Area Forecast Discussion
Issued by NWS Austin/San Antonio, TX

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FXUS64 KEWX 122339
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
539 PM CST Fri Jan 12 2018

.AVIATION...
Surface high pressure continues to settle into the area and as it
does so winds will continue to become less. Should see speeds for the
valid TAF period remain below 8 knots for the most part out of a
general northeast direction. Skies will remain mostly clear with VFR
prevailing.

&&

.PREV DISCUSSION... /ISSUED 254 PM CST Fri Jan 12 2018/

SHORT TERM (Tonight through Saturday Night)...
Upper air analysis from this morning depicted broad troughing off to
our east across the southern US with northwesterly flow aloft over
south central Texas. At the surface, high pressure has taken hold
across Texas, yielding sunny skies across the region. Temperatures
today are much cooler than yesterday and below climatological
normals. The cooling trend will continue as another surge of cold and
dry high pressure comes into our area overnight tonight into early
Saturday. A freeze is expected across much of the Hill Country and
eastern parts of South Central Texas overnight into Saturday morning.
Saturday will feature below normal temperatures and sunny skies. For
Saturday night, light winds, low dew points, and clear skies will
allow for efficient radiational cooling and a hard freeze is expected
across most areas.

LONG TERM (Sunday through Friday)...
The surface high moves off to the east allowing for a brief warming
trend Sunday into Monday with temperatures returning to near normal
on Monday. Moisture gradually increases bringing an increase in
clouds throughout the day on Monday.

The focus then shifts to Monday night into Tuesday. An upper level
shortwave rotating through the larger trough sends another surge of
arctic air south with the cold front moving into the Hill Country
around midnight Monday night and the remainder of South Central Texas
overnight into early Tuesday morning. Isentropic lift rapidly
develops over the frontal surface as moisture levels increase
further. Rain should begin overspreading the region from north to
south beginning on Monday evening. Models differ slightly on the
timing and intensity of the freezing temperatures Tuesday morning,
with the 12z Euro showing colder temperatures than the 12z GFS, but
both models show surface temperatures falling below freezing Tuesday
morning across the Hill Country and our northern counties with the
freezing line extending southward to roughly a Rocksprings to San
Antonio to Gonzales line by noon on Tuesday. In regards to
precipitation type, the GFS shows a warm layer aloft, suggesting a
transition to freezing rain or sleet for areas where surface
temperatures drop to or below freezing. The Euro shows an entire
sub freezing temperature profile for the northern counties,
suggesting the possibility of snow at these locations. At this point,
think freezing rain or sleet is more likely than snow. Temperatures
will not warm up much, if at all, on Tuesday afternoon. It is quite
possible temperatures in parts of the Hill Country and Central Texas
never rise above freezing, maintaining a chance for wintry
precipitation through the afternoon. Isentropic lift shuts off as a
slightly drier airmass filters in, ending the precipitation by
Tuesday night. Accumulated rainfall at this point is uncertain. The
12z GFS is much wetter than the 12z Euro. GFS ensemble mean shows up
to half an inch of QPF, with the Euro maintaining much less amounts.
At the present moment, WPC shows up to about a quarter of an inch.
Any possible icing amounts are even more uncertain, especially given
the fact that we are still about 3 and half days out from the event.
The primary question will be how much moisture remains in place after
surface temperatures drop below freezing.

Weak ridging and a drier airmass keep Wednesday precipitation free
with well below normal temperatures. The GFS and the Euro disagree
on Thursday and Friday. The GFS shows a warming trend with a chance
for showers Thursday and Friday. The Euro shows an upper level
trough digging into Texas Thursday night into Friday, yielding
another chance at a possible winter weather event. At this time, the
grids reflect more of the GFS solution.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              29  49  30  56  38 /   0   0   0   0   0
Austin Bergstrom Intl Airport  27  49  25  55  36 /   0   0   0   0   0
New Braunfels Muni Airport     28  51  27  56  37 /   0   0   0   0   0
Burnet Muni Airport            25  47  26  53  35 /   0   0   0   0   0
Del Rio Intl Airport           33  57  31  57  39 /   0   0   0   0   0
Georgetown Muni Airport        26  47  25  54  37 /   0   0   0   0   0
Hondo Muni Airport             29  56  27  58  39 /   0   0   0   0   0
San Marcos Muni Airport        28  50  27  56  37 /   0   0   0   0   0
La Grange - Fayette Regional   28  50  27  55  36 /   0   0   0   0   0
San Antonio Intl Airport       31  53  30  56  40 /   0   0   0   0   0
Stinson Muni Airport           31  54  30  57  41 /   0   0   0   0   0

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

Mesoscale/Aviation...Hampshire
Synoptic/Grids...Oaks
Public Service/Data Collection...33



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