DFW FORECAST DISCUSSION
521
FXUS64 KFWD 131242 AAA
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
642 AM CST Thu Dec 13 2018

...Winter Weather Advisory West...Strong Gusty Winds to Impact
Much of the Region...

.AVIATION...
/12Z TAFs/

A powerful storm system will bring low ceilings, thunderstorm
chances, and strong gusty winds.

Patchy showers this morning are helping to moisten the low-levels,
and transient MVFR ceilings will become more persistent as the
day progresses. MVFR visibility in the soupier air across Central
Texas will improve as the boundary layer warms and wind speeds
increase.

As an intense cyclone approaches from the west, strong forcing for
ascent will clutter the radar scope with convection. There will be
a window for thunder late in the day, but the stronger updrafts
will be east of the Metroplex TAF sites into the evening hours.
Steadily veering winds will become westerly and increase in speed
toward nightfall. Speeds will continue to increase tonight, and
the wind speed criterion for an airport weather warning may be met
during the overnight hours. Despite the intensity of the wind, the
direction should pose minimal crosswind issues.

Snow is expected on the higher terrain of western North Texas, but
as the 850mb low reaches the I-35 corridor this evening, the
distance between the snow production zone aloft and the surface
will increase. As a result, little if any snow is expected at our
TAF sites. Any ambitious wet flakes that do reach the surface
would quickly melt, and no impacts from wintry precipitation are
expected. The snow potential will end during the daylight hours of
Friday morning. Although the rain will taper off, the gusty winds
will continue throughout the day.

25

&&

.SHORT TERM... /Issued 457 AM CST Thu Dec 13 2018/
/Today Through Friday/

The strongest storm system in recent memory will impact North and
Central Texas today and tonight. Severe thunderstorms will be
possible in the warm air ahead of a cold front that will usher in
extraordinarily gusty winds. Although the postfrontal air will not
be particularly cold, intense dynamic cooling within the core of
the cyclone will produce some snowfall, particularly across the
higher terrain of western North Texas where a winter weather
advisory will be in effect.

While this storm system is still intensifying in New Mexico, an
unrelated shearing impulse is tracking through the eastern half
of the state. Although it has been gradually dissolving into the
subtropical jet downstream of the more potent low, it has
maintained enough fortitude to generate some elevated
thunderstorms within North Texas. However, its potency has been
most effectively realized with the richer moisture across
Southeast Texas, the ongoing complex within which will track into
East Texas this morning. Outside of the isolated storms, the
echoes within North and Central Texas have yielded little rainfall
at the surface overnight, but this has served to moisten the
column in advance of the stronger system still upstream.

The remainder of the morning will be the proverbial calm before
the storm. But as strong forcing for ascent begins spreading
across the region this afternoon, showers and thunderstorms will
proliferate across North and Central Texas. The most favorable
area for strong/severe thunderstorms will be across our eastern
and northeastern zones, but the boundary layer instability will
likely be tempered by the morning precipitation and lingering
cloud cover. While the best shear will lag well behind the best
instability, the increasing vorticity as the synoptic-scale
cyclone approaches will enhance updraft strength and hail growth
potential. However, the quality of boundary layer parcels will be
diminished both by morning rain cooling the near-surface air and
the initial system`s disruption to the low-level pressure fields.
Based on the track of the surface low, our northeast zones will
see the greatest backing of the surface winds, but the tornadic
potential will be dampened by the quality of the boundary layer
parcels and the rapid storm motion that will tend to limit the
lifespan of individual updrafts. An attendant wind threat may
result from the downward transport of stronger momentum aloft.

Within the northwest quadrant of the approaching cyclone, winds at
the 925mb level will exceed 50kts. Some of this postfrontal
momentum will reach the surface in our western zones this
afternoon, then spread across the region from west to east through
tonight. Although the track of the surface low, which will be
right through the heart of our CWA, will initially limit the wind
speeds, the strong winds will eventually envelop the bulk of the
region. The wind advisory area will remain intact; high wind
warning criteria (sustained 40 mph, gusts to 58 mph) are unlikely
to be met. While these winds will be of lesser strength than those
that accompany a severe thunderstorm, the duration of the winds
today will increase the likelihood of downed tree limbs, power
outages, and other wind damage. Secure any outdoor furniture and
holiday decorations.

Most winter weather events in North Texas involve an arctic air
mass, but this event will be entirely driven by dynamic cooling
within the core of this powerful cyclone. Despite the intensity of
the system, the snowfall forecast is by no means a slam dunk; it
may be more akin to a blindfolded slam dunk that could go terribly
wrong. Guidance is in remarkably good agreement with the large
scale features, including the track of the 850mb low. But with a
potentially unprecedented scenario within the working history of
numerical guidance, this consensus can`t preclude a deviation in
the eventual path. Regardless, confidence continues to improve in
accumulating snow in our western zones late this afternoon and
into tonight.

