Special Weather Statement, NWS Austin/San Antonio, TX

Area Forecast Discussion...UPDATED
National Weather Service Austin/San Antonio TX
636 AM CDT Tue May 18 2021

.AVIATION (12Z TAFS)... /NEW/

Plenty of uncertainly still exists and this forecast package is yet
again a low confidence forecast. Nevertheless, I have attempted to
pin down the key time frames for thunderstorms at all 4 terminals.
The use of PROB30s and TEMPO groups was necessary to get a better
picture of the most likely timeframe for thunderstorms. Expecting
MVFR CIGs at AUS and potentially SAT/SSF this morning through midday.
We should see VFR CIGs return by midday, with the threat for
thunderstorms increasing. The first round looks to try and develop
between 18Z and 23Z, with the second, perhaps heavier round after
01Z. Once again, this is a fairly low confidence forecast, but the
one thing I am confident about is ceilings being around MVFR or VFR
unless within a thunderstorm, and even then, IFR CIGs should be
limited to just a few hours. Del Rio should see slightly different
timing than the I-35 sites, with VFR CIGs by mid morning and
continuing throughout the day. Thunderstorms will work their way into
the picture by 00Z.

Morris

&&

.PREV DISCUSSION... /ISSUED 303 AM CDT Tue May 18 2021/

SHORT TERM (Today through Wednesday)...
..STRONG TO SEVERE THUNDERSTORMS ALONG WITH FLASH FLOODING APPEAR
LIKELY OVER THE NEXT SEVERAL DAYS...

South-Central Texas will remain within a fairly stormy pattern as
southwesterly flow aloft continues for the foreseeable future. Water
vapor imagery shows an H5 low centered over south-central Colorado
as of 07Z. This upper low will continue to slowly slide eastward,
before becoming an open wave by 00Z Thursday over western Kansas. A
series of mid-level shortwaves will continue to orbit the periphery
of this upper low, resulting in several rounds of strong to severe
thunderstorms capable of large hail, damaging winds, and most
concerning of all, heavy rains and flash flooding.

For the near term and the rest of tonight, convection remains
possible, especially over the Rio Grande Plains and Southern Edwards
Plateau, but a rather stout cap around 700 mb, with temps of 10 to
12 C appears to be squashing any sort of convection that attempts to
develop. This will likely continue through early morning in the
absence of a meaningful mid-level lifting mechanism, which is
currently sparking off convection near the Midland-Odessa area. A
stronger shortwave looks to round the base of the trough and eject
out over the western Hill Country and Southern Edwards Plateau by
mid to late Tuesday morning, which will be our next trigger for
possible storm development by late Tuesday morning/early afternoon.
With deep layer shear of 40-50 kts, MLCAPE of 2500-3500 J/kg, and
mid level lapse rates still on the order of 7-7.5 C/km, any storms
that form could become strong to severe with large hail and damaging
wind gusts. Flash flooding will become a bigger concern toward
Tuesday evening, especially if storms can organize and grow into
clusters or an MCS. PWATs of 1.5-2" will be common, with rainfall
rates of 1-2" per hour or higher within the heaviest storms. The
forecast still remains fairly complicated due to convective
processes that are harder to pinpoint in a setup like this.
Something to note: the WPC Excessive Rainfall Outlook for day 1 puts
the entire I-35 corridor including the San Antonio Metro within a
slight risk, along with portions of Williamson and Lee County within
a moderate risk for flash flooding. Rainfall totals of 1-2" with
isolated higher totals in excess of 3" are possible over the I-35
corridor and Coastal Plains.

