Tornado and Flash Flooding in Texas, April 20 to 21, 2023

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
348 PM CDT Thu Apr 20 2023

...New MESOSCALE UPDATE...

.SEVERE THUNDERSTORM WATCH 155 IN EFFECT FOR MOST OF SOUTH-CENTRAL
TEXAS UNTIL 10 PM CDT TONIGHT...

We continue to monitor the potential for strong to severe
thunderstorms this afternoon and this evening in advance of an
approaching cold front and dryline. Latest surface observations show
a wavy cold front extending from the DFW Metroplex southwest to near
Lampasas, prior to jogging west toward the Ozona vicinity. A dryline
extends due south from the Ozona vicinity into Coahuila. An extension
of said dryline appears to lead the cold front through the Sonora and
Junction vicinities, prior fusing with the boundary near Lampasas.
The most robust storms in the region have been observed in this area
over the last 30-45 minutes, likely due to the combination of lift
from both surface boundaries. This is one area we will closely
monitor moving deeper into the afternoon and evening, as the
boundaries are expected to gradually sag south into our southern
Edwards Plateau and Hill Country counties.

Further south and east, we continue to monitor for the potential of
prefrontal severe thunderstorm development throughout the I-35
Corridor and Coastal Plains. Dew points have soared into the low to
mid 70s over this portion of the CWA, with soundings from the AUS and
SAT showing minimal inhibition. Showers and storms are thus possible
over this region as well moving through the remainder of the
afternoon hours. Short term model guidance continues to hint at
scattered intensification of storms in this region through sunset.

Finally, outside potential exists for a storm to develop on the
Serranias del Burro Mountains through the early evening. It's
possible that one of said storms could make it to the Rio Grande and
our southwestern zones if this occurs. Radar trends will be closely
monitored. The Storm Prediction Center has issued Severe Thunderstorm
Watch 155 until 10 PM this evening for all of our counties except Val
Verde in light of all of the aforementioned potential. Large hail
exceeding quarter size and damaging wind gusts are the primary
concerns in any severe thunderstorms, though a few instances of very
large (> 2 inch) hail are possible. This potential would be highest
in cellular storms. Have multiple ways of receiving warning
information through the remainder of the afternoon and evening hours.

&&

.SHORT TERM...
(Tonight through Friday night)
Issued at 1242 PM CDT Thu Apr 20 2023

Mostly cloudy skies and southerly winds prevail across the area
early this afternoon. Temperatures are in the lower 80s across most
locations with some shower activity noted near Bexar County and east
of the I35 corridor. Aircraft soundings from AUS show a decent cap
that remains near 725 mb and this is likely preventing deep updrafts
from occurring. However, the HRRR continues to want to strengthen
this activity later today which likely points to the cap either
eroding or surface temperatures warming beyond the point where the
cap matters. The storm environment is unstable this afternoon with
CAPE values approaching 3000 J/kg. Effective bulk shear is near 30
knots which would support some severe threat if deep convection
becomes established. The main threats this afternoon would be hail
and damaging winds as helicity values are low.

The attention after the afternoon activity then focuses on a
potential line of showers and storms along a boundary to our
northwest. Morning high-res guidance shows this activity moving
through the CWA late this afternoon through the evening hours from
northwest to southeast. Guidance has shown a bit higher coverage of
activity to the west and the latest Day 1 Outlook from SPC accounts
for this with the slight risk now almost to the Rio Grande. Large
hail and damaging winds will continue to be the main risks for
severe convection. Low-level winds pick up slightly this evening,
and while we can't completely rule out a tornado or two, overall
chances are low. Rainfall amounts for most locations will average
below an inch, but some spots may pick up 1-3 inches with isolated
higher amounts. This could lead to a localized minor flooding threat
from the heavier totals.

Most of the shower and thunderstorm activity will push east of the
area by 1 AM. Lows tonight will be in the 60s. There could be a few
residual showers or storms in the east tomorrow morning, but
otherwise, a dry Friday is expected with highs ranging from the
lower 80s in the northeast to the lower 90s in the southwest. Mostly
clear skies and lows in the 50s to lower 60s returns Friday night.

