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

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485
FXUS64 KFWD 171155
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
555 AM CST Sun Dec 17 2017


.AVIATION...
/12z TAFs/

Very poor flying conditions are ongoing across our area TAF sites,
with IFR and LIFR cigs commonplace with occasional visibility
reductions down to around 1-3SM. Current observations across the
Metroplex reveal vsbys have settled to VFR, but would anticipate
some occasional dips over the next few hours. Incoming high cloud
cover may help limit additional radiative cooling atop the low
stratus deck, so prospects for widespread dense fog at the
Metroplex sites seems low through the rest of the morning. Waco
may flirt with slightly worse cigs/vsbys for the next few hours.

Given how deeply saturated things remain based on our ascending
balloon, went ahead and extended MVFR cigs through the mid-
afternoon hours as it`s hard to imagine insolation will be able to
rapidly chew away at this 2000+ foot deck of moisture. It`s
conceivable that these TAFs are even a bit too optimistic, as some
high-resolution guidance insists that IFR cigs may remain
pervasive across all terminals well into the afternoon. Opted to
show a return to VFR, however, as strengthening westerlies just
off the surface should encourage some drier air to filter in
through the day.

As things cool off tonight, anticipate IFR/LIFR cigs to re-develop
across much of the region with copious moisture remaining in the
lowest 2 kft. We`ve kept vsbys MVFR for now, but if strong
radiational cooling can occur under lingering cirrus, dense fog
would be a distinct possibility.

Carlaw

&&


.SHORT TERM... /Issued 349 AM CST Sun Dec 17 2017/
/Today and Tonight/

Brief ridging aloft will take place today as one upper low lifts
to the northeast and another deepens across the Desert Southwest.
The low levels will remain near saturation this morning following
Saturday`s rain event which will result in areas of low clouds
and fog. For the most part the wind north of I-20 will be strong
enough to keep the fog from becoming dense. However, the
visibility south of I-20 will occasionally fall below 1/2 mile,
especially in areas that lost the high cloud cover the quickest.
Therefore, we will issue a Dense Fog Advisory through late
morning generally south of I-20 and west of I-45. The clouds and
fog will begin to lift and clear just before midday as some drier
air mixes down. Increasing high clouds from the west may also
begin to improve conditions after sunrise so some adjustments to
the Advisory area may be needed as the morning progresses.

Temperatures today will be a bit above normal since the morning
will be starting off in the 40s and a bit of sun will return this
afternoon. High temperatures will range from the middle 50s in
the northeast to the mid 60s in the west. Temperatures tonight
will be similar to last night since the low levels will remain
near saturation and clouds will be abundant. Expect overnight
lows to remain generally in the 40s except for the northwest
where upper 30s will be likely due to some partial clearing.

Some patchy fog will likely develop again overnight since the low
levels will remain moist and wind speeds will be fairly light.
Locations that do temporarily clear out in the evening will be
the most likely to see fog.

79

&&

.LONG TERM... /Issued 349 AM CST Sun Dec 17 2017/
/Monday and Beyond/

Our next weather maker--a potent shortwave--is presently spiraling
into the Gulf of California early this morning and will be
spreading showers across the parched Desert Southwest today.
Impressively, this system will manage to squeeze every bit of
moisture out of the sub half-inch PWAT air present over this part
of the CONUS, indicative of the prolific ascent associated with
it.

An initial wave of mid-level forcing for ascent is slated to drift
across the region as early as Monday morning. As this occurs,
elevated showers and perhaps some thunderstorms should
materialize, in greatest coverage and quantity over the
southeastern half of the forecast area through Monday afternoon.
Since this will initially be fairly diffuse lift with the main
upper-low and trough axis still just entering New Mexico,
precipitation coverage isn`t expected to be all that robust, and
we`ll cap PoPs at 50% for the time being.

Around Tuesday morning, this trough axis will be swinging into the
Texas Panhandle, arcing southwestward across the Trans-Pecos
region of southwest Texas. If this sounds at all like deja vu,
it`s because a very similar scenario played out last evening and
overnight as a disturbance brought beneficial rains to a large
chunk of the region. This incoming system may actually be just a
bit more dynamic and cooler aloft, which may spell a higher chance
for thunderstorms.

