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

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FXUS64 KFWD 161815 AAC
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1215 PM CST Sat Dec 16 2017


.AVIATION...

The upper level low across the Trans-Pecos Valley is leading to a
deep layer of moisture surging northward into North and Central
Texas through the day today. The effects of the low can already be
seen with the increase in cloud cover across the region.

A 120+ kt jet streak in the vicinity of KDRT to KABI is leading
to an increase in turbulence between FL250 to FL300. The jet
streak is expected to continue moving east/northeast through the
afternoon, and would not be surprised if turbulence reports pick
up across North and Central Texas.

The atmosphere will continue to saturate from the top to bottom,
leading to MVFR cigs for Waco around 22Z and the Dallas/Fort Worth
airports by 00Z. As lift and moisture increase, rain will
overspread the region this afternoon into the evening. Light rain
showers have already developed south of Waco, and further development
is expected as large scale ascent approaches the region. The
heaviest of the precipitation will begin at Waco around 00Z
Sunday, and continue northward into the Dallas/Fort Worth by 03Z,
with IFR likely during the heaviest of the precipitation. Although
mid and upper level moisture will scour out after midnight, the
low levels will remain saturated, leading to an extended period of
IFR across the region through the early morning hours.

Low levels will dry out by Sunday afternoon as the influence of
the upper level low decreases. Winds will remain southerly
throughout this forecast cycle across all TAF cites. Winds will
slowly diminish to around 5-7 kts tonight, and continue through
the tomorrow afternoon.


Hernandez

&&


.UPDATE... /Issued 1051 AM CST Sat Dec 16 2017/

The upper level low currently located over the Trans-Pecos Valley
is quickly moving to the northeast this morning. The effects of
the low can already be seen across the region, with an increase in
cloud cover across Central Texas. Southerly winds will continue to
advect the clouds northward throughout the day, and as further
moistening occurs, precipitation is expected to develop across Central
Texas and spread north/northeastward by this afternoon. Can`t
rule out an isolated thunderstorm as there is some elevated
instability, especially east of I-35 and south of I-20.

Given the quick progression of the low to the northeast,
precipitation will end from west to east late tonight as much
drier air moves in. Highs today will remain in the low 60s across
North Texas, and 50s are expected across Central Texas where
cloud cover and precipitation will keep temperatures slightly
cooler.


Hernandez

&&

.SHORT TERM... /Issued 353 AM CST Sat Dec 16 2017/
/Today and Tonight/

The extended dry period should come to an end for much of North
and Central Texas today as an upper low rotates northward across
the Trans-Pecos. Moisture and lift associated with this system
will initially bring increasing mid and high clouds this morning
followed by scattered to numerous showers this afternoon. The best
moisture and lift will reside across the southern/southeastern
zones where we will carry the highest PoPs. Although surface based
instability will be negligible, some weak mid level instability
will be present which may result in a few intermittent lightning
strikes. The favored rain area will shift to the eastern zones
this evening as the upper low lifts into the Eastern Texas
Panhandle. Precipitation will end quickly from west to east
tonight when much drier air moves in from the southwest. Total
rainfall from this system will range from only a few hundredths of
an inch across the west/northwest zones to nearly an inch across
the southeast.

The extensive cloud cover today will offset increasing low level
warm air advection some and keep temperatures near or slightly
below seasonal normals with highs generally in the 50s. Although
the rain will move out tonight, the low levels will remain near
saturation which will result in some low clouds and even some
patchy fog. Overnight lows will be considerably warmer than they
have been and generally in the lower and middle 40s.

We were concerned over the past few days that the fire threat
might become elevated across the northern half of the region this
afternoon. It now appears the clouds and moisture return will be
sufficient to limit fire growth. However, fine dead fuels will
remain abundant and winds will increase this afternoon so some
small fire starts will still be possible.

79

&&

.LONG TERM... /Issued 353 AM CST Sat Dec 16 2017/
/Sunday and Beyond/

The vigorous shortwave which will be responsible for Saturday
night`s rainfall will be quickly peeling away from the region by
early Sunday morning. This will leave North and Central Texas in a
subsident wake as an incoming shortwave ridge axis fosters rapid
mid-level height rises through the day. While no precipitation is
expected on Sunday, copious amounts of moisture will remain
trapped beneath a strong inversion just about 2000 feet off the
surface resulting in pervasive low cloud cover and fog during the
morning. Subsiding and drying air will, however, eventually erode
the low cloud deck across most of the region during the
afternoon, allowing temperatures to rise into the upper 60s to
near 70 degrees out west, and into the lower to middle 60s
elsewhere.

