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

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FXUS64 KEWX 111150

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
650 AM CDT Tue May 11 2021

The cold front looks to dig in a bit deeper this morning, so have
left in IFR cigs for the morning at SAT/SSF. At AUS there is a bit of
light fog and lower CIGS of 200-500 ft so the improvements are pushed
back into the early afternoon. There`s low confidence that the front
is expected to wash out along I-35 sites, but the 06Z NAM backed up
the 00Z trend for AUS. Will offer just a hint of the front weakening
in the early afternoon, but am beginning to think the latest HRRR
solution might be more correct in only showing a brief wind response
at SAT/SSF and no response at AUS. If so, there could be lower IFR
type cig conditions lasting along I-35 for much of the day. Will
concentrate late afternoon and early evening for the best chance of
thunder, with mainly showers before and after.


.PREV DISCUSSION... /ISSUED 333 AM CDT Tue May 11 2021/

SHORT TERM (Today through Wednesday)...
Another Highway 90 and Winter Gardens round of storms continues to
chip away at our more drought-prone areas. MRMS indicated a swath of
2-3 inch rains from Brackettville to Hondo and over the La Pryor,
Loma Vista and Carrizo Springs areas. Not a whole lot of ground
truth reports to analyze in real-time but a USGS gauge just NE of
Hondo indicated a 2.3 inch amount that was a good deal higher than
the corresponding MRMS amount for the area. This underestimation was
probably localized due to the EWX radar going down and the lone DFX
contribution had hail contamination corrections to be made before
the storms transitioned to heavy rain producers as they tracked
east. All this to say, that there will probably be a couple 4 inch
measurements coming in from the 7 am volunteer reports. This would
have likely created some temporary run-off issues along and just
north of Highway 90, but overall, river responses have been mild so
far. However, a repeat of this scale of event for today could make
the river responses much more dramatic as half the creek channels in
this area are often dry more that they are running. A look at the
convective allowing model sim-ref products would suggest that the
heaviest rains for today/tonight will focus south of Hwy 90 and east
of Hwy 281, thus targeting areas that are either less saturated from
not getting the heavy rain totals over the past 7 days, or are less
flood prone due to terrain flatness and more porous soil types. Thus
while PoPs remain high in the today/tonight periods, the higher QPF
amounts are expected to be mainly 1-2 inches with perhaps an isolated
spot seeing 3+, most likely over the Rio Grande Plains/Winter
Gardens. This warrants the mention of a locally heavy rain wording in
the HWO, but there are low concerns for significant run-off issues.

As with the event from Monday night, there remains a threat for
severe weather with hail becoming the higher threat for most of the
area. Areas that manage to stay south of quasi-stationary front will
have a threat for damaging winds. The front surged into the San
Antonio metro area overnight, but convective allowing models all
show at least a subtle erosion of the front. The new 00z Tech-WRF is
most bullish and indicates a stronger storm complex to be possible
over the Nrn Hill Country and moving progressively SE along a well
defined cold front. It seems unlikely the frontal layer around
Austin and points north will simply wash away in the next several
hours, as the highs forecast for today are only going into the 70-75
range. Still, the areas along and S of I-10 would probably be able
to utilize this Tech-WRF model run as a reasonable expectation of
how severe or how heavy the rain will be, which is to say that I
think the output might prove to be slightly overdone. Certainly
there are good signals that the areas east of I-35 should have at
least one heavy round of rain in the afternoon/evening whereas they
mostly missed out from yesterday`s storms.

Our surface wind forecast directions are a bit simplified to show a
subtle return to southerlies over the I-10 corridor, so perhaps the
12Z updates will show a more precise timing of the fropa. Once the
front is through, we should see a nice cool-down into the upper 50s
and 60s. The MOS guidances trended cooled for the 00Z runs so there
may be some additional adjustment needed for tonight`s forecast
lows. The forecast highs for Wednesday are already appropriately
cool and we might even flirt with a few record low max records. The
PoPs for wednesday have been nudged down off the blends as the QPF
values in the deterministic runs seemed to suggest a lot of areas
with trace to 0.01 inch type rains.

LONG TERM (Wednesday Night through Monday)...
On Wednesday evening, low chances of showers and thunderstorms linger
near the Rio Grande due to convection possibly coming off the
Serranias del Burro and near the Coastal Plains being closer to the
frontal forcing.

Surface and upper level ridging bring fair weather for Thursday and
Friday. Cold advection ends, however, below normal temperatures close
out the week.

A wet pattern returns for the weekend into next week. The ridging at
the surface and aloft shifts to the east allowing a series of mid
level impulses to move over Texas while the low level jet
strengthens. Forcing by the impulses and low level jet on a moist
airmass generates renewed rounds of showers and thunderstorms
beginning Saturday and continuing through middle to late next week
when a deeper upper level trough moves over Texas. It`s too early for
a determination of storm severity or rainfall amounts, however,
being May and considering recent history, there is a potential of
strong to severe storms and locally heavy rains. Forecasts over the
next several days will provide more details as the models come more
into focus.


Austin Camp Mabry              75  62  68  58  74 /  80  70  40  10  -
Austin Bergstrom Intl Airport  77  62  68  57  74 /  80  70  40  10  -
New Braunfels Muni Airport     81  64  70  60  77 /  70  70  40  10  -
Burnet Muni Airport            73  58  65  55  72 /  90  70  40  10  -
Del Rio Intl Airport           83  68  74  62  78 /  70  60  60  20  10
Georgetown Muni Airport        74  60  66  57  74 /  90  70  40  10  -
Hondo Muni Airport             84  65  74  60  77 /  70  70  50  10  -
San Marcos Muni Airport        79  63  69  58  75 /  70  70  40  10  -
La Grange - Fayette Regional   83  66  71  60  77 /  60  70  50  20  -
San Antonio Intl Airport       82  65  71  60  75 /  70  70  40  10  -
Stinson Muni Airport           84  66  73  62  77 /  60  70  50  10  -




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