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

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FXUS64 KFWD 021815 AAA
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1215 PM CST Fri Dec 2 2016

.AVIATION...
/18Z TAFs/

Much of West Texas is blanketed with MVFR/IFR ceilings, the
eastern extent of which includes Cisco and Comanche. Along the
I-35 corridor, the lowest few thousand feet of the atmosphere
remains quite dry, but westerly flow within the 750-850mb layer
has guided this West Texas moisture in our direction. VFR ceilings
around 7kft AGL will steadily lower as this layer moistens
further, and weak lift within it will allow for a few sprinkles
of light rain this afternoon. However, the first round of rain
showers approaching Austin at midday will take until sunset to
reach the Metroplex. This activity will be free of thunder, and
in-flight icing will be the primary concern. But with better
instability to our west and southwest, there may be some impacts
to west departures and arrivals through the Glen Rose VORTAC.

Showers will be ongoing this evening as a LLJ strengthens, both
processes further lowering ceilings overnight. Another round of
elevated convection will approach from the southwest before
daybreak. With steeper lapse rates aloft, these updrafts may be
of sufficient strength to produce lightning. Supersaturation near
the top of the nocturnal boundary layer will result in IFR
ceilings beneath the convective cells. Although the intensity of
the activity will wane by midday Saturday, showers will continue
during the afternoon hours. Ceilings and visibility will
deteriorate further, and IFR conditions will prevail through the
extended portion of the DFW TAF.

A break in the precipitation may begin Saturday night and continue
on Sunday before the next round arrives Sunday night into Monday.

25

&&

.PREV DISCUSSION... /ISSUED 346 AM CST Fri Dec 2 2016/
Early morning water vapor imagery depicts our main weather feature
for the next few days in the form of a deepening upper trough
across the southwestern US. Models are all in reasonably good
agreement on handling this feature which will become a cutoff low
over Mexico before lifting northeastward into TX in the next 72
hours. Widespread rainfall will occur over most of North and
Central Texas from this evening through Monday due to this storm
system with embedded thunderstorms possible at times also. While
little in the way of hazardous weather is expected, we`ll need to
keep an eye on rainfall totals across parts of Central TX where
several inches of rain are possible over this 4-day span.

In advance of the deepening upper trough, stronger low-level flow
has begun advecting abundant moisture into South TX so far this
morning. This initial northward moisture surge will be west of
I-35, but will continue to spread northeastward today. Go ahead
and say goodbye to the sun this afternoon as it will be a rare
sight for the next few days as cloudiness overspread the entire
area. In this warm advection regime, isolated/scattered showers
should begin developing during the afternoon hours with a larger
swath of showers overspreading the area later this evening. Have
sped up the arrival of this wave a bit over the previous forecast
with some of the high-res guidance being a bit more aggressive on
the northward progression. This batch will be the first of a few
waves of showers and isolated storms that we`ll see over the next
few days. In addition, with rain and clouds lingering through
early next week, there will be little in the way of temperature
fluctuation with diurnal temperature changes of only a few degrees
likely from Friday night through Monday. Most areas will hover in
the mid 40s to low 50s.

After the initial activity this afternoon and evening, we`ll
probably see a bit of a lull in the rain before a swath of precip
arrives early Saturday morning. This Saturday morning wave should
be some of the most widespread rainfall with some of the highest
totals throughout the entire course of the event. Very strong
isentropic ascent will overspread the entire area and PW values
will be up to 2-3 sigma above normal. Very efficient rainfall
should be the result, especially across Central TX where some
elevated instability due to slightly steeper lapse rates should be
in place. This instability should also be sufficient to produce
some isolated lightning strikes, but lightning will be hard to
come by in general. The main area to watch during the Saturday
morning time frame will be locations south of a Killeen to Canton
line where around 3" of rainfall may accumulate in a fairly short
time. The good news is that these are the areas that remain in
abnormally dry or borderline drought conditions so they should be
able to withstand a decent amount of rainfall without many issues.

As the upper low continues digging southeastward into Mexico late
Saturday, the northern stream trough from which it was cut off
will eject to our east which should actually result in some weak
subsidence across parts of North Texas later Saturday and into
Sunday. This won`t necessarily be the case across or Central TX
areas where some warm advection above the low-level colder air
will continue to result in scattered showers, although expect the
rain totals to be considerably less than the Saturday morning
activity. Much of North TX may actually wind up being fairly dry
for a large portion of Sunday although there should still be
plenty of clouds around.

The final wave of rain will occur on Monday as the upper low
ejects northeastward and stronger dynamic forcing overspreads the
area. Cooler mid-level temperatures will mean an increase in
elevated instability so Monday will actually be the highest chance
for thunder across the entire area. Additional widespread showers
and isolated storms are likely throughout the day Monday. Once
again, we will need to keep an eye on a heavy rain potential
mostly across our southeast areas. Any heavy rain will be highly
sensitive to the track of the upper low and the resultant surface
low that is expected to develop across southeast TX. When all is
said and done by Monday night, would not be surprised to see some
rain totals of up to 6" in our southeast (although keep in mind
this will be spread across multiple days) with amounts tapering
off to as low as an inch or so across our northwestern areas.

With skies finally clearing out Tuesday, temperatures will bounce
back mostly into the 60s with weak southerly flow resuming through
the low-levels. Moisture return will be slow, however. As the
upper low that plagued the area with rain finally departs to the
east, another strong upper trough will follow closely behind and
Arctic air will spill southward through the Plains behind a very
strong cold front. At this point, it appears the cold front will
arrive in North TX during the day Wednesday. Since moisture will
be limited as mentioned earlier, the potential for showers and
storms will be quite low with this front. However, will continue
a mention of low PoPs across our eastern counties where moisture
content will be slightly higher. Some very strong pressure rises
should follow resulting in very windy conditions along and behind
the front as surface high pressure builds into the Southern
Plains.

Cold air and high pressure will settle into the area for the
remainder of the week. Have gone with the coldest guidance and
even undercut it by a few degrees in some spots especially for
Thursday morning`s lows. It`s a safe bet that these will be the
coldest temperatures of the season so far with lows in the 20s for
most locations. Continued north winds should result in some wind
chill values in the teens, as well. Highs on Thursday should only be
in the 40s. A slight warming trend will ensue on Friday and
Saturday as weak southerly flow resumes.

-Stalley

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    63  48  50  46  53 /  40  80  80  60  20
Waco                61  49  50  47  53 /  60  90  90  70  50
Paris               61  47  49  45  52 /  20  80  90  70  20
Denton              62  47  49  46  53 /  40  80  80  60  20
McKinney            62  48  49  46  52 /  40  80  80  60  20
Dallas              63  49  50  46  54 /  40  80  80  60  20
Terrell             62  48  50  46  53 /  40  80  90  70  30
Corsicana           62  49  50  47  53 /  50  90  90  80  40
Temple              61  49  51  47  53 /  60  90  90  80  60
Mineral Wells       62  47  49  46  51 /  50  80  80  60  20

&&

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

$$

25/82



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