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Area forecast discussion 
National Weather Service Austin/San Antonio Texas
717 PM CDT Monday may 25 2015

Aviation.../00z tafs/
storms have cleared all taf sites...but will include thunderstorms in the vicinity through
01z for Sat/ssf/aus in case backbuilding storms produce a rogue
lightning strike. A mesoscale low has formed behind this line...which
will cause variable winds below 10 kts at Sat/ssf/aus for the next
few hours...will handle this with amendments if they increase
above 10 kts. Dry air evident on WV imagery will help dry out the
middle and upper levels...but moist low level southeast flow will allow VFR
ceilings to lower to IFR by 7z at Sat/ssf/aus and MVFR by 9z at drt.
Model guidance suggests LIFR ceilings could develop between 10-14z at
Sat/ssf/aus...but will reevaluate this with the 06z taf package.

&&

Previous discussion... /issued 447 PM CDT Monday may 25 2015/

.Significant flash flooding ongoing across the northern and
central parts of south central Texas...

Short term (today through tuesday)...
a strong negatively tilted upper level trough has caused a
significant severe weather event that has already produced a few
tornadoes in The Hill Country and widespread flash flooding in The
Hill Country and I-35 corridor is ongoing this afternoon. A flash
flood emergency has also been issued for eastern Williamson and
Travis counties after 3 to 5 inches of rain fell in two hours. The
primary threat for the next several hours will continue to be
significant flash and river flooding...with the latter continuing
into at least the overnight hours. Rainfall rates up to 4 inches
per hour have occurred this afternoon...but these rates should
begin to decrease slightly as these storms continue to merge into
a line over the I-35 corridor from the San Antonio metropolitan and areas
north before pushing east across our County Warning Area. Dry air moving in from
the west evident on WV imagery should continue to cause precipitable water
values to decrease and end the heavy rainfall threat from west to
east as we move into the evening and overnight hours. Thus...the
Flash Flood Watch has been cancelled for our westernmost
counties...but continues for the vast majority of south central
Texas through 1 am CDT Tuesday.

Although shear values have decreased slightly from the 50-55 knots
seen earlier this afternoon in The Hill Country...helicity values
above 200-300 m^2/s^2 and low LCLs along and east of I-35 will
likely continue the tornado threat for this area for at least the
next few hours. Tornadoes will be possible within more discrete
cells within the line of storms currently centered over the I-35
Corridor North of San Antonio and expanding into NE Texas and southeast
OK. Thus...a Tornado Watch continues until 10 PM CDT for the
eastern Hill Country...and all counties along and north of the
I-10 corridor and along and east of the I-35 corridor northeast of
the San Antonio metropolitan.

Dry air will graciously fill into the entire County Warning Area by early Tuesday
morning...but another weak shortwave rotating around the
persistent longwave trough to our west may trigger isolated
thunderstorms Tuesday afternoon in the serrenias del burro that
move into the Rio Grande plains. Elsewhere...ample boundary layer
and soil moisture with daytime temperatures rising into the upper
80s may still allow for very isolated afternoon showers and
thunderstorms...but dry air should prevent development. In
addition...weak shortwave riding should begin to build in by late
Tuesday night to limit our rain chances.

Long term (tuesday night through sunday)...
a brief period of shortwave ridging with relatively dry air will
continue through early Thursday...giving the region a much needed
break from the heavy rainfall pattern we have been in for several
weeks for a few days. However...will still carry isolated
afternoon showers and thunderstorms for Wednesday due to ample
boundary layer moisture and soil moisture with highs nearing 90
degrees. Another upper level trough approaches the region on
Thursday to break down this ridge...increasing the coverage of
afternoon showers and thunderstorms...particularly in the Rio
Grande plains. Models generally agree that the main energy from
this upper level trough should remain to our north...but also
suggest that it will not be very progressive and stick around
until at least Sunday. Considering that a few weak boundaries may
slip into the region to provide an additional focus for
convection...rainfall chances will likely continue through Friday
and the weekend.

&&

Preliminary point temps/pops...
Austin Camp Mabry 81 70 88 73 89 / 80 30 20 20 20
Austin Bergstrom international Airport 81 70 88 73 89 / 80 30 20 20 20
New Braunfels Muni Airport 82 71 88 73 89 / 70 30 20 20 20
Burnet Muni Airport 80 69 87 72 88 / 80 20 20 20 20
del Rio International Airport 90 71 91 74 91 / 60 10 10 20 20
Georgetown Muni Airport 80 70 87 71 88 / 80 30 20 20 20
Hondo Muni Airport 84 72 87 74 89 / 60 10 20 20 20
San Marcos Muni Airport 81 71 87 73 89 / 70 30 20 20 20
La Grange - Fayette regional 82 70 88 75 90 / 80 50 20 20 20
San Antonio International Airport 83 73 88 74 89 / 70 30 20 20 20
Stinson Muni Airport 82 73 88 74 89 / 70 30 20 20 20

&&

Ewx watches/warnings/advisories...
Flash Flood Watch until 1 am CDT Tuesday for the following
counties: Atascosa...Bandera...Bastrop...Bexar...blanco...Burnet...
Caldwell...Comal...De Witt...Fayette...Gillespie...Gonzales...
Guadalupe...Hays...Karnes...Kendall...Kerr...Lavaca...Lee...Llano...
Travis...Williamson...Wilson.

&&

$$

Mesoscale/aviation...01
synoptic/grids...99
public service/data collection...33

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