Our next possible round of storms looks to arrive sometime late
Tuesday evening or early Wednesday morning, but exactly where storms
form is the biggest question. Another mid-level shortwave may move
through South-Central Texas late Tuesday evening through early
Wednesday morning, with thunderstorms attempting to organize over
central Texas and then push southeastward, forming into a complex by
very early Wednesday morning. The 06Z HRRR is very concerning in
it's depiction of a complex organizing and moving through the Hill
Country, then I-35 corridor by sunrise Wednesday. Very heavy rainfall
which could lead to flash flooding is expected during the day
Wednesday. Training of storms and slow storm movement will likely
result in flooding rather quickly, especially for areas that have
already seen heavy rainfall over the last few days. The day 2 ERO
places the entire I-35 corridor including the Austin and San Antonio
Metros within a moderate risk for flash flooding. Rainfall totals of
2-5" with isolated totals greater than 6" are possible on Wednesday
alone if latest guidance comes to fruition. A flash flood watch is in
effect for the eastern 2/3rds of our CWA through 18Z Thursday, with
rainfall totals of 3-7" and isolated totals up to 10" possible.

LONG TERM (Wednesday Night through Monday)...
The pattern aloft over the CONUS will be dominated by an upper level
ridge over the southeastern states mid to late week that drifts to
the Mississippi Valley over the weekend with an upper level trough
centered over the Great Basin mid week through the weekend. Closer
to our area, an upper level low develops over southern Texas into
northeastern Mexico mid to late week, then gets stretched and weakens
over the weekend. An unseasonably moist airmass remains, especially
over our eastern areas due to an upper level flow from the western
Caribbean and a lower level flow from the Gulf of Mexico. Strong
upward forcing of this unseasonably moist airmass will allow for
more rounds of mainly diurnally driven showers and thunderstorms
especially over our eastern areas along with a threat of locally
heavy rains and additional flooding. There is also a possibility of
a few strong to perhaps severe storms due to MUCAPE of 500 to 1500
J/kg. Temperatures will average below normal due to the extensive
cloudiness and areas of rain.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              84  69  79  66  78 /  90  70  90  70  70
Austin Bergstrom Intl Airport  84  69  80  66  79 /  90  70  90  70  70
New Braunfels Muni Airport     86  70  79  66  81 /  90  60  90  70  60
Burnet Muni Airport            81  67  79  64  78 /  80  60  70  60  50
Del Rio Intl Airport           93  72  86  67  86 /  20  30  50  10  -
Georgetown Muni Airport        82  68  79  65  78 /  90  70  80  70  70
Hondo Muni Airport             85  70  80  65  82 /  60  60  80  50  30
San Marcos Muni Airport        85  70  80  65  80 /  90  70  90  70  60
La Grange - Fayette Regional   86  72  81  69  81 /  70  70  90  80  80
San Antonio Intl Airport       84  70  79  66  79 /  80  60  90  60  50
Stinson Muni Airport           86  72  80  67  81 /  80  60  90  60  50

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Thursday afternoon for Atascosa-Bandera-
Bastrop-Bexar-Blanco-Burnet-Caldwell-Comal-De Witt-Fayette-Frio-
Gillespie-Gonzales-Guadalupe-Hays-Karnes-Kendall-Kerr-Lavaca-Lee-
Llano-Medina-Travis-Williamson-Wilson.

&&

Short-Term/Aviation...Morris
Long-Term...04

Hazardous Weather Outlook
National Weather Service Austin/San Antonio TX
354 AM CDT Tue May 18 2021

TXZ171>173-183>194-202>209-217>225-228-190900-
Llano-Burnet-Williamson-Val Verde-Edwards-Real-Kerr-Bandera-
Gillespie-Kendall-Blanco-Hays-Travis-Bastrop-Lee-Kinney-Uvalde-
Medina-Bexar-Comal-Guadalupe-Caldwell-Fayette-Maverick-Zavala-
Frio-Atascosa-Wilson-Karnes-Gonzales-De Witt-Lavaca-Dimmit-
354 AM CDT Tue May 18 2021

This Hazardous Weather Outlook is for South-Central Texas..

.DAY ONE...Today And Tonight.

Strong to severe thunderstorms are expected this afternoon and
evening, with large hail and damaging winds possible. Very heavy
rainfall is expected with any storms that form. All of South-
Central Texas is within a Slight Risk (Level 2 of 5) for severe
storms Tuesday afternoon and evening.