&&

.LONG TERM...
(Saturday through Thursday)
Issued at 1242 PM CDT Thu Apr 20 2023

Synopsis: Behind Friday's departed cold front, predominantly west to
northwest flow will prevail in the mid and upper levels through the
majority of the period. Conditions will remain unsettled, with
mentionable precipitation probabilities and shower/storm mentions
remaining in the forecast through the middle portions of next week.
While each day won't feature severe thunderstorm potential, an
isolated strong to severe storm can't be ruled out from San Antonio
and points westward on Saturday. Largely southerly to southeasterly
925-850 mb flow will prevail Sunday and Monday, bringing plenty of
Gulf moisture northward atop of a cooler boundary layer under the
influence of easterly winds. This will help to drive additional
precipitation chances through the beginning of next week. An upper
disturbance is forecast to eject from the central Rocky Mountains on
Tuesday, encouraging surface cyclogenesis to our north in the Texas
Panhandle/Red River vicinity. This will place the majority of South-
Central Texas to the east of a sharpening dryline. The combination
of the advancing dryline and favorable forcing aloft could support
more vigorous thunderstorm development during this portion of the
period. While still six days out, it's possible that some of this
activity could become strong to severe.

Saturday: Winds will be quick to turn southeasterly behind the
Friday cold front, bringing moisture and attendant instability
quickly back into portions of the area. Guidance continues to
suggest that the greatest instability will pool over our
southwestern zones, where as much as 1000-1500 J/kg of CAPE could
materialize in the mixed layer. The combination of said instability
and sufficient shear aloft could support some better-organized
convection during the afternoon hours. It's possible that some of
said activity could reach marginally severe levels, with large hail
and damaging winds being the primary concerns. The Storm Prediction
Center thus continues to include the majority of the southern
Edwards Plateau, Rio Grande Plains, and portions of western Hill
Country (including the city of San Antonio) in a marginal (level
1/5) risk for severe weather in its day three convective outlook.

Sunday Through Monday: Thanks to continued low level moistening, and
the jet stream in place overhead, shower activity will continue.
Whether thunderstorms mix in with the showers remains uncertain. The
global ensembles disagree regarding the amount of instability
available for storms on each afternoon, with GEFS members coming in
considerably more aggressive than their ECMWF counterparts. Trends
regarding instability will need to be monitored in the coming
forecast cycles, as ample deep layer wind shear would support some
potential for stronger thunderstorms during this portion of the
period. As things currently stand, predominantly rain showers and
isolated non-severe thunderstorms are expected through this portion
of the period.

Tuesday: More robust thunderstorm activity is possible along and
east of an advancing surface dryline during the afternoon hours,
particularly as the shortwave discussed in the synopsis begins to
eject into the Southern Plains. The combination of lift provided by
the dryline/approaching wave, ample instability, and sufficient bulk
shear would likely support some strong to severe storm potential,
particularly if the best forcing arrives during peak heating. Given
uncertainties regarding said timing, it's much too early to offer
any specifics regarding potential hazards. SPC has introduced a 15%
risk area for severe thunderstorms across most of South-Central
Texas in light of this potential.

Remainder Of The Period: Isolated storms could persist across the
far northeast on Wednesday, but it appears a drying trend will begin
to establish itself by late next week/the conclusion of the long
term period.

