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Area forecast discussion 
National Weather Service Austin/San Antonio Texas
216 am CDT Monday Apr 27 2015

Update...
we have allowed the Severe Thunderstorm Watch to expire. The line
of thunderstorms has moved east of the area and severe storms are
no longer expected in the area generally west of I-35. The Tornado
Watch continues over the south and east as the line moves into
that area.

&&

Previous discussion... /issued 137 am CDT Monday Apr 27 2015/

Update...
to issue Tornado Watch 125 for much of our southern County Warning Area. A line of
severe thunderstorms will continue to move toward the east this
morning. We have seen strong low level rotation in some of these
storms and issued some tornado warnings. Expect these storms to
continue with a tornado threat until later this morning.

Previous discussion... /issued 1249 am CDT Monday Apr 27 2015/

Aviation... /06z taf update/
squall line bearing down on the I-35 terminals at 36 knots will
have the prevailing wind direction out of tolerance in about 30
minutes. Updated tafs will be issued shortly to add a prevailing
westerly to northwesterly direction as was shown in the earlier
issuances. Thunderstorm gusts to 40 knots are possible and an
Airport weather warning is likely in the next half hour at kaus.
The storms will pass through rather quickly and improving
conditions should develop between 645 and 730z. Some lingering
MVFR ceilings are possible after the storms clear...but later updates
may show the lower clouds to be VFR. Several hours of VFR skies
are expected for Monday...but a reinforcing front could bring ceilings
down late Monday night.

Previous discussion... /issued 1135 PM CDT sun Apr 26 2015/

Update...
to expand the Severe Thunderstorm Watch to Maverick and Zavala
counties. The line of thunderstorms moving through our western
area is expanding to the south and severe storms may move into
northern Maverick and Zavala counties during the next couple of
hours. We have expanded the watch to those counties. We will see
how things develop over the next few hours to determine if we need
to expand the watch farther east. New zones will be out shortly.

Previous discussion... /issued 1002 PM CDT sun Apr 26 2015/

Update...
Severe Thunderstorm Watch continues across the Rio Grande plains
southern Edwards Plateau and areas along and west of I-35
corridor(this includes Austin and San Antonio metropolitan areas) through
2 am Monday. Latest images from area radars show a line of strong
to severe storms extending from central Texas southwestward to
Val Verde County. This line will continue to advance to the
southeast late this evening and affect the I-35 corridor by
midnight tonight. The main threats associated with these storms
will be thunderstorm damaging winds up to 70 miles per hour and large hail up
to 2 inches in diameter. However...can not rule an isolated
tornado. The line of storms will continue to push to the southeast
and affect the southeastern counties from 3 to 5 am Monday. Since
the line of storms is forecast to quickly move across the
area...widespread flooding is not expected. However...there is a
slight chance for isolated locally heavy rain across portions of
the southeast that could result in minor flooding across area
roads...poor drainage areas and small creeks.

Weather conditions will improve from west to east with some
lingering shower/thunderstorm activity across the far east
counties by middle morning Monday.

Previous discussion... /issued 718 PM CDT sun Apr 26 2015/

Aviation... /00z taf update/
another round of strong to severe storms are expected to impact
I-35 terminals this evening. Ahead of this development a gradual
lowering into MVFR...then IFR clouds should develop just before
a broken line of convection moves into I-35. Convective outflows
should occasionally lift ceilings to VFR near the storms...but will
keep skies at mainly IFR. Improvements into MVFR and VFR are
expected between 12z and 15z. Meanwhile VFR skies and a subtle
overnight wind shift is all that is accounted for at drt...as most
models keep the convection just to their NE.

Previous discussion... /issued 249 PM CDT sun Apr 26 2015/

Isolated severe potential the rest of the afternoon becoming
more widespread overnight as severe line of storms expected...

Short term (tonight through Monday night)...

For now through 7pm...
supercells are beginning to fire along the dry line which is spanning
from near del Rio...east of Junction...and north into central
Texas. Continued low level moisture flow from the southeast and a very
unstable environment will result in supercell development as far
south as the South Edwards plateau this afternoon. Satellite
imagery has been indicating the best instability and low level
convergence to be isolated to the dry line itself...which was well
handled in the 12z NAM solutions this morning.

