Flash Flooding and Flooding in Texas, September 10, 2024

Area Forecast Discussion
National Weather Service Brownsville TX
629 AM CDT Tue Sep 10 2024

...New AVIATION...

.SHORT TERM...
(Today through Wednesday)
Issued at 516 AM CDT Tue Sep 10 2024

Key Messages:

* Morning commute to be impacted by moderate to heavy rainfall
  associated with Tropical Storm Francine

* Moderate to heavy rain with embedded thunder will continue
  through this morning before tapering off and coming to an end
  from west to east later this afternoon; some showers could
  linger into this evening (especially along the island and near
  the coast)

* Heaviest rainfall will be concentrated over the southeastern
  portions of the area (i.e. Cameron and Willacy Counties)

* Hazardous marine conditions to continue through Wednesday night

Multiple hazards continue for portions of Deep South Texas and the
Rio Grande Valley through the short term forecast period in
conjunction with Tropical Storm Francine. The latest intermediate
advisory from the National Hurricane Center (NHC) stated that
Francine was located about 125 miles south-southeast from the mouth
of the Rio Grande and was moving north-northwest at 5 mph. Francine
has a central minimum pressure of 992 millibars with max sustained
winds of 65 mph. Francine is expected to continue moving north-
northwest through this morning, before turning to the northeast this
afternoon/evening.

Several hi-res CAM guidances suggest that another round of moderate
to heavy rain showers (with some embedded thunder) are expected to
impact the eastern half of our local forecast area overnight tonight
into the morning hours today. The greatest coverage of this moderate
to heavy rainfall will be located along our immediate coastal
counties. This activity is expected to persist through the morning
hours before coming to an end from west to east this afternoon. As
of this update, most of the rainfall has been confined to the
southeastern portions of our area (i.e. Cameron County). In fact,
there has been a rather tight rainfall gradient over Cameron
County where anywhere from 0.5 inches has fallen in the Harlingen
area to around 4 inches in Brownsville. Just to the south of
Brownsville, into northeastern Mexico, anywhere from 5-7 inches of
rain has fallen thus far with locally higher amounts. Another
1 to 2 inches with locally higher amounts is expected through
this morning before things wind down this afternoon. Again, most
of the QPF is expected to fall over/along our immediate coastal
counties (Cameron, Willacy, and Kenedy).

With EL-LCL mean cloud layer winds around 25 knots, these showers
and embedded storms are moving. However, given the high
atmospheric moisture content (deep tropical moisture in place),
localized flooding is possible, especially where showers/storms
train. The Weather Prediction Center (WPC) has maintained much of
Deep South Texas and the Rio Grande Valley under a Marginal Risk
for excessive rainfall in it's Excessive Rainfall Outlook (ERO).
Over the past 24 hours, there has been a downtrend in QPF amounts.
The one concern that we continue to investigate is the low to mid
level dry air entrainment into the south-southeastern portions of
TS Francine as well as some wind shear that could stymie it's
development. A Flood Watch remains in effect for Cameron and
Willacy Counties. We canceled the Flood Advisory for Kenedy
County, as little to no rain has fallen there up to this point and
the soils there will be able to handle the projected rainfall to
come.

In addition to the flooding potential from excessive rainfall,
marine/coastal hazards remain in play. Winds have trended down some
slightly over the past 24 hours. Nonetheless, with an
enhanced/strong enough pressure gradient in place, it's expected to
remain breezy through the day today with winds out of the north-
northeast 10-20 mph, gusting up to 30-35 mph inland. Further
offshore, stronger winds are expected with gusts ranging between 40-
60 mph. Therefore, Tropical Storm Watches and Warnings remain in
effect as tropical storm conditions are possible through today over
the Gulf waters.

Winds are expected to continue mainly out of the northeast this
morning before becoming more northerly this afternoon. Given the
situation, high seas and surfs remain in play. Therefore, a High
Risk for Rip Currents and High Surf Advisory remains in effect
through Wednesday evening. A Coastal Flood Advisory also remains
in effect through this afternoon/evening.

The flood threat will improve this afternoon from west to east as
Francine continues to track northward. However, the marine/coastal
hazards will linger. Another concern is the fact that Francine is
moving rather slow over the Gulf waters.

