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

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FXUS64 KEWX 190211

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
911 PM CDT Wed Oct 18 2017

Only made minor adjustments to the overnight grids to account for
high level clouds continuing to stream across South-Central Texas.
In addition, some of the high-res guidance is showing possible patchy
fog farther west than what the current forecast showed and made that
adjustment. Only other changes were to the hourly grids based on
current trends.


.PREV DISCUSSION... /ISSUED 704 PM CDT Wed Oct 18 2017/

VFR conditions will prevail this evening and for at least most of the
overnight hours as weak southerly flow causes a gradual return of
moisture to the region. As is typically the case on nights where we
transition back to southerly flow, models are exhibiting a large
range of solutions for tonight`s ceiling forecast with the NAM
depicting VLIFR conditions by sunrise and the GFS keeping all sites
VFR. Model streamlines (including the NAM`s) do not show a strong
onshore component to the low-level flow tonight and winds are weak,
which would make VFR conditions or patchy dense fog the most likely
scenarios. For now, we have kept the previous forecast of MVFR
ceilings at SAT and SSF where low ceilings are most likely due to
their closer proximity to the Gulf Coast while preserving VFR
conditions at AUS and DRT. However, forecast confidence is fairly low
at the current moment on what ceilings will look like at sunrise.

Regardless of whether low ceilings and visibilities develop, all
sites should be VFR by 18Z with southeasterly 5-10 knots winds at the
I-35 sites and 10-15 knot winds with gusts to 20 knots at DRT.
Isolated showers will be possible in the Coastal Plains tomorrow
afternoon, but they are currently not expected to affect the I-35
sites. Low IFR to MVFR ceilings should develop late tomorrow evening
into the overnight hours as stronger onshore flow streams even more
moisture into the region, with LIFR ceilings possible.

PREV DISCUSSION... /ISSUED 323 PM CDT Wed Oct 18 2017/

SHORT TERM (Tonight through Thursday Night)...
Very low pwat values will remain over South TX through this evening
as low level winds along the coast remains mainly alongshore through
late this afternoon. Surface to H850 winds should turn se this
evening as an upper level disturbance approaches out of Nrn Mexico,
but as shown in the CRP VAD winds, sfc to boundary layer flow
bringing Gulf moisture onshore will be slow to develop. Will expect a
late arrival of low clouds and possible fog with some areas along
I-35 not expected to see low level saturation until daybreak. Thus
the low level moisture could mix out quickly in the late morning over
the few central counties that see a low ceiling or patchy fog. By
Thursday afternoon, eastern counties will see rapidly deepening
moisture and perhaps enough instability contribution from the
approaching disturbance to generate some showers and perhaps an
Isolated storm or two. CAPE values should generally stay below 1800

With a slow trend on the returning southerly winds, the overnight
lows could end up being extra cool again in the protected valleys,
but not as dramatic as from those of this morning due to increasing
high clouds. Lowering diurnal ranges are expected for

LONG TERM (Friday through Wednesday)...
Upstream upper troughing is depicted as coming into the Southern
Plains states with a positive tilt which could make it more efficient
in generating rain chances in advance of the next significant front
expected Saturday night. Rain chances could thus be more independent
of diurnal trends by Friday and possibly even less dependent on the
moisture convergent axis that is forecast to stay mainly east of
Highway 281.

A deeper upstream trough replaces the relaxing upper level feature
during the day Saturday and could potentially create a brief dry-line
environment. Confidence is low on this period and will show PoPs
undercutting guidances Saturday into Saturday evening. Late Saturday
evening should show PoPs ramping up over the nw counties, with likely
PoPs along the front expected early Sunday morning. With all the
deterministic models showing good amplitude with the trough and a
similar timing, will also trim the timing to show reduced rain
chances from west to east during the day Sunday. A few strong storms
are possible overnight, but the late night timing of the front
should be a negating factor. Expected rainfall potential at this
point look to be mostly in the 1/4 to 1/2 inch range.

The sharpening of the trough could lead to some gusty winds to 30 mph
with the front, then a rapid decrease in wind and another chilly
overnight low for Monday morning. Surface high pressure keeps the
diurnal range high through Monday night, and another front reinforces
the cool and dry pattern on Tuesday.


Austin Camp Mabry              59  82  65  81  70 /   0  20  20  30  30
Austin Bergstrom Intl Airport  56  81  65  81  69 /   0  20  20  40  20
New Braunfels Muni Airport     58  81  65  82  68 /   0  20  20  30  20
Burnet Muni Airport            56  79  64  79  67 /   0  10  20  30  30
Del Rio Intl Airport           60  79  66  82  68 /   0   0  20  10  -
Georgetown Muni Airport        56  80  64  80  68 /   0  10  20  40  30
Hondo Muni Airport             56  83  67  84  69 /   0  -   20  20  20
San Marcos Muni Airport        58  81  65  82  69 /   0  20  20  40  20
La Grange - Fayette Regional   60  84  66  83  69 /   0  20  20  40  20
San Antonio Intl Airport       60  81  67  82  70 /   0  10  20  20  20
Stinson Muni Airport           59  83  68  84  70 /   0  10  20  20  20




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