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

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FXUS64 KEWX 282357
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
657 PM CDT Fri Apr 28 2017

.UPDATE...
See below for the 00z aviation forecast...

&&

.AVIATION...

Interstate 35 sites...

Currently all three I-35 TAF sites are VFR and should remain VFR for
the remainder of the afternoon and evening hours with southerly winds
up to 10-15 knots. MVFR CIGs are expected to develop around or
shortly after midnight. Guidance suggests the possibility of IFR CIGs
developing overnight but didn`t have high enough confidence in that
solution, so kept them at MVFR for now. VFR CIGs should return by
early afternoon on Saturday. Only possible noteworthy aviation
impacts for the forecast period will be Saturday evening into Saturday
night as a cold front is expected to move through the region,
generating showers and thunderstorms. Exact timing and
intensity/coverage of these showers and storms are still unclear at
this point, but heavy rain, lightning, hail and gusty winds are all
possible with these storms. Gusty northwesterly winds will be ushered
in behind the cold frontal passage late Saturday night into Sunday
morning.


Del Rio...

VFR conditions are forecasted for the entirety of the forecast
period. Southeasterly winds will veer more southerly tomorrow
afternoon before shifting to the northwest following a cold frontal
passage between 21z and 23z Saturday afternoon. Gusts up to 20-25
knots will be possible tomorrow afternoon and evening in wake of the
cold front.

&&

.PREV DISCUSSION... /ISSUED 246 PM CDT Fri Apr 28 2017/

SHORT TERM (Tonight through Saturday Night)...
Warm and humid across South Central Texas this afternoon. The center
of the upper level low near the Four Corners this afternoon will dig
southeast through New Mexico tonight and Saturday then east through
the Panhandle Saturday night. Stratus will re-develop late this
evening beneath a developing cap. Beneath the cap a few showers will
be possible by Saturday morning across the eastern half of the CWA.
The main question continues to be how much the cap will erode during
the day on Saturday. As continues to be the case, GFS point forecast
soundings completely erode the cap along and east of the I-35 corridor
Saturday afternoon. We continue to lean towards a slightly stronger
cap this far south as the rest of the models indicate and slower
erosion during the day on Saturday, awaiting for the main forcing
Saturday night. However, if a storm were to develop Saturday
afternoon steep mid level lapse yielding very high MUCAPE values of
3500-4500 J/KG combined with 0-6km shear values of 40-50 kts could
produce an isolated severe storm Saturday afternoon.

The greatest chance of strong to severe thunderstorms continues to
be Saturday night, as the cold front catching up to the dry line over
the area and PVA and cooling in the base of the trough overcome the
cap. It appears storms will develop southward into portions of the
Hill Country and I-35 corridor between 8 PM and midnight and east of
the I-35 corridor between 10 PM and 2 AM. Given the aforementioned
CAPE values pooled ahead of the front over the area and increasing
deep layer shear values to 50-60 kts large hail appears to be at
least equal if not slightly greater than the damaging wind threat for
the duration of the event. A much lower, but non-zero, threat for an
isolated tornado exists across the northeast CWA.

The are some indications in some of the hi resolution models
guidance that the cold front could race eastward east of I-35 and
undercut some of the deeper convection after midnight. However for
now SPC Day 2 Convection Outlook continues portions of the CWA near
and north of I-10 in a Slight Risk for severe storms, with portions
of the far northeast CWA in an Enhanced Risk. Have added mention of
severe into the forecast for this general area Saturday night.

Due to the progressive nature of this system rainfall amounts across
the central and eastern CWA look to average 1/4 to 1 inch limiting
any potential for widespread flooding concerns. Isolated pockets of
1-2 inches can not be completely ruled out north of I-10. Farther
west toward the Rio Grande rainfall amounts drop off to nothing.

LONG TERM (Sunday through Friday)...
Clearing with windy northwest winds developing during the day on
Sunday. This could lead to elevated to near critical fire weather
conditions Sunday afternoon across western areas that do not receive
much rain with this system.

Below normal lows will occur again on Monday morning behind the
front, with lows into the upper 40s and low 50s in many areas. A
warming trend will take place Monday afternoon through Wednesday.
Another upper level system and surface cold front looks to impact
the area Wednesday and Wednesday night. ECMWF continues to be slightly
more progressive than the GFS with the main chance for rain coming
with the front during the day on Wednesday while the GFS is indicating
Wednesday night. Cooler with below normal temperatures behind this
second front.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              76  88  57  75  52 /  20  40  70  10   0
Austin Bergstrom Intl Airport  76  88  57  75  49 /  20  30  70  10   0
New Braunfels Muni Airport     75  88  57  77  49 /  20  30  70  -    0
Burnet Muni Airport            73  84  52  72  48 /  20  40  70  -    0
Del Rio Intl Airport           71  91  54  79  54 /   0  10  20   0   0
Georgetown Muni Airport        75  86  55  73  50 /  20  50  70  10   0
Hondo Muni Airport             74  91  55  79  48 /  20  20  50   0   0
San Marcos Muni Airport        75  88  56  76  49 /  20  30  70  -    0
La Grange - Fayette Regional   76  87  59  75  51 /  20  20  80  10   0
San Antonio Intl Airport       75  89  57  78  52 /  20  20  60  -    0
Stinson Muni Airport           75  91  58  79  52 /  20  20  60  -    0

&&

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

$$

Mesoscale/Aviation...Williams
Synoptic/Grids...Allen



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