Area Forecast Discussion
Issued by NWS Corpus Christi, TX

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505
FXUS64 KCRP 200701
AFDCRP

Area Forecast Discussion
National Weather Service Corpus Christi TX
101 AM CST Mon Feb 20 2017

.DISCUSSION...

Convection continues across western portions of South Texas early
this morning. With strong capping remaining over eastern portions
of the area and a large cold pool now in place for west and
central areas, risk for severe thunderstorms is decreasing
significantly. Will let the severe thunderstorm watch expire at
1am. Could still see an elevated stronger storm, mainly a hail
threat through the night. With the decreasing severe risk, the
other concern becomes the heavy rain potential. With very high
moisture levels, showers that do develop overnight could produce
briefly heavy rains, however widespread heavy rainfall is not
anticipated at this time.

&&

.PREVIOUS DISCUSSION... /Issued 1148 PM CST Sun Feb 19 2017/

DISCUSSION...

Updated for 06z aviation.

AVIATION...

Primary concern remains the possibility of convection impacting
the terminals. Earlier line of storms has moved northeast of the
region. Also, eastward progression has been a bit slower with
stronger capping along the Coastal Bend. However, renewed
development of storms has occurred over the Rio Grande and,
according to hi-res models, should move east during the overnight.
In general, have accounted for storm chances with TEMPO groups...
initially in LRD and closer to CRP/VCT between 9z and 12z.
Lingering showers may occur past 12z but should dissipate by
midday. VFR conditions expected by late morning and should
continue into the afternoon. Brief gusty winds and reduced vsbys
will be possible in and around thunderstorms. Outside of the
storms, expect mostly a mix of VFR/MVFR ceilings.

PREVIOUS DISCUSSION... /Issued 857 PM CST Sun Feb 19 2017/

DISCUSSION...

Severe thunderstorm watch number 36 is in effect until 1am for the
Rio Grande Plains and Brush Country. Strong to severe line of
convection continues to march eastward this evening. Lower
confidence exists as to whether or not convection will be severe
as it reaches the coast. Will assess trends over the next few
hours.

PREVIOUS DISCUSSION... /Issued 558 PM CST Sun Feb 19 2017/

DISCUSSION...

Updated for 00z aviation.

AVIATION...

Chances for showers and thunderstorms will be the main aviation
concern tonight into Monday morning as upper level trough
approaches South Texas. Thunderstorm chances were based primarily
on latest hi-res model guidance. Best chances early this evening
will be at LRD, spreading eastward as the evening/overnight
progresses. Gusty winds and locally reduced vsbys will be possible
if thunderstorm directly impacts taf site. Showers and storms may
linger across the eastern taf sites into the mid morning hours on
Monday before pushing out of the area. Outside of thunderstorms,
expected a mix of MVFR/VFR ceilings.

PREVIOUS DISCUSSION... /Issued 402 PM CST Sun Feb 19 2017/

SHORT TERM (Tonight through Monday Night)...

Main forecast challenges reside around convective and heavy rain
chances tonight into Monday.

Clearing has taken place this afternoon across the Brush Country
resulting in an increasingly unstable airmass. As of writing,
MLCAPES were analyzed to be around 1500 to 2000 j/kg across the
Brush Country with rapidly diminishing CIN values as sfc temps
continue to warm to near 90 F. Convection is expected to develop
across either the W Brush Country or along/just west of the Rio
Grande either late this afternoon or early this evening.
Convective temps may be difficult to reach across the Brush
Country late this afternoon due to earlier delay in clearing
skies. However...a mid level vort max (over the Durango/Chihuahua
MX state line) rounding the base of an unusually deep trough will
advect NE towards the region over the next several
hours...resulting in an increase in large scale ascent. This
increase in ascent should be enough to result in rapid convective
development early this evening. Best time frame for convective
development across the Brush Country is roughly from 6pm onward. A
few light showers may occur before the greater lift arrives.

Initial convection/brief supercells should quickly develop into
clusters or a broken line segment given quasi veer-back-veer
forecast wind profiles across Webb County. Strong damaging
downbursts will be primary storm threats given inverted V
soundings and stronger winds aloft. Isolated large hail will be
possible with updrafts that develop within the greatest DCAPE
areas /prog to be draped across the Brush Country/.

