Area Forecast Discussion
Issued by NWS San Angelo, TX

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FXUS64 KSJT 221701
AFDSJT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN ANGELO TX
1101 AM CST MON DEC 22 2014

.AVIATION...
/18Z TAFS/

A cold front will move through the area today, with winds becoming
north to northwest at 8 to 12 knots. An upper level disturbance will
result in showers after midnight, first at the northern sites, then
spreading south through the morning hours Tuesday. The possibility
exists that some showers will develop late this evening, but
confidence is not high enough to include in the current TAF package.
MVFR ceilings will develop first at KABI around 10z, then spread
south to the Interstate corridor around 15z. The MVFR ceilings and
shower activity are forecast to persist through the end of the TAF
period.

&&

.PREV DISCUSSION... /ISSUED 600 AM CST Mon Dec 22 2014/

AVIATION...
/12Z TAFS/
Main impacts this morning are LIFR CIGs at KSOA...and IFR CIGs at
KJCT. These CIGs should improve to VFR from 15Z to 17Z as dry west
winds surge into the area this morning. Winds will gradually increase
from the Northwest at 12G20Kts through noon today. There is a chance
for light rain after 06z tonight as a strong upper level
disturbance and reinforcing cold front moves into the area. CIGs
tonight should stay remain VFR.

PREV DISCUSSION... /ISSUED 402 AM CST MON DEC 22 2014/

SHORT TERM...
(Today and Tonight)

Mostly sunny today with slightly elevated fire weather conditions
this afternoon as a weak front moves into the area. Then a
chance for light rain tonight as a strong upper level disturbance
interacts with another much stronger cold front. See fire weather
discussion below.

Meanwhile, a strong upper level jet (ULJ) max of 110Kts at 300
millibars, diving southeast across the Intermountain West today,
will carve out a deep upper level trough across the Southern
Plains and West Central Texas tonight. This places WC TX in the
favored left exit region of the ULJ max. As the 110 KT upper
level jet max/upper trough interacts with another stronger
reinforcing surface cold front, this interaction should result in
strong frontogenetic lift tonight across the area, resulting in
the development of light rain. Models do differ on the placement
and timing of the light rain. Blended the different model
solutions by going with a gradient of higher PoPs north and lower
PoPs south.

As the atmosphere undergoes rapid cooling and saturation aloft
towards 12z/6AM Tuesday, this could result in an atmospheric profile
that could support snow in the higher elevations northwest of a
line from Buffalo Gap to Sterling City by 12z. However, no snow
was mentioned since most of it should melt as temperatures remain
above freezing. For more details on the snow please read the
longer term discussion below.

26

LONG TERM...
(Tuesday through Sunday)

A potent mid-latitude trough will be moving slowly across the
Plains over the next few days. By Tuesday morning, the center of
this cyclone is progged to be in the vicinity of IA, with a strong
shortwave rounding the southern periphery of the trough. A cold
front will be making its way south across West Central TX early
Tuesday, providing a focus for low-level lift. Analysis of cross-
section data indicates a strong ribbon of frontogenesis
along/behind the surface cold front, juxtaposed with a swath of
negative equivalent potential vorticity (EPV), indicating elevated
instability. This forcing for ascent, coupled with additional lift
associated with the left exit region of the approaching upper
tropospheric jet streak, will provide a decent chance for
measurable precip across the area through early Tuesday afternoon.

While the usual question is "How much rain will we get?", the more
interesting question is "Will all of this precipitation fall as
rain?". Surface temperatures are forecast to be around 40 degrees
early Tuesday morning, with point soundings indicating at least
3000 ft of above freezing temperatures in the surface layer. This
layer is also dry, however, and diabatic cooling via wet-bulb
processes could lower surface temps sufficiently to see a little
snow mixed in with this rain. This remains a low probability event
and with temperatures in the upper 30s to lower 40s, impacts will
be nil. The best opportunity for any snow flakes would likely be
in the higher terrain of the Big Country or western Concho Valley
(primarily along Callahan Divide). As stated, no accumulations are
anticipated and given the low probability, snow will not be
mentioned at this time. We`ll certainly be watching how this
system evolves. High temperatures will be around 50 degrees.

Precip will end from west to east on Tuesday, giving way to a
subsident regime in the wake of the departing trough. We`ll see
clear skies and dry weather for Christmas Eve travel plans.
Continued cold advection via northwest winds will keep
temperatures in the low to mid 50s. Temperatures Wednesday night
are forecast to fall into the low to mid 30s as winds decrease and
back to the west. Temperatures will rebound nicely for Christmas
Day as southwesterly low-level winds increase in response to rapid
lee cyclogenesis. Southwest winds will increase to 15-25 mph and
will result in much warmer temps aloft. Temperatures at 850 mb are
progged to range from 11-14C by Thursday afternoon, which would
support max temperatures well into the 60s. Temps were bumped up a
few degrees from the previous forecast and now range from the mid
to upper 60s.

Medium range forecast models continued to show another trough
moving across the southern Rockies Thursday night into Friday.
This will drive another cold front south across West Central TX,
but moisture return is currently expected to be insufficient to
support rain chances. Highs on Friday should fall back into the
50s with overnight lows down near 30 degrees by Saturday morning.
This may provide San Angelo with its first freeze of December
2014. Cool temperatures are expected again on Saturday in the wake
of this cold front with dry weather continuing into the weekend.

Johnson

FIRE WEATHER...
A surface trough over the region will slide east this morning and
will be followed by a weak cold front shifting the southwest
winds to the northwest at 10 to 15 mph today. The combination of
low afternoon relative humidities, mild temperatures and a
moderate breeze could result in a slightly elevated fire weather
conditions this afternoon.

Elevated fire weather conditions are possible across West Central
Texas on Thursday/Christmas Day as a strong surface trough results
in southwest winds of 20 mph with gusts to near 30 mph. Relative
humidity values will be near 20 percent.

&&

.PRELIMINARY POINT TEMPS/POPS...
Abilene  65  41  49  33  52 /   5  50  30   0   0
San Angelo  71  41  50  33  54 /   5  30  30   5   0
Junction  72  43  52  32  54 /   0  20  20   5   0

&&

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

$$

99/99/99




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