Area Forecast Discussion
Issued by NWS San Angelo, TX

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FXUS64 KSJT 261136
AFDSJT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN ANGELO TX
536 AM CST Fri Dec 26 2014

.AVIATION...
/12Z TAFS/

High cloudiness passing across the area is making it difficult to
see exactly what the low clouds are doing underneath. But appears
that MVFR ceilings are moving towards the southern terminals of
Junction /KJCT/, Sonora /KSOA/ and Brady /KBBD/. Whatever MVFR
cigs do move in should be relatively brief, with clearing skies
across most of the area by mid morning. VFR conditions with gusty
southerly winds will prevail through the afternoon. A strong cold
front will shift winds around to the north tonight, reaching the
I-20 corridor by mid evening and then moving south across the
remainder of the terminal locations before sunrise Sunday morning.


&&

.PREV DISCUSSION... /ISSUED 350 AM CST FRI DEC 26 2014/

SHORT TERM...
(Today and tonight)

One last warm day in store for West Central Texas before much
colder conditions move in. Upper level trough axis moves across
the Southern Rockies by late this afternoon, allowing a cold front
to drop south through Oklahoma and the Texas Panhandle. This front
will reach the northwest Big Country northwest of Abilene by late
this afternoon, but well after the time of max heating. With skies
clearing and more sunshine this afternoon, and with 850mb
temperatures climbing another 1-3 degrees C, highs this afternoon
should be warmer than what we saw on Christmas Day. Took
temperatures up to near 70 or into the lower 70s, but would not be
surprised if a few locations were still even warmer and made a run
at 75.

Cold front reaches to near a San Angelo to Brownwood line by
midnight and clears the southern counties along I-10 by sunrise.
Much colder conditions tonight, although cloud cover and gusty
winds will slow the temperature drop. Another approaching
shortwave will try to squeeze a little precip out across the area
mainly after midnight. Lift and moisture are both a little scarce,
so anything that falls will be light. Kept the mention of a
rain/snow mix across portions of the extreme northwest Big Country
north of Sweetwater. Soundings suggest that getting snow will be
difficult, but not out of the question. However, given this
difficulty and the low chances of occurrence and the fact that
temperatures will be marginal at best as well, opted to pull the
mention out of Sweetwater and trimmed it back across other
portions of the area. Basically confined to Fisher County at this
point.

07

LONG TERM...
(Saturday through Thursday)

A rather deep, positively tilted trough will be to our west
Saturday  morning with strong southwesterly flow aloft over West
Central TX. The cold front is expected to be clearing the area
around or just after sunrise, with brisk north winds advecting
much colder air into the region throughout the day. Model cross-
sections indicate a band of frontogenesis in the 850-700 mb layer.
Weak isentropic ascent will also be present, mainly early in the
day. This should provide enough lift to squeeze out some very
light precipitation across the CWA. Point soundings show a deep
warm layer (~3C) from 850-700 mb and with surface temps at or
above freezing, the probability for wintry precipitation is low.
Thus, precip was kept as light rain for all zones. Temperatures
will remain quite cool, however, topping out in the 40s in most
areas.

Isentropic ascent associated with the trailing trough axis may
provide an opportunity for light rain Saturday night, primarily
south and west of San Angelo. Overnight lows are expected to dip
below freeze across all of West Central TX Sunday morning, with
skies clearing late and winds diminishing as the trough axis
passes to our east. Quasi-zonal flow aloft on Sunday will back
to a more southwesterly direction by early next week as a large
trough digs over the western half of the CONUS. We`ll see
moderating temps and dry conditions Sunday and Monday, with highs
approaching 60 degrees Monday afternoon. By Tuesday, however, the
bottom falls out.

An arctic airmass is poised to arrive in West Central TX Monday
night and Tuesday, bringing much colder temperatures and a slight
chance for wintry precipitation to the region. High temperatures
on  Tuesday are forecast to range from near freezing along I-20
to the lower 40s along I-10. Even colder temperatures are
anticipated on Wednesday with afternoon highs in the low to mid
30s. Overnight lows during this period are forecast to be in the
low to mid 20s.

A period of light precipitation is expected Tuesday and Tuesday
night as isentropic ascent develops over the shallow arctic
airmass. Point soundings depict a saturated profile around -20C
which should support some dendrite formation. However, a deep dry
layer is evident between 750 mb and 500 mb. Even if ice crystals
do form, it is expected that they will sublimate while falling
through the dry layer and the duration of such top-down moistening
is not expected to persist long enough to saturate. This suggests
the favored precip type would be a mixture of rain or freezing
rain (depending on the surface temperature and sleet (mainly in
the north and/or overnight. Based on recent progs, QPF amounts are
on the light side, so significant travel problems are not
anticipated. That said, a lot can change over the next few days so
we`ll continue to monitor this scenario closely.

As the cold airmass deepens on Wednesday, isentropic ascent will
diminish. With the primary closed still well off to the west,
PoPs were not included for Wednesday and Wednesday night. We
should still see a lot of cloud cover with sub-freezing wind
chills through the day. By Thursday, the upper-low over the Four
Corners begins to move east and should hasten the development of
precipitation across the region. Low-level southerly flow and
southwesterly mid-level winds imply warm advection and should
preclude wintry precipitation with this wave. Temperatures will
slowly moderate Thursday and Friday with a slight chance of rain
showers both days.

Johnson

&&

.PRELIMINARY POINT TEMPS/POPS...
Abilene  69  34  42  27  53 /   5  20  20  10   5
San Angelo  71  35  47  28  56 /   5  20  10  10   5
Junction  71  43  51  31  56 /   5  10  10  10   5

&&

.SJT WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

07





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