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

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FXUS64 KFWD 181823
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
1223 PM CST Mon Dec 18 2017


.AVIATION...
/18Z TAFs/
Obviously a challenging aviation forecast, with IFR (and in some
areas, LIFR) conditions persisting across North and Central Texas
at midday. With a slack pressure gradient providing little
appreciable surface/low layer mechanical mixing, and no
convective turnover occurring, stayed pretty pessimistic on the
improvements to both ceilings and visibilities through the next 18
hours. Kept IFR conditions going at each of the TAF sites, though
was probably the most optimistic at both KDFW and KDAL, where a
relative maximum in observed cigs/vsbys currently exists.

Still expect some light stratiform - with embedded showery - precipitation
to start breaking out across Central Texas later tonight as lift
associated with the strong upper low over the Southwest U.S.
builds across the area. Began mentioning -RA at KACT beginning at
05z, but held off on any precipitation in the Metroplex TAFs
until 13z. Will be taking a closer look at this timing with the
remaining 12z and 18z guidance that`s coming in, and may adjust
this onset timing with the 21z amendments.

Bradshaw

&&


.UPDATE... /Issued 1110 AM CST Mon Dec 18 2017/
Dense fog persists over western portions of our county warning
area this morning, while conditions have improved towards the
east. Based on surface observations and forecast soundings from
the RAP and HRRR, opted to extend the Dense Fog Advisory for areas
generally west of Interstate 45 and south of Interstate 20 through
1 PM.

For the rest of today, main change to previous forecast was to
lower high temperatureconditionsat least IFR conditionsAt every location,
short- term guidance
s across much of the area by a few degrees. Visible satellite
suggests that North and Central Texas should remain socked in with
cloud cover for the rest of the day, with only weak warm air
advection (southeasterly winds, but less than 10 knots).
Otherwise, only minor tweaks to PoPs were made. Updated products
already sent.

37

&&

.SHORT TERM... /Issued 347 AM CST Mon Dec 18 2017/
/Today and Tonight/

Damp and dreary weather will continue across the entire region
with persistent low level warm air advection ahead of an
approaching upper low currently rotating across the Arizona/Mexico
border. Areas of dense fog and drizzle will continue through the
morning with nearly all locations experiencing temporary
visibility reductions below 1/2 mile. Therefore, we will expand
and extend the current dense fog advisory to include the entire
forecast area through late morning. Wind speeds and some meager
mixing should improve all visibility above 1 mile by midday but
the low levels will remain very moist so low clouds will linger
through the day and into the night. Fog will also develop again
tonight due warm/moist advection over a relatively cool ground.

The extensive cloud cover will keep temperatures down today but
low level warm air advection should allow for some slow warming
with afternoon highs 5 to 10 degrees warmer than at sunrise. On
the flip side, cooling will be slow tonight due to the clouds and
warm air advection, so overnight temperatures will be quite mild
for mid December with lows mainly in the 50s.

Large scale lift is just now making it into Far West Texas and
will most likely not arrive in North and Central Texas until
tonight. Although the widespread drizzle will make it appear as if
it has rained all morning, most areas will not see measurable
rainfall until tonight or even Tuesday. However, the southeast
zones may see some warm air advection showers late this afternoon.
Have included some isolated thunderstorms this afternoon and
tonight due to increasing mid level lapse rates.

79

&&

.LONG TERM... /Issued 347 AM CST Mon Dec 18 2017/
/Tuesday through Christmas Eve/

Tuesday morning`s upper-air charts will look surprisingly similar
to this past Saturday`s with a potent shortwave barreling through
the New Mexico Bootheel and into the Trans-Pecos. While some
diffuse forcing for ascent will be ongoing early Tuesday morning,
lift will increase markedly into the late-morning and afternoon
hours as 50-80 m/12hr height falls and diffluent flow overspreads
the region. From a visual standpoint, this next incoming wave--
which is presently pinwheeling across the Mexican state of Sonora
--is impressive, and managed to effectively wring out all the
available moisture across southern Arizona resulting in upwards of
a half foot of snowfall across the higher elevations.

