592
FXCA62 TJSJ 280736
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
336 AM AST Tue May 28 2024

.SYNOPSIS...

Marginally unstable weather conditions are expected to persist
across the region through the next several days. A southeasterly
wind flow will continue to bring plenty of tropical moisture from
the warmer Caribbean waters, enhancing the potential for strong
showers and thunderstorms over the interior and northwestern
Puerto Rico each afternoon.A relatively drier airmass is expected
to filter into the region by the end of the workweek, promoting
stable weather conditions and warm to hot temperatures.

&&

.SHORT TERM...Today through Thursday...

Overnight observations from satellite and radar have indicated
persistent variable to mostly cloudy skies across the region.
Scattered showers with isolated thunderstorms and areas of
persistent light rains were observed across the local waters, with
some moving over the US Virgin Islands, Vieques, Culebra, and Eastern
Puerto Rico. So far, the highest rainfall totals of up to three
quarters of an inch were observed over western Saint Thomas.
Overnight low temperatures ranged from the upper 60s across higher
elevations to the lower 80s across eastern and southern Puerto Rico
and the local islands. Winds were generally light and variable in
Puerto Rico, influenced by land breezes, and from the east-northeast
across the US Virgin Islands.

As the tropical wave moves away from the region, moisture levels
will gradually decrease over the next few days. However, persistent
moisture will keep precipitable water (PWAT) values well above the
climatological thresholds, exceeding 2.0 inches through at least
late Wednesday night. By Thursday, PWAT values are expected to drop
to more typical levels, around 1.8 inches. Meanwhile, the upper-
level trough will start lifting northward and weakening. Still, it
will remain close enough to hold a mid-to-upper-level ridge to the
south-southeast, maintaining marginally unstable conditions. Jet
dynamics aloft and 500 mb temperatures between -6 to -7 degrees
Celsius support this instability. As the ridge moves closer,
conditions will likely become less conducive to deep convective
development by midweek due to the entrainment of drier air and
higher-than-normal 500 mb temperatures. East to east-southeast winds
at 5-15 mph will continue, resulting in a relatively weak steering
flow.

Weather conditions will gradually transition to a more typical
seasonal shower pattern, driven by diurnal heating and local
effects, with enhanced afternoon convective development over the
interior and west to northwestern Puerto Rico each day. Afternoon
convection is also possible elsewhere, and similar to the potential
impacts from overnight and early morning showers affecting eastern
Puerto Rico and the U.S. Virgin Islands, the expected flooding risk
is minimal. Recent GOES-16 Geostationary Lightning Mapper (GLM)
data, along with the Galvez-Davidson Index and the latest model
guidance, indicate the potential for scattered thunderstorms, some
capable of producing heavy rain or scattered shallow convection with
isolated thunderstorms. This scenario raises concerns about elevated
flooding risks in urban areas, roads, small streams, and washes.
Given the current soil saturation levels between 90-100%, isolated
flash floods and mudslides in steep terrain are also possible.
Although the intensity and spread of the expected activity may
decrease, the flood risk will persist throughout the forecast
period.

Despite persistent cloud cover and high rain probabilities, warmer-
than-normal conditions will continue. Coastal and urban areas are
expected to have lows in the lower 80s, while higher elevations may
see lows in the upper 60s to around 70 degrees. In contrast, highs
will range from the lower 90s in lower elevations to the upper 70s
in higher elevations. With ample moisture present, heat indices may
exceed 108 degrees Fahrenheit in many coastal and urban areas, with
some locations experiencing brief periods of 112 degrees or higher.
These warm conditions pose a significant health risk, prompting the
issuance of Excessive Heat Warnings/Advisories. Residents and
visitors should take necessary precautions, particularly those
engaging in outdoor activities.

.LONG TERM...Friday through Tuesday...

A mid-to-upper level ridge moving from the south-southeast into the
local area will promote relatively drier weather conditions and
warmer temperatures. Nonetheless, light wind flow from the southeast
in combination with daytime heating and local effects will enhance
showers and thunderstorm development across the northwestern
portions of Puerto Rico each afternoon. Despite of the presence of
the ridge, tropical moisture being pulled from the warm Caribbean
Waters will approach the local area by late Saturday. The latest
Precipitable Water Content Analysis suggest values from 1.9 to 2.45
through the period. Due to the expected conditions, there is a
limited to elevated risk of urban and small stream flooding and
isolated flash flooding mainly for central, northern and
northwestern Puerto Rico. By Sunday into Tuesday a weak surface
trough is forecast to develop west of our area, increasing once
again the potential for moderate to heavy rainfall across the
islands. Warmer temperatures are expected to persist through the
entire period with values between the upper 80s to low 90s along the
coastal and urban areas of the islands. Heat indices are likely to
surpass the mid 100s, resulting in excessive heat conditions for the
islands.

&&

.AVIATION...

(06z TAFs)

SHRA/TSRA will result in brief MVFR conds at TJSJ/USVI terminals
through 28/14Z. Afternoon SHRA/TSRA will generate MVFR to brief IFR
conditions across TJSJ/TJBQ between 28/16-22Z, followed by VCTS
after 28/22Z.  VCSH possible across USVI terminals throughout the
period. Light and variable winds across PR terminals and E-ENE winds
at 5-10 knots across USVI terminals. Winds will increase to 10-15
knots after 28/14Z, with sea breeze variations and higher gusts near
SHRA/TSRA.

&&

.MARINE...

A moderate east to southeast wind flow will prevail across the region
over the next few days. Showers and isolated thunderstorms associated
with the trailing moisture of a departing tropical wave and a surface
trough east of us will continue to affect the regional waters and
local passages overnight. So far, the next tropical wave, located in
the central tropical Atlantic near 47W, is forecast to approach the
regional waters by mid-week, with its bulk of moisture well to the
south across the Caribbean Sea.

&&

.BEACH FORECAST...

There is a low risk of rip currents across all beaches of Puerto
Rico and the U.S. Virgin Islands tonight. Moderate to heavy
showers and isolated thunderstorms are possible each afternoon
mainly across the northwestern waters of Puerto Rico.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning from 10 AM this morning to 6 PM AST
     Wednesday for PRZ001-005-008.

     Heat Advisory from 10 AM this morning to 6 PM AST Wednesday for
     PRZ002-003-007-010>013.

VI...Heat Advisory from 10 AM this morning to 6 PM AST Wednesday for
     VIZ001-002.

AM...None.
&&

$$

SHORT TERM...ICP
LONG TERM....GRS
AVIATION...ICP
MARINE...CAM