Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 300542 AAB

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1242 AM CDT Tue May 30 2017

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


.AVIATION...Light and variable winds with some passing high clouds
prevail across deep south Texas early this morning. MVFR conditions
may develop for a few hours before dawn due to light winds,
relatively clear skies and moist soils from recent heavy rains to
allow patchy fog to develop across portions of the Rio Grande
valley. Otherwise, VFR conditions expected to prevail for the next
24 hours. Light northeast winds later this morning will veer to
the east this afternoon.


.PREV DISCUSSION... /issued 652 PM CDT Mon May 29 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Satellite images and surface observations indicate
low to mid level clouds across portions of the lower Texas coast
and the northeast Mexican coast this evening as well as across
portions of the CWA this evening. Ceilings were near 1300ft at
KT65 to near 4500ft at KHRL. Expect MVFR conditions to develop
late tonight into early Tues morning as light winds and relatively
clear skies allow patchy fog to develop across portions of the Rio
Grande valley.

PREV DISCUSSION... /issued 251 PM CDT Mon May 29 2017/
SHORT TERM (Tonight through Tuesday Night):Last night`s MCS did a
number on the state of our atmosphere significantly lowering our
instability and dropping dew points and turning our winds around
to the north-northeast. High clouds just starting to thin out with
some lower level cumulus developing with the residual low level
moisture. Westerly flow aloft and weak impulses still within the
subtropical jet stream and the stationary front over South Central
Texas remain in place so can not rule out an isolated shower or
thunderstorms ovenight. Latest satellite loops do show convection
beginning to redevelop over the Sierra Madre well north and west
of our CWA as the instability recovers in those areas. Locally,
with instability remaining low and a low level inversion in place
will keep our rain chances low overnight. Sufficient thinning of
the high overcast this evening may combine with the wet soil to
produce some patchy fog overnight...that is if convection does not
develop. Temperatures to remain on the cooler side in the lower
70s instead of the upper 70s that we had much of last week.

Tuesday/Tuesday night...Deep South Texas will continue to under the
influence of the weak westerlies flow aloft and a continued moist
atmosphere. The frontal boundary north of the region remains intact,
albeit in a weaken state. Weak impulses within the westerly flow and
daytime heating be sufficient to increase the instability allowing
for an isolated shower or storm. Models continue to hold off on any
significant rainfall chances until Tuesday night and Wednesday when a
stronger disturbance approaches from the west. Model guidance show a
slight recovery on high temperatures Tuesday and the latest forecast
reflects this trend.

LONG TERM (Wednesday through Monday): For the time being...the
very beginning of the long term looks to be the most active of the
six-day period. Synoptically...a broad mid/upper level trough over
northwest Mexico will be providing a diffluent pattern aloft
across southern TX while a stationary front is forecast to be
draped across south central TX just to the north of the forecast
area. Due to the synoptic setup, along with precipitable water
values approaching two inches, the Weather Prediction Center has
placed all of the Rio Grande Valley in a marginal risk for
excessive rainfall for Wed as widespread showers/thunderstorms
with heavy rainfall are expected. The Storm Prediction Center also
has much of the forecast area in a marginal risk for severe
weather at the same time. It seems the GFS medium-range model may
be having trouble resolving precipitation for Wed as it was off
to a poor start with actual conditions this morning. The ECMWF
looks as if it is handling the setup better and shows a greater
concentration of precip/heavy rain over the northern portion of
the forecast area Wed afternoon/night, which seems reasonable
given its proximity to the boundary to our north. High
temperatures on Wed should be near normal, if not well below
normal, in response to anticipated cloud cover/precipitation.

For Thu/Fri, the Rio Grande Valley looks to be impacted by short-
wave ridging that will develop just to the west of the region.
Though moisture will still be fairly abundant across the area
during this time, the weak ridge combined with general
southeasterly onshore flow, most likely will enable a sea breeze
pattern to develop that will lead to isolated/widely scattered
showers/thunderstorms mainly over the eastern half of the forecast
area. However, this doesn`t mean chances of precip will be zero
over the western half as residual moisture will combine with
daytime heating for the possibility of isolated
showers/thunderstorms there.

Though not necessarily completely precipitation-free, Sat/Sun look
to be down days precipitation-wise as strong upper ridging
develops over northwest Mexico and the aforementioned trough
ejects to the northeast into the central portion of the country.
The bottom-line for these two days will be low end chances of
precipitation with high temperatures rising above seasonal
averages once again.

Monday, the last day of the long term, is definitely a wild card.
Though the GFS and ECMWF generally agree on a fairly strong ridge
remaining over northwest Mexico, the GFS pushes a strong front
(relatively speaking for the first of June) through the Rio
Grande Valley while the ECMWF is not showing anything like this at
that time. Given the residual moisture that will be in place
across the region, the low end chances of precipitation currently
called for on Monday seem fairly reasonable at this time and will
not deviate from this.

MARINE (Tonight through Tuesday night): At 2 pm Buoy 42020
reported southeast winds around 6 knots and seas of 6.6 feet with
a period of 8 seconds. Winds and seas continue to lower from the
strong east outflow from the early morning thunderstorm complex.
The interrupted southeast pressure gradient is slowly recovering
and should return later tonight or early Tuesday. Light to
moderate southeast winds and slight to moderate sea should resume
and continue into Tuesday night. isolated showers or a
thunderstorm is still possible at any time with better chances
Tuesday night.

Wednesday through Saturday: Modest winds/seas to prevail Wed/Thu
before winds/seas pick up some Thu night and rise into the Small
Craft Should Exercise Caution level. Winds/seas to become
modest/fall below headline criteria once again on Fri then
continue into Sat.




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