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

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FXUS64 KEWX 161946

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
246 PM CDT Sun May 16 2021

.SHORT TERM (Tonight through Monday Night)...
The big winner for the rainfall today has been folks down along the
Coastal Plains and then along the I-35 corridor through Austin. One
LCRA hydromet gauge in North Austin has reported 3.13 inches of
rainfall since midnight. For the rest of the afternoon expect focus
of the rainfall to continue to shift to the north and east with
mainly light to moderate showers, and a few rumbles of thunder
continuing for the I-35 corridor. Mostly sunny skies across the area
for the late morning and these early afternoon hours may help spark
off additional isolated showers and a few storms thanks to the lift
cause by daytime heating. The first big forecast question for the
short term will be whether or not any Serranias del Burro (SdB)
storms can get going this afternoon and if those move more to the
southeast parallel to the border or move more easterly into the
area. There are indications of some initial development on the KDFX
radar. Models in general though have backed off on the SdB storms
with a weak cap holding them at bay. If a storm can get going in the
6pm to 8pm time frame there is plenty of instability for them to tap
into, and large hail is likely the main hazard.

While the precipitation today was cause by the next upper level
disturbance in what promises to be a week`s worth of rain, and aided
by a mesoscale convective vortex that developed along the Texas
Coast, the main weather story - the large upper level low pressure
system continues to move from California to Arizona today. It will
progress eastward slowly through the period and be centered over the
Four Corners Region by the end of the short term period.

Monday presents several forecasting challenges. While the upper
trough progresses closer to Texas a sharp dryline will form out west
during the day. While the upper low will help create broad lift
across the state of the Texas, the dryline will be the only true
focus mechanism. Southerly flow will continue at the surface with
southwesterly to westerly flow aloft. Model soundings for late
tomorrow across the area show very impressive instability - with
CAPE values over 3000 j/KG. At the same time lapse rates of over 8
C/km. The two main questions for tomorrow will be first whether the
weaker shear (only 30 to 40 knots) and modest capping inversion
present in some models will help prevent convection and second if
convection can form where will it initiate and how will it move?

In general think Monday will be mostly dry-ish and warm day with
highs reaching to near normal in the mid to upper 80s. Some showers
may be around during the morning across the Hill Country and Austin
Metro area thanks to dying Mesoscale Convective Systems (MCS) across
North and West Texas tonight. From there a lot of questions remain.
The likely outcome will be isolated SdB storms late in the afternoon
and then an MCS approaching our area late Monday night into Tuesday
morning. While the mesoscale models would be where a meteorologist
would turn in this case they tend to struggle with this step up for
one simple reason. While there are ample ingredients in the
atmosphere for showers and storms there is nothing to focus them
other than the dryline out west. Taking a look at the 5 model Storm
Prediction Center High Resolution Ensemble Forecast (HREF) each
model has a slightly different solution. In general what most models
agree on is an MCS or possibly 2 forming off of the dryline late
tomorrow. This is the area SPC now highlights with an Enhanced Risk
around Lubbock. Less certain are our SdB supercells from West of Del
Rio. Only 3 of the 5 show those. If they can form though, they
promise to be big hailers that could sustain themselves all the way
into the I-35 corridor as an MCS into early Tuesday morning. Other
model solutions show various storm complexes moving into the Hill
Country, multiple rounds of storms out west, and storms coming from
North Texas southward.

Piecing this all together, think the day on Monday will be mostly
shower activity, with a few thunderstorms possible, while the
evening and overnight periods into Tuesday morning will be more
active. The PoPs reflect this trend, increasing significantly after
7pm tomorrow evening. Precipitable Water values continue to run
above seasonal norms, in the 1.3 to 1.5 inch range. As we saw today,
if a storm becomes slow moving or can tap into the moist atmosphere
it can produce a quick 1-3 inches of rainfall and cause localized


.LONG TERM (Tuesday through Sunday)...
An upper low will move slowly from the Four Corners region to the
northern Plains from Tuesday through Thursday. This will keep our CWA
in unstable southwesterly flow during this time period. Meanwhile the
low level flow will be southeasterly keeping a warm, moist airmass in
place. This combination will lead to multiple rounds of showers and
thunderstorms. Deep moist flow will have PW values from 1.5 to 2.0
inches over the eastern half of the forecast area. This will result
in chances for locally heavy rain and possible flash flooding. The
best chance for heavy rain looks to be Wednesday and Wednesday night.
In addition to the flood threat there will also be a chance that some
storms will be strong to severe both Tuesday and Wednesday. Another
upper low will move onto the west coast for the latter part of the
week. The low level flow will briefly turn easterly Thursday, but
turn back to the southeast Thursday night. Little drying will take
place. Perturbations in the upper flow will trigger convection each
day Friday and Saturday. Sunday an upper level ridge may build far
enough west to suppress any convection, but we will still carry a
slight chance.


Austin Camp Mabry              70  85  72  83  69 /  30  30  40  90  70
Austin Bergstrom Intl Airport  70  85  72  84  69 /  30  30  40  90  70
New Braunfels Muni Airport     71  87  72  86  69 /  30  20  30  80  70
Burnet Muni Airport            69  84  70  81  66 /  30  30  60  90  60
Del Rio Intl Airport           72  93  72  93  70 /  20  10  40  30  30
Georgetown Muni Airport        69  84  71  82  68 /  30  40  50  90  70
Hondo Muni Airport             71  86  72  85  69 /  20  20  30  80  50
San Marcos Muni Airport        71  86  72  84  68 /  30  20  30  80  70
La Grange - Fayette Regional   70  85  73  86  71 /  40  50  20  70  70
San Antonio Intl Airport       72  86  72  84  69 /  30  20  30  80  60
Stinson Muni Airport           73  88  73  86  70 /  30  20  20  80  60




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