818
FXCA62 TJSJ 150914
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
514 AM AST Sun Sep 15 2024

.SYNOPSIS...
A tropical wave with axis now crossing Puerto Rico will exit the
area later today. Showers and isolated thunderstorms will persist
across the coastal waters and local passages with some affecting
the south and east coastal areas of Puerto Rico during the rest
of the early morning. A drier airmass will move in from the east
and spread across the region by this afternoon. This should then
limit most of the afternoon convection to portions of central and
west to northwest Puerto Rico where localized active weather and
periods of heavy rains may be experienced. Hot and humid conditions
will continue into early next week with an elevated to significant
heat risk possible across portions of the islands. The prevailing
southeasterly winds will become more easterly by Monday as a surface
high pressure ridge will build north of the region.

&&

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

Clusters of strong showers developed across southeastern sections of
Puerto Rico, resulting in periods of heavy rain early this morning
over Ponce and Peuelas. Another band of showers affected the
eastern portions of the island; however, Doppler radar did not
indicate significant rainfall accumulations in that region.
Overnight temperatures were slightly warmer than usual due to the
prevailing southeasterly wind flow, with minimum temperatures in the
low 80s across coastal areas, while cooler conditions were observed
in the higher elevations.

For the remainder of the period, ridging aloft will promote more
stable weather, characterized by drier air aloft and warmer mid-
level temperatures. A broad surface high pressure system will
continue to dominate the area, sustaining the southeasterly wind
flow over the islands. Satellite-derived GOES imagery indicates a
decent amount of tropical moisture with precipitable water (PWAT)
values near 2.0 inches for this morning and early afternoon.
Therefore, isolated to scattered showers are forecast, mainly
affecting the eastern sections of Puerto Rico.

By this afternoon and continuing into Monday, a drying trend is
expected as the southeasterly winds provided by the surface ridge
bring in a mass of air containing Saharan dust particles, as
suggested by the NASA Goddard Earth Observing System Model V5.
According to global model guidance, PWAT values will drop two
standard deviations below climatological normals, leading to stable
conditions with minimal instability. This will result in clear skies
and increased solar radiation, contributing to warmer days with
elevated heat indices, particularly across coastal and urban areas.
Forecast 925 mb temperature trends suggest a warming pattern through
at least Tuesday. Although Tuesday will remain warm, pockets of
shallow moisture will increase cloudiness, potentially triggering
afternoon convection and isolated showers.

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

An overall dry and stable weather pattern is forecast at least
through Wednesday with winds becoming more easterly and gradually
diminish. A slight increase in trade wind moisture is forecast by
Thursday and rest of the workweek as winds become more east
northeast and bands of shallow moisture is brought in across the
area from the central and northeast Atlantic. Thereafter the local
winds become light and variable with a northeasterly component as
the surface high pressure ridge relaxes north of the region. Recent
guidance continue to suggest a retrograding Tutt will deepen and
sinks southwards between Puerto Rico and the northern Leeward
islands by Friday into the weekend. This in turn will destabilize
the upper levels and favor better chance for early morning and
afternoon convection across the region over the weekend. Activity
however should be more localized and not widespread. However, good
daytime heating and local effects will support enhanced afternoon
convection in some areas across portions of central interior and
west sections of Puerto Rico each afternoon. Lesser activity is so
far forecast around the U.S.Virgin islands where mostly isolated
showers will be possible for the most part.

The prevailing surface features will be a building surface high
pressure over the west and southwest Atlantic, interacting with
what is now Tropical Storm Gordon. This is still forecast to move
well northeast of the region over the Central Atlantic and have
no direct impacts on the islands. Under this expected pattern, a
mix of sunshine and clouds and variable weather conditions are
forecast, with an overall drier airmass to remain in place, along
with some traces of suspended dust particulates. The occasional
passage of bands and shallow pockets of moisture from time to time
can be expected across the region. This will be sufficient to aid
in the development of nocturnal and diurnal convection each day.
This scenario along with the intense daytime heating will lead to
periods of enhanced afternoon convection in isolated areas especially
across portions of Puerto Rico each day.

For the most part, the winds are expected to be fairly light and
therefore any locally and diurnally induced afternoon activity will
will be sufficient to produce slow-moving showers and thunderstorm
with locally heavy rains. This may lead to periods of elevated flood
threat especially across the central and west to southwest sections
of Puerto Rico.

&&

.AVIATION...
(06Z)
VFR conditions will persist with vicinity showers (VCSH) expected
near TJSJ and TIST until 15/14Z. Winds will increase around
15/15Z, shifting more from the southeast (SE) with gusty
conditions and sea breeze variations. Thunderstorms (TSRA) and
showers (SHRA) are forecast across northwestern Puerto Rico,
particularly near TJBQ, which will result in lower ceilings and
reduced visibility (VIS) due to convective activity.

&&

.MARINE...
A weak surface high pressure pattern will remain in place across the
western Atlantic, while a tropical wave crosses the regional waters
today. This pattern will promote gentle to moderate southeasterly
winds with occasional passing showers and isolated thunderstorms with
brief gusty winds over the coastal waters and local passages. Overall
seas will range between 2 to 4 feet occasionally up to 5 feet over
the offshore Atlantic waters and passages and between 1 to 3 feet
elsewhere.

&&

.BEACH FORECAST...
Beachgoers can expect a low risk of rip currents along most of the
local beaches of the USVI and PR. Winds along the coastal areas will
be between 10 to 15 mph with sea breeze variations and occasionally
higher gusts during the afternoon hours.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 11 AM this morning to 5 PM AST this afternoon
     for PRZ001>003-005-007-008-010>013.

VI...Heat Advisory from 11 AM this morning to 5 PM AST this afternoon
     for VIZ001-002.

AM...None.
&&

$$

SHORT TERM...LIS
LONG TERM....RAM
AVIATION...LIS