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

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FXUS64 KFWD 051153 AAA

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
553 AM CST Mon Dec 5 2016

/12Z TAFs/

Another challenging aviation forecast this morning as low level
warm advection ahead of our main upper trough lifting out today
provides plenty of lift for increasing -RA/-SHRA/and isolated TSRA
today. In addition, the shallow cool boundary layer will continue
to saturate with light N/NE winds through the lowest 1000-1500
feet. This will make forecasting cigs, vsbys, and any
precipitation or drizzle challenging.

For now, there`s enough mixing with the increasing lift approaching
the area to start off MVFR with some light showers increasing this
morning at all airports. As weak low level wind fields remain out
of the N/NE and a gradual light rain/drizzle occur through this
afternoon, saturation of the sub-cloud layer commence. IFR cigs
will return by 21z most locations, then lower into LIFR and
possibly some FG/BR by this evening, as subsidence occurs behind
the departing mid level trough with light and at times variable
winds 5 knots or less.

A few high-resolution models do try to filter in some dry air from
the north by morning, but with the boundary layer remaining
moisture rich with light winds, will carry more pessimistic
conditions through 12z Tuesday.



.PREV DISCUSSION... /ISSUED 340 AM CST Mon Dec 5 2016/
Morning water vapor imagery depicts the upper low (which has
affected our weather since Saturday) spiraling northeastward into
Texas. As it does, our final wave of showers and possibly a couple
embedded thunderstorms will accompany it`s departure. Light rain
is ongoing across most of South and Central Texas this morning and
this activity should expand northeastward in advance of the upper
trough. Low-level warm advection will overspread the region with
ascent being aided by the approaching vort max. Instability
remains limited, but pockets of stronger cooling aloft associated
with the cold-core low may be able to provide enough buoyancy for
some isolated thunder throughout the day, but expect the bulk of
the activity to be light rain showers. Instability increases from
North to South, so Central TX will have the better shot at a few
rumbles of thunder. Wrap-around precipitation should be pivoting
out of the forecast area to the northeast around midnight.

The trickiest part of this forecast has actually become the
tonight and Tuesday periods. As clouds decrease tonight with the
departing upper low, cooling should occur rapidly where skies
become clear. Some patchy fog will be possible as low-level
moisture remains in addition to saturated grounds due to recent
rain. To complicate matters further, a weak surface low will drift
southeastward through the forecast area associated with a
secondary shortwave rotating through northwest flow aloft. As this
low shifts south of the area around daybreak, a cold front will
push through the region. This front looks a bit stronger than
what I`ve expected the past few days, and there should actually
be a decent shot of cooler air behind it. In addition, with low-
level moisture still in place, the sharp frontal inversion should
result in a saturated layer in the lowest ~100mb so widespread low
clouds will be likely through at least midday Tuesday. Some steep
near-surface lapse rates in this saturated layer should result in
some patchy drizzle behind the front as well, and have included
this mention area-wide. Drier air will work into the area during
the afternoon hours eliminating the drizzle potential from north
to south, and even resulting in some clearing across parts of
North TX. Since most of the day is expected to be cloudy and there
will be decent cold advection behind the front, have undercut high
temperature guidance by a few degrees for Tuesday afternoon.

On Tuesday night and Wednesday, low-level winds will come around
to the east/southeast allowing some better moisture to quickly
return to the area. Warm advection and isentropic ascent within
this moisture axis should result in some light showers across East
TX throughout the day Wednesday. In the meantime, the broad trough
across the entire CONUS will continue to deepen and cold air will
funnel southward through the Plains setting the stage for our
long-awaited Arctic front on Wednesday night.

Fortunately, models have finally come into better agreement on the
frontal passage timing with the GFS moving into line with the
later ECMWF solution. A pre-frontal trough will slide through the
forecast area Wednesday afternoon causing winds to turn to the
north in advance of the true cold front. The true front will plow
into North TX Tuesday evening and clear our southern zones just
after midnight. Strong winds of 20-30 mph and higher gusts should
accompany this frontal passage which will result in very strong
cold advection. Temperatures should fall into the upper 20s and
low 30s by Thursday morning, and with winds expected to remain
gusty well behind the frontal zone, wind chills in the teens and
low 20s can be expected for most areas. As strong cold advection
continues through the day, many locations will not make it out of
the 30s except for some of our Central TX counties that may reach
the low 40s. Have continued to undercut all temperature guidance
as raw models and MOS struggle with extreme changes and large
departures from normal.

Winds will lessen Thursday night and Friday morning as surface
high pressure becomes centered over southern OK. Light north winds
will be in place, and with clear skies and very dry surface air
(dewpoints in the low teens), favorable radiational cooling should
cause temperatures to plummet. Have drastically undercut guidance
once again on Friday morning with my only fear being that I didn`t
go cold enough. Current forecast lows at DFW and Waco are around
20 degrees which would be the coldest temps since January 2015.

High pressure will shift eastward on Friday and Saturday allowing
a slight warming trend to begin. Highs on Friday should return to
the 40s; 50s should return by Saturday along with better moisture
as well. Strong warm/moist advection within a 40kt low-level jet
could result in some scattered showers across East TX Saturday
afternoon and into Sunday, therefore have continued some low PoPs
but no thunder mention. The GFS and ECMWF both suggest that some
widespread rain could occur on Sunday and Monday along a quasi-
stationary frontal zone draped through the plains, but it appears
most of this activity would be to our east with much of North and
Central TX left in a drier airmass.



Dallas-Ft. Worth    50  43  51  43  55 /  90  30  10   5  10
Waco                50  42  53  44  60 /  90  20  10   5  10
Paris               48  44  51  43  52 /  90  60  10   5  20
Denton              50  41  51  40  56 /  80  20  10   5  10
McKinney            49  43  50  42  56 /  90  40  10   5  20
Dallas              50  44  51  44  56 /  90  30  10   5  10
Terrell             50  43  52  44  58 /  90  40  10   5  20
Corsicana           50  44  53  46  60 /  90  30  10   5  20
Temple              50  44  52  43  61 /  80  10  10   5  10
Mineral Wells       50  41  49  39  56 /  70  10  10   5  10




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