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Area Forecast Discussion (AFD)

Issued by Fort Worth/Dallas, TX (FWD)

FXUS64 KFWD 091755

Area Forecast Discussion
National Weather Service Fort Worth TX
1155 AM CST Sun Dec 9 2018

/18Z TAFs/

Much drier air has filtered in across North and Central Texas as 
a surface high pressure takes hold across Oklahoma. This dry air 
led to a quick dispersion of the MVFR ceilings, and has left all 
North Texas TAF sites in the VFR category. Waco will likely 
remain in MVFR for a few more hours before joining the rest of 
the terminals in VFR. With no major modification in the airmass, 
expect VFR to remain across North Texas through the next 24-30 
hours. The threat for fog across the TAF terminals will be limited
due to strong 10-15 kt winds just above the surface. Lighter 
winds are expected across the Bowie cornerpost, leading to a 
higher fog potential. 



.UPDATE... /Issued 1045 AM CST Sun Dec 9 2018/

Surface high pressure has settled across Oklahoma, filtering in 
much drier air into the region. Latest satellite trends indicate a
slightly faster clearing trend than previously expected. Given 
this earlier clearing trend, bumped expected high temperatures up 
by several degrees in many locations, especially where the sun has
been shining for several hours. 

All other weather elements remain unchanged, with northerly to
northwesterly winds continuing through today. 



.SHORT TERM... /Issued 329 AM CST Sun Dec 9 2018/
/Today Through Tonight/

Though overall sensible weather has quieted down compared to 24
hours ago with regard to precipitation, challenges are still 
presenting themselves in the short term forecast. The primary
challenge today is timing the clearing of stratus currently over
much of the area this morning. This will likely play a role on
high temperatures today. 

Increasing subsidence in wake of our recent system will enhance 
top-down drying later this morning into the afternoon. The one 
issue will be residual moisture trapped beneath a strengthening 
subsidence inversion based near 800mb per the latest NAM/RAP 
forecast soundings. In addition, it appears little in the way of 
low level warm advection is expected within the 925mb-850mb layer. 
With these processes in mind, I feel model blend highs are too warm 
with insolation and thermal advection lacking and stratus lingering 
through peak heating, which this time of year usually occurs
in the early afternoon hours due to shorter days. Look for mainly
a mostly cloudy and brisk morning with partial clearing later in 
the afternoon hours. Highs will struggle into the lower to mid 40s. 
The unidirectional north flow in of 15-20 knots underneath the 
frontal inversion will likely easily transfer to the surface with 
brisk north winds 10-15 mph continuing until the boundary layer can 
cool and decouple late this afternoon or early evening. 

Winds will diminish rapidly after nightfall, as a 1030mb surface 
high slides south across West-Central Texas. Winds will back 
around to the west-northwest and diminish quickly to around 5 mph 
or less overnight and may actually become calm across our far 
west and northwest counties and protected main-stem river valleys
during the pre-dawn hours Monday morning. Strong radiational 
cooling will set up with lows Monday morning tanking into the mid
20s to lower 30s across the entire CWA. 

This brings me to the concern for the formation of freezing fog
forming during the pre-dawn hours along/north of I-20 and along/west
of I-35/35E and possibly protected river valleys across the 
remainder of the CWA. Surface dew point temperatures will likely 
be in the upper 20s and lower 30s from DFW and points west and 
north of the immediate DFW Metroplex. The crossover temp method 
(which supports fog development if the following nights low 
temperatures fall to below afternoon surface dew point 
temperatures) indicates favorable trends supportive of fog or 
freezing fog formation. In addition, forecast soundings show 
increasing mixing ratio/wet-bulb values from the surface to 950mb 
with favorable hydro-lapse rates for fog as well. 

