mineral Wells 61 49 53 35 48 / 0 20 70 60 30


Winter Storm Watch from Thursday afternoon through Friday
morning for TXZ100-101-115-116-129-130-141-142.



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