Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 201716

Area Forecast Discussion
National Weather Service Brownsville TX
1216 PM CDT Sat May 20 2017

.DISCUSSION...Updated for latest aviation discussion below.


.AVIATION...Coastal counties are seeing VFR conditions, while
patchy MVFR ceilings continue further inland. Expect the MVFR
ceilings to rise to VFR within the next hour or two. Moderate
southeast winds continue today, with higher gusts more likely at
KBRO and KHRL due to a tighter pressure gradient. Stratus deck
expected to build in again this evening and continue through the
early morning hours of Sunday. Model guidance indicates ceilings
lowering to IFR, but confidence is low in that scenario given the
setup, and have opted to go with MVFR ceilings for the time
being. A cold front continues to push south across the state today
with showers and thunderstorms firing up along and ahead of it.
While showers and thunderstorms are not expected this afternoon
and tonight at area terminals, some activity could move into and
develop across the western and northern zones late this evening
and tonight. Rain chances increase tomorrow.


.PREV DISCUSSION... /issued 635 AM CDT Sat May 20 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Patchy MVFR ceilings have been breaking up just before
sunrise and will continue to dissipate or rise through the
morning. VFR is expected to prevail much of the day with SCT to
BKN cumulus mixed with high clouds. A cold front over Northern
Texas pushes south tonight with stratus developing and lowering
this evening. Convection in the Valley is not expected at time but
some thunderstorms could form over the Higher terrain of NE Mexico
and drift into the Western areas overnight. A better chance of
rain is expected Sunday. Southeast wind remain moderate with gusts
of 20-25knots expected this afternoon.

PREV DISCUSSION... /issued 443 AM CDT Sat May 20 2017/

SHORT TERM (Today through Sunday):  Upper low/trough currently moving
through the Central Plains will work its way into the Upper
Mississippi Valley Sunday. At the surface the associated cold front
is noted over Northwest Texas and will make its way slowly Southeast
ending up along or just north of our CWA border. With the front
remaining well to our north today, no significant disturbance with
the weak SW flow aloft and still a relatively dry elevated mixed
layer between 500-700mb, convection will be hard to come by.
Forecast continues to maintain very low pops across the NW
ranchlands this afternoon despite the reasoning above. This is for
any convection that develops over the Sierra Madres and drifts
into the ranchlands. Confidence is very low for the daytime
activity with slightly better confidence for evening and overnight
activity as the EML erodes as moisture begins to show signs of
deepening at least across the NW counties. Temperatures today will
be similar to the last few days with 90s to near 100 out west
along the Rio Grande. Pressure gradient is not as robust today and
with the upper low/trough lifting northeast a lighter breeze is

Tonight and Sunday rain chances increase but still favor Northern
and Western Zones as the front pulls up stationary just north of our
CWA. Winds back east and moisture values reach their peak. With
limited forcing and little to no upper support pops remain only in
the 30 percent range across the West and 20 percent in the East. Any
chance of strong to severe thunderstorms (this evening/tonight) is
fading with SPC backing off on the areal coverage of a Marginal risk
area now favoring the extreme NW areas in the vicinity of the
frontal boundary. Overnights to remain 5 to 8 degrees above
normal with cloudy skies and the persistent southeast-east winds.
Highs on Sunday are expected to a few degrees lower especially
north and west due to more cloud cover and the added moisture.

LONG TERM (Sunday Night through Friday):  Unsettled conditions
are expected for the first portion of the long-term period, with
benign weather from Wednesday night onward.

A stationary surface front is forecast to be draped across South
TX Sunday evening, though most likely to the north of the CWA, per
latest WPC prog charts. GFS/ECM/NAM pretty consistent with pooling
respectable column moisture values (PW`s of 1.7-1.8", especially
west and north) ahead of this boundary. Meanwhile, aloft, a
modest 500-mb shortwave, currently analyzed just off central Baja,
will be moving through central TX, providing some added lift in
addition to the boundary. Upshot is generally scattered coverage
precip, especially after midnight. ECM in particular showing ~1"
totals near Zapata, so this may bear watching. SPC also painting
the far northern tier of the CWA with Marginal Risk for severe
weather, as that area will lie closer to the southern jet stream
over central TX with associated deep-layer shear possibly enough
for some discrete supercells. These could initiate either near the
front (if it sags far enough south), or, more likely, over the
higher terrain upstream in Mexico with the passage of the mid-
level shortwave.

Models indicating some drying aloft Monday into Monday night in
the wake of the shortwave, so PoP`s will be down a bit, to the
widely scattered/scattered category. Will have to rely on diurnal
heating or sea-breeze boundary (which will be possible in the
modest SE background flow) to get things going. Temps continue to
run a few degrees above normal.

On Tuesday, pre-frontal moisture begins to pool over the area
again as the next, stronger cold front moves into central TX. GFS
indicates PW values as high as 2" by evening. GFS also about 6h
faster vs. ECM in bringing front through the CWA from the north
during the Tuesday evening hours. Strong omega/deep RH, possibly
aided by some weak short-waves, indicated on time-height sections
leads to some 1-2" rainfall totals over the mid-Lower Valley in
the GFS guidance, but ECM not nearly as bullish, as it develops
the bulk of the precip south of the International Border.
Situation bears watching but obviously some uncertainty, plus
soils can handle a fair amount of rain with FFG values in the 4-6"

Fairly rapid drying behind the front for Wednesday, with NW flow
aloft and drier air working in at the surface. Current temp
forecast is for low-mid 80s F, which combined with afternoon
dewpoints in the 50s-mid 60s and northerly breezes, will feel
quite pleasant. Short-wave ridge moves overhead on Thursday, with
onshore flow redeveloping and temps recovering to just below
normal. Dry conditions continue into Friday and Friday night with
mid-level subtropical ridge building northward over the area. SE
winds begin to crank up again with surface high parked over the
northern Gulf.

MARINE (Now through Sunday):  The pressure gradient across the
Western Gulf of Mexico is expected to slacken off over the next 36
hours as an upper low pressure trough over the Central Plains moves
northeast and its associated cold front approaches South Central
Texas. Moderate Southeast winds and seas persist today and begin
weakening tonight. The front is expected to stall north of the
Coastal waters and Deep South Texas Sunday with winds turning more
easterly and weakening further. Seas to respond and lower below 5
feet. Will maintain wording for small craft exercise caution today
with conditions improving late this afternoon and this evening.

Sunday Night through Wednesday Night: Mainly moderate E-SE flow
persists through Tuesday night, as frontal boundaries across Texas
keep the pressure gradient relatively slack. Seas also stay at
moderate levels, around 4 ft., during this period. Stronger
northerly winds behind a frontal passage early Wednesday morning
likely to lead to a period of Small Craft Exercise Caution
conditions, though at this time, it appears that seas will not
build above 5 ft. Chances for showers and thunderstorms will also
exist through much of the period, with localized downpours and
strong winds possible, especially Sunday night and Tuesday night.




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