Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Austin/San Antonio Texas
342 am CDT Tuesday Mar 31 2015
Short term (today through wednesday)...
skies are mostly cloudy at the present time with low clouds once
again draping across the area. The forecast for clouds should be
mostly persistent with morning clouds...and partly to mostly
cloudy skies this afternoon. Highs today will top out in the upper
70s to lower 80s...which should generally be about a 1 to 3 degree
increase versus what occurred yesterday.
The overnight water vapor imagery shows a small...but strong upper
level low just west of Baja California. The progressive pattern should bring
this system near the region by Wednesday morning with increasing
probability of precipitation leading up to Wednesday. For this afternoon through late
evening...the dryline is expected to be near Val Verde County but
the best eastward surge to the boundary is expected to be to our
north and this is where the best mesoscale lift should be located.
Overall...the chances of isolated convection developing near the
dryline in our County Warning Area is quite low. If any storms do develop...cape
values will be around 1200 j/kg with 0-6 km shear values near 40
knots. These parameters would support an isolated to multicellular
Mode...with perhaps an isolated mesocyclone. The middle-levels of the
atmosphere look to be quite dry with the forecast soundings taking
the shape of an inverted v. If any storms do develop /again the
chances are low/ the main hazard would be damaging winds. Storm Prediction Center has
placed Val Verde and some of Edwards County in the marginal risk
of severe weather and this seems reasonable.
As the upper low approaches the area by Wednesday morning...all
guidance suggests it will be weakening as it does so.
Nevertheless...weak lift will support better rain chances as the
lift moves from west to east across the area during the day
Wednesday. Guidance is in poor agreement with rain amounts with
the system and this is likely due to the handling of the weakening
system. If the system leans on the slightly stronger side...then
some areas could pick up a quick 3/4 of an inch of rain.
However...if the system is nearly gone after its trip over the
higher terrain of Mexico...then rainfall will be more scattered
and generally less than 1/4 of an inch. Some of the WRF members
are supporting the higher amounts...with the sref mean in the 1/10
to 1/4 inch range. This is most likely due to the NAM members as
the operational NAM supports little precipitation. Will have to watch the
system on water vapor as it travels over northern Mexico to see
how fast the weakening occurs. Cape and shear values Wednesday
afternoon should be lower than today and am not expecting much in
the way of strong storms.
Long term (wednesday night through monday)...
rain chances should end from west to east by Thursday morning. The
dryline will continue to be near our western counties...but bulges
well north of the region and am not expecting any convection on
Thursday. The focus of the forecast then shifts to the passage of
a cold front Friday afternoon. A trough will pass north of the
region Thursday...sending a cold front south through the plains.
This front should arrive to the northern zones early Friday
afternoon...quickly moving through the County Warning Area by Friday evening. The
500mb flow across Texas at the time will be nearly zonal...
providing little upper support for lift coincident with the frontal passage.
However...the 850 front will be almost directly on top of the
surface front. Frontogenesis charts show instability above this
steep frontal wedge and this should be sufficient for the
production of scattered showers and thunderstorms near the
frontal boundary Friday afternoon. Cape values will be near 2500
j/kg with 0-6 km shear values near 35 to 40 knots. It is tough to
say as of now whether storms will be rooted along or behind the
fast moving cold front. If storms are able to get going ahead of
the front...they could pose a hail risk...but current thinking is
the front should quickly race ahead of developing
storms...limiting the severe potential.
A weak disturbance embedded in the nearly zonal flow will keep the
active weather going for Easter Sunday. The best chances of rain
with this system will be east of the County Warning Area...but enough moisture
should be lingering behind the Friday system for scattered showers
and storms on Sunday. One or two strong storms could be possible
Sunday as well...but overall chances should be limited given
cooler weather expected for the weekend behind the Friday cold
front. Rain chances should diminish by Monday.
Preliminary point temps/pops...
Austin Camp Mabry 82 64 78 64 83 / 10 20 40 20 10
Austin Bergstrom international Airport 82 63 79 63 82 / 10 20 50 20 10
New Braunfels Muni Airport 82 63 79 63 84 / 10 20 50 10 10
Burnet Muni Airport 81 62 78 62 84 / 10 30 30 10 10
del Rio International Airport 81 63 83 62 92 / 10 40 20 10 0
Georgetown Muni Airport 81 64 77 63 82 / 10 20 30 10 10
Hondo Muni Airport 80 63 79 62 86 / 10 30 30 10 10
San Marcos Muni Airport 82 64 79 63 82 / 10 20 50 20 10
La Grange - Fayette regional 83 65 79 64 82 / 10 20 50 20 10
San Antonio International Airport 81 64 78 64 84 / 10 30 50 10 10
Stinson Muni Airport 81 64 79 64 85 / 10 30 50 10 10