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

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FXUS64 KEWX 202349 AAA

Area Forecast Discussion...UPDATED
National Weather Service Austin/San Antonio TX
649 PM CDT Sun May 20 2018

Please see the 00Z aviation forecast discussion below.


A complex of thunderstorms will continue to shift east and south of
the region, with most activity expected to remain to the east of our
TAF sites for the remainder of this evening. However, based on
current radar trends, we did mention VCTS at SAT and TSRA at SSF
through 01Z. The wind direction at SAT and SSF will be very
complicated due to MCV formation ongoing over southern Bexar county
and a wake low over Medina county. Will go with some gusty north
winds at SAT and SSF for the next few hours given winds will
primarily be influenced by the departing convection. Winds should
then generally trend to a more east to southeast direction through
late evening. Low clouds should also re-develop given recent rains
and some brief clearing of high clouds. We expect MVFR cigs around
06Z along I-35, with IFR clouds and fog developing closer to 09Z.
Slow improvement is then in store along I-35 tomorrow afternoon. At
DRT, MVFR should develop around 09Z and continue through the morning
hours. Improvement back to VFR is then expected by early afternoon.


.PREV DISCUSSION... /ISSUED 602 PM CDT Sun May 20 2018/

Made some adjustments to the PoP/Wx/QPF grids through 12Z tomorrow
based off evolution of radar trends and latest HRRR runs. This
remaining line of heavy rainfall should progress east southeast
producing rainfall rates of over an inch per hour but in western
areas, a well worked over atmosphere is likely preventing re-
development. Much farther west, convection is taking place over the
Mexican mountains but just about all hi-res models keep this activity
well west of the Rio Grande. Thus lowered PoPs substantially after
00Z in the west while exiting the current system mostly out of the
CWA by 03Z.

PREV DISCUSSION... /ISSUED 308 PM CDT Sun May 20 2018/

SHORT TERM (Tonight through Monday Night)...
Showers and thunderstorms continue as a slow moving outflow boundary
continues to move southward and provide lift for further cell
development. Currently, the boundary appears to be located south of
Del Rio, to San Antonio and extended northeast to just south of La
Grange. The majority of convection has been either confined to the
boundary or developed ahead of the boundary as is the case currently.

The most significant complex, located in Uvalde county as of 245PM,
is riding the aforementioned boundary as well as producing its own
healthy outflow, which is providing focused lifting along the
interaction point of both boundaries. Ahead of this complex a
gradually increasing PWAT environment (~1.6-1.8") exists as well as
convergent 850 mb flow and moisture flux. The hi-res suite has
largely whiffed on depicting this part of the line of convection
today and only the most recent runs of the HRRR (16-17Z) have begun
to resolve this feature. While still overdoing convection, it
continues this complex along the southward moving boundary, missing
San Antonio metro to the south, and progressing southeastward into
CRPs CWA. This is largely due to a weakening low level flow regime as
well as diminishing instability availability, yet the high moisture
content and continued convergent orientation to the low level flow
regime should keep this as a heavy rainfall concern throughout the
rest of its passage through the southern CWA.

Farther east, stronger southerly 850 mb flow will continue and shift
to develop a more eastward component into the evening hours which
will continue strong convergence along the outflow oriented more
southwest to northeast in that area. Additionally, even higher PWATs
of over 2" east of the I-35 corridor will continue the possibility of
heavy rainfall well into the evening. While heavily leaning on the
recent HRRR runs for short term guidance, the ensemble precip
accumulation zeroes in on Lee/Fayette counties and eastward seeing
the most rainfall through 12Z tomorrow morning. Looking at radar
trends as of 3PM, this would seem reasonable as further cell
development continues to the southeast and trains over this area.
Ensemble guidance shows 4-5 inches for a storm total in these areas
which could produce some minor flooding concerns but should stay
below FFG values in those areas thankfully.

As a result, believe a further 1-3 inches are possible for a line
from Eagle Pass to San Antonio to Giddings with isolated amounts of
5+ inches still possible for Lee/Fayette counties as well as parts of
Dimmitt, Zavala, Frio, Medina, and Atascosa counties. North of the
boundary, the majority of the rainfall should be completed for the
evening. Some CAMs are continuing to produce convection on the
Mexican side of the Rio Grande but the well worked over environment
from the boundary passage should keep any additional development at
bay. Thus, overnight, have PoPs gradually decreasing through
midnight, and decreasing substantially after midnight with only low
end chance PoPs for the extreme southeast zones.

Also PoPs for Monday will depend on destabilization efficiency and
most likely be isolated, thus have kept in a broad brush 30 PoP with
a slight preference to the Rio Grande Plains.

LONG TERM (Tuesday through Sunday)...
A much more benign pattern is in store for the extended as a ridge
axis builds over the central United States and blocks energy from
impacting the region from a broad low out west. With afternoon highs
in the upper 80s and 90s and dewpoints in the 60s throughout the
week, can`t rule out isolated afternoon showers and thunderstorm
possibilities but chances should be pretty low as the ridge continues
to build and dry out our air mass. Only included slight chance or
sub-20 percent chances throughout the week beyond Monday.


Austin Camp Mabry              70  88  70  90  70 /  30  20  10  10   0
Austin Bergstrom Intl Airport  70  89  69  90  68 /  30  20  10  20   0
New Braunfels Muni Airport     70  88  69  89  68 /  50  30  10  10  -
Burnet Muni Airport            67  86  67  88  68 /  20  20  10  10   0
Del Rio Intl Airport           71  88  71  89  72 /  30  30  20  10  -
Georgetown Muni Airport        68  88  68  89  69 /  20  20  10  10   0
Hondo Muni Airport             70  88  69  90  69 /  20  30  10  10  -
San Marcos Muni Airport        69  88  69  90  69 /  50  20  10  10   0
La Grange - Fayette Regional   71  89  71  90  70 /  50  20  10  20  -
San Antonio Intl Airport       71  88  71  89  71 /  50  30  10  10  -
Stinson Muni Airport           72  89  71  90  71 /  60  30  10  10  -




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