With some uncertainty on how all these variables play out, we will
play it conservative on the freezing fog potential but introduce 
at least patchy mention just west through north of the immediate
DFW Metroplex. This will need to be analyzed closely through the
day into this evening, as any freezing fog could cause a concern
for some very light icing on elevated surfaces such as bridge and
overpasses across the region during the morning rush hour on



.LONG TERM... /Issued 329 AM CST Sun Dec 9 2018/
/Monday Onward/

As discussed above, some patchy freezing fog may linger into early
Monday across our northwestern zones before quickly mixing out by
mid morning. The remainder of the day should feature a warm-up 
into the 50s following a few days of colder temperatures. A fairly
potent shortwave will actually be tracking southward through the 
area during the day within highly amplified flow. However, due to 
the absence of any moisture, this feature will largely go 
unnoticed from a sensible weather standpoint. 

The main focus through the extended forecast is the next storm
system that will affect the Plains through the middle and end of 
the week. Model solutions indicate this should consist of 3 
distinct shortwaves, a couple of which may phase at some point. 
However, as one should expect, the track/depth/timing of these 
features remains wildly inconsistent among guidance 4-5 days into 
the forecast. While there has been and will continue to be 
substantial disagreement on the finer details of this system, 
there are some conclusions that we can safely draw based on 
overall run-to-run consistency and trends. 

On Tuesday, a fairly dense canopy of cirrus should overspread the
area ahead of the leading disturbance. However, the warming trend
should continue due to increased southerly flow and warm 
advection through the low levels. Tuesday night, stratus should 
make a return following an early-week hiatus. Increased cloud 
cover should hold low temperatures substantially higher, likely in
the mid/upper 40s for Wednesday morning. By Wednesday afternoon, 
moisture may become sufficiently deep to support some warm 
advection showers, particularly east of I-35, as the main area of 
ascent sweeps through the area with this lead wave. However, the 
strongest ascent will remain well north with the better moisture 
quality farther south, so any rain coverage should be relatively 
meager. This disturbance will draw a weak cold front into the area
before it stalls and loses its identity later Wednesday evening. 

The next shortwave, or potentially a pair of phasing shortwaves,
should arrive quickly on the heels of the departing lead
disturbance. Trends suggest this resultant system should be 
much deeper and more potent than its predecessor, with a strong 
surface cyclone developing across northwestern Texas by Thursday 
morning. This setup would favor and area of showers and isolated 
storms across our eastern zones on through Thursday as a warm 
front lifts northward. The system's dryline would zip through the 
area during the day, while a strong cold front plows through 
shortly thereafter. The main feature of note with this system may 
end up being the winds. Due to rapid surface cyclogenesis 
occurring ahead of the vigorous upper shortwave, very breezy 
conditions would be possible should this solution materialize. 
Models are suggesting potentially a 50-60 kt northerly 850mb jet 
on the back edge of this system, which could easily mix down some 
40+ mph non-convective gusts on Thursday afternoon through 
Thursday night behind the cold front. In addition, strong 
conveyor belts within this cyclone should allow plenty of moisture
to wrap around its western flank, resulting in post-frontal 
deformation zone precipitation. At this time, the progressive 
nature of model solutions indicate precipitation would end well 
before sub-freezing air can arrive in north Texas behind the cold
front. However, the wintry weather potential will still need to 
be monitored in the coming days in case models trend towards a 
slower or more negatively-tilted upper pattern. In wake of this 
mid/late week system, relatively benign conditions should prevail 
into next weekend with temperatures hovering generally around 
seasonal normals. 



Dallas-Ft. Worth    48  30  54  35  61 /   0   0   0   0   0 
Waco                50  29  56  33  61 /   0   0   0   0   0 
Paris               45  30  51  30  57 /   0   0   0   0   0 
Denton              48  27  55  33  61 /   0   0   0   0   0 
McKinney            47  29  52  33  59 /   0   0   0   0   0 
Dallas              50  32  54  35  61 /   0   0   0   0   0 
Terrell             49  30  53  33  60 /   0   0   0   0   0 
Corsicana           49  32  52  33  58 /   0   0   0   0   0 
Temple              49  31  56  33  61 /   0   0   0   0   0 
Mineral Wells       48  28  56  32  61 /   0   0   0   0   0