Area Forecast Discussion
Issued by NWS Austin/San Antonio, TX
FXUS64 KEWX 181657
Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1057 AM CST Wed Jan 18 2017
Rain has moved east of the TAF sites as of now and will not mention
anything in the TAFs any longer. IFR and low MVFR ceilings should be
in place for much of the day if not all the day as low-level
moisture remains quite high. Will have to watch for the potential of
fog tomorrow morning at the I-35 sites given the light winds and
saturated grounds. Will go 2sm in the TAFs for now and may need to go
longer if confidence improves. VFR should return by tomorrow
afternoon across all sites as drier air filters into the region.
.PREV DISCUSSION... /ISSUED 1020 AM CST Wed Jan 18 2017/
Widespread showers with isolated to scattered thunderstorms continue
to move northeast producing observed rainfall rates of up to 1 inch
per hour with overall totals remaining below 2 inches. These rain
rates have caused some minor flooding issues and low water crossing
closures in the typical spots, particularly in parts of Hays, Travis,
Williamson, Bastrop, and Caldwell Counties that have received 2-4
inches of rain over the past 24 hours. Although the upper level low
will remain to our northwest over the next 18-24 hours, the shortwave
and associated isentropic ascent that is responsible for our ongoing
storms will continue to progress east-northeast as the cold front
that is along the Hwy 59/I-69 corridor in east Texas pushes further
east. This will allow for isentropic descent to occur on the backside
of the shortwave the should keep rain chances low for the second
half of the afternoon and evening across the entire region.
Rain chances are still expected to increase after Midnight along and
east of I-35 as one final shortwave rotates around the upper level
low to cause a brief period of isentropic ascent. Most of this should
just be light to moderate shower activity to generally keep rain
totals overnight and tomorrow morning below a tenth of an inch with
perhaps a few spots in the far southeast Coastal Plains counties
closer to a quarter of an inch. Temperature and dew points have been
adjusted slightly based on current observational and model trends,
but are generally on track for highs in the mid 50s to lower 60s.
PREV DISCUSSION... /ISSUED 529 AM CST Wed Jan 18 2017/
12Z Aviation Update Below.
Scattered SHRAs and isolated TSTMs will impact the eastern half of
the CWA through the morning hours, including the I-35 corridor and
SAT/AUS. Activity should move east of the I-35 corridor 16Z-18Z.
Ceilings currently LIFR to IFR across most of the area, with the
exception of the Rio Grande and DRT where ceilings have risen to VFR.
The ceilings at SAT will rise to MVFR after 18Z but may take longer
at AUS, and have not indicated this happening until after 00Z. A
window of VFR conditions may develop at SAT/AUS after 06Z before
patchy fog develops early Thu morning. N to NW wind 5-10 kts
diminishing this evening.
PREV DISCUSSION... /ISSUED 413 AM CST Wed Jan 18 2017/
SHORT TERM (Today through Thursday)...
Pwat values over the Coastal Prairies jumped up to 1.7+ inches,
leading to some continued heavy downpours in spots overnight. A more
organized cluster of storms closer to the frontal boundary and the
deepest moisture should help preclude a training pattern over South
Central Texas this morning, but repeated rounds of moderate showers
and occasional brief heavy downpours should be expected for the rest
of the morning. Narrow CAPE and deep tropical layer up to 500 mbs on
sounding data suggests continued isolated heavy downpours of 1 to 2
inches east of I-35 from 6 am to noon with swrly winds aloft
bringing in drier mid level air by midday. Given the saturation of
soils over the sern counties, will go ahead and include locally heavy
downpours for much of the Coastal Prairies.
The deepest swath of moisture is forecast to shift east late today,
but only barely and a lobe of the SW-NE oriented oval upper low will
pivot east for 00z Thursday, leaving good chances for rain over at
least our eastern counties through tonight. By early Thursday, the
base of the upper trough begins lifting NE across TX with increased
mixing of drier westerly winds to promote stability. Expect to see
the rains come to an end over all our counties by 18z Thursday. The
westerly winds should mix a swly wind to the surface in the
afternoon and allow for a nice warm-up.
LONG TERM (Thursday Night through Tuesday)...
An active upper level pattern is expected Friday through the weekend,
capped off by a high amplitude system to bring our next weather
hazard into play Sunday.
A strong upper jet over the Pacific digs an upstream trough right on
the heels of the departing upper low and helps to maintain southerly
surface winds for Thursday night. Current model depictions show only
a modest pressure gradient as surface troughing over North TX
broadens to account for the upstream energy. This leaves modest
southerly surface winds over our sern counties with shallow moisture
spreading back north. Expect to see some light drizzle or sprinkles
and perhaps some fog over southeast counties Friday morning should
the pressure gradient weaken enough.
A weak shortwave passes well north of the area, but the strong upper
jet deepens a broader and stronger upper low to approach TX by
Saturday. Low level moisture is expected to be limited for late in
the day Friday to preclude deeper convection in the daytime, but the
increase in moisture and dynamics aloft Friday night may be enough
to promote some weak elevated thunderstorms over Central TX and parts
of the Hill Country. Light elevated precipitation remains a
possibility over northern counties into Saturday night as the broader
upper low continues to deepen over TX. The low becomes well formed
over TX and the Southern Plains by 00z Sunday, with drier air aloft
clearing rain chances west to east Saturday Night.
By 12z Sunday deterministic ECM/GFS solutions show the upper low is
rather deep over the Ark-la-tex and is still being deepened by a
strong jet streak with the GFS showing 140 knots on the backside of
the low at 300 mbs. Winds are unidirectional from from the jet stream
to the surface, and this should bring a formidable Pacific style
front with very strong winds mixing down from several thousand feet
up. The GFS shows that 850 mb winds ranging from 50-70 knots at 850
mbs which could potentially mix some 50 knot gusts to the surface. A
wind advisory at least is expected to be in effect as we get closer
to the this event. Should the stronger and better positioned GFS
upper low verify, High Wind Warning may be needed. Mid level drying
should also take humidities to near or below 20 percent and which
could lead to near critical fire weather conditions. There are weaker
solutions for the upper low, such as from the CMC, that will keep
confidence in the low to moderate range, so will continue to monitor
and suggest the potential for a high end level wind advisory in the
A dry and somewhat less eventful pattern follows for Monday through
much of next week, with a cold front of moderate strength suggested
for Tuesday night.
.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry 56 50 76 52 77 / 80 20 - - 10
Austin Bergstrom Intl Airport 56 49 75 49 76 / 90 20 10 - 10
New Braunfels Muni Airport 58 50 75 51 76 / 70 20 10 - 20
Burnet Muni Airport 55 46 74 48 75 / 60 10 - 0 10
Del Rio Intl Airport 61 45 74 45 75 / 20 - 0 0 0
Georgetown Muni Airport 55 48 74 49 75 / 80 10 - - 10
Hondo Muni Airport 61 47 75 48 77 / 50 10 - - 10
San Marcos Muni Airport 57 50 74 51 75 / 80 20 10 - 10
La Grange - Fayette Regional 56 52 72 53 75 / 90 40 30 - 20
San Antonio Intl Airport 61 51 75 51 76 / 60 20 10 - 20
Stinson Muni Airport 61 50 75 50 77 / 60 20 10 - 20
Public Service/Data Collection...YB