A flash flood watch is also in effect for the eastern half of
South-Central Texas...or along and east of a line from Mountain
Home to Dilley. The heavy rain threat will greatly increase this
evening as storms organize out west and move through the region
late tonight.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

Several rounds of showers and thunderstorms are possible through
the week, especially Wednesday through Thursday. Thunderstorms
could be strong to severe at times, and there is a Marginal Risk
(Level 1 of 5) for severe storms Wednesday for most of South-
Central Texas.

A flash flood watch is in effect east of a Mountain Home to
Dilley line through 1 PM CDT Thursday. Daily rainfall amounts of
1 to 4 inches are possible, and portions of Central Texas could
end up with multiple day rainfall totals up to 10 inches. These
heavy rainfall totals will likely send some area rivers and
streams into minor or moderate flood stages.

.SPOTTER INFORMATION STATEMENT...

Spotters are encouraged to report severe weather and/or flash flooding
to the National Weather Service.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE AUSTIN/SAN ANTONIO TX
951 AM CDT TUE MAY 18 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0100 AM     NON-TSTM WND DMG DILLEY                  28.67N 99.17W
05/18/2021                   FRIO               TX   PUBLIC

            THE FRIO COUNTY EMERGENCY MANAGER REPORTED
            SOME MINOR WIND DAMAGE IN DILLEY. PICTURES
            SHOWED MINOR ROOF DAMAGE AND A TREE BLOWN
            OVER. TIME OF THE DAMAGE IS ESTIMATED. THE
            DAMAGE APPEARS TO HAVE BEEN CAUSED BY A WAKE
            LOW ON THE BACKSIDE OF STORM COMPLEX THAT
            MOVED THROUGH EARLIER IN THE NIGHT.

&&

PAY

Special Weather Statement
National Weather Service Austin/San Antonio TX
956 AM CDT Tue May 18 2021

TXZ184>186-203-181545-
Uvalde TX-Kerr TX-Edwards TX-Real TX-
956 AM CDT Tue May 18 2021

...SIGNIFICANT WEATHER ADVISORY FOR WEST CENTRAL KERR...NORTH CENTRAL
UVALDE...EASTERN EDWARDS AND REAL COUNTIES UNTIL 1045 AM CDT...

At 955 AM CDT, Doppler radar was tracking a strong thunderstorm near
Camp Wood, or 18 miles west of Leakey, moving north at 35 mph.

Penny size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Leakey, Camp Wood, Barksdale, Garner State Park, Tuff, Rio Frio,
Reagan Wells, Vance, Lost Maples State Natural Area, Prade Ranch and
Devil's Sinkhole State Natural Area.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 2995 9956 2991 9958 2990 9960 2984 9960
      2945 9980 2967 10017 3015 10012
TIME...MOT...LOC 1455Z 198DEG 31KT 2967 10006

MO

Special Weather Statement
National Weather Service Austin/San Antonio TX
1029 AM CDT Tue May 18 2021

TXZ185-186-188-181615-
Gillespie TX-Kerr TX-Real TX-
1029 AM CDT Tue May 18 2021

...SIGNIFICANT WEATHER ADVISORY FOR NORTHWESTERN KERR AND WEST
CENTRAL GILLESPIE COUNTIES UNTIL 1115 AM CDT...

At 1028 AM CDT, Doppler radar was tracking a strong thunderstorm 8
miles southwest of Kerr Wildlife Management Area, or 20 miles north
of Vanderpool, moving northeast at 45 mph.

Penny size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Mountain Home, Kerr Wildlife Management Area, Harper, I-10 Near The
Kimble-Kerr County Line and The Intersection Of I-10 And Us 290.

LAT...LON 3030 9921 2991 9958 3001 9976 3029 9960
      3029 9930 3034 9930
TIME...MOT...LOC 1528Z 213DEG 39KT 3003 9963

MO