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Area forecast discussion 
National Weather Service Austin/San Antonio Texas
844 am CST Monday Dec 29 2014

Update...
with dense fog observed along and south of the Edwards
Plateau...went ahead and added fog mention into the forecast from
15z to 18z for these areas. Fog should begin to lift by 16-17z
with the Rio Grande River hanging on maybe an hour or so longer.
Everything should clear out by 18z however.

&&

Previous discussion... /issued 551 am CST Monday Dec 29 2014/

Aviation...
low levels are near saturation and still expecting some MVFR fog
to form in the next hour or two...but am starting to lose
confidence. Any lower visible should only last a few hours as
temperatures climb rapidly after sunrise. The rest of today and
tonight will be VFR with light winds. Ceilings will lower to MVFR late
Tuesday morning as a cold front moves into the region.

Previous discussion... /issued 437 am CST Monday Dec 29 2014/

Short term (today through tuesday)...

Freeze occurring early this morning generally along and north of
U.S. 90/I-10...with the potential for temperatures across southern areas
to briefly dip to freezing this morning. Patchy ground fog will be
possible early this morning along and southwest of a del Rio to
Hondo to Pleasanton line. A light southwest wind...sunny skies and
dry airmass will promote a large diurnal swing in temperatures for late
December...with highs rebounding into the low 60s for most
locations. Not as cool tonight...but a potential light freeze
across The Hill Country and some lower elevations east of the
Escarpment.

The next cold front...with Arctic origins...will move through
south-central Texas on Tuesday. Cooler on Tuesday...especially
across northern areas with falling afternoon temperatures and
highs only in the middle to upper 40s...middle 50s south.

Long term (tuesday night through sunday)...

Increasing confidence in freezing rain for The Hill Country Wednesday
and Thursday mornings...

The Arctic airmass will entrench itself across the region Tuesday
night through Thursday. 1058mb surface high Tuesday across Montana
modifying to 1045mb Wednesday across western Kansas gives US a good
indication of how dense the airmass is. Eventually an upper
level low is forecast to cut off across Southern California
Tuesday night. As this happens weak isentropic ascent over the
shallow...cold airmass develops across the western half of the County Warning Area
Tuesday night into Wednesday. There are some differences between
the GFS and nam12 with how fast moistening into the lowest levels
takes place...with the nam12 very aggressive with moistening and
developing quantitative precipitation forecast late Tuesday afternoon and evening. GFS maintains a
significant dry layer below 900mb for most of the region...except
along the Rio Grande...until Wednesday night at which time moistening
towards the surface takes place...but generates thick cloud cover
with the isentropic ascent above this. European model (ecmwf) is in between...and have
followed for now with this solution. This will result in patchy light
rain and drizzle developing Tuesday night across the Rio Grande up
into the Northwest Hill country...spreading slowly east during the
day on Wednesday. There is the potential for patchy freezing
drizzle and patchy freezing rain to develop early Wednesday
morning along and north of a Rocksprings to blanco to Burnet line.
Accumulations would be very light Wednesday morning.

Given the dense nature of the airmass mentioned above...and thick
overcast conditions...we have lowered high temperatures on Wednesday...with
little diurnal range expected. Most locations over The Hill
Country will not making it out of the middle 30s and locations along
the I-35 corridor from San Antonio will not make it out of the
upper 30s.

The upper level low is forecast to move east into Arizona new
years evening night into New Years Day. The isentropic upglide over
the cold airmass strengthens west to east during this time. In
addition...GFS and European model (ecmwf) are both indicating deeper middle level
forcing in the southwest flow aloft moving through the northwest
County Warning Area Thursday. Forecast soundings indicate cooling and saturation
does take place in the upper cloud layer to support ice Crystal
initiation. However...the warm layer depth below increases
significantly during this time and warms to over 5 degree c...resulting
in melting of any ice crystals. This will set up the potential for
a freezing rain event across much of The Hill Country Wednesday
night through Thursday morning...and coming very close to the I-35
corridor Thursday morning.

We have extended the freezing rain line slightly farther east
from the previous package for late Wednesday night into Thursday
morning...now along and north of a Comstock to Boerne to
Georgetown line (and including far northern areas of Bexar County
and western areas of Comal...Hays...and Travis counties). Light
accumulations are likely across this area early Thursday
morning...which would result in travel impacts. Will continue to
watch temperature forecast trends closely over the next couple of
days...and if cooler trends continue then could be impacts along
the I-35 corridor from San Antonio to Austin. Also lowered highs
on New Years Day...only in the upper 30s to around 40 over The
Hill Country and I-35 corridor. Also...while not officially
forecast...there is the potential for sleet to mix in across
northern areas of The Hill Country Wednesday and Wednesday night.
Will be issuing and Special Weather Statement highlighting the potential for freezing
rain midweek.

The upper level low moves across southern New Mexico Thursday
night and Friday and through the Southern Plains Friday night
through Saturday night. Forcing peaks across south-central Texas
Friday and Friday night. Starting Thursday afternoon and
continuing into Friday night surface temperatures and warm cloud layer
temperatures would result in all liquid. There will also be the potential
for embedded thunder Thursday night through Friday night as
elevated lapse rates steepen.

Clearing and drying takes place over the weekend with seasonable
temperatures

Jr

&&

Preliminary point temps/pops...
Austin Camp Mabry 61 36 51 35 38 / 0 0 10 10 20
Austin Bergstrom international Airport 60 32 51 35 39 / 0 0 10 10 20
New Braunfels Muni Airport 62 35 53 36 39 / 0 0 10 10 20
Burnet Muni Airport 61 33 46 31 35 / 0 0 10 10 20
del Rio International Airport 61 38 57 37 41 / 0 0 10 30 20
Georgetown Muni Airport 60 33 48 33 37 / 0 0 10 10 20
Hondo Muni Airport 62 34 57 36 40 / 0 0 10 20 20
San Marcos Muni Airport 61 34 53 35 39 / 0 0 10 10 20
La Grange - Fayette regional 59 36 54 37 41 / 0 0 10 10 20
San Antonio International Airport 62 37 55 37 39 / 0 0 10 10 20
Stinson Muni Airport 62 37 56 38 40 / 0 0 10 10 20

&&

Ewx watches/warnings/advisories...
freeze warning until 9 am CST this morning for the following
counties: Atascosa...Delaware Witt...Fayette...frio...Gonzales...Karnes...
Lavaca...Uvalde...Wilson...Zavala.

&&

$$

Mesoscale/aviation...03
synoptic/grids...04
public service/data collection...00

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