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

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FXUS64 KEWX 252351
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
651 PM CDT Wed Apr 25 2018

.UPDATE...
Please see the 00Z aviation forecast discussion below.

&&

.AVIATION...
An area of rain along with a few embedded thunderstorms continues
along the I-35 corridor early this evening. We will mention
prevailing light rain along with VCTS for the I-35 sites for the next
1-2 hours. Rain will then shift east of the I-35 corridor, leaving
plenty of cloud cover in it`s wake. As of now, it appears cloud bases
will be just above MVFR. Clouds will clear from north to south
tomorrow as winds generally remain from the north or light and
variable. Late in the period, mainly high clouds are anticipated as
winds transition to a more southeasterly direction. For DRT, low
clouds with bases just above MVFR are in store overnight. We expect a
slow clearing trend through the morning hours as winds become light
and variable. Southeast winds should return by early evening.

&&

.PREV DISCUSSION... /ISSUED 235 PM CDT Wed Apr 25 2018/

SHORT TERM (Tonight through Thursday Night)...
As of 2PM this afternoon, a multi-cell cluster of storms was pushing
eastward along the southern Edwards Plateau into the Hill Country.
These storms have been mainly riding the cold front as it pushes
slowly southward and interacting with 2000+ j/kg of MLCAPE and
favorable 925-850mb moisture flux convergence. A special 18Z KDRT
sounding launched just ahead of the cold front shows limited
elevated instability behind the front thus expect the weaker elevated
convection well behind the cell cluster to remain rather benign other
than some occasional CG strikes.

Latest HRRR brings this cluster across Bexar county between 4-7 PM
with the front and then favoring the southern third of the CWA after
00Z. Extrapolating this motion through 06Z, the majority, if not all
of the precip should be clear of the CWA with predominantly north
winds taking over, initially 10-15 mph but then weakening to 5-10
towards tomorrow morning.

Morning lows tomorrow will be cooler, ranging from the mid to upper
40s in the Hill Country and Plateau and mid to upper 50s in areas
south. Thursday expected to be quite a pleasant day with mostly clear
skies by the afternoon and highs in the mid to upper 70s. Winds
should be light and variable as surface high pressure sets over
the area.

LONG TERM (Friday through Wednesday)...
The main closed low responsible for today`s activity will continue to
swing east into the southeast CONUS leaving Texas under a temporary
ridge axis before a weak shortwave moves over Friday. GFS/ECMWF keep
this passage dry with a weak cold front that stalls along our
northern border with SJT.

Moving into the weekend, a textbook omega block pattern sets up by
Sunday morning with a large ridge over the central US and two closed
lows in the Pacific Northwest and over New England. This will shift
east by early next week and bring back PoP chances to the area and
southwest flow returns and low level moisture flux returns to the
region.

By mid-week, the aforementioned Pacific NW low will be replaced by a
deep trough axis over the Rockies and will likely be influenced by
multiple embedded shortwaves from Tuesday through the rest of the
week. GFS/ECMWF both highlight Thursday as a cold front quickly
pushes south into the area.

PREV DISCUSSION... /ISSUED 1235 PM CDT Wed Apr 25 2018/

AVIATION...
/18Z TAFS/
A cold front is now affecting the southern Edwards Plateau. The
frontal boundary is forecast to push to the southeast this afternoon
and affect much of the area, specially the I-35 terminals from 20Z
through 23Z. Have included TEMPO groups for all area sites to account
for the potential of TSRA. Once the cold front moves across/wind
shift takes place, northwest to north winds of 10 to 15 knots are
expected with gusts up to 20 knots early this evening. There is a
chance for cigs to go down to MVFR conditions during heaviest storms
and for much of the overnight hours for KSSF. Austin area airports
should remain VFR after the passage of the front.

PREV DISCUSSION... /ISSUED 932 AM CDT Wed Apr 25 2018/

MORNING UPDATE...
The latest SPC Day 1 Convective Outlook has expanded the marginal
risk for severe storms farther northeast which now includes San
Antonio. RAP/HRRR trends indicate increasing SBCAPE along the Rio
Grande Plains and especially on the Mexican side of the border where
3500+ j/kg have been advertised. Additionally, the 12Z KDRT sounding
shows steep mid level lapse rates and favorable low level moisture
flux convergence into the southward moving cold front. Effective
shear values of up to 50 kts will also help to maintain updrafts as
they get going along the front.

