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

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FXUS64 KEWX 261133
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
633 AM CDT Wed Apr 26 2017

.AVIATION DISCUSSION...FOR 12Z TAF PERIOD...
MVFR CIGs currently over the I-35 terminals will erode in the first
few hours of the TAF as a frontal boundary moves across. Winds will
shift from southerly this morning to northwesterly and increase to
15-25 kts with gusts up to 30 kts possible, especially at KAUS. The
wind shift has already impacted KDRT. Winds should subside by dark
tonight, calming to under 10 kts generally.

&&

.PREV DISCUSSION... /ISSUED 427 AM CDT Wed Apr 26 2017/

SHORT TERM (Today through Thursday)...

Main hazard highlight today will be heightened wildfire danger as
critical conditions of 10-20 percent humidity and sustained winds of
15-25 mph with gusts to 35 mph are expected to occur this afternoon
into the early evening. A red flag warning has been issued for the
Rio Grande Plains, Southern Edwards Plateau, portions of the Hill
Country and even into the US highway 281 corridor including San
Antonio. Please see the fire weather section below for more details
on this hazard.

Gravity waves can be seen on local radars entering the Hill Country
this morning ahead of a dryline/boundary situated from San Angelo NE
towards Dallas. A few showers are occurring along and behind the
boundary but this activity is expected to remain north as front moves
south this morning and afternoon. Very dry air is expected to advect
in behind the dryline passage with dewpoints falling into the upper
to mid 30s this afternoon. With afternoon temperatures reaching into
the upper 80s to near 90, relative humidity values will fall to
10-20 percent. Combination of a tightening pressure gradient and
downward momentum transfer of winds in the first 4-5 kft will support
sustained surface winds of 15-25 and gusts to 35 mph. This
combination will result in enhanced wildfire spreadability, if one
was to occur. A wind advisory will need to watched for if sustained
winds become greater than 25 mph. Current guidance suggests that this
will be locally possible in the typical windier spots but felt the
overall spatiotemporal aspect of elevated winds was marginal to
warrant a wind advisory. Again, one may be necessary on the morning
update if the pressure gradient becomes stronger than anticipated.

Perhaps a rouge shower could skirt Burnet to Williamson this morning
but limited to no rainfall is generally expected with the passage of
the boundary today with substantial cloud clearing as PWATs fall to
0.3-0.5 inches this afternoon. The winds will decrease overnight into
Thursday morning and allow for a cooler morning given the drier air.
However, a rebound back to near normal temperatures is expected in
the afternoon with mid 80s to low 90s.

LONG TERM (Friday through Tuesday)...

Moisture will return quickly by Friday as well-above normal
temperatures return. However, a pattern change over the weekend will
favor greater rain chances Saturday afternoon through early Sunday
morning as a cold front moves through. The greatest rain chances
will be late Saturday night into early Sunday. Cooler and dry conditions
are expected to start next week.

Moisture levels will rapidly increase overnight Thursday into Friday
morning with dewpoints reaching back into the low 60s west and 70s in
the coastal plains. This moisture surge could help some drizzle/mist
to develop Friday morning underneath a stout capping inversion. This
capping inversion should remain strong enough Friday afternoon to
prevent any storms to develop per model sounding indications at this
stage. However, if a storm were to break through the cap, it would go
strong to severe quickly. This however, appears very conditional and
a low chance. Temperatures Friday will also be back to well-above
normal readings like today and yesterday.

A strong shortwave trough is well advertised across numerous global
models to shift across north Texas Saturday into Sunday. A strong
SW to NE oriented belt of 65 knot H5 winds will shift across the Big
Bend of Texas and force a dryline to shift east towards the I-35
corridor through the day. A modest warm sector along and east of the
dryline characterized by upper 60 to low 70 dewpoints, temperatures
reaching into the mid 80s, MLCAPE >3500 J/Kg, 0-6km shear of 35-45
knots, and vertical totals near 30C. The ultimate question is will
the cap break Saturday afternoon to allow for stronger convection?
Around 30 meter H5 height falls are progged to occur from 18Z to 00Z
Saturday afternoon and this, or the potential dryline convergence,
could help foster enough forcing to either push through the cap or
erode it after 4pm along and east of I-35. Interestingly, the latest
EC run indicates greater QPF at the 00Z timestep than previous runs
and aligns better with the GFS during this time frame. Far north and
north east areas of south-central Texas are being highlighted in the
SPC Day 4 outlook. This potential hazard will need to be watched
closely.

By late Saturday night into Sunday morning, the dryline shifts
farther east towards the coastal plains as greater forcing finally
pivots across north-central Texas. Models have come into better
agreement that greater rain and storm coverage will occur during this
time frame with upscale growth into a line or rain shield as it
pushes south and east into the coastal plains. Have increased rain
chances over previous forecast with 40-60% values for I-35 areas and
southeast. The cold front should be through entire region Sunday
morning with clearing conditions behind it. Tranquil and cooler
weather will occur to start the week with a slow warm up into mid
week.

FIRE WEATHER...

Combination of humidity values dropping to the 10-20 percent range
this afternoon along with sustained winds of 15-25 mph and gusts to
near 35 mph at times, a red flag warning has been issued along and
west of the US Highway 281 corridor. This includes portions of the
Hill Country, southern Edwards Plateau, and the Rio Grande Plains.
This area also includes Bexar County and the San Antonio Metro area.
While fuel dryness remains near average for this time of year, the
1-hour and 10-hour fuel moisture has dropped over the past week with
the warm temperatures and lack of recent rainfall. The red flag
warning is in effect from noon through 7pm. Humidity values will
recover through the evening and winds will decrease - allowing for
fire spreadability potential to decrease after 7pm.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              89  54  85  69  92 /  10   0   0  10  10
Austin Bergstrom Intl Airport  89  50  84  70  91 /  10   0   0  10  10
New Braunfels Muni Airport     89  50  85  70  92 /  -    0   0  10  10
Burnet Muni Airport            83  49  82  68  88 /  -    0   0  -   10
Del Rio Intl Airport           90  56  90  69  95 /   0   0   0   0  -
Georgetown Muni Airport        84  50  83  70  90 /  10   0   0  10  10
Hondo Muni Airport             90  49  88  68  95 /   0   0   0  -   10
San Marcos Muni Airport        89  50  85  70  92 /  -    0   0  10  10
La Grange - Fayette Regional   89  51  85  69  91 /  10   0   0  10  10
San Antonio Intl Airport       90  53  86  70  93 /   0   0   0  10  10
Stinson Muni Airport           92  53  86  70  93 /   0   0   0  10  10

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
&&

$$

Mesoscale/Aviation...TB3
Synoptic/Grids...Allen



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