Convective segments/clusters should shift ENE by late in the
evening and slowly weaken as diurnal warming/associated CAPE
values decrease and CIN increases from E to W. However...strong to
isolated severe storms may still approach the coastal counties
and Victoria area before weakening. Focus then turns to potential
for moderate to heavy rain event. After midnight, guidance
indicates the H25 jet may start to split resulting in an increase
in upper level divergence. An area of very high deep layer
moisture depth (by mid Feb standards) is also prog to exist
across this same area. These features combined with an increase in
large scale ascent as primary mid/upper level trough axis slowly
approach from the far west should result in an increase in
moderate to at times heavy rainfall...particularly from Port
Aransas northward to the Victoria area (and points east). While
widespread flash flooding is not expected at this time...isolated
localized flooding may occur along with urban and small stream
flooding. Flash Flood Guidance values remain high across these
areas as well (1 hour FFG of around 2.5 inches, with 3 and 6 hrly
FFG values around 3 inches or greater). As such...have held off on
a Flash Flood Watch at this time after coordinating with WFOs EWX
and HGX.

Back edge of heaviest precip should be along/offshore around
sunrise (if not sooner). However...mid/upper level trough axis
will still be to the west of the Coastal Plains with a high
moisture depth prevailing. Some CAM guidance...and to an extent
the ECMWF...want to develop precip across the Coastal Counties to
Victoria during the morning...and as such I have maintained high
POPs for these areas. Critical moisture values in the H85 to H7
layer shift east by Mon afternoon resulting in a decrease in
precip. H5 trough axis still remains along/just west of the Rio
Grande in the afternoon...and with some diurnal warming...isolated
to scattered convection /small showers and storms/ may develop due
to the elevated instability and continued ascent.

Trough axis finally begins to shift east faster by late Mon
Night...with isolated elevated convection remaining possible Mon
night until trough axis shifts east.

Went below guidance for min temps tonight due to expected
convective cold pool impacts on area temps. May still be too warm
across Coastal Plains. Went on the cooler end of guidance Monday
due to expected persistent cloud cover through the day.

Thanks to HGX...EWX...and BRO for excellent coordination today.

LONG TERM (Tuesday through Sunday)...

Deterministic GFS/NAM/ECMWF generally agree with the axis of the
upper low moving across the CWA Tuesday resulting in significant
subsidence/drying. GFS deterministic predict PWAT values below
normal over the CWA Wednesday/Thursday. Increasing onshore flow
expected late Wednesday through Thursday in response to both a
developing sfc low/trough to the lee of the Rockies/Southern Plains
owing to increasing zonal upper flow and a somewhat small amplitude
disturbance (GFS ensemble mean/GFS and ECMWF deterministic.) As the
upper system moves east, the deterministic runs predict a front to
move across the CWA Friday. The combination of near surface moisture
from onshore flow yet dry aloft (GFS deterministic soundings) may
result in radiational fog early Friday morning. Elevated fire
weather conditions might occur over portions of the CWA Friday
afternoon.

MARINE...Moderate to strong SE flow should weaken this evening
and especially overnight. Areas of fog possible along the coast,
especially this evening. Strong convection and high chances of
showers for late tonight through especially Monday morning.

&&

.PRELIMINARY POINT TEMPS/POPS...
Corpus Christi    77  56  79  57  80  /  80  20  10  10  10
Victoria          75  57  78  54  79  /  90  20  10  10  10
Laredo            78  54  83  55  87  /  20  10  10  10   0
Alice             78  55  82  54  84  /  40  10  10  10   0
Rockport          74  58  76  60  76  /  90  20  10  10  10
Cotulla           78  53  83  52  87  /  20  10  10  10   0
Kingsville        79  56  81  55  84  /  70  20  10  10   0
Navy Corpus       74  58  76  61  75  /  90  20  10  10  10

&&

.CRP WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory until 3 AM CST early this morning For the
     following zones: Waters from Baffin Bay to Port Aransas
     from 20 to 60 NM...Waters from Port Aransas to Matagorda
     Ship Channel from 20 to 60 NM.

&&

$$

PZ/83...SHORT TERM



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