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

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FXUS64 KFWD 110558
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
1158 PM CST Mon Dec 10 2018

.AVIATION...
/6Z TAFS/

Surface high pressure continues to drift eastward tonight and
light southerly winds are now in place across the region.
Southerly flow will increase during the day on Tuesday with high
clouds overspreading the region. A fast moving disturbance will be
spreading into northwest Mexico late Tuesday night with low level
moisture rapidly spreading northward into Texas. MVFR cigs will
likely spread north out of central Texas late Tuesday night into
Wednesday morning, so we`ve added a BKN015 to the extended portion
of the DFW TAF.

Dunn

&&

.SHORT TERM... /Issued 321 PM CST Mon Dec 10 2018/
/Tonight through Tuesday/

The weather should remain relatively quiet during the next 24
hours. Morning surface analysis revealed a ridge of high pressure
across much of the southern two-thirds of the Continental United
States. This high pressure will remain in control tonight, leading
to calm, clear, and cool conditions across North and Central
Texas. Temperatures should be a bit warmer than last night as
winds shift from the south. Lows tonight will be in the mid to
upper 30s most places, though a few sheltered locations may see
their overnight lows fall to near the freezing mark.

High pressure will break down and shift to the east on Tuesday as
an upper-level trough propagates through the Northern Rockies.
Strong differential cyclonic vorticity advection over the Central
and Northern Rockies will lead to pressure falls across the lee
of the Rockies and the Southern High Plains. The resultant
isallobaric wind response will produce strengthening southerly
flow (i.e. warm air advection). High temperatures will be about
5-10 degrees warmer than today across the region, with highs
likely breaking into the 60s across our entire forecast area.
Winds will be a little on the breezier side at about 10-15 MPH,
with perhaps a few gusts close to 20 MPH. Later in the day, a
cirrus canopy should begin to infiltrate our western and
especially our northwestern counties (mostly likely after high
temperatures are already reached) as a 90+ KT subtropical jet
begins to impinge upon the South-Central United States, with this
cirrus spreading east into the overnight hours.

Godwin

&&

.LONG TERM... /Issued 321 PM CST Mon Dec 10 2018/
/Tuesday Night into Early Next Week/

The main focus for the long-term period is the potential for a
very dynamic, impactful storm system during the latter half of
this week, especially Thursday into early Friday. This system has
the possibility to be multi-faceted in terms of impacts, but there
is considerable uncertainty with regard to the evolution of the
system and the resultant impacts to North and Central Texas.
Regardless, we are closely monitoring the potential for the
following:

1) Strong winds (anywhere from a west-northwest to a northerly
direction) Thursday into early Friday across much of the region
(medium-high confidence).

2) Any potential for snowfall late Thursday into early Friday
primarily across the western half of North Texas (low confidence).

3) The potential for strong storms Thursday towards East Texas
(low confidence).

Now for the details:

The evolution of the mid/upper pattern, and its impact on
pertinent surface features (and sensible weather), will rely on
the complex interplay of at least two shortwave troughs
approaching the southern Plains through mid/late week. Initially,
in the wake of a low-amplitude 500mb ridge sliding east, return
flow across Texas will transport 40s/50s dew points, increasing
temperatures, and some pockets of low-level stratus northward
across our area Wednesday.

While our local weather should be relatively quiescent through
Wednesday night, the aforementioned shortwave impulses upstream
should begin to interact to set the stage for at least some
impactful weather across North and Central Texas starting on
Thursday.

Medium-range guidance is in relatively good agreement on the
general idea that a strong 120+ kt north/northwesterly upper jet
will allow a sharp shortwave trough to dig south/southeast across
the southern High Plains and generally towards the Red River
Valley into the day Thursday. Meanwhile, a secondary, weaker
southern-stream impulse is expected to lift east/northeast across
Texas through early Thursday. This initial impulse and related
warm-advection to its east is expected to generate at least a few
shallow showers during the morning hours, generally along/east of
the I-35 corridor.

/*Strong Storm Potential*/
While likely the lowest potential impact, any strong storms would
be the first of the three facets mentioned above to affect our
region. Strengthening kinematic profiles (characterized by
southerly surface winds veering to robust west/southwest mid-level
flow) would offer a conditional environment favorable for
organization of deeper, sustained updrafts. However, there are
several negative influences on strong/severe weather potential: 1)
Passage of the initial wave Thursday morning likely veering low-
level flow some (reducing convergence), 2) only modest levels of
return moisture, and 3) the possibility of a relatively early
passage of a dryline/front that would minimize time for any
destabilization, especially given expected cloud cover early
within the warm conveyor. In turn, any potential will likely be
confined towards Paris/Sulphur Springs/Palestine through the
morning/afternoon, and surface-based buoyancy appears limited even
this far east. Still, there could be some potential for a few
stronger convective gusts in any thunderstorms, and we`ll continue
to monitor the possibility that a slower progression of the
referenced surface/mid-level features could slightly boost
chances for a few stronger storms across East Texas.

