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Area forecast discussion
National Weather Service Austin/San Antonio Texas
656 PM CDT sun Sep 24 2017

Aviation.../00z_tafs/
isolated convection that has been affecting the I-35 taf sites is in
the process of dissipating with additional development not expected.
VFR conditions are expected at all sites this evening as southeast
5-10 knot flow persists with some brief stronger gusts during the
early evening hours. Moist onshore flow will allow for MVFR ceilings
at Sat by 5z and ssf by 7z, earlier than model guidance suggests but
a bit later than the past few evenings. Winds will continue to be
weaker at aus to delay the onset of MVFR ceilings or patchy fog until
closer to 9z, with IFR visibilities and ceilings between 11-15z. VFR
conditions should return between 17-18z at the I-35 sites, with
isolated to scattered convection expected through the afternoon.

Shower activity near drt is ongoing, so we will continue to carry
vcsh there through 2z. The ceiling forecast is trickier at drt due to
having more midlevel cloud cover ahead of the trough and front that
is located further west. MVFR ceilings should develop late in the
overnight hours and may persist into the early afternoon hours unlike
today. Shower activity should be more widespread tomorrow afternoon
than it was today, but we kept drt at vcsh instead of prevailing
showers. Shower and thunderstorm chances should increase into the
evening hours just beyond the end of their taf period.

&&

Previous discussion... /issued 324 PM CDT sun Sep 24 2017/

Short term (tonight through Monday evening)...

The long awaited milder weather pattern is on the way as a tropical
storm influenced air mass looks to cloud up the skies for much of the
day Monday with a cooling trend going forward. High temps should be
4-8 degrees cooler than those from today.

The bigger story is that a significant Pacific tropical storm related
rain event is going to unfold which will be further elaborated in
the next long term section. Scattered showers continue to stream over
the coastal prairies into areas along Hwy 281 and are peaking at the
current time due to good insolation and pwat values of around 1.7
inches. Higher amounts of cloud cover have already taken over the
areas west of Hwy 83 where intermittent showers and pwat values
around 2.0 inches exist. Initial rains are expected to create only
isolated and minor run-off concerns through midday Monday, although a
few of the higher resolution models yesterday signaled an isolated
pocket of heavy rain already into the drt area by daybreak Monday. By
Monday afternoon, a flash flood advisory will likely be needed as areas along and
west of Hwy 83 begin to see scattered slow moving heavy showers and
storms with some potential MCV activity going into the evening hours.

Long term (monday night through sunday)...
by late Monday night, the full impact of Tropical Storm Pilar will be
felt over our western counties and the pattern is expected to be near
stationary through Wednesday night as a slow moving upper trough
pulls the moisture over our western counties like a conveyor belt.

A persistent complex of convection is expected in the early morning
hours Tuesday with areas of 4+ inch rains possible mainly along and
west of Hwy 83. In general, the western half of south central Texas
has been well below normals in terms of late Summer/early fall
precip. The first 24-hr impacts and flash flood threats are thus
expected to be localized. However, the multi-day rain event has the
potential to last into at least Thursday morning, leading to a high
chance for river rises along and west of Highway 281 with the rivers
near Highway 83 possibly receiving the most water.

By 12z Wednesday, the cold front associated with the upper trough
should enter the southern Edwards Plateau counties, while a backdoor
fropa is expected to spread into the I-35 corridor from the east.
With constant training of convection and tropical air expected to
continue moving through the Highway 83 corridor, the cold front
progress may be upended slightly, as model data shows a kink in the
surface pressure patterns over the Rio Grande plains and srn Edwards
Plateau into Wednesday evening.

Models have trended more progressive with the front Wednesday,
resulting in lower high temps by this time. By late Wednesday night,
the continued fetch of moisture from Pilar is forecast to be
weakened, as the NHC has the system weakened to a depression and near
the coast of Mexico over 24 hours earlier. Storm total rains to
justify the anticipated flash flood advisory watch is thus concentrated from 12z
Monday through 12z Thursday, with common amounts introduced in the
3-6 inch range and higher isolated totals around 8 inches. Rainfall
amounts expected after this period should generally be less than 1/2
inch given the latest model handling of the cold front.

Rain chances in a more modest overrunning pattern should continue to
be high through Friday, especially over the areas to the west where
frontal depth is more shallow. The mid level pattern is significantly
compromised by next weekend, but there remains abundant moisture and
possibly a portion of elevated dynamics left from the remnants of
Pilar possibly continuing light rain chances. Will keep with a milder
and more moist pattern for late next week.

&&

Preliminary point temps/pops...
Austin Camp Mabry 72 89 73 87 72 / 10 20 20 50 50
Austin Bergstrom Intl Airport 71 89 72 86 71 / 10 30 20 50 40
New Braunfels Muni Airport 72 88 73 86 72 / 20 40 20 50 50
Burnet Muni Airport 70 86 71 83 70 / 10 20 20 50 60
del Rio Intl Airport 74 87 73 84 71 / 60 70 80 80 80
Georgetown Muni Airport 71 88 72 86 71 / 10 20 20 40 40
Hondo Muni Airport 73 91 74 87 73 / 30 40 50 70 70
San Marcos Muni Airport 72 88 73 86 71 / 10 30 20 50 50
La Grange - Fayette regional 72 89 73 89 73 / 10 30 20 30 30
San Antonio Intl Airport 73 88 74 85 73 / 30 40 40 60 60
Stinson Muni Airport 74 89 75 86 74 / 30 40 40 60 60

&&

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

$$

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