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Area forecast discussion
National Weather Service Austin/San Antonio Texas
1233 PM CDT Thu may 25 2017

Aviation...
VFR conditions will prevail through the afternoon and evening at all
area airports. Winds will be from the south at 10 to 15 kts with
gusts up to 25 kts during the afternoon. Low cigs will develop around
midnight in Austin and San Antonio and by around 10z at drt. All
sites will improve to VFR by around noon Friday.

&&

Previous discussion... /issued 422 am CDT Thu may 25 2017/

Short term (today through friday)...
another unseasonably cool evening and morning has occurred across
south central Texas given light and variable winds under clear skies.
Temperatures have fallen into the upper 50s in The Hill Country with
60s elsewhere, and they may continue to fall a few more degrees to
once again approach record lows at aus (58 in 1975). Southerly flow
will return this morning to advect low-level moisture back into the
region, but skies will generally remain clear under the influence of
a mid-to-upper level ridge passing over Texas. High temperatures will
climb into the lower 90s across most of the region with some mid to
upper 90s in the Rio Grande plains, several degrees warmer than
yesterday. Gusty southerly winds to 25 mph will be possible during
the evening hours with low clouds returning after midnight to keep
low temperatures up in the lower to mid 70s across the region. These
clouds should lift by late Friday morning to allow temperatures to
warm back up near today's highs, but with a significant increase in
humidity that will cause heat indices to climb into the 100-105
degree range across the region. The dryline will push into western
Val Verde County to create elevated fire weather concerns, but
capping aloft will prevent any convection from developing.

Long term (friday night through wednesday)...
Saturday will generally be very similar to Friday across most of our
area aside from a slight increase in temperatures and heat indices
during the afternoon. Thus, Saturday will generally feel like the
hottest day of the period with a few isolated heat indices above 105
degrees and temperatures above 100 degrees in the Rio Grande plains.
This slight increase in temperatures may be enough to help force
convection past the cap along the dryline which should be located
from western Val Verde County to somewhere between Brady and San
Angelo just west of our County Warning Area. Any very isolated thunderstorms that
form would be able to take advantage of steep mid-level lapse rates
and shear values around 30-40 knots that may be marginally supportive
of supercells to cause a very conditional threat for large hail and
damaging winds. Storm Prediction Center has outlined our Edwards Plateau and western Hill
Country counties in a slight risk with a marginal risk approaching
the I-35 corridor for any cells that are able to move into our area.
However, it is important to note that this is a low probability, high
impact type of risk contingent on the cap breaking which typically
has not occurred over the past few weeks here without significant
synoptic forcing (like what we saw on tuesday).

The cap will not be able to prevent convection from developing by
Sunday afternoon over the Edwards Plateau and western Hill Country as
a trough digging through The Rockies will help push a cold front
into region. Strong to severe storms may be possible during the
afternoon and evening hours as shear will be a bit higher, but
instability a bit lower given an increase in moisture aloft. These
storms may evolve into an mesoscale convective system that pushes through the I-35 corridor
overnight before dissipating east of the area. If this scenario plays
out, the I-35 corridor would see a greater threat of damaging winds
than locally heavy rainfall. However, the strongest dynamic forcing
associated with the trough will be well north/NE of the region and the
cap aloft may prevent a significant complex from pushing through. In
that scenario, a stalling front and remnant outflow boundaries could
provide a focus for Monday morning convection along the Escarpment
and I-35 corridor where locally heavy rainfall would be a greater
threat than severe storms. Thus, we have 40-60 probability of precipitation across the
entire area during both the overnight Sunday into Monday and Monday
morning and afternoon periods as rain will likely occur in one if not
both of these two periods, with the progressive mesoscale convective system scenario likely
causing a break in the action for most of the day on Memorial Day.

There is even greater uncertainty over the western portions of our
County Warning Area including the Rio Grande plains for what will occur Sunday night
into Monday where a progressive mesoscale convective system appears less likely. Thus, a
slow-moving complex capable of producing locally heavy rainfall is a
distinct possibility per the gefs ensembles and several operational
models, which could be assisted by a remnant circulation/mesoscale convective vortex left
behind if a progressive mesoscale convective system was able to develop and push through the
I-35 corridor. Given a weak low level jet that should be perpendicular to the
Escarpment and stalled front west of San Antonio, heavy rainfall
certainly definitely appears more likely for these areas at the
current moment on Sunday night into Monday then it does further
northeast of San Antonio. The key take home message is that everyone
across south central Texas should be weather aware particularly for
the second half of this Memorial Day weekend as the forecast comes
into clearer focus.

The model consensus suggests the wet and relatively cool period will
continue into at least the middle of next week as a steady stream of
shortwaves track over a stalled or remnant frontal boundary and set
of outflows. It is hard to depict confidence in any one period past
Memorial Day, but for now have gone with 50 pops on Tuesday and
Wednesday with a slight decrease in pops towards the end of next
week. However, it is often difficult to predict when these wet
periods will end as we transition out of Spring into Summer, so the
current forecast banks on model guidance under forecasting the
ability of this wet period to persist longer than it is thinking. At
the very least, this wetter and cooler than normal period should
mitigate drought and fire weather concerns moving into early Summer.
However, increasingly saturated soils and elevated streams and rivers
from heavy rainfall may lead to an increasing flash flood threat
through the second half of the forecast period with additional
events.

&&

Preliminary point temps/pops...
Austin Camp Mabry 74 93 76 94 75 / - - 10 10 10
Austin Bergstrom Intl Airport 74 93 76 95 75 / - - 10 10 10
New Braunfels Muni Airport 73 93 76 94 75 / - - 10 10 10
Burnet Muni Airport 72 94 75 93 73 / - 0 - 10 20
del Rio Intl Airport 75 96 78 97 77 / 0 0 - 10 10
Georgetown Muni Airport 74 93 76 94 74 / - - - 10 20
Hondo Muni Airport 74 94 77 97 76 / 0 - - 10 10
San Marcos Muni Airport 74 93 77 95 75 / - - 10 10 10
La Grange - Fayette regional 75 92 77 94 77 / 0 - 10 10 10
San Antonio Intl Airport 75 94 77 95 76 / - - 10 10 10
Stinson Muni Airport 74 95 77 97 76 / - - 10 10 10

&&

Ewx watches/warnings/advisories...
none.
&&

$$

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