Latest HREF/TTU-WRF continue to favor the Rio Grande Plains this
afternoon and produce a somewhat linear complex of multi-cell
clusters beginning at or shortly after 18Z out west. Given the
aforementioned parameters, there will be increased potential for hail
up to quarter size and strong straight line winds as the cold front
continues south throughout the day.

PREV DISCUSSION... /ISSUED 633 AM CDT Wed Apr 25 2018/

AVIATION...
MVFR ceilings are encroaching SSF and should be at DRT shortly. Think
most of the other sites will remain VFR this morning, but MVFR could
side swipe SAT later. The cold front will move through the area early
this afternoon, shifting winds to the north with speeds around 15
knots. Latest runs of the high-res models have shifted the precip
east a bit and actually leave DRT mostly dry. The ultimate areas of
rainfall will be highly dependent on where activity develops and will
mention VCTS at all sites this afternoon but expect future fine
tuning to prevail thunder at some sites when it becomes apparent
where this will be. Dry air will slowly filter in after 00z with
clearing skies expected overnight.

PREV DISCUSSION... /ISSUED 607 AM CDT Wed Apr 25 2018/

UPDATE...
Patchy fog is spreading into our southeastern counties. We have
updated to include patchy fog this morning in the region.

PREV DISCUSSION... /ISSUED 338 AM CDT Wed Apr 25 2018/

SHORT TERM (Today through Thursday)...
Upper air analysis last evening showed a trough over the southeastern
US and a ridge over the west with northwesterly flow across Texas. At
the surface, a front was stretched across Texas north of our CWA from
Texarkana to Denton to San Angelo to Midland. Ahead of the front
winds were from the southeast across our forecast area. During this
period an upper level low will move out of the Central Plains to the
lower Mississippi River Valley bringing the frontal boundary through
our CWA. The front should be through our area by this evening and
move away overnight. This system will bring chances for showers and
thunderstorms with the best chance across the western half of the
area. Some strong storms could form over the mountains in Mexico and
move across the Rio Grande this afternoon and evening. These storms
could produce large hail and strong winds. SPC has this area in a
marginal risk for severe thunderstorms. A chance for rain will
linger tonight, but then drier air will move in on northerly winds
Thursday. Temperatures will also be down a few degrees Thursday.

LONG TERM (Thursday Night through Tuesday)...
The upper level flow will remain northwesterly through Friday.
Another weak front will move through Friday, but there won`t be much
return moisture between the two fronts. So, this second front will
likely move through dry. The upper level flow will be essentially
zonal over the weekend and the weather will be dry. Models show
activity developing along the dryline Monday and Tuesday with chances
for thunderstorms moving across our CWA both days. Given model
performance over the past few weeks, we have reduced model POPs for
this period since we don`t have much confidence at that time period.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              51  80  56  80  56 /  30   0   0   0   0
Austin Bergstrom Intl Airport  51  79  53  79  53 /  30   0   0   0   0
New Braunfels Muni Airport     53  79  53  80  55 /  60   0   0   0   0
Burnet Muni Airport            47  78  52  77  52 /  20   0   0   0   0
Del Rio Intl Airport           54  82  59  81  59 /  20   0  10  10  -
Georgetown Muni Airport        48  79  51  78  52 /  30   0   0   0   0
Hondo Muni Airport             53  79  55  81  56 /  50   0  -    0   0
San Marcos Muni Airport        52  79  53  80  53 /  50   0   0   0   0
La Grange - Fayette Regional   54  76  53  79  54 /  50  -    0   0   0
San Antonio Intl Airport       54  78  56  80  57 /  60   0   0   0   0
Stinson Muni Airport           56  78  57  80  58 /  60  -    0   0   0

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

Mesoscale/Aviation...24
Synoptic/Grids...Treadway
Public Service/Data Collection...YB



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