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

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FXUS64 KEWX 191202
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
602 AM CST Mon Feb 19 2018

.AVIATION... /12Z TAFS/

Will maintain a downward trend toward ifr cigs at 13Z along I-35 and
at 14Z at DRT, but 50 knot winds at 850 mb have limited decoupling
below the boundary layer so far. Shower activity in the short-term is
trending down, and would leave out the mention of the late morning vcsh
if not for the latest run of the Texas Tech WRF regenerating midday
activity along I-35. Several midday to late afternoon hours of mvfr
cigs should continue again with a possible mix out in the late
afternoon to early evening. Most of the finer res models regenerate
another round of Edwards Plateau convection in the early evening with
DRT probably impacted in similar fashion to the way it was overnight.
Models are less distict on timing of any rain/thunderstorm chances at
I-35; will assume a similar trend of what happened overnight with the
light shower activity currently over the Rio Grande Plains expected
to be stronger for late tonight.

&&

.PREV DISCUSSION... /ISSUED 436 AM CST Mon Feb 19 2018/

SHORT TERM (Today through Wednesday)...

Main weather highlight continues to be centered on multiple rounds of heavy
rain expected to impact portions of the region starting late tonight
and over the next 48 hours into Wednesday evening. There appears to
be three semi-distinct rounds of heavy rain and thunderstorms. The
first will be tonight into Tuesday morning out west near the Rio
Grande and Edwards Plateau, the second will be along and east of I-35
in the Coastal Plains late Tuesday afternoon and evening, and the
third will be a transition of convection from the second that
interacts with the cold front from late Tuesday night into Wednesday
morning. Overall rainfall accumulations are expected to range from
1-3 inches with isolated amounts of 4-5 inches possible. Rainfall
rates at times could be heavy enough to cause localized flash
flooding. A flash flood watch may be needed Tuesday night into
Wednesday afternoon for this hazard for portions of the Hill Country,
the I-35 corridor, and portions of the Coastal Plains. The impact
window will close early Wednesday evening as the front shifts through
the region.

For this morning, a strung out line of showers and isolated
thunderstorms is located across the Southern Edwards Plateau and
just NW of the Hill Country as a compact but potent H5 impulse shifts
across the Big Bend of Texas. This impulse was analyzed best by the
NAM and the next 24-48 hours are weighted 50% towards the NAM with
25% GFS and 25% ECMWF. For the rest of today, expect the showers
in the Hill Country to linger through the morning but then slowly
dissipate near noon. Cloudy and warm conditions are expected for the
day with isolated shower coverage while south winds continue to
stream in additional moisture as PWATs climb into the 1.3-1.6" range.

The first round of storms and heavy rain is likely this evening and
into Tuesday morning across the Rio Grande and Southern Edwards
Plateau area as NAM, GFS, ECMWF, and hi-res models all agree on a
semi-organized complex shifting from Mexico into Texas. Some storms
associated with this complex could be strong to marginally severe
with pockets of gusty winds, small hail, and heavy rainfall. This
activity should then shift northeast across the Hill Country through
early Tuesday morning but stay north and west of the metro areas.
Drizzle to scattered light showers will be more likely for the metro
areas Tuesday morning like many of the previous mornings.

By Tuesday afternoon and evening, a transition will take place as
greater 700-300H layer vertical velocities and DPVA shift over to
just east of the I-35 corridor as enhanced mid- to upper-level lift
is implied per NAM/GFS solutions. Hi-res models are also in agreement
that a stream of enhanced showers and elevated thunderstorms could
form in a narrow band. The questions will be how unstable will the
mid-levels be and where will the band be located. Am currently siding
more with NAM soundings which show more tampered mid- and upper-
level instability with moist adiabatic lapse rates vs. the higher
lapse rates of the GFS soundings. NAM and TTU-WRF place the band
across the Coastal Plains with GFS just slightly farther west but
still east of the I-35 corridor. Impacts could remain limited during
this second round if instability remains weak and rainfall rates
remain low in lower vulnerable geographic locations east of I-35.

However, a transition will occur Tuesday night into the third and
likely highest impact window early Wednesday afternoon. Greater mid
and upper-level divergence is progged to overspread Texas as a H25
RRQ of a jet streak strengthens in response to stronger ejecting
shortwave trough impulses connect into the base of a longwave trough
over the Southern Plains. As a result of dynamic cooling from the
vertical motion, MUCAPE values should increase and likely allow for
more convective rainfall rates as thunderstorms develop over the
region. Given near S/SW unidirectional low-level flow, some training
of thunderstorms will be possible. Another concern is that the
placement of this training could be near or over the I-35 corridor
through Wednesday morning and cause local flooding impacts. Furthermore,
a few storms could be strong to marginally severe initially before
growing upscale. This convection will then intersect a cold front
moving from the north, further aiding in rain and thunderstorms
through mid-day Wednesday. Once the front passes through Wednesday
afternoon, convective rates will decrease and the strong thunderstorm
risk will diminish. However, rain will linger behind the front as
continued southwest H85-H5 flow persists with isentropic upglide.

LONG TERM (Thursday through Sunday)...

Cool and damp conditions will start the extended period Thursday,
followed by a warm front on Friday with continued scattered showers
and isolated thunderstorms each day into the weekend.

A near permanent trough axis will be anchored over the western CONUS
as deep southwest flow continues over Texas through late week and the
weekend. This pattern will help support rain and isolated thunderstorm
chances most periods as moisture returns by Friday and remains
through the weekend. Pattern is more indicative of an El Nino type
with an active southern branch jet stream with embedded troughs
supporting lift for shower development. Both GFS and ECWMF indicate
this active pattern to continue even mid- to late-next week. This
will be quite welcomed to help alleviate some of the drought
conditions.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              75  65  72  59  64 /  20  50  80  80 100
Austin Bergstrom Intl Airport  76  65  72  61  66 /  20  40  70  80 100
New Braunfels Muni Airport     77  65  73  62  69 /  20  40  60  80  90
Burnet Muni Airport            74  64  71  54  58 /  30  70  80  80 100
Del Rio Intl Airport           81  63  81  58  71 /  10  60  10  70  40
Georgetown Muni Airport        74  66  71  56  59 /  30  60  80  80 100
Hondo Muni Airport             82  66  77  62  72 /  20  40  40  70  80
San Marcos Muni Airport        76  65  73  62  67 /  20  40  60  80 100
La Grange - Fayette Regional   77  67  75  65  71 /  20  30  70  80 100
San Antonio Intl Airport       78  66  74  63  69 /  20  40  50  80  90
Stinson Muni Airport           80  66  75  64  73 /  20  30  40  80  90

&&

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

$$

Mesoscale/Aviation...Oaks
Synoptic/Grids...Allen



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