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

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FXUS64 KEWX 281809 AAA

Area Forecast Discussion...UPDATED
National Weather Service Austin/San Antonio TX
109 PM CDT Fri Apr 28 2017

/18Z TAFS/
Low clouds continue to break with VFR expected across area terminals
through late this evening. Low clouds will come back into the area
around midnight tonight and linger for much of Saturday for MVFR
conditions. Some weather models suggest for IFR cigs to develop
overnight into Saturday morning along the I-35 sites, however,
prevailing winds are expected to be around 10 knots to limit lowering
cigs. Will monitor closely and adjust TAFS in future issuances if
trends continue.

A strong cold front is expected to move across the area Saturday
evening. Strong to severe storms are expected to be moving along the
boundary and having a direct impact to areas along and east of
Interstate 35 including both Austin and San Antonio airports. The
timing of the storms along the I-35 sites is from 00Z TO 06Z Sunday.
Main hazard will be large hail and damaging straight-line wind

Across KDRT, VFR conditions are expected through the forecast period.
Will turn breezy and gusty overnight.


.PREV DISCUSSION... /ISSUED 638 AM CDT Fri Apr 28 2017/

MVFR CIGs will mix out to few-sct VFR level midday into afternoon.
MVFR CIGs will redevelop this evening into overnight and mix out
again midday Saturday. CIGS may lower to IFR, however, moderate low
level jet will tend to keep CIGS from falling much. S to SE winds 7
to 17 KTS will prevail. Some gusts to 28 KTS are possible.

PREV DISCUSSION... /ISSUED 417 AM CDT Fri Apr 28 2017/

SHORT TERM (Today through Saturday Night)...
Returning Gulf moisture has allowed low stratus to spread over most
of South Central Texas tonight, keeping temperatures up in the mid
60s to lower 70s. Winds have remained around 10-15 mph due in part
to a 40 knot southwesterly low-level jet. A broad trough centered
over the Colorado Rockies is helping a surface low develop over
northwest Texas that will track into southern Oklahoma later today.
This increased pressure gradient will continue to keep southerly
winds up around 15 mph with gusts to 25 mph this afternoon as peak
mixing and heating occurs. A dryline extending south from the surface
low will push into our westernmost counties later this afternoon,
but a strong capping inversion and lack of forcing aloft should
prevent deep convection today. The main weather impact for today
will be the heat as low clouds will generally lift by the afternoon
to allow for highs in the lower to mid 90s everywhere except for the
higher elevations in the Hill Country where highs will be in the
upper 80s. Relative humidity values will be more like June than mid-
April, allowing heat index values to climb into the upper 90s across
the region. These will be the highest values we have seen since last
fall, so hydration during outdoor activities this afternoon will be
very important. Low temperatures Saturday morning will be in the mid
70s for most spots, near record values for this time of year.

The trough centered over Colorado will split into two cutoff lows,
with the stronger of the pair digging from the Four Corners region
into New Mexico Friday night into Saturday before ejecting NW over
the Texas panhandle Saturday night as it becomes negatively tilted.
This will help promote the formation of a much stronger surface low
along a stalled front in northwest Texas or Oklahoma, allowing
southerly winds to remain around 15 mph with gusts to 25 mph and a
relatively strong cold front to push through the region Saturday
night. Low clouds should persist deeper into the afternoon tomorrow
to keep temperatures down a bit in the upper 80s and lower 90s for
most spots except in the Rio Grande Plains which will see upper 90s.
This would allow the cap to hold through most of Saturday afternoon
ahead of the dryline which will be pushing into our western counties
late in the afternoon or early evening. However, if the cap is able
to erode and deep convection forms as the GFS suggests may occur,
lapse rates will be very steep (supporting CAPE in excess of 3000
J/kg) and 40 knot shear will support supercellular storm modes
capable of producing strong to severe thunderstorms whose primary
threats would be large hail and damaging winds. However, we continue
to lean towards a stronger cap as indicated previously as the GFS is
likely eroding it too fast as it has in other events this spring.

The greatest chance of strong to severe thunderstorms for most of the
region continues to be very late Saturday afternoon into the evening
and early overnight hours. The surface dryline or front combined
with PVA and cooling due to height falls aloft associated with the
trough will easily overcome any capping inversion that remains. Given
that shear will remain parallel to the surface boundary instead of
being line-perpendicular, a broken line of strong to severe storms
currently appears to be more likely than a single MCS or squall line.
Thus, large hail appears to be at least equal if not greater than
the damaging wind threat for the duration of the event, but storms
may eventually line out as the front approaches to transition the
threat into more of of a damaging straight-line wind threat. Rain
amounts should average below one inch, but isolated amounts of 2-3
inches will be possible for any areas that get storms ahead of any
broken or solid line that develops. Nevertheless, the flash flood
threat should be low given the progressive nature of the storms.

LONG TERM (Sunday through Thursday)...
Showers and thunderstorms should clear our easternmost counties by
Sunday morning as skies clear and high pressure builds in. Although
Sunday will be pleasant from a temperature standpoint in the 70s, a
strong pressure gradient will allow for W-NW winds sustained at 15-20
mph with gusts of 25-35 mph to make the day less pleasant. There
will be fire weather concerns in our western counties if they do not
receive wetting rainfall on Saturday night given these strong winds
combined with relative humidity values below 20 percent. Winds will
calm down considerably by Sunday night to set up a cool Monday
morning with lows in the mid 40s to lower 50s. Temperatures will warm
back to normal for Monday and slightly above normal for Tuesday and
Wednesday before another cold front moves through the region
Wednesday afternoon or evening to bring the area another chance of
showers and thunderstorms. Temperatures should cool down slightly
behind the front for the second half of next week before warming up
for the weekend as a ridge of high pressure moves over the region.


Austin Camp Mabry              76  89  58  75  51 /  10  30  60  10   0
Austin Bergstrom Intl Airport  76  89  57  74  48 /  10  20  70  10   0
New Braunfels Muni Airport     75  89  57  75  49 /  10  20  60  10   0
Burnet Muni Airport            73  85  53  71  48 /  10  30  60  -    0
Del Rio Intl Airport           70  93  55  79  53 /  10  10  20   0   0
Georgetown Muni Airport        75  86  56  72  48 /  10  40  60  10   0
Hondo Muni Airport             74  93  56  78  48 /  10  20  50  -    0
San Marcos Muni Airport        76  88  57  74  49 /  10  20  70  10   0
La Grange - Fayette Regional   76  88  59  74  49 /  10  20  70  10   0
San Antonio Intl Airport       75  90  57  76  50 /  10  20  60  10   0
Stinson Muni Airport           75  92  58  77  51 /  10  20  50  10   0




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