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Area forecast discussion 
National Weather Service Austin/San Antonio Texas
953 am CDT Sat may 23 2015

Update...
biggest change to the forecast was to up rain chances across the
Rio Grande plains by 10 percent this afternoon. Also adjusted
highs for today across The Hill Country up a degree given current
observations. Otherwise only minor changes made and trended all
hourly grids based on morning surface observations.

&&

Discussion...
all of the morning mesoscale/high-res models continue to trend
earlier for the line of showers and thunderstorms this evening.
Most have it developing over the del Rio area around 5 PM and
getting into the I-35 corridor between 9 and midnight. Heavy rains
and severe weather are still looking likely with this system...and
will look in depth into that in the afternoon package and
discussion. For the rest of today scattered showers and
thunderstorms should increase in coverage this afternoon ahead of
the line.

Will continue to stress that anyone on local rivers and lakes this
Holiday weekend...today through Monday...need to pay very close
attention to the weather and any warnings that are issued. Have
multiple ways to receive weather information...have a plan to
seek shelter or get to safety should severe weather or flooding
occur...and always remember turn around...dont drown.

&&

Previous discussion... /issued 653 am CDT Sat may 23 2015/

Aviation... /12z taf update/
active convection is impacting aus at the time of taf issuance.
The scattered showers and thunderstorms look to be pulling north
of the Sat/ssf vcntys for a few hours and could lift north of the
aus area in the next hour or two as well. Deep tropical moisture
and another ripple of energy near drt could begin to impact the
I-35 sites again by 15z. Convection should be scattered so timing
is in low confidence for today. Confidence increases for the
prevailing thunder period...although the time window may be able
to compress once the much anticipated storm complex begins to take
shape. After the complex is speculated to pass east of I-35 toward
daybreak...will continue prevailing showers...with some possible
further improvements to follow during the daylight hours Sunday.
Will keep conditions fairly pessimistic in case the system slows
down.

Previous discussion... /issued 314 am CDT Sat may 23 2015/

Short term (today through sunday)...
moderate to heavy rainfall is getting an early start with
convection increasing west of Highway 83. This is all associated
with increasing lift ahead of the main upper low which is still
over Utah/Arizona per 00z upper air analysis and latest water
vapor imagery. Rainfall rates with this activity are generally at
1-2 inches per hour and we have already issued one Flood Advisory.
The hrrr is zeroing in on this area where rain is currently
increasing and is coming up with some high totals this morning.
Because of this early threat of heavy rainfall...will go ahead
and start the Flash Flood Watch now. This will be a bit too early
for the eastern zones...but want to avoid confusion with a
segmented product. High-res model guidance continues to show the
western County Warning Area as the favored area for convection later this morning
before spreading across the area this afternoon. Rain looks likely
for today with 60/70 probability of precipitation across the County Warning Area. Individual cells could
contain heavy rainfall and some training of cells is also
possible.

The main forcing with this initial wave of lift associated with
the upper low will begin to arrive to the western zones late this
afternoon and evening. Confidence is high that a line of showers
and thunderstorms will develop with this lift and stretch across a
large portion of Texas. There are some discrepancies with the
guidance on the exact timing of this with most models somewhere
between the 21z to 03z time frame...with some of the high-res
guidance being the earliest. This line is expected to slowly March
through the County Warning Area overnight and is expected to average a quick 1-3
inches of rain as it moves across. Overnight rain on top of any
rainfall that occurs today will not be able to soak into the
ground...as soils remain quite saturated from a very wet Spring.
Most of the rain will runoff into creeks...streams and rivers and
flooding is once again likely.

Although flooding rains is of the highest concern on this Holiday
weekend...there is the threat of severe weather as well. Severe
parameters are favorable for severe storms...but am concerned with
the higher coverage of storms and the effect that could have by
cluttering up the near-storm environment. However...cant rule out
bowing segments within the main line that could produce some
severe winds. Also...low-level winds will increase and some
turning in the profile will pose an isolated tornado threat with
any stronger line segments as well as the main line moves through
the area.