&&

.AVIATION...
(18Z TAFS)
Issued at 1201 PM CDT Thu Apr 20 2023

Scattered showers continue near the San Antonio sites and east of
AUS and will continue through the afternoon. Will handle this with
VCSH for now. Otherwise, MVFR ceilings continue with VFR likely later
this afternoon. More widespread and robust convection is expected
this evening at the I35 sites. Will handle this with VCTS with TEMPO
groups when prevailing thunder is most likely. After storms move out,
northerly winds and MVFR ceilings can be expected once again.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              82  65  82  54 /  70  80  20   0
Austin Bergstrom Intl Airport  82  63  81  53 /  70  70  20   0
New Braunfels Muni Airport     83  66  84  55 /  70  70  10   0
Burnet Muni Airport            82  62  79  52 /  60  80  10   0
Del Rio Intl Airport           91  68  91  61 /  20  10   0   0
Georgetown Muni Airport        82  62  79  52 /  60  90  20   0
Hondo Muni Airport             85  66  86  56 /  60  40  10   0
San Marcos Muni Airport        82  65  82  54 /  70  70  10   0
La Grange - Fayette Regional   80  67  79  53 /  70  70  20   0
San Antonio Intl Airport       83  67  84  57 /  70  70  10   0
Stinson Muni Airport           85  68  86  58 /  70  60  10   0

&&

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

Short-Term...29
Mesoanalysis...Quigley
Long-Term...Quigley
Aviation...29

Hazardous Weather Outlook
National Weather Service Austin/San Antonio TX
232 PM CDT Thu Apr 20 2023

TXZ171>173-183>194-202>209-217>225-228-211945-
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-
232 PM CDT Thu Apr 20 2023

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

.DAY ONE...Tonight.

Severe thunderstorms are possible across all of south central
Texas this afternoon and evening, especially from the Hill Country
eastward through the Interstate 35 corridor and across the
coastal plains. While large hail and damaging winds gusts are
favored, a tornado can't be ruled out.

A Severe Thunderstorm Watch is in effect for most of the area
through 10 PM.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Elevated fire weather conditions are possible Friday afternoon
across the southern Edwards Plateau and Rio Grande Plains due to
gusty north winds and low humidity.

A few strong storms remain possible across the Rio Grande plains
and southern Edwards Plateau Saturday afternoon. Large hail is the
primary severe weather concern at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated.

Special Weather Statement
National Weather Service Austin/San Antonio TX
332 PM CDT Thu Apr 20 2023

TXZ202-202115-
Kinney TX-
332 PM CDT Thu Apr 20 2023

...A strong thunderstorm will impact portions of southeastern Kinney
County through 415 PM CDT...

At 332 PM CDT, Doppler radar was tracking a strong thunderstorm over
Spofford, moving east at 20 mph.

HAZARD...Winds in excess of 40 mph and nickel size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Brackettville, Spofford, Anacacho, Turkey Mountain, Fort Clark
Springs and Laughlin AFB Aux Field.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

A Severe Thunderstorm Watch remains in effect until 1000 PM CDT for
south central Texas.

&&

LAT...LON 2913 10052 2928 10054 2945 10011 2909 10011
      2908 10024
TIME...MOT...LOC 2032Z 270DEG 16KT 2920 10038

MAX HAIL SIZE...0.88 IN
MAX WIND GUST...40 MPH

GALE

Special Weather Statement
National Weather Service Austin/San Antonio TX
359 PM CDT Thu Apr 20 2023

TXZ173-192-202145-
Williamson TX-Travis TX-
359 PM CDT Thu Apr 20 2023

...A strong thunderstorm will impact portions of western Williamson
and northwestern Travis Counties through 445 PM CDT...

At 359 PM CDT, Doppler radar was tracking a strong thunderstorm over
Leander, or near Cedar Park, moving north at 20 mph.

HAZARD...Winds in excess of 40 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Cedar Park, Georgetown, Serenada, Anderson Mill, Leander, Lago Vista,
Liberty Hill, Florence, Georgetown Dam, Sun City, Mahomet, Andice,
Jonestown and Seward Junction.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

A Severe Thunderstorm Watch remains in effect until 1000 PM CDT for
south central Texas.

&&

LAT...LON 3080 9764 3045 9782 3050 9802 3078 9797
      3086 9789
TIME...MOT...LOC 2059Z 196DEG 17KT 3055 9790

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...40 MPH

QUIGLEY