Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 171653 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1153 AM CDT Sun Sep 17 2017

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

.AVIATION...Satellite images and surface observations indicate low
to mid level clouds across the CWA late this morning. Ceilings
were near 3500ft at KHRL to near 3900ft at KPIL. Expect VFR
conditions to prevail across the Rio Grande valley and northern
ranchlands this afternoon into this evening as the 500mb ridge
across south Texas and the western Gulf of Mexico provides
subsidence across the area. Isolated showers may develop along
the seabreeze front across the coastal sections of the CWA this

.PREV DISCUSSION... /issued 623 AM CDT Sun Sep 17 2017/
DISCUSSION...Updated for latest aviation discussion below.

AVIATION...An axis of MVFR visibilities in light fog/mist has
developed from KHBV down toward KHRL early this morning. Not
expecting this to impact KBRO or KMFE, per GOES-16 fog product
trends. HRRR suggests that this (likely shallow) mist will
dissipate by 14Z, or shortly after sunrise.

Mostly favorable flying conditions expected for the daytime hours
today. Light onshore flow will persist with a sea-breeze expected
to develop during the afternoon. Could see some brief MVFR CIG`s
late morning, but duration should be short prior to the ceiling
mixing to VFR. Very isolated showers along the sea-breeze front
are possible, but areal coverage too low to mention in TAF.

With low-level moisture on the increase over the next 24 hours,
and light winds expected again overnight Sunday into Monday, will
have to watch for MVFR conditions developing again. BUFKIT
soundings from RAP and NAM models, as well as SREF probabilities,
suggest that this would take the form of lower ceilings instead of

PREV DISCUSSION... /issued 354 AM CDT Sun Sep 17 2017/

SHORT TERM (Now through Monday): In the near term, went ahead
and added patchy fog wording to portions of the northern
ranchlands (mainly Jim Hogg and Brooks counties) through 14Z with
calm winds in place and visibility already down to 7SM at KHBV and
KBKS. Not expecting this to work down into the Rio Grande Valley
at this time, though some shallow field fog is possible.

Column moisture will be on a gradual upswing today, as the 500-mb
high center over the western Gulf wobbles eastward, opening up
light, southerly mid-level flow.  The gradual nature of the moisture
return should keep land areas precip-free today, though. Otherwise,
another seasonally hot and humid day is in store.  See no reason why
McAllen wouldn`t approach 100F again, as it did yesterday. Afternoon
heat indices will be in the 103-108F along the Rio Grande.

For Sunday night, lower to mid-level moisture continues to increase,
with PW values of 2"+ overspreading the entire CWA by daybreak
Monday.  Models indicating scattered coverage of showers farther
offshore with isolated coastal activity.  GFS and NAM differ a bit
on the depth of the moisture return, with NAM`s deeper humidity
leading to somewhat higher PoP`s (though still not particularly
impressive with the upper high`s influence still being felt). Pretty
good signal in all of the available model guidance for sea-breeze
showers and thunderstorms to develop on Monday around mid-day into
the afternoon.  Coverage to remain mostly isolated except perhaps
scattered in the corridor roughly between Highways 77 and 281 where
moisture convergence looks to be enhanced.  Temps will be very
similar to Sunday`s.

LONG TERM (Monday Night through Saturday): Weak 500mb ridging will
remain in place over the western Gulf of Mexico and south Texas
through much of the period. Low level moisture with precipitable
waters waffling around two inches will support a slight chance of
showers and thunderstorms across portions of deep south Texas
through the period. The combination of mid-level ridging overhead
and a more west to northwesterly flow aloft should support marine
showers in the morning and afternoon seabreeze convection, mainly
across the coastal sections, each day. Near to above normal
temperatures will continue across the area through the forecast
period. High temperatures each day will range from the upper 80s
at the beaches, low to mid 90s near the coast and the upper 90s to
around 100 inland with a mix of sun and clouds. Heat index values
expected between 100 to 105 degrees each afternoon. Overnight low
temperatures will generally be in the 70s under partly to mostly
cloudy skies.

MARINE (Now through Monday): Light to moderate onshore flow to
persist through the short-term. Buoys in the north central Gulf
currently reporting small ESE swell of 1-2 ft. Wind waves will not
be able to add much to this total. A few downpours are possible
on Sunday night into Monday morning, mainly for the Gulf waters
from 20- 60nm out.

Monday Night through Thursday...High pressure across the Gulf
will maintain light to moderate onshore winds and low to moderate
seas through the period. A few showers may develop along the coast
from time to time.



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