161
FXCA62 TJSJ 212040
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
440 PM AST Tue May 21 2024

.SYNOPSIS...

A wet and unstable weather pattern is expected to begin on
Wednesday, as a trough in all levels approaches the Hispaniola
from the south. This trough in combination with moisture from a
trade wind perturbation will enhance the potential for heavy rains
and thunderstorms through Friday. Therefore, the risk for
urban and stream flooding is elevated.

&&

.SHORT TERM...Tonight through Thursday...

Mostly clear to partly cloudy skies were observed across the
local islands. By mid-morning temperatures began to rise due to
the combination of southeast winds and warm moist air. As a result
above- normal temperatures were observed across the coastal and
urban areas. The Luis Munoz Marin International Airport in San
Juan weather station reported 95 degrees Fahrenheit, which ties
the old record of 95 degrees in 2005. An Excessive Heat Warning
was in effect for the northern, western and southwestern coast of
Puerto Rico and St. Croix. A Heat Advisory was in effect
elsewhere. In addition a maximum heat index of 116 degrees
Fahrenheit was reported in Hatillo by a local emergency manager at
around noon. Despite the warm and hot conditions, moderate to
locally heavy showers were seen across the northwestern and north-
central municipalities of Puerto Rico. Doppler radar estimated
rainfall accumulations of 0.5 to 1.0 inches across the mentioned
sectors.

The ongoing shower activity across western Puerto Rico is expected
to end by sunset. Overall, tranquil weather conditions are
anticipated during the overnight hours for all the local islands. By
Wednesday onward, an approaching deep-layer trough aloft will
increase instability across the local area. Sufficient moisture from
a trade wind perturbation and tropical moisture pulled from northern
South America with precipitable water content above 2.4 inches will
enhance the rainfall activity across the region through the rest of
the period. This event is expected to start by Wednesday afternoon
and lasting through the end of the workweek. Squally weather
conditions and thunderstorms are anticipated during this period.
Therefore, a Hydrologic Outlook (ESFSJU) was issued for Wednesday
through Friday for Puerto Rico and the Virgin Islands due to an
increased risk of flooding.

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

PREV DISCUSSION.../issued 416 AM AST Mon May 20 2024/

An unsettled weather environment is expected to persist on Friday,
with a surface low now centered north of Hispaniola, and still under
the influence of a mid to upper level trough. The low will drive the
winds from the south at the surface, and from the southwest a little
bit above that. Pulses of clouds, showers and thunderstorms are
expected to move from the Caribbean Sea into portions of Puerto Rico
and the U.S. Virgin Islands. The Galvez-Davison Index indicate the
potential for scattered thunderstorms and heavy rain at times. It is
possible that these thunderstorms will bring gusty winds at times.

On Saturday, the islands will be in between a moist channel near the
Mona Passage, and a mid level high pressure centered just to the
east of the Leeward Islands. This high will push some drier air
closer to the Virgin Islands, possibly with some Saharan dust.
Nevertheless, sunshine will begin to peek across the region, and
with all the available moisture, convection will fire up in the
afternoon, favoring portions of eastern and northeastern Puerto
Rico, then moving into the waters of Vieques, Culebra and the Virgin
Islands. With enough sunshine, heat indices will creep again above
100 F.

The pattern becomes more seasonal on Sunday and early in the
workweek. The trough will gradually lose influence in the region,
while a mid to upper ridge gets established across the northeastern
Caribbean. The pressure gradient will remain weak on Sunday, with a
low level steering flow out of the southeast on Sunday, at around 6-
9 kts, and recovering speeds on Monday and Tuesday, generally from
the east at 10 to 12 knots. Each of these days, enough sunshine will
trigger afternoon convection, with the potential for heavy showers
and isolated thunderstorms across the interior, northwestern and
north-central Puerto Rico. Passing showers will move across the
Virgin Islands and eastern Puerto Rico at times too.

In summary, the main risk on Friday and Saturday will be flooding
and mudslides, as well as rapid river rises in areas that receive
heavy rain. Then, for Sunday and early next week, the flooding
threat will be limited for the northwestern quadrant of Puerto Rico.
It will become hot too, as heat indices go above 102 degrees each
afternoon.

&&

.AVIATION...
(18Z TAFS)

Mainly VFR conditions expected to prevail. TSRA will develop across
the interior, N and NW PR thru 21/23z. This activity may impact
TJBQ, where TS are possible to bring periods of reduced VIS and low
ceilings. Winds will blow from SE at 8-15 kts, with stronger wind
gusts and sea breezes variations. Unsettled weather conditions will
develop tomorrow, especially aft 22/16z, affecting the local flying
area.

&&

.MARINE...

The interaction of an induced surface trough to the west and a
weakening surface high pressure to the east will promote moderate
east-southeast winds through midweek. As the surface trough moves
closer to the region expect increasing shower and thunderstorm
activity across the local waters from Wednesday onward. This trough
may increase winds across the Mona Passage and Atlantic waters late
Wednesday night into Friday. Winds will slow down again by the
weekend.

&&

.BEACH FORECAST...

Increasing winds across southwestern Puerto Rico and near St.
Croix will make the rip current risk to reach moderate for St.
Croix by late tonight into Wednesday. Elsewhere, a low rip
current risk will continue.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning until 6 PM AST this evening for PRZ001-
     005-008-010-011.

     Heat Advisory until 6 PM AST this evening for PRZ002-003-012-013.

VI...Excessive Heat Warning until 6 PM AST this evening for VIZ002.

     Heat Advisory until 6 PM AST this evening for VIZ001.

AM...None.
&&

$$

SHORT TERM...RAM
LONG TERM....GRS
AVIATION...CAM
PUBLIC DESK...MRR