261
FXUS64 KBRO 172028
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
328 PM CDT Mon Jun 17 2024

...Rain Is Coming, but How Much and Where the Most Falls Remains
Uncertain...

...New SHORT TERM, LONG TERM, MARINE, HYDROLOGY...

.SHORT TERM...
(Tonight through Tuesday night)
Issued at 220 PM CDT Mon Jun 17 2024

Key Messages:

* Coastal Flood Watch in effect for the barrier island and Coastal
  Kenedy, Willacy, and Cameron Counties late Tuesday night into
  Thursday

* High Risk for rip currents begins this evening

* Rounds of showers and thunderstorms are expected beginning tonight
  and persisting through Tuesday

* Heavy rainfall could result in flash and areal flooding across
  parts of Deep South Texas and the Rio Grande Valley

* Tropical cyclone developing over the Southwestern portions of the
  Gulf of Mexico will bring much-needed, beneficial rains to the
  region

The main focus during the short term forecast period continues to
center around a developing tropical cyclone over the Southwestern
Gulf of Mexico. Latest forecast from the National Hurricane Center
(NHC) has it that a Tropical Depression or Tropical Storm could
develop by midweek over the bay of Campeche. While the core of this
tropical entity is expected to track towards the western Gulf Coast
and into Mexico, a flux of deep tropical moisture courtesy of the
Central Atlantic Gyre (CAG), will envelope the region in the form of
waves/rounds of showers and thunderstorms.

Precipitable water (PWAT) values are forecasted to increase from
about 2 inches to 2.5 inches by Tuesday night. To further put this
into perspective, that`s an increase from +1 to + 2 STDEVs above
normal to +3 to +4 STDEVs above normal. These as astronomically high
PWAT values and goes to show how much water vapor content will be in
the atmosphere. Additionally, the Integrated Water Vapor Transport
(IVT) values are expected to be +1 to +2 STDEVs above normal. The
influx of moisture in the days ahead will almost serve or act like
an Atmospheric River (AR) from the Gulf of Mexico into
South/Southeast Texas.

Given the tropical environment in place, rainfall rates are expected
to very efficient, that capable of addressing the D0 (Abnormally
Dry) to D1 (Moderate Drought) situation over the region. Year to
date precipitation deficits will likely flip over to a surplus
during/after this event.

That said, the concern for flooding and flash flooding exists.
Despite the high RFC flash flood guidance (FFGs) over the area,
given the efficient rainfall rates on top of this area being a
floodplain, raises the prospects for flooding and/or flash flooding
across the region. The other concern are the marine hazards. Large
swells amid gusty winds over the Gulf of Mexico will track towards
the South/Southeast Texas Coast beginning as early as tonight.
Coastal flooding could become a concern as early as Tuesday night.
As a result and after collaboration with our neighboring offices,
we`ve decided to issue a High Risk for rip currents beginning this
evening and a Coast Flood Watch beginning Tuesday night.

For tonight, expect another warm and muggy night with another round
of isolated showers and storms. Overnight lows are expected to be in
the mid 70s to near 80F degrees.

During the day on Tuesday, max sfc differential heating coupled with
continued influx of deep tropical moisture could help to ignite some
diurnal showers and thunderstorms across the region. Currently have
30-50% chance PoPs over the western half of our CWFA and categorical
PoPs (55-80%) over the eastern half of the CWFA. Coverage is
expected to be isolated to scattered in nature. High temperatures
will be slightly cooler than today thanks to increased cloud
coverage and the possibility of showers and thunderstorms. Daytime
highs are expected to be in the lower 90s east to upper 90s west.

Tuesday night will feature another warm and muggy night with the
chance again for another round of showers and thunderstorms.
Overnight lows will be similar to tonight with values in the mid 70s
to near 80F degrees.

