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

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FXUS64 KFWD 210013 AAA
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
613 PM CST Sat Jan 20 2018

.AVIATION...
/00Z TAFs/

Although VFR conditions persist late this afternoon, MVFR and IFR
cigs will return this evening. IFR ceilings may occur between
03-08Z, but otherwise MVFR ceilings are expected during the
overnight hours. Breezy south winds of 12-18 kts with gusts up to
25 kts will prevail through the night. Patchy drizzle and light
fog are possible again tonight, but the stronger winds should
prevent widespread heavy fog.

An upper level trough will move into the Southern Plains on
Sunday, sending a dryline across the region. Showers and
thunderstorms are expected to occur along and ahead of the dryline
during the afternoon hours with the best rain chances east of the
TAF sites. At this time, will continue the VCSH mention between
17-20Z or 21Z, but it`s possible a thunderstorm could occur VC
KDAL near the end of this window. However, the broken line of
storms to the east may impact arrivals and departures in that
direction.

The main impact in the afternoon and evening hours will come from
the winds. The winds will shift to the southwest and increase to
around sustained 17-20 kts with gusts of 25-30 kts. This will
result in several hours of crosswind issues between about
17Z-02Z. During the evening hours, the winds will become more
westerly behind the dryline, but should decrease in speed to
around 15 kts.

JLDunn

&&

.SHORT TERM... /Issued 337 PM CST Sat Jan 20 2018/
/Through Tonight/

Similar to yesterday, we`ll start this evening off under mostly
clear skies along and west of I-35, with cloudy conditions east
of there. Later this evening, a strong upper storm system will be
moving through the southwestern U.S. which will result in
increasing southerly low level flow across North Texas tonight.
This 40 kt low level jet will help rich moisture spread back to
the west after dark. As it does, we should see skies quickly
become overcast across much of the region. Cloud heights will
lower again through the nighttime and areas of drizzle or fog may
once again be an issue particularly in our eastern counties.
Stronger forcing for ascent ahead of the main trough is expected
to spread into North Texas very late tonight into Sunday morning.
This may be sufficient for a few light rain showers, again mainly
in the east, toward early Sunday morning. We`ll have 20% PoPs
across the east to account for this.

Given the increase in cloud cover and breezy southerly winds,
overnight lows are only expected to fall into the 50s areawide.
Some of our eastern and southeastern counties may be near 60
degrees for low temperatures tonight.

Dunn

&&

.LONG TERM... /Issued 337 PM CST Sat Jan 20 2018/
/Sunday through Friday/

Gulf moisture will continue surging northward ahead of an
approaching surface dryline and vigorous upper trough Sunday
morning. The strong low level warm and moist advection will likely
result in a chance for a few showers, east of U.S. 281. In
addition, some patchy fog and drizzle can`t be ruled out areawide
through mid-morning. Afterward, a vigorous mid level shortwave
trough will lift east-northeast out of the Southern Rockies and
over the Central and Southern High Plains in the afternoon hours.
The resultant height falls shift east and mixing of strong west-
southwest winds above the surface will help the surface dryline
mix rapidly through the northwestern half of the CWA by early-mid
afternoon, with gusty west-southwest winds, warm temperatures, and
rapidly falling humidity values west of this feature. Meanwhile,
to the east of the dryline will be a modestly capped atmosphere
through early afternoon before increasing large- scale ascent
helps to gradually break the cap by mid-late afternoon in the warm
and moist sector over our eastern counties. Highs between 70-75
degrees will likely occur with some modest downsloping component,
to more 65-70 degrees east of the dryline where cloud cover will
be more persistent with streamer showers initially occurring
beneath the elevated cap.

A marginal risk for severe storms remains possible in the moist
sector across the southeast third of our CWA and can`t argue this
premise for an isolated severe storm. The main wildcard is how
rich and deep the low level moisture will be and actual instability
that will be available when convection develops in response to the
vigorous shortwave aloft and it`s associated surface dryline. The
left exit region of a 130-150 knot upper jet max arriving overhead
will lend to effective bulk shear of 40-50 knots from the southwest,
with impressive 0-1 km south-southwest bulk shear exceeding 40
knots! The shear and lift are not in question, but the instability
and quality moisture depth for deep updrafts will determine our
potential for severe weather.

What I expect is a multi-cellular cluster of showers and storms to
develop by 2-3 pm and after near I-35 and points east, then
develop into a low storm top QLCS or squall line along the
progressive dryline boundary just before 6 pm Sunday evening.
Storms will be fast-moving to the northeast, so even with limited
instability 500-700 J/KG or less, strong to isolated damaging
winds with brief severe storms is likely to be the main threat.
Now if temperatures warm via the low level warm advection beneath
the persistent cloud cover more than expected, then some hail and
even an isolated tornado would be possible considering the 3km
NAM and WRF forecasted curved low level hodographs and increased
SBCAPE values, so we`ll be monitoring this situation very very
closely, as will the Storm Prediction Center.

This system isn`t long removed from the extreme cold and winter
weather seen this past week being we`re still in January.
Nevertheless, severe storms as seen in the past can arise anytime
of the year. Otherwise, several counties in the Fire Weather Watch
along and west of U.S. 281 were upgraded to a Red Flag Warning
and may eventually need a Wind Advisory issued on later shifts as
good downward momentum occurs under near unidirectional west-
southwest flow aloft. A few counties were borderline and left them
in the Fire Weather Watch in case the surface dryline mixes faster
eastward than currently forecast. Somervell and Cooke counties
did appear to stay above critical humidity values and took them
out of any fire weather highlights. Nevertheless, some elevated
grass fire danger will likely occur elsewhere once the surface
dryline passes to the east.

Storms will exit the far eastern counties likely by mid evening
Sunday, as a pacific cold front overtakes the dryline and the
shortwave aloft continues occluding into an upper low and lifting
northeast away from the area. Cooler, but not much colder air
will arrive behind this cold front, with a reinforcing shot of a
combination of mountain and Pacific air arriving Tuesday night.
Mostly sunny and near or slightly above seasonal temperatures
will occur the first half of the week as zonal flow or shortwave
ridging aloft occur. Another vigorous longwave trough will
organize across the Western U.S. late in the week with the return
of breezy southerly winds, Gulf moisture, and slightly warming
temperatures once again. The increasing warm advection ahead of
this trough may produce a few mornings of advection fog and
drizzle once again, as well as a chance for showers and possibly
an isolated thunderstorm by Friday.

05/

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    59  71  42  60  36 /  10  30  10   0   0
Waco                58  69  41  61  31 /  20  40  20   0   0
Paris               55  68  42  55  34 /  20  70  40   0   0
Denton              56  71  39  58  31 /  10  30   5   0   0
McKinney            57  70  40  57  31 /  10  40  20   0   0
Dallas              59  70  42  59  37 /  10  40  10   0   0
Terrell             58  70  42  58  33 /  20  60  30   0   0
Corsicana           59  68  43  59  35 /  20  60  40   0   0
Temple              57  70  40  62  33 /  20  50  20   0   0
Mineral Wells       53  71  37  59  31 /   5  10   0   0   0

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 1 PM to 9 PM CST Sunday for TXZ100-101-115-
116-129-130-141.

Fire Weather Watch from Sunday afternoon through Sunday evening
for TXZ091-102-117-131-132-142-143.

&&

$$

82/30



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