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

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000
FXUS64 KEWX 100504
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1104 PM CST Tue Jan 9 2018

.UPDATE...
Please see the 06Z aviation forecast discussion below.

&&

.AVIATION...
An area of fog and low clouds currently east of the I-35 corridor
will continue to spread westward overnight. We have opted to start
cigs and fog a little earlier than the previous forecast and will
mention LIFR conditions starting 08-09Z at SAT, AUS and SSF. Based on
current data, we also expect some VLIFR cigs and possibly vsbys at
AUS and SAT early Wednesday morning. Conditions will likely remain
VLIFR or IFR along I-35 through most of Wednesday morning. We then
expect some improving conditions by early afternoon as daytime
heating and gusty south winds promote low-level mixing. Out west at
DRT, VFR conditions are forecast through the period with continue
east to southeast winds.

&&

.PREV DISCUSSION... /ISSUED 1033 PM CST Tue Jan 9 2018/

UPDATE... /DENSE FOG TONIGHT/

A Dense Fog Advy was issued for 09Z-16Z for areas along and east of a
line from Liberty Hill to Hondo to Dilley. Rolling advection fog is
likely based on finer res model depictions showing a high chance of
dense fog after midnight. Mostly light fog has already formed over
the Coastal Prairies with downstream dewpoints currently running
around 5 to 10 degrees cooler along the I-35 Corridor. With the
locally drier air temperatures may hit overnight minimums at around
midnight and level off through the rest of the morning as the
temperature and moisture gradient advances inland. Some of the finer
res models, especially the NAM indicate a good chance of drizzle over
the dense fog area. Not much support for this is shown in the rapid
refresh runs, so the occurrence of drizzle may be a brief one.

PREV DISCUSSION... /ISSUED 638 PM CST Tue Jan 9 2018/

UPDATE...
Please see the 00Z aviation forecast discussion below.

AVIATION...
VFR conditions are in place this evening as light south to southeast
winds continue and SCT-BKN high clouds move in from the west. High
clouds are expected to remain intact overnight and this will have an
impact on fog and low cloud development. At the present, it appears
some fog will initially develop along the I-35 corridor between
06-09Z with vsbys initially low end MVFR - IFR. An area of low clouds
is then set to expand with LIFR conditions expected along I-35 around
12Z. LIFR will continue through mid-morning when conditions improve
into IFR. Daytime heating and mixing should promote continued
improvement to MVFR by early afternoon, with VFR forecast by mid-late
afternoon. VFR conditions are expected through the period at DRT as
it appears low clouds will not move into the Rio Grande plains.

PREV DISCUSSION... /ISSUED 254 PM CST Tue Jan 9 2018/

SHORT TERM (Tonight through Wednesday Night)...
As of 2PM this afternoon, GOES-E visible imagery showed some high
cirrus moving in from the west, winds were beginning to increase into
the 10-15 mph range on the Plateau and Rio Grande Plains due to a
enhanced surface pressure gradient from troughing in the Texas
Panhandle, and highs were ranging from the mid 60s to low 70s. At the
H5 level, a ridge axis was situated over the region which will slowly
shift eastward over the next 36 hours.

Dewpoints are expected to increase overnight and into tomorrow
morning as low level winds shift back to the southeast. Hi-res models
show dewpoint depressions falling to below a degree along and east of
the I-35 corridor tomorrow morning and develop fairly widespread fog
across these areas through about 15Z. After this fog dissipates, the
eastern third of the CWA should remain mostly cloudy, preventing
highs from reaching as high as locations under less cloud coverage in
the western 2/3. Afternoon winds will also increase to 10-20 mph on
the Plateau and Hill Country as a surface low develops off the lee of
the Rockies and is pushed southeast along the advancing cold front.

LONG TERM (Thursday through Tuesday)...
A series of frontal systems will be the theme of the long term period
as the first of these fronts reaching the CWA by Thursday morning.
This frontal passage is quite strongly forced by 30-40 kt 925-850 mb
winds behind it and should push through quickly late Thursday morning
and early afternoon. The biggest concern should be the wind potential
as MOS and deterministic guidance indicate afternoon sustained winds
of 20-25 mph winds out of the north with gust potential to 30 mph.
Our typically windy spots in these north winds scenarios (along and
just east of the I-35 corridor) may see gust potential over 30 mph
during the afternoon. Kept PoPs east of the I-35 corridor with the
fropa due to meager moisture in place.

Temperatures will fall into the low 30s on the Plateau, Hill Country
and Austin areas Friday morning and combined with the still elevated
winds, will likely result in wind chills in the 20s by early Friday
morning.

Friday should be cool with continued north breezy winds, but another
cold front is progged to approach late Friday night into early
Saturday morning, knocking on the northern CWA border by 12Z
Saturday. While not as dynamically impressive, reinforcing cold air
will keep highs on Saturday and Sunday only in the 40s/50s with a
broad surface high over the eastern half of the state. This high
should shift off to the east quickly Sunday evening and allow low
level winds to shift back to the south and southeast by Monday
morning.

Monday is the next progged cold front per the GFS and Monday
night/Tuesday morning per the ECMWF. Both models agree overall with a
fairly standard overrunning pattern and decent low level moisture
enough to increase PoPs Monday into Monday evening with an expanding
area of QPF across the CWA. Temperature guidance does drop
temperatures into the 30s Tuesday morning but at this time, keeps all
areas above freezing thus not expecting anything other than light
rain for now. Will keep an eye on this evolution moving forward
however.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              44  69  54  61  35 /   0   0  10  10   0
Austin Bergstrom Intl Airport  42  69  53  63  34 /   0  10  10  10   0
New Braunfels Muni Airport     41  71  53  66  35 /   0  10  10  10   0
Burnet Muni Airport            42  69  49  56  31 /   0   0  10  10   0
Del Rio Intl Airport           44  71  48  65  35 /   0   0   0   0   0
Georgetown Muni Airport        43  68  53  58  32 /   0   0  10  10   0
Hondo Muni Airport             41  71  49  66  35 /   0   0  10  10   0
San Marcos Muni Airport        42  70  53  64  35 /   0  10  10  10   0
La Grange - Fayette Regional   45  67  56  65  34 /   0  10  10  30  -
San Antonio Intl Airport       44  71  53  65  37 /   0  -   10  10   0
Stinson Muni Airport           43  72  53  66  38 /   0  10  10  10   0

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory from 3 AM to 10 AM CST Wednesday for the
following counties: Atascosa...Bastrop...Bexar...Caldwell...Comal...
De Witt...Fayette...Frio...Gonzales...Guadalupe...Hays...Karnes...
Lavaca...Lee...Medina...Travis...Williamson...Wilson.

&&

$$

Mesoscale/Aviation...24
Synoptic/Grids...Oaks
Public Service/Data Collection...YB



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