Visible satellite and surface analysis shows cumulus development
extending as far south as Kerrville and this Point North and east
will be the focus for isolated supercell development through the
rest of the peak heating hours this afternoon. It is difficult to
ascertain the likelihood of development as the dryline is
interacting with the moist Montana air mass resulting in uneven mixing
from north to south and resultant bulging in the dryline. If any
parcels are able to lift above the boundary layer and tap into the
elevated instability and strong middle level lapse rates...very large
hail and damaging straight line winds could rapidly develop.
Tornadic potential will also be possible for cells in the better
moisture environment farther displaced from the dry line.

For 7pm through midnight...
upper level divergence will intensify in the exit region of the
upper level jet beginning to spread into west central Texas and
will interact with the dryline...which may slightly recede this
evening. Upscale growth to a severe mesoscale convective system is
expected along the dryline...which should be roughly where it is
currently located...if not 25 miles or so farther west and begin
to propagate east southeastward.

Midnight through 4am...
the intense squall line should begin impacting the northern zones
including the Austin metropolitan area by midnight or 1am with damaging
straight line winds up to 70 miles per hour and possibly higher depending on
how much instability remains in place during the overnight hours.
Did notice some hints at middle level lapse rates relaxing just a bit
so hopefully the wind potential will remain at or below the
current thinking. Quarter sized hail will also be possible and
potentially some larger hail in the more intense updrafts of the
line. As the line continues its progression south and east of
Austin by 2 to 4 am...the mature line should continue to evolve
into a wind and heavy rain threat and less of a hail threat.
While the movement of the line should be rather quick...rain rates
should increase as the line begins to ingest more moist boundary
layer air. This would result in some locally heavy rain that may
result in localized flash flooding along and east of the i35
corridor where more saturated soils are present from previous
rains over the last week.

Tomorrow morning and into the afternoon hours...
the upper low responsible for the severe episode today will shift
eastward into the Red River valley while a cold front begins to
drop down into central Texas. The severe threat will shift to the
extreme eastern zones and by middle-day should be mostly out of the
area. As the cold front approaches...there still exists a slight
chance for thunder but GFS/Euro are both not advertising much of
note with that activity.

Late Monday night and into Tuesday morning...
the cold front will begin to move through the area overnight
Monday and result in much cooler temperatures moving into the long
term period. Much drier air will filter in as winds shift to the
north and clear out the pop chances.

Tb3

Long term (tuesday through sunday)...

Benign weather conditions are expected for the extended forecast
period. Northwest flow aloft will dominate through Saturday for cool
nights and mild afternoons. A weak cold front is expected to move
across south central Texas Thursday afternoon but dissipates before
reaching the coastal plains. Both GFS and ecm concur with this
solution. This will re-enforce dry and pleasant weather on
Friday. An upper level trough is forecast to push from the Pacific
northwest into the northern rockies by Sunday. At the
surface...southerly winds and moisture returns across the area as
the upper level trough pushes to the east. This translate to a
slight chance for showers and thunderstorms across the east half
of south central Texas Sunday afternoon and across the Rio Grande
for the evening period. Temperatures will remain close to normal
levels through the period with highs in the lower 80s and lows in
the lower 60s.

Ob17

&&

Preliminary point temps/pops...
Austin Camp Mabry 84 59 73 52 76 / 50 20 10 - 0
Austin Bergstrom international Airport 83 58 72 51 75 / 50 10 10 - 0
New Braunfels Muni Airport 84 59 74 51 76 / 50 - 10 - 0
Burnet Muni Airport 82 56 70 49 75 / 50 30 10 - 0
del Rio International Airport 86 59 76 52 79 / 10 0 0 0 0
Georgetown Muni Airport 83 57 71 49 75 / 50 20 10 - 0
Hondo Muni Airport 84 59 75 50 76 / 50 0 - 0 0
San Marcos Muni Airport 83 60 73 50 75 / 50 10 10 - 0
La Grange - Fayette regional 83 61 73 51 75 / 60 20 10 - 0
San Antonio International Airport 84 60 74 52 76 / 50 - - 0 0
Stinson Muni Airport 84 61 74 53 76 / 50 - - 0 0

&&

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

$$

Mesoscale/aviation...04
synoptic/grids...05
public service/data collection...01

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