Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 201911
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
211 PM CDT Sun Aug 20 2017

.SHORT TERM (Tonight through Monday Night): Few changes to the big
picture of the previous forecast: Rain free overnight, followed by
the potential for just enough moisture arriving on the front end of
the east-central Gulf upper level gyre Monday to kick off isolated
sea breeze showers and perhaps thunderstorm.  Typically, this
forecast could be generalized - but with the Eclipse Across America
the big story Monday, even in an area like the Valley where only
partiality (50 percent or so) is expected, there is a keen interest
in cloud cover and precipitation, and we`ll address this below. With
the loss of heating Monday night, mainly clear skies and light to
calm winds return, though there could be a few late night lower
clouds returning in preferred area.  For daybreak temperatures,
nudged down both Monday and Tuesday morning given the near calm
winds, mostly clear skies, and lengthening night (just a month from
the equinox).

For the big eclipse day: The trick is predicting when the sea breeze
will push in, and if/where it can activate showers and perhaps a
storm that would block out the sun during and toward the peak or
partiality (a crescent sun),which is around 115 PM here. Unlike
today, low to mid level flow is easterly and a touch lighter.
Today`s sea breeze pushed roughly along US 77/IH 69E fairly quickly,
between noon and 1 PM. Tomorrow could be an hour slower, and with
the slightly higher moisture content especially between 850 and 600
mb the potential for 40-50 percent cloud cover and a few towers is
greater. At this point, due to remaining uncertainty held sky cover
at 3 hour intervals.  As mentioned yesterday, and a bit more
confidently today, the best viewing at peak appears to be in two
areas:  South Padre Island and perhaps Port Isabel/Bayview, *if* the
sea breeze can push far enough west, and the upper
Valley/Ranchlands. For the mid Valley (US 281/IH 69C area), not
expecting the rain to reach which is good news, but enough beefiness
of the clouds should make viewing off an on as the crescent sun goes
in and out of the cumulus.  From Brownsville-Harlingen-Raymondville,
the key will be if any showers/storms develop between 1130 and 1 and
leave mid/high level clouds behind. If this happens, mid/high level
`debris` will spread west and southwest, and could dim or blot out
the sun even where it doesn`t rain, such as the Weslaco/Donna area.

Other than that, minimal "dimming" shouldn`t have much impact on
temperatures here, but did slow the rise just a bit between 11 and 1
PM. Maximum temperatures should still reach triple digits along/west
of US 281/IH 69C.

.LONG TERM (Tuesday through Sunday): GFS/ECMWF are in general
agreement Tue/Wed that a mid/upper level inverted trough will be
over the northwest Gulf with northerly flow aloft over deep south
TX. Sufficient moisture over the eastern-third of the Rio Grande
Valley will lead to isolated showers, and possibly a thunderstorm,
along the sea breeze these two afternoon.

For Tuesday night/Wednesday, both models show the remnants of
what was Tropical Storm Harvey re-emerging on the western side of
the Yucutan peninsula and re-energizing. In the Wednesday through
Friday timeframe, the ECMWF re-energizes Harvey`s remnants more so
that the GFS, and takes a center of circulation westward across the
south central Bay of Campeche to the central east coast of Mexico,
while the GFS grinds a re-energized, but weaker circulation than the
ECMWF, westward along the extreme southern Bay of Campeche. The
bottom-line for the forecast area looks to be a significant increase
in moisture from the southeast Thursday night through Sunday with
precipitable water values rising to above 2.3 inches. As
expected, the more northerly track of Harvey`s re- energized
remnants leads to higher chances of precipitation over the
forecast area, while chances are somewhat lower per the GFS`s
farther south track. Chances of precipition throughout the
extended forecast appear on track and will not make any
significant changes from the previous forecast cycle. Temperature-
wise, daytime highs look to remain at least somewhat above normal
each day, except Friday/Saturday when they may actually run
closer to normal due to increased moisture/cloud cover.

&&

.MARINE (Tonight through Monday Night): Light winds and slight seas
dominate the afternoon, a nice break from the active southeast wind
wave pattern that dominated last week and held on a little bit (2 to
3 feet) for part of Saturday. This trend wil continue through Monday
night with the surface ridge parked across the entire Gulf; in fact,
afternoon winds will tend to turn easterly, even across the Gulf.
The only concern will be isolated late night through afternoon
showers and perhaps a thunderstorm over the Gulf waters, tending to
favor the outer waters after the sea breeze gets active.  Not
expecting enough to alter boating or fishing plans, but something to
keep an eye on.  For Laguna Madre, the usual afternoon sea breeze
pickup but nothing more than 15 knots at times; early mornings
should be nearly calm and smooth. Isolated Sea breeze showers and
perhaps a storm could impact part of the Bay and the northern
portion of Laguna Madre by the land cut mainly between 11-noon and 1
or 2 PM Monday.

Tuesday through Thursday: Surface high pressure over the northwest
Gulf of Mexico will maintain modest winds/seas across the waters
through the forecast period. Sufficient moisture will remain across
the waters for isolated showers and thunderstorms.

&&

.CLIMATE...Interesting statistic reached on Saturday: McAllen/Miller
International Airport reached 70 days of 100 or higher in 2017,
placing 2017 as #4 all-time behind only 2016 (90 days) and 2009 and
1998 (tied at 78).  Today should make 71, and more heat this week
may push the number close to 75.  Late week/weekend potential for
rain may provide a break, and how the pattern evolves thereafter
will determine if 78 days becomes third-ranked in August, September,
or not at all.  Stay tuned!

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&

$$

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