It`s not often we talk about the elevation of the snow line in a
winter weather forecast, but it will be a critical piece of the
puzzle today. The favored areas for snow at the surface will
initially be the higher terrain of the Big Country to our west,
then gradually spread east. Even if snow is mentioned in the
forecast for your area, rain may still be the dominant
precipitation type, particularly early in the event and along the
fringes of the wintry weather potential. Heavier bursts of snow
may accompany the core of the low, but surface temperatures above
freezing and low snow/water ratios should keep totals under our
warning criteria (4 inches in 12 hours), hence the advisory rather
than a warning. East of the advisory area, snow production aloft
will continue within the core of the cyclone, but warmer
temperatures within a mile of the surface will enhance melting,
and little if any snow should reach the surface along and east of
the I-35 corridor.

For areas that see significant snow, visibility will be limited
with strong wind scattering the falling snow, but once on the
ground, the heavy wet snow will be less willing to blow around.
Lightning has already accompanied some of the snow in West Texas,
and thundersnow will be possible within North Texas as well. These
heavier snow showers will be more likely to accumulate and may
even do so on roadways. Roads that accumulate snow, particularly
those more heavily travelled, should remain slushy. However, the
snow cover may allow temperatures to dip below freezing Friday
morning, and the morning commute could feature some icy bridges
and overpasses in the advisory area.

25

&&

.LONG TERM... /Issued 457 AM CST Thu Dec 13 2018/
/Friday Night Through Wednesday/

Our vigorous low pressure system will continue to move readily
east into the Lower Mississippi Valley Friday night, then across
the Southern Appalachians and southeast states on Saturday. The
residual and deep cyclonic north-northwest flow in wake of this
system will easily be able to translate momentum down to the
surface and maintain gusty northwest winds 15-25 mph over most of
the CWA Friday evening, before decoupling of the much drier
airmass and a slow-relaxing pressure gradient allows winds speeds
to diminish by Saturday morning. The northwest winds will remain
brisk east of I-35 on Saturday, as similar processes of mixing
keep speeds up to at least 10-15 mph. Otherwise, ridging aloft and
associated strong subsidence will allow for plentiful sunshine,
though colder conditions this weekend with lows in mainly in the
30s with highs struggling into the 50s Saturday, but modifying
into the lower-mid 60s on Sunday. One quandary will be if we have
any lingering effects from snowfall over our current Winter
Weather Advisory area and impacts on both low and high
temperatures. For now, I did go slightly cooler, but not much
cooler without really knowing how this all plays out.

Moving into the early part of next week, a strong northern stream
shortwave disturbance will move southeast from the Central/Northern
Plains into the Mississippi and Tennessee Valleys. This system
will dampen the shortwave ridge overhead and allow for a
reinforcing shot of cooler, but not extremely cold air southward.
Meanwhile, mid level energy will amplify southward across the
Eastern Pacific Ocean early next week and push a broad and weak
lead shortwave within the southern branch of the westerlies toward
West Texas. The reinforcing cooler airmass from the north and
increasing mid-high cloudiness arriving from the west will likely
mean slightly cooler days, especially across the northeast half
of the CWA Monday-Tuesday. Modified low level moisture return and
the return of southerly breezes will help low temperatures remain
more mild between 35-45 degrees.

Otherwise, I did leave some low chances for showers in the mid-
late week period, as divergent southwest flow aloft occurs
overhead in advance of an organizing system over the Rockies and
Desert Southwest. Medium range models and their ensemble members
continue to struggle with the energy within the main trough to our
west. In addition, uncertainty on our airmass modification and
resulting instability has me leaving any mention of thunderstorms
out of the forecast(for now). We`ll focus more on this system this
weekend and beyond once we have our current system long gone. It
does appear it will be much milder mid-late next week with no
concern for any wintry precipitation at this time. South-southwest
low level winds, with plentiful cloud cover associated with the
developing southwest flow aloft will generally hold low
temperatures in the 40s, while high temperatures only modify into
the upper 50s to mid 60s.

05/

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    62  40  49  38  59 /  80  90  70   5   0
Waco                63  39  49  38  59 /  50  80  50   5   0
Paris               63  44  49  38  56 /  90  90  70  30   0
Denton              60  39  49  36  59 /  80  90  70   5   0
McKinney            63  40  48  37  56 /  90  90  70  10   0
Dallas              64  40  49  38  59 /  80  90  60  10   0
Terrell             65  42  50  38  57 /  80  80  60  10   0
Corsicana           65  40  50  38  56 /  50  80  50   5   0
Temple              63  37  49  38  59 /  30  80  40   0   0
Mineral Wells       55  35  44  33  56 /  80  80  60   5   0

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 3 PM this afternoon to noon CST Friday for
TXZ092>095-103>107-118>121-133-134-145-146-158>161-174-175.

Winter Weather Advisory from 3 PM this afternoon to 9 AM CST
Friday for TXZ100-101-115-116-129-130-141>143-156.

Wind Advisory from noon today to noon CST Friday for TXZ091-
100>102-115>117-129>132-141>144-156-157.

&&

$$

25/05

NWS FWD Office Area Forecast Discussion