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

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FXUS64 KFWD 070046 AAA
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
646 PM CST Wed Dec 6 2017

.AVIATION...
/00Z TAFs/

VFR conditions will prevail with ceilings generally FL100-200.
Light winds will reorient themselves to the north on Thursday.

Bands of rain, some mixed with sleet, continue to stream across
Central Texas. The bulk of this precipitation will remain south of
Waco tonight. As a deepening shortwave, now over Colorado,
approaches on Thursday, lift will steadily increase within the
lingering mid clouds. Despite the intensity of this upglide, which
will be sufficient to produce snow crystals a few miles above our
heads, additional dry advection within the sub-cloud layer will
likely be too much to overcome. However, in Central Texas, where
precipitation has been working against the dry advection within
the postfrontal layer, a sprinkle (perhaps with an unmelted flake
or two) will be possible Thursday afternoon/evening. At this time,
the potential is too low to include in the Waco TAF, but the
strength of the lift will continue to be assessed for subsequent
issuances. Fall streaks (sublimating snow) may be visible in the
Metroplex, creating areas of turbulence, but precipitation at the
surface is much more unlikely as far north as the I-20 corridor.

Although northerly momentum and its associated dry advection will
continue in the lowest 2 miles above the surface, the broad
surface high settling into the region will assure generally light
(westerly/northwesterly) winds at the surface overnight. The
direction will eventually veer to the north early Thursday as a
reinforcing shot of cold air arrives in advance of the upper
trough. Wind speeds will increase by mid-morning, with frequent
gusts in excess of 25kts during the daylight hours Thursday.

25

&&

.SHORT TERM... /Issued 338 PM CST Wed Dec 6 2017/
/Tonight and Tomorrow/

During the day today we received several reports of sleet (mixed
with rain) across Central Texas from Comanche to Anderson County
and south. The rain/sleet mix across our southern counties will
continue to diminish from north to south this evening and tonight,
and all precipitation is expected to end before daybreak
tomorrow. No accumulations or impacts from sleet is expected, and
none have been reported today.

Cloudy skies will remain across most of the region tonight with
the exception being north of Interstate 20. In particular as you
get closer to the Red River, thinning clouds are expected but
should not become completely clear as a steady stream of dense
high cloud continues in advance of the arrival of our next upper
level trough. As the trough approaches from the northwest late
tonight, it will start to push some cooler and drier air into the
region. Most of the cooler air will arrive during the day
tomorrow, but the balance between some cold air advection late
tonight and cloud cover makes for a challenging overnight
temperature forecast. Across Central Texas, dense cloud cover is
expected to keep overnight lows slightly warmer in the mid to
upper 30s. This allows for about a 5-10 degree temperature drop
from current afternoon readings as the rain clears the area.
However, along and north of Interstate 20, overnight lows will
likely be cooler due to thinner cloud cover and the arrival of the
cooler air. For this area, will advertise lows ranging from the
upper 20s to mid 30s with the higher readings in the immediate DFW
Metroplex.

Cold air advection will increase during the day on Thursday, and
unfortunately wind speeds will also increase as another
front/surface high brings reinforcing cold air to the region. It
will be a cloudy and breezy day for most of the region, but
again the cloud cover may thin late in the day near the Red River
as the upper level trough arrives. High temperatures tomorrow
will range from the upper 30s in the north to upper 40s in the
south, and high temperatures may have to be decreased for parts of
Central Texas for reasons discussed below.

The main challenge for tomorrow is the small potential for a light
rain/snow mix across parts of the region during the afternoon and
evening hours. The approaching upper level trough is strong and
will spread considerable lift across the region. Isentropic charts
indicate isentropic lift above about 600 mb will increase during
the afternoon hours, in particular south of Interstate 20. With
some possible weak elevated instability, it is possible we could
see isolated to scattered elevated showers develop. Cold air
advection during the day is expected to further dry out the lower
levels of the atmosphere which will work against any precipitation
reaching the ground, but prefer to carry a low chance for a light
rain/snow mix during the afternoon hours south of Interstate 20.
Forecast soundings and wet bulb temperature profiles indicate
snow could be the main winter precip type (mixed with rain) but
some sleet pellets could also occur. The surface and very near
surface layer are both expected to remain above freezing, but IF
any snow makes it down to near the surface, there`s a decent
chance it will melt. IF any snow survives the trip to the surface,
no accumulations are expected due to melting at the surface. If
we do get elevated convective development (or virga due to
evaporation depending on the dryness below the sub cloud layer),
evaporative cooling would be another factor into cooler daytime
temperatures across part of all of Central Texas tomorrow
afternoon.

