571
FXCA62 TJSJ 222018
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
418 PM AST Fri Nov 22 2024

.SYNOPSIS...
The frequency of showers and thunderstorms will increase through
the next several days, maintaining an elevated risk of flooding
for the islands. Southerly winds on Saturday will maintain
high temperatures above normal. The risk of rip currents will stay
high through much of the upcoming workweek.

&&

.SHORT TERM...Tonight through Sunday...

Southerly wind flow led to higher temperatures compared to previous
days, with some stations in coastal and urban areas reporting
maximum temperatures in the lower 90s. Thunderstorm activity was
largely suppressed over land due to ridging aloft, which inhibited
significant convective development. However, a few thunderstorms
were detected over the Caribbean waters and the Mona Passage, as
an airmass with above-normal moisture continues to move
northward. An increase in shower and thunderstorm activity is
expected, particularly across southern Puerto Rico and the USVI,
as this moist airmass advances over the forecast area.

Weather conditions over the weekend will differ from previous days
as the moist airmass continues to linger over the northeastern
Caribbean. On Saturday, most precipitation is expected across the
interior and northwestern quadrant of Puerto Rico, driven by a
southeasterly steering wind flow. Uncertainty increases for Sunday
as a polar trough exiting the eastern seaboard approaches the
northern Caribbean. The degree of instability aloft will depend on
how far south and east the polar trough extends and the extent to
which it erodes the upper-level ridge.

Latest model guidance suggests most of the activity will focus on
Hispaniola and the Mona Passage, with some reaching the western
third of Puerto Rico. However, the extent of shower and thunderstorm
activity will depend on the positioning of low-level convergence
zones and the availability of favorable dynamics aloft. Regardless,
expect weather conditions to transition to a more unstable and
wetter pattern, increasing the potential for flooding, particularly
across interior and western portions of Puerto Rico. Continue to
monitor updates to the forecast throughout the weekend.

.LONG TERM...Monday through Friday.../from prev discussion/

At upper levels, the sub-tropical jet over Cuba will slide just
south of the polar jet well north of the Leeward Islands, This
sub-tropical jet which will more or less coincide with the
location of a cold front will be unable to come much closer to
the area due to high pressure that stretches from the Central
Caribbean to the tropical Atlantic along 20 north. The upper level
high will drift a little south during the period.

A perturbation in the lower levels mentioned above will approach
the area and cross the Windward Islands from the east early Sunday
morning to bring additional moisture to the area. This moisture
will move quickly past the forecast area to the south and feed the
developing north-south trough north of Panama while pulling drier
air in from the east over us on Monday. As the trough develops a
low over the eastern tip of Jamaica, winds will become more
southerly and bring more moisture from the southeast Caribbean for
continued shower activity. Then on Thursday and Friday as the low
near Jamaica extends a trough to the north northeast, a trough
will brush the northern part of our forecast area as an anti-
cyclonic eddy forms off the tip of Cabo Rojo, sustaining the
modest flow of moisture across the islands.

500 mb temperatures will be mostly minus 5 to minus 6 during the
period, so there will be a chance of thunderstorms--mainly during
the afternoons over Puerto Rico and over the local waters at
night. Although some local urban and small stream flooding could
develop, showers should move just fast enough to prevent major
river rises during the period. Nevertheless most areas will have
at least some rain each day.

&&

.AVIATION...
(18Z TAFs)

VFR conditions will prevail for all TAF sites for most of the
period. However, SHRA/TSRA is possible in the vicinity of TJPS/TJSJ
after 23/03z. Brief MVFR expected for these terminals, with ocnl
IFR. Light and variable winds from the SE/SSE below 10 kts will
persist with winds gradually shifting ESE by 23/18z.


&&

.MARINE...
Light to gentle east to southeasterly winds will prevail through
late Saturday afternoon. Pulses of the northerly swell will continue
to arrive, tonight, but conditions are likely to remain below small
craft advisory criteria. Another small long-period northerly swell is
expected to arrive Sunday morning.

&&

.BEACH FORECAST...
The buoy in San Juan indicates that the high rip current risk
remains high. Pulses from the swell will continue to arrive with a
moderate to high rip current risk for the north facing beaches in
Puerto Rico and the Virgin Islands. A long-period northerly swell
will arrive this weekend, maintaining the risk high for the
upcoming week.

&&

.CLIMATE...
Southerly winds on Saturday will maintain 925 mb temperatures
above normal. In fact, for San Juan Area, the record daily high for
tomorrow is 90F, and it is likely that the actual value will be
around this number. It is less likely that the record will be
broken in the Virgin Islands, but it will not be that far off in
Saint Croix, where the record daily high tomorrow is 91F.

&&


.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk until 6 PM AST this evening for PRZ001-002-
     005-008-012.

VI...None.
AM...None.
&&

$$

RADAR/BEACH/CLIMATE...ERG
MARINE...MRR
SHORT TERM DISCUSSION/AVIATION...CVB