With such a dynamic system, it`s no surprise that high-resolution
guidance continues to develop widespread showers and embedded
thunderstorms across North and Central Texas Tuesday and Tuesday
night. Of additional concern is the potential for some strong and
perhaps even a few marginally severe storms Tuesday and into
Tuesday evening as the mid-levels cool and lapse rates steepen.

As this system approaches, effective deep layer shear values
(taken from the effective inflow base as opposed to the surface)
are forecast to increase to 35-45 kts as elevated instability on
the order of 400-900 J/kg materializes--with the highest values
across our south and eastern counties. The latest NAM also
indicates that the warm sector may approach our far southeastern
row of counties (Milam to Anderson) Tuesday evening as dewpoints
in the mid-upper 60s return. As a result, believe there is a
potential for strong storms with a small hail threat for locales
south and east of a Goldthwaite to Paris line, with a somewhat
higher threat for a few marginally severe storms with quarter-
sized hail south and east of a Killeen to Waco to Athens line
where instability should be maximized. Should storms across our
far southeast manage to become surface-based, then strong wind
gusts would also become a concern in this area as a line of
showers and storms marches eastward Tuesday afternoon and evening.

Finally, while this system will be progressive, PWATs are forecast
to increase above 1.5" (some guidance suggests 1.75" isn`t out of
the question), which would rival/exceed record values for this
time of year. As a result, some embedded heavy rainfall rates are
expected, and some localized excessive runoff issues are possible
from the the DFW Metroplex and points south and east.

The next feature of interest, of course, is the strong cold front
which now looks to arrive into the region very late Thursday night
and into Friday morning. With some moisture return occurring ahead
of the front, sufficient instability will exist for isolated to
scattered thunderstorms with the front. With 500 mb flow
increasing to 50-60 kts, can`t discount the possibility of some
stronger thunderstorms, but the time of day should help limit the
amount of available surface-based instability.

Ensemble and deterministic global guidance continues to pare back
the potential development of a upper-low near Baja by Friday and
into Saturday. Fewer than about 10% of the available ensemble
members from the NAEFS (North American Ensemble Forecast
System...with membership from the GEFS and GEPS ensembles) still
develop this feature, with the remaining 90% carrying a
progressive open trough swiftly across the region. Thus, this
forecast will continue to paint a dry picture on Saturday and
Saturday night given the overwhelming support from medium-range
ensembles.

For Sunday, the latest ECMWF and Canadian interestingly slow the
initial cold front so much across our southeast, that return flow
ahead of it results in showers and thunderstorms, with
temperatures some 15-20 degrees above what the GFS indicates.
Given the tremendous troughing present to our west during this
time and the very shallow but dense nature to the cold air, it
seems difficult to imagine a scenario where the front wouldn`t
make it solidly through our entire forecast area.

Finally, while it`s apparent that a major ice and snow event is
becoming increasingly UNLIKELY for the Friday-Saturday period,
robust troughing and associated baroclinicity will likely be
lurking to our north by Sunday and into Christmas Day. The past
several runs of the GFS and Canadian have indicated the potential
for banded precipitation to develop in the cold airmass in
response to intense mid-level frontogenetic circulations. Should
precipitation develop, it would fall to the surface as light snow.
Far too much uncertainty exists in this potential, however, to
mention this in the official forecast grids at this time, but this
scenario certainly bears close attention.

Carlaw

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    53  58  49  64  51 /  20 100  50   0   0
Waco                54  61  48  67  50 /  60 100  20   0   0
Paris               51  58  53  62  49 /  20  90 100   5   0
Denton              51  57  45  63  49 /  20  90  60   0   0
McKinney            52  59  47  63  49 /  20 100  80   0   0
Dallas              54  59  50  64  51 /  30 100  60   0   0
Terrell             54  60  52  63  49 /  30 100  70   0   0
Corsicana           55  62  51  65  50 /  50 100  60   0   0
Temple              54  62  48  68  49 /  60 100  20   0   0
Mineral Wells       51  59  43  64  47 /  20  90  30   0   0

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 1 PM CST this afternoon for TXZ115>117-
129>134-141>146-156>161.

&&

$$

66


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