Temperatures are expected to continue to run cooler than normal for
early to mid September standards thanks to the extensive cloud cover
and rainfall. High temperatures are progged to reach into the 80s
across Deep South Texas with overnight lows in the low to mid 70s
through the short term period.

&&

.LONG TERM...
(Wednesday night through Monday)
Issued at 516 AM CDT Tue Sep 10 2024

The long-term forecast period will feature an uptick in precipitation
(PoP) chances and the potential return to an unsettled weather
pattern. We start of the long-term on Thursday where in the wake
of Tropical Cyclone Francine, dry and tranquil weather conditions
will continue. A weak cold front to the south helping to drive
some cooler air from the north and a ~1012 mb sfc high pressure
system, will result in enough subsidence to yield a dry and
tranquil weather pattern. This trend will continue into Friday.
Temperatures will also be warmer, but still near normal levels.
High temperatures are forecast to be in the 90s across Deep South
Texas and the Rio Grande Valley on both Thursday and Friday.

Saturday through the early parts of next week, we have to monitor
precipitation trends. Global forecast models and ensembles are
suggesting the potential return to and unsettled weather pattern
with day-to-day diurnal chances for showers and storms as
instability values increase from the south. Increased atmospheric
moisture content coupled with a seabreeze could help to ignite
these showers and storms. Given the chance for rainfall during
this time period, temperatures are expected to run near normal
levels with highs in the upper 80s to lower 90s, and overnight
lows in the mid 70s to near 80F.

&&

.AVIATION...
(12Z TAFS)
Issued at 628 AM CDT Tue Sep 10 2024

Through 12z Wednesday....

Key Aviation IDSS Messages for the 12z TAF cycle:

* MVFR-IFR conditions to persist through this morning in association
  with this round of moderate to heavy rainfall; worst conditions
  over KBRO and KHRL

* Rainfall to come to an end late morning into the afternoon hours
  from west to east with flying conditions improving back to VFR-MVFR

Moderate to heavy rainfall over the area has resulted in IFR
conditions over KBRO and KHRL as of this update. Expect for this
activity to continue through the morning hours. Later this morning
and especially this afternoon this activity should come to and end
from west to east. Flying conditions will improve back to VFR-
MVFR once the rain comes to an end.

North-northeast winds 10-20 kts with occasional gusts up to 25
kts will persist through the 06z TAF period with wind gusts
prospects diminishing later in the period.

&&

.MARINE...
Issued at 516 AM CDT Tue Sep 10 2024

Today through Wednesday night....Hazardous marine and coastal
conditions in connection to Tropical Cyclone Francine are
expected to continue through this period. An enhanced pressure
gradient amid Francine will result in moderate to strong winds
over the Gulf Waters. This will lead to high/rough seas including
the high risk for rip currents. Coastal flooding will remain a
concern through this evening.

Thursday through Tuesday....Favorable marine conditions return
during this period with low to moderate winds and low seas.

&&

.PRELIMINARY POINT TEMPS/POPS...
BROWNSVILLE             82  75  90  76 /  90  50  20  10
HARLINGEN               82  73  91  74 /  90  40  20  10
MCALLEN                 86  76  97  77 /  70  30  10  10
RIO GRANDE CITY         89  73  96  74 /  40  20  10  10
SOUTH PADRE ISLAND      84  78  86  80 /  90  70  30  20
BAYVIEW/PORT ISABEL     83  74  88  75 /  90  60  20  10

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...Tropical Storm Warning for TXZ354-355-454-455.

     Tropical Storm Watch for TXZ254-255-351-451.

     Flood Watch until 3 PM CDT this afternoon for TXZ254-255-354-355-
     454-455.

     Coastal Flood Advisory until 7 PM CDT this evening for TXZ451-
     454-455.

     High Rip Current Risk through Wednesday evening for TXZ451-454-
     455.

     High Surf Advisory until 7 PM CDT Wednesday for TXZ451-454-455.

GM...Tropical Storm Warning for GMZ130-132-150-155-170-175.

     Tropical Storm Watch for GMZ135.

&&

SHORT TERM...23-Evbuoma
LONG TERM....23-Evbuoma
AVIATION...23-Evbuoma