JLDunn

&&

.LONG TERM... /Issued 338 PM CST Wed Dec 6 2017/
/Thursday night through next Wednesday/

While generally more quiescent weather is in store for much of
the long term portion of the forecast, we`ll be keeping our eyes
on our far south and eastern counties into Thursday evening as
guidance is now offering some subtle hints at a continued
precipitation potential. Otherwise, a gradual warming trend is
expected into the weekend and early next week as above normal
temperatures return (some upper 60s to near 70 degree highs are
now in the forecast for Monday).

For Thursday evening-night: large scale height falls will
continue to envelope the region ahead of a sharp upper-level
trough axis which will pivot through the region Thursday night and
into Friday morning. Some moisture will remain in place above 700
mb per forecast soundings across the southeastern half of the
forecast area. The concerning aspect to this portion of the
forecast is that lift and elevated instability may still be
present above 700 mb within this region of lingering moisture. An
extremely tight packing of the isotherms ahead of the trough axis
should facilitate the development of a rather robust--yet narrow
and focused--band of frontogenesis between 500 and 700 mb across
our far south and eastern counties. Associated robust isentropic
ascent can be found in model guidance along the 310 K theta
surface within a zone of relatively steep lapse rates. While
coarse-resolution guidance remains quiet on the precip/QPF front,
we oftentimes see precipitation develop in these scenarios where
guidance has misdiagnosed the magnitude of moisture/instability
present aloft. Indeed, the 18z NAM is now showing a sliver of
MUCAPE above 600 mb during this time, which raises some concern
for the development of elevated convective elements. For this
forecast, we`ve added a mention of rain and snow south and east of
an Athens to Killeen line Thursday evening given this latest
guidance. We`ll need to closely monitor trends in model guidance
though, since if convective showers do materialize, wet bulb
temperatures would ultimately support an all snow precipitation
type as surface temperatures fall to near and below freezing.
Obviously, there are a lot of "IFs" in this portion of the
forecast given the subtleties involved, and as a result, we won`t
be showing any snow/sleet accumulations at this time given the
amount of uncertainty.

Otherwise, Friday morning is looking like it`ll be one of the
coldest mornings since this past January as temperatures dip into
the lower and mid 20s across the northern portions of the forecast
area, and into the upper 20s elsewhere. Some guidance is even
hinting that some isolated lows in the upper teens are possible in
the most sheltered and outlying areas, although there may be some
lingering high cloud cover to mute this potential somewhat. Still,
everyone is expected to experience sub-freezing temperatures by
Friday morning, so take care of any tender vegetation which may
have enjoyed the recent November warm spell.

Rather robust upglide will continue, even on the backside of the
departing trough on Friday, but with the best moisture sliding out
of the area, we`ll show a dry and sunnier forecast Friday
afternoon.

Temperatures will moderate nicely over the weekend with highs in
the lower to mid 60s anticipated on Sunday. Another cold front is
forecast to move through the region Monday afternoon and evening
with a reinforcing shot of somewhat cooler air for Tuesday. High
temperatures ahead of the front on Monday near 70 degrees are not
out of the question, especially across the southern half of the
region. Very limited moisture means this front will come through
dry.

Carlaw

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    36  43  28  55  36 /  10  10  10  10   0
Waco                36  46  26  55  35 /  20  20  10  10   0
Paris               32  42  23  48  31 /  10  10  10  10   0
Denton              29  42  22  53  33 /  10  10  10  10   0
McKinney            31  42  24  51  33 /  10  10  10  10   0
Dallas              37  44  28  53  37 /  10  10  10  10   0
Terrell             32  45  24  52  32 /  10  10  10  10   0
Corsicana           34  46  27  52  33 /  20  20  20   5   0
Temple              36  47  28  55  35 /  50  20  20  10   0
Mineral Wells       31  42  24  55  33 /  10  10  10  10   0

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

25/14



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