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

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FXUS64 KEWX 181827
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
127 PM CDT Thu Oct 18 2018

.UPDATE...
Please see the 18Z aviation forecast discussion below.

&&

.AVIATION...
MVFR cigs along with increasing coverage of showers can be expected
through the afternoon hours. After 23Z, we expect cigs to lower into
IFR as showers become more widespread. We could see an isolated
thunderstorm or two during the overnight hours, but our current
thinking is the better chances for storms will remain near the coast
overnight. Showers continue into Friday morning and based on current
data, we will keep IFR conditions intact well into tomorrow
afternoon.

&&

.PREV DISCUSSION... /ISSUED 906 AM CDT Thu Oct 18 2018/

UPDATE...
Area radars are currently showing light showers mainly across the
southern counties of south central Texas. However, a line of light
to isolated moderate showers just entered the southern parts of
Karnes and Dewitt counties while moving to the northwest. So far, the
rain rates are less than a one tenth of an inch.

Based on current radar and satellite data, the moderate to localized
heavy rainfall expected for this morning is been delayed for this
afternoon and evening per latest HiRes models and guidance. Still
expecting 1 to 2 inches with isolated 4 inches for the next 18 to 24
hours.

Will keep you updated as new information comes in and the shower
activity begins to impact the region.

PREV DISCUSSION... /ISSUED 410 AM CDT Thu Oct 18 2018/

SHORT TERM (Today through Friday)...
For today and into Friday a disturbance will rotate around the
western side of the upper level high situated across the Gulf Coast
interacting with very moist air to create more scattered light to
moderate showers across the area. The best chance of any of these
heavier showers will be later today, through the overnight hours into
Friday morning. With additional rainfall amounts of 1 to 2 inches possible,
with isolated totals up to 4 inches we have extended the Flash Flood
Watch through Friday morning. Beyond Friday morning the watch may
need to be reconfigured as the threat for heavier rain will shift
from across the Rio Grande Plains to focus on the Texas Hill Country.
This disturbance will drag in rich Gulf moisture and increase
precipitable water values to 1.9-2.1 inches for late today through
Friday (check out the Wednesday evening update AFD for a more in-
depth explanation on PW). These values are well into the 90th
percentile for this time of year and could translate into pockets of
heavier rainfall. Soils, especially across the Edwards Plateau (from
Real to Kerr counties) and Hill Country (from Gillespie to Llano to
Burnet counties), are so saturated that even 1 to 2 inches of rain
will cause increased runoff and could lead to additional river rises
or flooded road ways. Over the last 3 days anywhere between 4 and 12
inches of rainfall have fallen on the Edwards Plateau and Hill
Country so these areas remain the focus for the flash flood threat,
but moderate to heavy rainfall could work into the I-35 corridor
today and tonight as well leading to their inclusion in the watch.
Afternoon highs will continue to run about 10 degrees below seasonal
normals, with lows right around where they should be for mid October.

With the saturated soils across parts of South Central Texas and the
continued wet forecast this remains a dangerous situation as rainfall
from the last few days continues to work its way through area rivers.
The Lower Colorado River Authority announced that by mid-day Thursday
up to 8 flood gates at Mansfield Dam could be opened (4 are currently
releasing about 25,000 cfs down the Colorado) to move some of the
water from Lake Travis down the Colorado. Lake Travis, as of 3am is
at just over 700 feet msl, the 6th highest the lake has ever been
(Visit the NWS AHPS page and search for MSDT2 to view the other past
crests). The Devils River in Val Verde County, the Nueces River in
Dimmit County, and the Frio River in Frio County all continue to be
in flood this morning as well.

We urge residents and visitors to South Central Texas to monitor
closely the weather conditions for the next several days as any
rainfall amounts could lead to additional flash flooding or river
flooding. Also, those along the Colorado River should monitor
information coming from the Lower Colorado River Authority concerning
releases from area lakes and take needed actions to protect lives and
property along the Colorado.

LONG TERM (Friday Night through Wednesday)...
The longterm forecast is a mixed bag of good news and bad news. The
good news is that on Saturday a large trough will dig out of Canada
across the eastern half of the U.S. and drag a cold front through
Texas. While this will act as a focus for more rain during the day on
Saturday the front should clear out precipitation for Saturday night
through Sunday night, other than some isolated showers across the Rio
Grande Plains. This should give a good 24 hours for water to work
through area rivers, with no additional rainfall. The front will also
help to knock down temperatures that will have rebounded to afternoon
highs near 70 by Saturday (still below normal), back down into the
lower 60s. A lucky few may see some peaks of sun on Sunday though
mostly cloudy skies should prevail.

Now the bad news...there is more rain in the forecast for the first
half of next week. Both the ECMWF and GFS show the persistent upper
level low pressure system that has been sitting across the Southwest
U.S. finally ejected eastward across the Southern Plains. This in
itself will provide a catalyst for another round of potentially heavy
rainfall beginning on Monday and lasting through Wednesday night.
What bares watching is the possibility than a Pacific tropical system
gets pulled into the through as it traverses across Texas. This
system has yet to form, but is shown in both the ECMWF and GFS and
NHC currently has a 90% chance of formation along the southern
Mexican coast. The GFS has the trough picking up the tropical system
Wednesday into Thursday producing widespread heavy rainfall across
much of Texas. The ECMWF is faster with the main trough pulling it
through Texas as tropical system makes landfall. The difference here
will be the amount of moisture the trough has to work with. With all
of the Pacific moisture the GFS dumps 5-10 inches from San Angelo to
Abilene, while the ECMWF with the miss-match of trough first, then
tropical system shows only 2-4 inches. There are a lot of factors
that will come into play next week from the timing of the trough
moving eastward, to the location, strength, and timing of the yet-to-
form Pacific Tropical System, and how much the area gets to dry out
this weekend. How these come together will determine how much and
where the rain falls during the first half of next week and what
additional impacts will be felt across South Central Texas.

Probably the *best* news is that both long-term global models show
ridging trying to work into Texas behind the trough and a drier
forecast beginning Thursday of next week.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              58  57  63  59  67 /  70  60  80  50  50
Austin Bergstrom Intl Airport  59  57  64  59  66 /  60  60  70  50  50
New Braunfels Muni Airport     59  58  66  60  68 /  70  60  70  50  50
Burnet Muni Airport            56  54  61  56  65 /  80  80  90  60  50
Del Rio Intl Airport           58  57  68  61  67 /  70  70  40  40  60
Georgetown Muni Airport        57  55  61  56  65 /  70  70  80  60  50
Hondo Muni Airport             59  58  67  62  69 /  80  80  60  50  60
San Marcos Muni Airport        59  57  66  60  67 /  70  60  70  50  50
La Grange - Fayette Regional   64  60  70  60  68 /  60  50  70  60  40
San Antonio Intl Airport       59  59  67  61  68 /  80  70  70  50  50
Stinson Muni Airport           60  60  68  63  69 /  80  70  70  60  50

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Friday morning for Bandera-Bexar-Blanco-
Burnet-Comal-Edwards-Gillespie-Hays-Kendall-Kerr-Kinney-Llano-
Maverick-Medina-Real-Travis-Uvalde-Val Verde-Williamson-Zavala.

&&

$$

Aviation...Platt
Short-Term/Long-Term...17
Public Service/Data Collection...YB


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