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

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FXUS64 KEWX 171740
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1240 PM CDT Sat Jun 17 2017

.AVIATION...
A seasonally-strong pressure gradient will maintain moderate and
gusty winds at all TAF sites this afternoon. The gusts will drop off
with the end of daytime heating. Other than SCT-BKN cirrus from
thunderstorms in north Texas, look for MVFR stratocumulus clouds to
develop around 06-7Z and last until 15-16 at the I-35 TAF sites.
There will be no restrictions at KDRT...VFR there. Expect a repeat on
Sunday with hot near-surface conditions and moderate and gusty
southerly winds.

&&

.PREV DISCUSSION... /ISSUED 647 AM CDT Sat Jun 17 2017/

AVIATION.../12Z TAFS/
MVFR ceilings above 1400 feet currently prevail at the I-35 TAF
sites, but they are bit more intermittent at SAT where coverage
continues to oscillate between scattered and broken. Ceilings should
rapidly improve to VFR by 14Z at all sites this morning as southerly
(southeast at DRT) 10-15 knot winds gusting to 20-25 knots continue
through at least late afternoon if not into the evening hours. A
similar progression of redeveloping MVFR ceilings is expected for the
I-35 TAF sites around 8-9Z overnight before lifting to VFR by 15Z
tomorrow morning.

PREV DISCUSSION... /ISSUED 419 AM CDT Sat Jun 17 2017/

SHORT TERM (Today through Sunday)...
Low-level nocturnal stratus is in the midst of developing across the
eastern half of the area this morning. Should see these dissipate
late this morning leaving behind mostly sunny skies. The main
forecast problem in the short-term is going to be the temperature
forecast for today and whether or not our other climate sites besides
Del Rio hit the magical 37.78 degrees Celsius or better known as
100F. Model guidance is warmer than yesterday at the surface and
deterministic models are progging 850 temps to warm by 3-4C versus
yesterday. This is quite a jump in 24 hours with just southerly flow
at 850 mb. So the question is: why are the models showing this
temperature increase? Forecast soundings show the mixed layer
extending above 850 mb so the increase in 850 temps could be
partially blamed on higher surface temperatures, but the soundings
also show substantial warming at 800 mb which is above the mixed
layer.

At 500 mb there is a weak disturbance moving from north to south to
the south of us that was actually in our area today. It was just too
dry and capped to bring any rain chances. The center of the 500 mb
high is well to our west and is actually expected to weaken today as
an upper trough moves into the NW CONUS. One does not normally expect
a weakening ridge to the west to equate to warming surface
temperatures. However, the reason models are hitting on warmer
temperatures today is all due to that exiting shortwave to our south.
500 mb winds today behind this feature will transition from a NNW
direction to a NNE direction which favors subsidence. The sinking
motions in the atmosphere can easily be seen on a 700 mb omega plot.
To compare, omega values yesterday afternoon were near zero and
actually positive in some places which represents rising air. Today,
we should see -2 to -5 mb/s motions which represents strong
subsidence and sinking air.

So how does subsidence lead to warming temperatures? As air sinks,
the pressure increases and the ideal gas law tells us that within
constant volume, if pressure increases temperature has to as well.
This explains why we are expecting the jump in temperatures today.
With higher temperatures, we should see a little more mixing this
afternoon with slightly lower dewpoints. This should mean that
afternoon heat index values will be near yesterday`s for most of the
area except the southwest CWA which is expected to be above Advisory
criteria and issued an advisory for this area. The rest of the CWA
will be just under Heat Advisory Criteria and will handle that threat
with an SPS. After night time lows tonight in the middle to upper
70s across much of the area, highs tomorrow will be hot, but about
3-4 degrees cooler than today as the low-level thermal profile cools
with the loss of the increased subsidence.

LONG TERM (Sunday Night through Friday)...
On Monday, the previously mentioned trough moving into the NW CONUS
will be pushing east into the Ohio Valley and this should send a
weak frontal boundary into Texas Monday afternoon. The GFS/ECMWF both
stall this front near our northern border. This feature in
combination with little capping and abundant moisture could spark off
a couple of thunderstorms in the afternoon and perhaps lingering
into the evening hours. Will introduce a 20 PoP in the northern CWA
Monday afternoon for now, but these could be expanded in areal
coverage as well as extended into Monday evening with later forecasts.
Forecast soundings do show about 3000 J/kg of SBCAPE and an
inverted-v type sounding which could lead to the chance of
microbursts if stronger thunderstorms do develop. The lack of large-
scale forcing for ascent should keep the overall coverage of activity
low. Due to low probabilities and confidence will not mention
anything in the HWO yet.

The focus of the long-term forecast then shifts to the Gulf of Mexico
and the area of disturbed weather southeast of the Yucatan. The ECMWF
is the only model showing a lot of moisture moving into Texas, but
not one of the GFS ensemble members is showing any appreciable precip
late next week. Given the uncertainties, will have a 20 PoP in the
afternoons Thursday and Friday for the far eastern counties.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              76  98  75  96  75 /   0   0   0  10  10
Austin Bergstrom Intl Airport  75  98  74  95  73 /   0   0   0  10  10
New Braunfels Muni Airport     74  98  74  95  73 /   0   0   0  10  10
Burnet Muni Airport            74  95  73  93  72 /   0   0   0  20  10
Del Rio Intl Airport           77 103  76  99  76 /   0   0   0  -   10
Georgetown Muni Airport        75  97  74  94  73 /   0  -   -   20  10
Hondo Muni Airport             74 100  75  98  72 /   0   0   0  -   10
San Marcos Muni Airport        75  97  75  95  73 /   0   0   0  10  10
La Grange - Fayette Regional   76  96  75  95  75 /   0  -   -   10  10
San Antonio Intl Airport       75  99  76  96  75 /   0   0   0  10  10
Stinson Muni Airport           75  99  76  96  74 /   0   0   0  10  10

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 7 PM CDT this evening for the following
counties: Atascosa...Dimmit...Frio...Maverick...Zavala.

&&

$$

Mesoscale/Aviation...26
Synoptic/Grids...05
Public Service/Data Collection...YB



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