&&

.LONG TERM...
(Wednesday through next Monday)
Issued at 220 PM CDT Mon Jun 17 2024

Key Messages:
* Best opportunity for widespread rain is Wednesday, with local
  persistent downpours producing minor poor-drainage flooding

* Coastal flooding will be sufficient to reach into the dunes on
  the barrier island around high tide Wednesday and Thursday, and
  could be sufficient to cause minor damage to unprotected
  properties

* Small Craft should remain in port through at least Thursday due
  to rough and building seas and increasing winds

The focus remains on the expected development of the area of low
pressure which will likely be some type of hybrid system centered
east of southern Tamaulipas (La Pesca to Tampico) at the start of
the period, but with primary wind and coastal impacts well north
of the center, including the lower Valley. Models continue to be
in different camps, particularly the deterministic models,
regarding rainfall - with the GFS/Canadian showing a tight
gradient between potential widespread flooding rainfall across the
Coastal Bend northwest toward San Antonio, while the ECMWF is more
of a broad brush of increasing rain and QPF moving from east to
west...though still favoring higher amounts along/inland from thd
middle and upper Texas coast.

Timing of the rain early in the long term remains somewhat
uncertain as well, though all models seem a little delayed with
the potential "bigger" rains that could cause local urban/poor
drainage flooding, now from late morning through at least late
afternoon. Regardless, the theme of Wednesday being the "big" rain
day has not changed, and we`re still expecting a widespread totals
of 3 to 4 inches across the RGV, with up to 6 inches along the
northern tier (Hebbronville to Sarita) through midday Thursday.
The key to any flooding will be rainfall rates and rainfall
duration...and any slow-moving or persistent tropical banding
could easily drop 2 inches per hour for 3 hours and get the floods
going with up to 2 or 3 feet of water depth in poorly draining
locations.

For now, we`re holding off a bit on a Flood Watch given how dry
things are and the uncertainty of timing and eventual location of
heavier/flooding rainfall vs. welcome rain. We may reconsider this
evening/overnight tonight or Tuesday.

The progressive movement of the associated upper low into north
central Mexico Thursday should allow rains to taper off from east
to west during the afternoon...while this will be the period
(Wednesday night through Thursday/Thursday night) that headwaters
of the Rio Grande flowing into Falcon reservoir get their long-
awaited rains...which will ultimately take their levels "off the
floor" by the weekend after so many weeks at new historical lows.

For Friday, backside easterly flow combined with residual moisture
and region in between waves will keep things unsettled...and while
national blend of model rain chances seem a bit overcooked, the
idea of scattered-numerous showers and a few thunderstorms is
reasonable with notably lower area-wide rain totals. That said,
instability could kick off torrential rains which, if falling on
newly saturated ground, would cause additional hyper-local
flooding of poor drainage locations.

Saturday through Monday remain highly uncertain, but the bottom
line is mainly daytime rain chances continue, even as temperatures
climb back to just below average (averages range from 93 to 98
then). With plenty of residual moisture in place, any heating
could cause more torrential rainfall and local flooding just about
anywhere for short bursts of time.

Finally...we`ve issued a Coastal Flood Watch to begin at 1 AM
Wednesday and continue (for now) through 1 PM Thursday. This is
based on the expected rapid and persistent build up of onshore
wave energy via higher seas and swells, and model depiction of
rapid rises of water levels that could reach near 3 feet above
ground level near the shore line. This would ensure water into the
dunes early Wednesday and Thursday morning at high tide, and if
wave energy is strong enough, minor flood damage.

&&

.AVIATION...
(18Z TAFS)
Issued at 1227 PM CDT Mon Jun 17 2024

Through 18z Tuesday....VFR conditions are taking place at all of the
terminals under a FEW-SCT deck of cumulus to stratocumulus clouds
with bases between 3,000-6,000 feet AGL and unrestricted
visibilities. VFR conditions, by and large, are expected to
prevail through the 18z TAF cycle.

