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

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000
FXUS64 KFWD 271154
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
554 AM CST Sun Nov 27 2016

.AVIATION...
Moisture is and will continue to be on the increase across the
forecast area in advance of an eastward advancing upper level
trough over the Rockies. The warm/moist air advection has led to
low clouds and light fog across a good portion of the region,
with IFR CIGs reported throughout the DFW Metroplex this morning.
Visibilities should remain primarily in the 3 to 5 SM range for
the next few hours, with the lower conditions staying mainly east
of the DFW TRACON. Conditions have thus far remained MVFR in the
Waco area, but looking at some of the lower CIGs nearby, the
expectation is that KACT will experience IFR CIGs in the next few
hours. All locations will likely see improvement to MVFR during
the mid to late morning hours as the stronger 925MB winds mix
down, with VFR conditions developing area-wide this afternoon.

Two distinct shortwaves will rotate around the upper trough out
west. The first is currently over the Four-Corners and will eject
northeast into the Central Plains later today. This will send a
Pacific front east-southeastward across the Southern Plains and
West-Central TX late afternoon into the evening. The front will
encroach on the northwest DFW TRACON late evening, then push
across the Metroplex overnight. This will occur simultaneously
as a second shortwave piece of energy (rounding the base of the
upper trough) moves eastward across West Texas, spreading good
lift across North and Central TX. As a result, thunderstorms are
expected to develop quickly along the front after midnight CST,
impacting the northwest DFW TRACON 07-09Z, the DFW area terminals
09-12Z, then KACT 12-14Z.

The front will push through all areas by mid morning, ushering in
drier air and gusty southwest to west winds in its wake. Surface
velocities will initially veer to southwesterly at 15-20 KT
around 28/12Z, then westerly at 20-25KT at 28/18Z. This may
create some cross wind issues at DFW just beyond this TAF period
and will be something to keep an eye on after the front and
associated convection move east of the region.

30

&&


.PREV DISCUSSION... /ISSUED 448 AM CST Sun Nov 27 2016/
A deep upper trough has passed over the coastal mountain ranges
and will race to east through the Intermountain West over the next
24 hours. Rapid height falls will spread across the Lone Star
State as the initial shortwave associated with the system ejects
out of the Rockies today. This will induce upglide on the
moistening isentropic surfaces beneath it, and precipitation
processes will ensue near the top of the currently shallow
moisture layer in the vicinity of 850mb (generally 295-300K).
This precip mode will be sprinkles of light rain, entering our
western zones this morning.

The moisture flux within this layer will intensify this afternoon
at which point the 850mb winds may top 50kts. The warm advection
that results will likely keep the boundary layer capped and limit
the strength of wind gusts at the surface. Nonetheless, it will
certainly be breezy today, albeit under the speed criterion for a
wind advisory. The warm and moist advection will steadily
destabilize the boundary layer, but the cloud cover that this
advection assures will limit surface heating, likely keeping this
instability from being realized. Despite the capped boundary
layer, there may be some elevated showers this afternoon with
steepening mid-level lapse rates sufficient for embedded thunder.

However, the main event will be late tonight into Monday. The
intense 850mb flow will continue Sunday night as the approaching
lift from the longwave trough gradually erodes the inversion.
Although the instability will diminish within the nocturnal
boundary layer, it will remain unseasonably mild for late
November. Coupled with dew points in the 60s, unstable surface
parcels will likely be contributing to the depth and intensity of
any convection that develops in advance of the boundary. In this
low CAPE/high shear environment, individual updrafts may be
sheared to pieces before they are able to percolate much hail. But
if the deep-layer momentum they contain can reach the ground,
there will be an attendant wind threat with these otherwise
innocuous warm-advection echoes.

How much the disorganized activity associated with the large-
scale lift well ahead of the boundary disrupts the eventual linear
complex accompanying the front is the main forecast challenge.
The consensus among mesoscale guidance is the leading activity to
be limited in intensity and areal extent. This would ostensibly
increase the severe potential with the linear complex, but even
near-boundary updrafts that end up rotating may suffer from
excessive speed shear. While this may diminish the severe threat
overall, as long as the mean flow aloft remains nearly parallel to
the axis of convection, the downstream precipitation will not
interfere with the quality of the low-level inflow. This would
maintain a wind threat throughout the lifetime of the line, which
would likely impact the Metroplex during the predawn hours Monday
morning.

While training will occur throughout the event, including with the
main line itself, the progressive nature of the system as a whole
will limit the duration of precipitation at any given location.
The precipitation efficiency will be maximized in eastern zones
where the moisture depth and flux will allow for unseasonably high
rainfall rates. However, the combination of limited duration and
the considerable capacity of drought-parched soils, the training
is unlikely to result in significant flooding concerns.

A Pacific front will sweep through North and Central Texas on
Monday, shifting the wind to the west and bringing an end to rain.
This downslope warming will keep temperatures above normal on
Tuesday. But once the upper trough axis finally transits the area
(without any additional precip), continental polar air will surge
into the region. Noticeably cooler weather will follow on
Wednesday.

Shortwave ridging on Thursday will be short-lived as another upper
trough dives south along the West Coast, then deepens over the
Gulf of California this weekend. This will allow rich moisture
from deep in the tropics to spread across Texas, setting the stage
for a prolonged chilly rain event next weekend and into the first
full week of December. Although temperatures are expected to
remain safely above freezing during the event, the damp daytime
temperatures will be the coldest so far this season.

25

&&



.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    68  60  72  49  68 /  30  80  50   5   5
Waco                70  62  73  45  70 /  20  60  60   0   0
Paris               64  59  70  46  66 /  20  80  80   5   5
Denton              67  58  71  46  67 /  30  80  40   5   5
McKinney            66  60  71  46  67 /  30  80  60   5   5
Dallas              68  61  72  50  69 /  20  80  60   5   5
Terrell             68  62  72  47  68 /  20  80  70   5   5
Corsicana           69  62  73  47  70 /  10  70  70   5   5
Temple              71  61  72  45  71 /  10  60  60   0   0
Mineral Wells       69  55  70  44  67 /  20  60  30   0   5

&&

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

$$

30/25



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