By Sunday morning...the line of storms will be nearing the eastern
counties and should move out of the County Warning Area sometime around 18z. The
threat of flooding rains and some severe weather will continue
Sunday morning. By Sunday afternoon...cape values increase to
above 3000 j/kg which is somewhat initially concerning.
However...forecast soundings show decent amounts of drying aloft
behind the initial wave which should limit activity. Also...the
environment should be pretty worked over from the overnight and
morning activity. The one area to watch will be the western
counties where the most recovery time will be and some high-res
members are showing the potential for redevelopment Sunday
afternoon in this general area.

Rainfall amounts this morning through Sunday will average 1-4
inches with the higher amounts near the I-35 corridor. We must
continue to stress to anyone on local rivers and lakes this
Holiday weekend to pay very close attention to the weather and any
warnings that are issued. Have a plan in place to seek shelter
and always remember to turn around...dont drown.

Long term (sunday night through friday)...
should see a lull in activity early Sunday night...but this
should unfortunately be brief. Currently on water vapor
imagery...another impulse associated with the main upper low can
be seen near the Washington and Oregon border. 00z guidance seems
to have picked up on this feature quite well and brings it into
Texas for Monday /Memorial Day/. Subsequently...the 00z model
guidance has increased the quantitative precipitation forecast amounts forecasted for Monday
afternoon. The GFS is the highest of all guidance with another 2
inches of rain for the I-35 corridor. Went ahead and increased
rain totals in the official forecast...but not quite as high as
the GFS is advertising. If confidence increases that up to another
2 inches of rain will fall on Monday....the Flash Flood Watch will
need to be extended at a later time. Cape/shear values will
support the threat of severe weather as well. Forecast soundings
due show the threat for hail...damaging winds and possible
isolated tornadoes for Monday afternoon. The overall threat for
tornadoes will be highly dependent on the overall coverage of
activity and if any discrete supercells are available to develop.
Again...anyone outdoors on the Holiday need to make sure to have a
plan and be ready to seek shelter.

The upper air pattern becomes more zonal on Tuesday with weak
ridging on Wednesday. Overall coverage of activity should decrease
for mid-week. Probability of precipitation will slightly increase once again towards the
end of the work week as the next disturbance nears the area.

&&

Preliminary point temps/pops...
Austin Camp Mabry 81 68 81 71 82 / 60 90 60 40 60
Austin Bergstrom international Airport 81 68 81 70 82 / 60 90 70 40 60
New Braunfels Muni Airport 82 68 82 71 82 / 60 90 60 40 60
Burnet Muni Airport 79 66 80 69 81 / 60 90 60 30 50
del Rio International Airport 82 68 89 70 87 / 80 70 30 20 40
Georgetown Muni Airport 80 68 80 70 82 / 60 90 60 40 60
Hondo Muni Airport 82 66 84 70 82 / 70 90 50 30 60
San Marcos Muni Airport 81 68 80 70 82 / 60 90 60 40 60
La Grange - Fayette regional 83 69 80 71 82 / 40 80 80 50 70
San Antonio International Airport 82 68 82 72 83 / 60 90 60 30 60
Stinson Muni Airport 83 69 83 72 83 / 60 90 60 30 60

&&

Ewx watches/warnings/advisories...
Flash Flood Watch through Sunday evening for the following
counties: Atascosa...Bandera...Bastrop...Bexar...blanco...Burnet...
Caldwell...Comal...De Witt...Dimmit...Edwards...Fayette...frio...
Gillespie...Gonzales...Guadalupe...Hays...Karnes...Kendall...Kerr...
Kinney...Lavaca...Lee...Llano...Maverick...Medina...real...Travis...
Uvalde...Val Verde...Williamson...Wilson...Zavala.

&&

$$

Mesoscale/aviation...10
synoptic/grids...Treadway
public service/data collection...33

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