Another rather vigorous shortwave trough is then slated to
approach the region Monday afternoon, before departing to the east
by Wednesday. This system, which is presently dropping southward
along the California Coast early this morning, was initially
expected to fracture into two to perhaps three separate vorticity
centers, effectively diffusing available forcing for ascent over
the region and keeping precip chances relatively low. Recent
guidance, however, has all trended towards developing a much more
coherent singular upper-low center more or less intact. As a
result, much more lift and moisture appears like it will be
available Tuesday into Wednesday, and PoPs have been raised
significantly across the region. While some initial light warm
advection showers are possible as early as Monday afternoon across
parts of the region, the best chances for rainfall are slated to
arrive on Tuesday afternoon, and linger through the overnight
hours. Isolated thunder has also been added to the forecast given
the increased lift and much cooler mid-levels as the core of the
upper-low drifts overhead.

Not much will be shaking on the weather front on Wednesday as this
latest system departs to the east and lingering moisture is
gradually scoured out. Temperatures should manage to rise into the
mid 60s and perhaps into the 70s across Central Texas under
thinning cloud cover. Thursday also looks like it will feature
rather benign weather outside of some gusty southwesterly winds
ahead of the next set of potentially  weather feature(s) into the
Christmas weekend.

First, as we`ve been messaging over the last few days, run-to-run
model consistency regarding the progression, timing, and strength
of an incoming arctic cold front remains quite high during the
Thursday evening-overnight period. An initial sharp north wind
shift and subsequent cool-down is forecast to enter our
northwestern zones as early as Thursday evening before this
massive density gradient surges into the Gulf of Mexico on Friday
morning. Some light precipitation (non-frozen) is possible at the
surface ahead of the front across our southeastern counties
Thursday evening ahead of the surging wedge of arctic air.

So, piece one of the puzzle can be set down on the table: we`re
fairly confident that much colder air will be present in the
lowest 3-4 thousand feet or so, with wet bulb temperatures
certainly below freezing due to dewpoints in the teens and single
digits.

We can then start to fill in the border pieces somewhat, but
individual details regarding the progression of the all-important
close upper-low (or lack thereof) to our SW remains quite unclear.
We know that exceptional amplification to the +PNA ridge, which
has been a staple of the large-scale weather pattern of late,
should occur as the ridge axis builds through the Arctic Circle,
perhaps coming close to scraping 85 degrees north by next
weekend. We`re also pretty confident that a vort max associated
with energy currently nearing the Bering Sea should crest this
building ridge axis during the middle of next week. What happens
after this is up in the air, and ultimately boils down to where
this shortwave is in relation to the ridge axis as an anticyclonic
wave break occurs. Too far east--like the most recent 00z ECMWF
and last night`s GFS--and this shortwave trough simply slips
eastward, remaining an open wave as it quickly transits Texas sans
much fanfare at all. If this system hangs back just a few hundred
miles farther west as the 2PVU surface pinches off, and you`ve
got a developing closed upper-low near Baja/Sonora.

Roughly speaking, global ensemble and deterministic guidance seems
to be split nearly down the middle into these two camps (a third,
less supported solution is to retrograde the low so far out into
the Pacific it becomes a non-factor in our weather). Since
individual deterministic runs have been swinging back and forth
between these solutions wildly over the last few days and ensemble
guidance remains bifurcated, we can`t say which of these solutions
is the most probable at this juncture.

What we can say, however, is that the upcoming pattern will
support the potential for a Baja-type closed low to usher
moisture into the region up and over the arctic lower-level air,
which would bring a **potential** for impacts due to freezing
rain and sleet (to perhaps the southeastern half of the region)
and snow and sleet (north and west) to the region from Friday
night (December 22nd) through the Christmas weekend (25th). The
alternative being a cold but generally dry scenario through the
weekend.

At some point, ensemble guidance should begin to favor one of
these scenarios enough to begin pinpointing the most likely
upper-level evolution, but until then, nailing down specifics will
be much too premature.

Carlaw

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    60  44  64  46  67 /  40  80   0   5  20
Waco                55  42  66  47  66 /  80  90   0  10  30
Paris               59  42  60  44  65 /  10  90   5   5  20
Denton              60  42  64  43  67 /  20  80   0   5  10
McKinney            59  42  62  44  66 /  30  90   0   5  20
Dallas              60  45  64  47  67 /  40  90   0   5  20
Terrell             59  44  64  45  67 /  40  90   5   5  20
Corsicana           57  44  64  47  66 /  60 100   0  10  30
Temple              53  42  66  48  65 / 100  90   0  10  30
Mineral Wells       60  42  66  42  67 /  30  60   0   5  10

&&

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

$$

08


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