Area Forecast Discussion
Issued by NWS San Angelo, TX

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FXUS64 KSJT 280448 AAB
AFDSJT

Area Forecast Discussion...UPDATED
National Weather Service San Angelo TX
1148 PM CDT Mon Mar 27 2017

.AVIATION...
/06Z TAFS/

Patchy low cloud development is underway over the Hill Country,
and will quickly expand north/northwest across west central
Texas overnight. Expected timing of low cloud development is by
07Z at KJCT and KSOA, 08Z-09Z at KSJT, and by 10Z at KABI.
Ceilings are expected to be in the range of 1000-2000 ft, with our
southern terminals on the low end of that range. With the
exception of KSOA, which should have ceilings below 1000 ft for a
few hours early Tuesday morning, going with ceilings mostly above
the GFS and NAM MOS. Model MOS guidance sometimes has ceilings
too low at our TAF sites. Cloud ceilings will slowly rise
through late morning and into the afternoon, and should climb
above 3000 ft during the 18Z-21Z time frame.

Some uncertainty remains on placement and timing of thunderstorms
Tuesday and Tuesday evening. This will depend to an extent on
whether a lead wave ahead of the main upper low enters west
central Texas in the afternoon. At this time, believe any elevated
convection in the morning will occur west of our TAF sites.
Indications are for the dryline to mix east near or into our far
western border counties by late afternoon. Moisture should be
deep enough ahead of the dryline to limit its progression to the
east. Thunderstorm development is possible along the dryline, and
ahead of the dryline if a lead wave moves into our area.
Consensus of model data indicates an increased possibility of
thunderstorms at KABI after 21Z, where a VCTS group has been
included. Late in the evening, a Pacific cold front is forecast
to overtake the dryline west of KSJT, and model consensus
indicates rapid thunderstorm development (into a squall line)
along this boundary. The squall line will then move quickly east
across west central Texas overnight. Have enough confidence in
this scenario to carry TSRA at KSJT by 04Z, and at KABI by 05Z.
An attendent severe weather threat will exist with thunderstorms
Tuesday (especially in the afternoon) and Tuesday night.

East winds will veer to southeast overnight. On Tuesday, southeast
winds will veer to the south and become gusty by afternoon.

19

&&

.PREV DISCUSSION... /issued 602 PM CDT Mon Mar 27 2017/

AVIATION...
/00Z TAFS/

The main items of focus with the 00Z TAFs include lower cloud
ceilings overnight and Tuesday morning, increasing wind speeds
during the day Tuesday, and the possibility of thunderstorms
by Tuesday afternoon.

Generally clear skies and VFR conditions early tonight will be
followed by low cloud development from the south/southeast
overnight. Timing of low cloud development is by 06Z at KJCT and
KSOA, around 08Z at KSJT, and by 10Z at KABI. Ceilings are
expected to be in the range of 1000-2000 ft, with our southern
terminals on the low end of that range. With the exception of
KSOA, which should have ceilings below 1000 ft for a few hours
early Tuesday morning, going with ceilings above the GFS and
NAM MOS. Model MOS guidance sometimes has ceilings too low at
our TAF sites.

Cloud ceilings will slowly rise through late morning and into the
afternoon, but have some uncertainty at this time on how quickly
the ceilings will climb to 3000 ft. Leaning toward the 18-21Z for
a resumption of MVFR conditions, but this will assessed further
with the subsequent 00Z model data. Uncertainty also exists with
placement and timing of thunderstorm development on Tuesday. At
this time, believe any elevated convection in the morning will
occur west of our TAF sites. Thunderstorm development in the
afternoon could occur near the dryline, but also have uncertainty
on how far it will mix east (may only get into our far western
counties). With the uncertainty, holding off with TSRA mention in
the TAFs at this time. Also worth noting that an attendent severe
weather threat will exist with any thunderstorms which occur on
Tuesday, especially during the afternoon and evening.

Generally light winds will become east to southeast tonight. On
Tuesday, southeast winds will veer to the south and become gusty
by afternoon.

PREV DISCUSSION... /issued 355 PM CDT Mon Mar 27 2017/

SHORT TERM...
(Tonight and Tuesday)

Main forecast challenge is timing and coverage of
thunderstorms, some severe, for Tuesday afternoon and evening.

Weak ridging aloft this afternoon will head east overnight as a
strong upper low dives quickly southeastward toward west Texas
tomorrow. This evening...will see a low level jet develop around
midnight...bringing low clouds over much of the area by morning.
Difficulty with the thunderstorms tomorrow will be related to how
quickly low clouds burn off and how unstable the atmosphere will get
by the afternoon.  Short-range models develop a round of storms
across the Big Country and Concho Vqalley early...in the 1-3 pm time
frame, with another round of storms Tuesday night as the main upper
low moves east across the Panhandle.  Will likely see some severe
storms in the first round in the afternoon, with a squall line
overnight with hail and high winds possible. Have highest precip
chances overnight Tuesday.  Will have to watch close for afternoon
storm development, as timeing will be during afternoon commute.

08

LONG TERM...
(Tuesday Night through next Monday)

Tomorrow night will see an upper low over northeast New Mexico
moving southeast over the Panhandle with the associated trough
axis extending from the upper low to the Texas Big Bend region. A
triple point is expected to be situated just west of the Big
Country by tomorrow evening. Models differ on the position and
movement of a dryline Tuesday evening with the ECM having the
westernmost placement, west of the CWA. Unlike the ECM and the
GFS, the NAM has a well defined squall line aligned southwest to
northeast from central Crockett county to central Throckmorton
county by midnight Tuesday with the line moving east through all
of the forecast area by Wednesday morning. The remaining models
have a more diffuse line of storms with rainfall also ending over
our eastern counties by Wednesday morning. Substantial low-level
and deep-layer vertical shear are still expected to be supportive
of supercells capable of large hail, damaging winds, and tornadoes
Tuesday night. The northern two-thirds of the CWA remains in an
enhanced risk of severe storms tomorrow afternoon through tomorrow
night.

A dry forecast is on tap for the remainder of the week as upper
level ridging moves over the forecast area in the wake of the
departing upper trough. Afternoon highs will be in the upper 70s
to around 80 Wednesday, in the mid 70s Thursday, and in the low
80s Friday.

Another upper level low will move southeast from the Pacific
northwest on Thursday and over the Four Corners by Saturday
morning. This system will bring another chance for showers and
thunderstorms to most of the area through the coming weekend.

15

&&

.PRELIMINARY POINT TEMPS/POPS...
Abilene  59  79  55  78 /  10  40  60  10
San Angelo  60  83  55  82 /   5  40  70  10
Junction  64  81  56  81 /  20  40  80  10
Brownwood  62  78  56  80 /  10  40  70  10
Sweetwater  59  81  54  76 /   5  40  50  10
Ozona       62  81  54  79 /   5  30  60  10

&&

.SJT WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$



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