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Area forecast discussion
National Weather Service Austin/San Antonio Texas
1047 am CST Wed Dec 13 2017

Update...
high clouds continue to stream across most of south central Texas
aside from our northwestern-most areas this morning as an upper level
jet streak moves across the region. Aircraft soundings at Sat and drt
show that the atmosphere is nearly saturated at the jet layer been
200-325 mb (around 25-35 kft) with sat's saturated at 30 kft. Based
on the upper level dynamics, moisture, and upstream cloud cover, we
expect that these high clouds will continue to occur throughout most
of the afternoon hours aside from possibly our northwestern counties.
Therefore, we have decreased high temperatures a few degrees today
following short-term MOS trends to account for a small reduction in
solar insolation this afternoon. High temperatures should increase
into the lower to mid 60s across most of the region with a few spots
in the upper 60s to near 70 over our northern and northwestern-most
counties where the coverage of high clouds should be lower.

&&

Previous discussion... /issued 525 am CST Wed Dec 13 2017/

Aviation...
VFR skies will prevail today through Thursday morning with mainly
scattered-broken decks above 10k ft. Vrbl winds less than 5 kts increase to
S to SW 7 to 15 kts late morning into afternoon. Winds decrease to
variable less than 5 kts this evening and then northwest to north 7 to 12 kts on
Thursday.

Previous discussion... /issued 416 am CST Wed Dec 13 2017/

Short term (today through thursday)...
will be able to enjoy another mild day across south central Texas as
the 500 mb pattern overhead undergoes a transitional period characterized
by subtle ridging and westerly winds aloft. A pre-frontal low over
the northern half of the state will enhance the pgf for the northern
half of the County Warning Area this afternoon and likely result in some breezy
southwest winds of 15-20 mph.

After a slightly warmer morning Thursday, our next cold front will be
on approach from the north. While there is a bit of uncertainty in
the arrival time in the GFS/ECMWF, the front will serve to dry out
the low levels and seriously hinder the possibility for pops once it
arrives. Thus, have kept Thursday dry in the grids.

Long term (thursday night through tuesday)...
the long term period will begin with a cool Friday as a surface high
moves over the region. While this is occurring, 500 mb low pressure will
become cut off over the West Coast of Mexico and tap into some
Pacific moisture, effectively channeling this moisture into south
Texas. By Saturday, another shortwave will drop down across The
Rockies originating from the Pacific northwest and eject the Mexican
low northeast, evolving back into a negatively tilted shortwave across
the southern Continental U.S..

This very progressive evolution of the 500 mb pattern will result in
south central Texas seeing likely a quick shot at elevated showers
and possibly some thunderstorms over the course of the day Saturday.
While dynamically, this low is quite impressive, characterized by a
100+ kt 250 mb jet streak and a saturated column above 500 mb, low
level moisture is lacking, instability is nil, and precipitable water values only
slightly eclipse 1 inch. However, with 0-6 km bulk shear advertised
and plenty of dynamical lift, can't rule out some elevated
thunderstorms as this system quickly moves through Saturday. Also,
while the bulk of the moisture and lift is aloft, quantitative precipitation forecast expectations
are rather minor, with less than a quarter inch expected.

As this system clears out Sunday, another cold front is progged to
move through, dropping temperatures again into the mid 30s on the
plateau Monday morning. The aforementioned second shortwave will
then enter West Texas late Sunday night into Monday morning behind
the cold front and push eastward across the state. This is where
models begin to diverge as the European model (ecmwf) is significantly more
progressive with the evolution of the shortwave, and the GFS is much
slower. Thus have lower confidence in the forecast beyond Sunday.
However, next week does look warmer, likely in the mid 60s to low
70s, with the next significant cold front possibly arriving late in
the week.

&&

Preliminary point temps/pops...
Austin Camp Mabry 66 43 64 40 57 / 0 0 0 - -
Austin Bergstrom Intl Airport 64 39 62 40 57 / 0 0 0 - -
New Braunfels Muni Airport 64 42 64 41 58 / 0 0 0 10 10
Burnet Muni Airport 68 40 61 37 56 / 0 0 0 - -
del Rio Intl Airport 66 42 66 42 60 / 0 0 0 10 10
Georgetown Muni Airport 67 40 62 38 57 / 0 0 0 - -
Hondo Muni Airport 65 43 68 41 60 / 0 0 0 10 10
San Marcos Muni Airport 63 41 63 40 57 / 0 0 0 10 10
La Grange - Fayette regional 63 42 64 41 59 / 0 0 0 10 10
San Antonio Intl Airport 63 45 66 43 59 / 0 0 0 10 10
Stinson Muni Airport 64 44 67 43 60 / 0 0 0 10 10

&&

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

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