#644 Postby weatherdude1108 » Thu Dec 06, 2018 4:55 pm
000
FXUS64 KEWX 062141
AFDEWX
Area Forecast Discussion
National Weather Service Austin/San Antonio TX
341 PM CST Thu Dec 6 2018
.SHORT TERM (Tonight through Friday Night)...
Regional radars early this afternoon show isolated to scattered
showers drifting east from the Hill Country into Central Texas.
Overall rain accumulations from this activity so far have been
generally light and less than one tenth of an inch, but some of this
activity north of Interstate 10 has been able to take advantage of
some mid-level instability over Central Texas and has produced
isolated lightning strikes and locally higher totals approaching one
half inch. Afternoon SPC mesoanalysis shows a cold front stretching
from near Fort Stockton to Abilene to Wichita Falls with deeper
moisture (precipitable water values around 1.4-1.5 inches) nosing
into the Middle Texas coast. Strong isentropic ascent on the 295K
surface and this deeper moisture will result in periods of light rain
or drizzle persisting through the evening hours across the region.
Rainfall accumulations through the evening are expected to remain
light and generally under one quarter of an inch.
The cold front is expected to push through the Hill Country this
evening, reaching the northern counties of the forecast area by
midnight. As the front reaches the Interstate 35 corridor around
sunrise, embedded disturbances in the southwest flow aloft will
result in increasing shower and thunderstorm coverage along the
frontal boundary. Light to moderate rain during this time could
result in a challenging Friday morning commute for the Austin and San
Antonio metros and motorists should allow for extra time to make it
into work or school in the morning. Through sunrise, expect an
additional half inch of rain with more robust cells along the cold
front/Interstate 35 corridor capable of producing isolated 1-1.5
inch amounts.
The cold front will continue to sag south of Interstate 35 through
the morning hours, but low and mid level flow becoming oriented
increasingly parallel to the frontal boundary will result in the
front slowing or possibly stalling as it reaches the Coastal
Plains/southeastern counties of the forecast area. This will allow
for the first round of heavy rainfall during the afternoon hours into
the evening hours as nearly uniform flow above the surface results
in training of showers and thunderstorms south and east of the
I-35/I-37 corridor (or really, for areas along and just behind the
cold front). Cannot rule out a strong or severe thunderstorm
developing along the front Friday afternoon or evening, but with the
continued southward placement of the front expect thunderstorm
activity to be elevated with the overall severe risk low. Forecast
rain amounts in this area are in the 1 to 2 inch range during the day
Friday with lighter amounts expected farther west. Upper flow will
continue to amplify across the region will continue to amplify across
the region Friday afternoon and evening as a closed low now off the
California coast becomes an open wave over Arizona. Embedded
disturbances within this upper flow may actually allow a break in the
rainfall (or at least periods of lighter rain) during the late
morning/midday hours on Friday. This would depend on how much
subsidence is able to be realized behind stronger shortwaves lifting
out of Mexico during the day, but is worth noting as there is some
signal of this happening within the TTU-WRF, ARW, and NMM.
By Friday afternoon, the cold front is expected to have sagged as
far south as the Texas Coastal Bend but , but will likely be cancelled early based on
the speed of the overall storm system.
&&
.LONG TERM (Saturday through Thursday)...
Dry slotting associated with the passage of the shortwave trough
will result in rain shifting north and east of the region midday
Saturday with 10-20 MPH breezy north to northwest winds settling in
across the region in the wake of the front during the day. Surface
ridging building into the region Saturday night and Sunday will allow
for winds to begin to relax with deep northerly flow through the
atmospheric column keeping the region dry and cooler through the
middle of the week with highs in the 50s.
With the surface ridging settling near the region on Monday,
excellent conditions for radiational cooling may allow for a light
freeze across portions of the Hill Country both Monday and Tuesday
mornings. Otherwise, a gradual warming trend begins Monday through
the middle of the upcoming week with highs climbing back into the 60s
by Wednesday. Rain chances don`t look to return to the forecast
until the next disturbance approaches the region late next week.
&&
.HYDROLOGY...
December is not a common flash flooding month for South Central
Texas, with the last flash flood warning issued in the region during
the month of December occurring in 2002. However, the region is
coming out of a very wet fall with September 2018 being the wettest
September on record for the City of San Antonio. There have also been
several freezes around the region causing vegetation to go dormant,
which means that it is not able to absorb water. Both of these will
make it easier for any rain that falls to convert into runoff and
produce at least elevated flows along area creeks, streams, andincreasing forcing with the
approach of the shortwave trough from Arizona will result in a
secondary round of heavy rain developing along the Rio Grande and
sweeping west to east across South Central Texas Friday evening and
night. Heavy rain is still expected during this time though as
evaluation of several environmental characteristics for heavy rain do
indicate that strong warm advection will be present in the 850-700
MB layer (resulting in steepening lapse rates and enhanced
precipitation efficiency), there will be strong diffluence aloft with
the approach of the upper trough (providing extra lift), and high
precipitable water values in place (more than 130 percent of normal).
However, mean winds within the cloud bearing layer during this time
(850-300 MB) are in the 35-50 knot range which indicates that this
overall complex will be moving at a fast clip and this will help
limit higher rain totals. Expect the system to clear most of the
region between 3-6 AM on Saturday morning, with overall rain amounts
for this event (tonight through Saturday morning) in the 3-5 inch
range west of I-35/I-37. The Flash Flood Watch remains in effect for
portions of South Central Texas beginning midday Friday and
continuing into Saturday
rivers.
Forecasted rainfall across the Guadalupe, San Antonio, Colorado, and
Brazos River basins has resulted in forecasted rises info minor and
moderate for several locations within these basins. Two locations
within the Guadalupe Basin, the San Marcos River at Luling and the
Sandies Creek near Westhoff, are forecast to rise into major flood
stage. Fluctuations in where the heaviest rain will fall will
ultimately affect how high (and how quickly) these points rise and
residents who live near a river or waterway should make sure to stay
aware of changing conditions over the next 24 to 36 hours.
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