943
FXCA62 TJSJ 210723
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
323 AM AST Tue May 21 2024

.SYNOPSIS...
Another very hot day is expected across the islands, especially
along coastal and low-elevated areas. A wetter weather pattern is
expected to begin on Wednesday, as a surface and mid to upper
level trough approaches Hispaniola. Wednesday is expected to be
the wettest day, but showers and thunderstorms will continue to
stream into the islands through early in the weekend. The risk for
flooding will be elevated.

&&

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

Excessive heating due to the combination of warm air and
southeasterly surface wind flow will result hot heat indices for the
urban and coastal sections in Puerto Rico and the US Virgin Islands
during the later morning hours through the afternoon. Therefore
from 10am to 6pm local time, an Excessive Heat Warning is in
effect for the coastal areas of southwest to northwest Puerto Rico
to the San Juan Metro area and St. Croix. There is also a Heat
Advisory for northeastern to southeastern PR, also Vieques,
Culebra, and the northern USVI. Once all this heating occurs,
afternoon convection may develop near the San Juan Metropolitan
Area and the interior, with the northwest quadrant having a high
chance for thunderstorms after 3pm under the east-southeasterly
wind pattern. Downpours may result in ponding of water or flooding
of poorly drained areas in these locations.

Tomorrow onward, an approaching deep-layer trough aloft will
increase instability across the region. Additionally, moisture
from a trade wind perturbation and plenty of tropical moisture
lifted from the southern Caribbean will pool above-normal moisture
content over the islands. 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.
Currently, models suggest heavy rainfall is possible for most of
the region for Wednesday afternoon into Thursday, as winds
accelerate over night on Wednesday this will also bring the chance
for squally weather conditions. Thunderstorms will be seen during
this time as the strong winds are expected across the mid to
upper-levels. On the bright side, temperature will be noticeably
lower due to the increase of cloud coverage and showers.


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

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...
(06Z TAFS)
Mainly VFR conditions expected to prevail. After 16z an increase
of rain is expected across the Cordillera Central and around TJBQ,
where TS are possible to bring periods of reduced VIS and low
ceilings. Winds will from the SE at 8-13 kts, with stronger wind
gusts and sea breezes variations.

&&

.MARINE...
A surface high pressure across the central and northeast Atlantic
will drive the trade winds from the southeast at moderate speed.
A trough will develop south of Hispaniola, increasing shower and
thunderstorm activity across the local waters Wednesday onward. This
trough may increase winds across the Mona Passage and Atlantic waters
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. Elsewhere,
a low rip current risk will persist.


&&

.HYDROLOGY...
A wet pattern is expected to begin on Wednesday, with showers and
thunderstorms reaching the islands at times. So far, most rivers
are running near normal or above normal, hence any additional
rains could result in rapid river rises, as well as some flooding
and mudslides. Conditions will gradually dry out during the
weekend.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning from 10 AM this morning to 6 PM AST this
     evening for PRZ001-005-008-010-011.

     Heat Advisory from 10 AM this morning to 6 PM AST this evening
     for PRZ002-003-012-013.

VI...Excessive Heat Warning from 10 AM this morning to 6 PM AST this
     evening for VIZ002.

     Heat Advisory from 10 AM this morning to 6 PM AST this evening
     for VIZ001.

AM...None.
&&

$$

SHORT TERM...RC
LONG TERM....ERG