As the main lobe of vorticity approaches, anticipate widespread
showers and isolated thunderstorms across most of North and
Central Texas on Tuesday. As mid-level temperatures drop in
response to dynamic cooling, mid-level lapse rates may increase
to around 7 C/km which would facilitate the development of some
elevated instability (MUCAPEs on the order of 400-800 J/kg or so).
With effective bulk shear values increasing into the 35-40 kt
range, we`ll need to keep our eyes peeled for some potential
stronger storms with a small hail threat. At this juncture, it
appears that instability will be just a bit too meager to result
in a more appreciable threat for strong storms. Precipitation
chances will end swiftly from west to east by Tuesday evening and
overnight, with dry conditions returning to the region for
Wednesday.

Thursday is looking like it`ll feature plentiful high cloud cover
as moisture streams northward from the equatorial Pacific, but
breezy southerly winds should still allow temperatures to rise
into the upper 60s and lower 70s area-wide.

...Thursday night and into the Christmas Weekend...
Our much-advertised strong cold front still appears to be on
track to surge through North and Central Texas as early as
Thursday evening, but more likely overnight Thursday and into
Friday morning. You`ll definitely notice when this front passes as
winds will turn sharply out of the north and become gusty as the
rearward sloping wedge of cold air is driven towards the Gulf.

The biggest development of the night has been a marked shift in
both the available global deterministic and ensemble guidance
regarding the evolution of the upper-air pattern during the end of
next week and into the Christmas holiday weekend. As we discussed
last night, the main sets of guidance seemed to be split into two
camps, one favoring a more progressive synoptic pattern and the
others favoring the development of a closed Baja-type upper-low.
The former would favor a cold but mainly dry forecast, while the
latter would support widespread wintry precipitation across much
of Texas. The latest 00z guidance has now swung solidly in favor
of the former more progressive scenario, as energy carving over
the top of the West Coast ridge leaks just a bit too far east and
north, avoiding becoming cut-off from the mean flow. In fact, only
about 15-20% of the GEFS (GFS Ensemble) members and 5-10% of the
GEPS (Canadian Ensemble) from the latest run develop a Baja-type
upper-low late next week and into the weekend. The EPS (European
Ensemble) means also support an overall more progressive pattern
this time around, in line with the 00z deterministic guidance.

All of this to say that, for the time being, the preponderance of
evidence suggests that the most favored outcome is for a cold but
generally dry forecast (aside from some showers/storms ahead of
the front Thursday night-Friday), and we have removed the mention
of wintry precipitation for this forecast package.

Now, we`ll point out that even with the numerous perturbations to
the initial starting conditions (the "background"), ensemble
guidance typically remains underdispersive without statistical
post-processing to artificially inflate the probabilities. As a
result, the earlier quoted low percentages do not necessarily
translate into actual probabilities from a calibrated statistical
sense. Thus, there still remains a non-trivial potential for
a more wintry scenario to unfold in the coming days, but the 100+
model solutions we have available to us from ensemble guidance
right now is sending us a message that this scenario appears less
likely. Even in this drier regime, extended model guidance is
hinting that there is still a potential for perhaps some light
snow associated with intense mid-level frontogenesis on Sunday,
but this remains outside our current forecast window. Keep
checking back with us and your other trusted weather sources as we
refine the details of this complex weather scenario over the
coming days.

Carlaw

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    61  46  62  53  62 /   5   5  20  40  60
Waco                61  47  61  56  63 /   5  10  40  50  70
Paris               57  44  61  53  63 /  10   5  30  50  50
Denton              59  42  61  52  61 /   5   5  20  30  60
McKinney            58  43  61  53  61 /  10   5  20  40  60
Dallas              61  47  62  54  62 /  10   5  30  50  60
Terrell             60  47  62  54  63 /  10   5  40  50  70
Corsicana           61  47  62  56  64 /   5   5  40  50  70
Temple              62  48  61  56  64 /   5  10  40  50  70
Mineral Wells       63  42  61  50  62 /   5  10  20  30  60

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 11 AM CST this morning for TXZ130>134-
143>147-156>162-174-175.

&&

$$

90/79



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