/*Strong Wind Potential*/
The second issue with which to contend is the strengthening winds
which will be spreading east/southeast across North/Central Texas
through Thursday night. As the secondary, very energetic impulse
approaches the region, focused ascent along its leading edge
should bring a surface cyclone southeastward across parts of the
southern Plains on Thursday.

Where these features track will notably impact the wind direction
and max gust potential. Using the surface low as a simplified
proxy for the entire system, a more northerly/easterly track
(generally along/north of the Red River to the Arklatex) would
bring strong westerly winds behind the initial dryline/front during
the day, followed by strong north/northwesterly flow behind the
main cold front later in the day. This track would also place most
of our area under steeper low-level lapse rates, enhancing gust
potential. In fact, the upper-end of the gust potential could be
near 50 kt, based on some forecast thermal/kinematic profiles. A
more southerly/westerly track (generally south of I-20) would
confine steeper low-level lapse rates, a higher gust factor, and
the strongest overall flow more west of I-35 and south of I-20.
These are spatial details that are simply too difficult to assess
with much certainty around 60-72 hours in advance so will continue
to advertise strengthening northwesterly flow on Thursday
(especially towards western North Texas and Central Texas), with
gusts up to 35-40 kt. Either way, some impacts from windy
conditions are likely to be realized. Start planning to secure
loose decorations and other items before Thursday!

/*Wintry Weather Potential*/
The last, and most attention-grabbing (per usual), forecast factor
is the possibility of snow late Thursday into early Friday across
parts of the region. As the mid/upper low digs southeast across
the southern Plains on Thursday, an amplifying 700-500mb
frontogenetic circulation (aided by deformation processes) is
forecast to establish somewhere from portions of West Texas
eastward to the Sabine Valley late Thursday. Fairly robust mid-
level ascent associated with this band will likely produce a
generally north-south oriented corridor of at least moderate
stratiform precipitation that could develop east across parts of
Central/North Texas through Thursday night.

With regards to the low-level thermal profile, if you`re a snow
lover, the northern stream is doing no favor. The reason is that
any lead impulses across the central US will generally be of
Pacific origin, with a resultant lack of very cold surface air
over the Plains. Therefore, as any frontogenetic band shifts
across the region, surface temperatures are generally expected to
be in the upper 30s/lower 40s Thursday night. Despite notably
cooling temperatures aloft within the 850-700mb cold conveyor
trailing the mid-level low, the lowest several thousand feet
should be above freezing through the night. Therefore, will
maintain only rain in the forecast at this time.

The largest caveat is that vertical motion in the mid levels may
be fairly robust within this potential comma-head band. If and
where precipitation rates are heavy enough, diabatic effects
(i.e., melting snow locally cooling the environment) may be
sufficient to bring snow to the surface. Medium-range guidance is
hinting at this potential by showing a small "blob" of mid 30s
surface temperatures underneath the modeled deformation band,
which the GFS/ECMWF both shift across parts of North Texas through
Thursday night. It is certainly plausible that some snow could be
found embedded within this band. However, due to the noted
concerns from a lack of antecedent cold air, it will likely
require moderate/heavy precipitation rates to locally cool the
environment enough for surface snow. As mentioned, some guidance
indeed shows this potential over parts of our area, but a look at
ensemble guidance shows the sensitivity of surface precipitation
type to when/where the mid-level system closes off/deepens, as
this will be where precipitation rates would be high enough to
overcome remaining low-level warm air.

With all of that said, this would likely be a focused area, and a
wide range of solutions is plausible with this event. Indeed, the
15Z SREF shows only one outlier solution with appreciable
accumulating snowfall across North Texas. So it`s certainly not an
easy forecast, but the current set-up generally warrants
continuing only a liquid forecast. Still, there exists a very low,
but non-negligible, potential for a locally impactful event late
Thursday into the overnight. Continue to check back with us, local
media, and other trusted weather sources for the latest.

After this system, a progressive pattern persists across the
southern US, with a warming/drying trend into the upcoming
weekend. Thereafter, precipitation chances look to increase
slightly during the first half of next week, as another southern
stream system approaches.

Picca

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    37  63  48  63  52 /   0   0   0  10  20
Waco                35  62  48  65  54 /   0   0   0  20  20
Paris               34  60  45  60  52 /   0   0   0  20  40
Denton              33  63  46  63  50 /   0   0   0  10  20
McKinney            34  62  47  62  52 /   0   0   0  10  30
Dallas              37  63  49  64  53 /   0   0   0  10  30
Terrell             34  62  47  63  53 /   0   0   0  20  40
Corsicana           37  62  47  64  54 /   0   0   0  20  30
Temple              35  61  47  65  52 /   0   0   0  20  20
Mineral Wells       34  65  45  63  47 /   0   0   0  10  10

&&

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

$$


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