Area Forecast Discussion
Issued by NWS San Angelo, TX

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FXUS64 KSJT 170439
AFDSJT

Area Forecast Discussion
National Weather Service San Angelo TX
1039 PM CST Sat Dec 16 2017

.AVIATION...
/06Z TAFS/

IFR to LIFR CIGs expected for tonight into Sunday morning for all
TAF sites with a shallow layer of moisture near the ground. May
see some MVFR fog develop tonight at most sites. KBBD could go to
IFR in VIS due to fog. These CIGs should lift between 18Z and 21Z
to VFR.

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&&

.PREV DISCUSSION... /issued 813 PM CST Sat Dec 16 2017/

UPDATE...
Removed evening rain chances. Added patchy fog to the southern 2
tiers of counties, including the I-10 corridor.

DISCUSSION...
Western edge of rain band has moved east of West Central Texas at
8 PM as center axis of the upper trough also moved east of the
region. Thus removed tonight`s rain chances. Added patchy fog to
southern counties as wind speed will become light in that region
by morning and vegetation was wet due to the light rain earlier
today.

PREV DISCUSSION... /issued 546 PM CST Sat Dec 16 2017/

AVIATION...
/00Z TAFS/

Most of the light rain will move out of the area by early evening.
CIGs will vary between MVFR and IFR through tonight. May also see
VFR CIGs briefly. Visibilities may lower to MVFR in light fog
also tonight. Expect improvement after 15Z on Sunday as low clouds
burn off. Surface winds should continue to diminish after 06z.

PREV DISCUSSION... /issued 321 PM CST Sat Dec 16 2017/

SHORT TERM...
(Tonight and Sunday)

Patchy rain is occurring this afternoon across our area along with
widespread cloud cover, in association with an upper low and trough
moving across the Big Bend region. The upper trough will lift out to
the northeast across our area early tonight, as an upstream low and
trough dive southeast into the northern Baja region. Rain chances
will shut off as the aforementioned trough axis swings through. Low
cloud cover should linger until late tonight/early Sunday morning,
when boundary layer flow starts to veer to the southwest. Not that
much of a temperature dropoff is anticipated tonight, with overnight
lows expected to range from the upper 30s across our western and
southern counties, to the lower 40s in the eastern Big Country and
Heartland areas.

Mostly sunny skies and pleasant conditions are expected on Sunday
with southwest and south winds remaining generally under 10 mph.
A weak cold frontal boundary should sag south into our far northern
counties by late afternoon or early evening. Afternoon highs Sunday
are expected to be in the mid 60s to around 70.

LONG TERM...
(Sunday night through Saturday)

Sharply colder by Christmas Weekend...

Another upper level low will approach the area from the Desert
Southwest on Monday and will bring another chance of
precipitation to the area Monday night and especially Tuesday.
Most of the area should see some rain with this system but
southeast sections will have the best chance and will have highest
POPs across this area on Tuesday. Temperatures will be warm
through midweek, with highs mainly in the 60s and overnight lows
in the 40s.

Significant changes are anticipated towards the end of the week
and into Christmas weekend as the medium range models continue to
show a very strong cold front moving across the area Thursday
night, with an arctic airmass becoming entrenched across the area
through Christmas day. Forecast highs are only expected to be in
the 30s Friday through Sunday, with lows in the 20s.

However, there is considerable model uncertainty concerning
precipitation chances and amounts for next weekend. The GFS
continues to be the wetter model as it closes off an upper low
over the Desert Southwest by Friday and then slowly moves it east
across the area through Christmas Day. The ECMWF shows a much more
progressive trough, with little if any precipitation affecting
the area. For now, will side closer to the wetter GFS solution
and have POPs across the area Friday through Sunday. Temperatures
will be cold enough for winter precipitation, starting off as
rain on Friday and transitioning to a mix of rain, sleet and snow
by Friday night and Saturday. Temperatures look cold enough for a
transition to all snow by late Saturday night into Christmas eve.

We would like to stress that confidence regarding winter precipitation
and amounts remains low at this time due to the model discrepancies
with the upper pattern but is high concerning the arrival of
much colder air by next weekend into Christmas Day.

&&

.PRELIMINARY POINT TEMPS/POPS...
Abilene  40  67  41  62 /  30   0   5   5
San Angelo  39  69  42  65 /  30   0   5  10
Junction  39  68  44  66 /  30   0  10  20
Brownwood  40  68  43  64 /  50   0   5  10
Sweetwater  39  65  40  61 /  20   5   5   5
Ozona       37  67  41  63 /  20   0   5   5

&&

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

$$

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