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

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FXUS64 KFWD 180947
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
347 AM CST Mon Dec 18 2017


.SHORT TERM...
/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...
/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

&&


.AVIATION... /Issued 1157 PM CST Sun Dec 17 2017/
Difficult aviation forecast with respect to low clouds and fog
potential tonight. Fog has become dense in some areas, but since
low clouds have already settled in over the area, visibilities are
most impacted at stations with temperatures in the mid 40s which
are lower than the average mid 40s dewpoints in the region. Thus
fog is more advective in nature, and the heat of the Metroplex is
likely to keep DFW and DAL visibility higher than GKY which
cleared out earlier this evening and is in the low 40s. If stratus
were to build down to the surface, these visibilities may
diminish areawide. Monitoring temperatures will likely be the key
to what occurs.

Regardless of the difficult visibility forecast it is a given
that CIGs will be in the LIFR to IFR category through the night
and into the morning. Believe the RAP and NAM have the right idea
with the inversion remaining strong through the day tomorrow so
will show IFR CIGs prevailing through mid afternoon with perhaps a
brief period of MVFR by late afternoon and evening. By Monday
evening, stronger south winds near the surface will bring
warm/moist advection to the area and CIGS/VSBY will fall again to
LIFR or possibly VLIFR.

Very light southeast winds will become southerly tomorrow
afternoon at speeds of 5-10kt. Still have opted not to mention any
showers in the TAF through this set of TAFs, but rain/storms may
be needed by Tuesday morning.

TR.92

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    60  53  58  49  64 /  10  30 100  50   0
Waco                59  54  61  48  67 /  20  50 100  20   0
Paris               58  51  58  53  62 /  10  40  90 100   5
Denton              58  51  57  45  63 /  10  30  90  60   0
McKinney            59  52  59  47  63 /  10  30 100  80   0
Dallas              60  54  59  50  64 /  10  40 100  60   0
Terrell             59  54  60  52  63 /  10  40 100  70   0
Corsicana           60  55  62  51  65 /  20  40 100  60   0
Temple              60  54  62  48  68 /  20  60 100  20   0
Mineral Wells       60  51  59  43  64 /  10  20  90  30   0

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 11 AM CST this morning for TXZ091>095-
100>107-115>123-129>135-141>148-156>162-174-175.

&&

$$

90/79


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