The main concern during the 18z period will be the risk for showers
and thunderstorms developing (mainly during the afternoon and
overnight hours). Forecast models continue to suggest the
potential for showers and thunderstorms developing later this
evening/tonight and then again during the day on Tuesday. These
showers and thunderstorms are associated with a plume of deep
tropical moisture advecting from a developing tropical cyclone
over the Southwestern Gulf of Mexico. Any storms that do move over
a TAF site will have the capabilities of briefly reducing cigs
and/or visibilities down to IFR levels. Showers and storms will be
isolated through tonight becoming more isolated to scattered
during the day on Tuesday. Have Prob30 groups in the TAFs to
reflect this potential. Otherwise, expect for VFR conditions to
prevail.

Winds will be out of the east 7-15 kts through this afternoon into
this evening with occasional gusts between 20-25 kts. Winds will
subside tonight out of the east at 5-10 kts. During the day on
Tuesday, winds are expected to shift out of the northeast at 5-10
kts.

&&

.MARINE...
Issued at 220 PM CDT Mon Jun 17 2024

Tonight through Tuesday Night: Adverse to hazardous marine
conditions are expected tonight through Tuesday night. An enhanced
pressure gradient with the developing tropical cyclone over the
Southwestern Gulf, will result in increased winds and swells and
thus elevated seas. That will trigger a high risk for rip currents
beginning this evening. Additionally, coastal flood potential is
increasing. As a result, a Coastal Flood Watch has been issued for
Tuesday night. Small Craft Advisory conditions are expected
tonight through Tuesday night.

Wednesday through Saturday Night...All eyes are on the tropics
Wednesday into Thursday for potential warnings - either tropical
or even gale - with the tightening gradient well north of any
center of circulation/low pressure moving into central/southern
Tamaulipas. Gradient gets a boost from the "heat ridge" over the
eastern/southeastern U.S. and it covers the entire Texas Gulf
waters. Pretty good chance that from Wednesday through Thursday
we`ll see near or just above gale-force gusts persisting even
after any "landfall" of the system to our south...and there`s some
likelihood that 20 knot and gusty winds could sustain themselves
through Saturday. Regardless of the winds, seas will build to
10-12 feet Wednesday and continue into Thursday before gradually
subsiding but still remain above 7 feet through Friday...and
perhaps longer if some incoming models are correct showing
additional low pressure waves developing/ejecting into the Bay of
Campeche.

Bottom line? Small craft should remain in port through at least
Thursday.


&&

.HYDROLOGY...
Issued at 220 PM CDT Mon Jun 17 2024

As mentioned above, reservoirs that feed the Lower Rio Grande
remain at a combined historical low since both were constituted
(created and filled) back in 1954 (Falcon) and 1971 (Amistad).
Values today were at 9.27 percent at Falcon, and 18.96 percent
(total share) at Amistad. How much rainfall reaches Amistad`s
inflow region remains in question, but current forecasts suggest 5
to 10 inches, locally 15 inches, to fall into the headwaters that
feed the Rio Grande into Falcon by the end of the week.

This type of efficient rain is sure to create sufficient rises
"off the floor" of the historic record lows...though combined
values and even those at Falcon will still be very low compared
with long-term averages. For more information, go to ibwc.gov or
waterdata.ibwc.gov.



&&

.PRELIMINARY POINT TEMPS/POPS...
BROWNSVILLE             81  91  79  90 /  40  80  90 100
HARLINGEN               77  92  76  88 /  40  70  80 100
MCALLEN                 80  94  79  87 /  20  60  70 100
RIO GRANDE CITY         78  95  78  85 /   0  40  50  90
SOUTH PADRE ISLAND      83  87  81  86 /  50  80  90 100
BAYVIEW/PORT ISABEL     80  89  78  88 /  50  80  90 100

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...Coastal Flood Watch from late Tuesday night through Thursday
     afternoon for TXZ351-354-355-451-454-455.

     High Rip Current Risk from 7 PM CDT this evening through Tuesday
     evening for TXZ451-454-455.

GM...None.
&&

$$

SHORT TERM...23-Evbuoma
LONG TERM....52-Goldsmith
HYDROLOGY...52-Goldsmith