232
FXCA62 TJSJ 260930
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
530 AM AST Sun May 26 2024

.SYNOPSIS...

East to southeast winds, normal moisture content, and daytime
heating will lead to an increase in the heat indices across most
coastal and urban areas today. An Excessive Heat Warning and Heat
Advisory are in effect. Please refer to the latest heat product
(NPWSJU) for more information. A tropical wave will cross the
eastern Caribbean from later today through Monday. Shower and
thunderstorm activity will gradually increase across the local
waters and islands from around noon today. Normal to above normal
precipitable water content should persist through the long term
period, and afternoon convection is expected each day in the usual
areas.


&&

.SHORT TERM...Today through Tuesday...

Fairly tranquil conditions prevailed during the overnight hours.
Doppler radar detected a few isolated showers, but mostly across the
Caribbean waters. GOES-16 Total Precipitable Water (PWAT) satellite
imagery indicates slightly drier air settling over the forecast
area, with PWAT values ranging from 1.5 to 1.6 inches. Therefore,
similar weather conditions are expected to persist through the
morning hours, with a few isolated showers over the local waters
reaching windward coastal areas at times, but resulting in minimal
rainfall accumulations.

This slot of drier air is associated with subsidence ahead of the
tropical wave currently located at 61.7W. Although moisture content
will be reduced throughout the day, model guidance suggests that
maximum temperatures will be slightly higher today compared to the
previous day. The combination of east-southeast winds, high
temperatures, and sufficient moisture could lead to heat index
values exceeding Heat Warning criteria in several locations across
the forecast area. Therefore, an Excessive Heat Warning has been
issued for most municipalities across northern Puerto Rico and St.
Croix, with a Heat Advisory in effect for the rest of the urban and
coastal areas.

The deep-layered trough over Hispaniola persists, with a weak jet at
250 mb providing ventilation aloft across the northwestern quadrant
of the CWA. Shower activity this afternoon is not expected to be
widespread; however, any strong shower development across the
interior that moves over western/northwestern Puerto Rico could tap
into favorable jet dynamics aloft, potentially promoting another
round of thunderstorm activity in that area. By late afternoon and
early Monday, the leading edge of the tropical wave will begin to
filter into the eastern portions of the forecast area, continuing
its westward movement on Monday. Trailing moisture behind the
tropical wave will persist on Tuesday, maintaining PWAT values above
2.3 inches for the remainder of the period, potentially reaching
nearly 2.5 inches at timesexceeding the climatological maximum
value for this time of year.

The GOES-16 Geostationary Lightning Mapper (GLM) currently detects a
significant amount of lightning activity over the Leeward Islands
associated with this tropical wave. Meanwhile, at upper levels, the
deep-layered trough appears to weaken in the next few days, but some
jet dynamics will persist in the vicinity, along with sufficiently
cool 500 mb temperatures, promoting marginal instability across the
area through early this week. Therefore, as the tropical wave moves
over the area, it could interact with sufficient instability at
upper levels, leading to an increased flooding risk. Continue to
monitor the forecast as the tropical wave crosses the region in the
next few days.

.LONG TERM...Wednesday through Sunday...

The long-term forecast remains on track. A mid-to upper-level
ridge will hold for most of the period over the eastern Caribbean,
resulting in drier air aloft and warmer 500 mb temperatures.
However, enough low-level moisture will persist through the
forecast period as the precipitable water content (PWAT) remains
around 2.00 inches or higher through Sunday. Given the expected
conditions, the combination of low-level moisture and local
diurnal effects will enhance shower and thunderstorm development,
especially in the afternoon hours over the interior and western PR.
As the surface high pressure builds over the central Atlantic,
the pressure gradient will decrease, and veering winds will focus
the shower activity over the northwestern quadrant and the San
Juan Metro area. Given the expected conditions, there is a limited
to moderate flood threat for each afternoon, and mainly over
the interior and northern half of Puerto Rico.

From Friday to Sunday, a similar weather pattern will persist as
the surface high pressure over the central Atlantic interacts
with another high building over the western Atlantic, and in
between a weak surface trough is expected to develop north of the
islands. In response, increasing moisture and southeasterly winds
will prevail across the region. Under this influence, warmer
conditions with heat indices between 102-110 degrees Fahrenheit
or higher are expected, resulting in excessive heat conditions for
the northern coastal and urban areas.


&&

.AVIATION...

(06z TAFs)

Mostly VFR conditions will prevail today.  However, clouds and
mountain obscuration could develop along the interior PR this
afternoon with SHRA/VCTS possible at TJBQ thru 26/22z. The frequency
of SHRA and isold TSRA will increase near TJSJ/TIST/TISX after
26/21z due to an approaching tropical wave. Winds will be from the
ESE at 9-13 knots aft 26/13z.


&&

.MARINE...

A moderate east to southeast wind flow will prevail across the region
over the next few days. A tropical wave will move across the Caribbean
waters later today, increasing shower and thunderstorm activity
through at least Monday across the local waters.


&&

.BEACH FORECAST...

There is a low risk of rip currents today across all beaches of
Puerto Rico and the U.S. Virgin Islands. Isolated thunderstorms
are possible with the arrival of tropical wave during the
afternoon.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning from 10 AM this morning to 4 PM AST this
     afternoon for PRZ001-005-008.

     Heat Advisory from 10 AM this morning to 4 PM AST this afternoon
     for PRZ002-003-007-010>013.

VI...Excessive Heat Warning from 10 AM this morning to 4 PM AST this
     afternoon for VIZ002.

     Heat Advisory from 10 AM this morning to 4 PM AST this afternoon
     for VIZ001.

AM...None.
&&

$$

SHORT TERM...CVB
LONG TERM....DSR