584
FXCA62 TJSJ 150831
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
431 AM AST Wed May 15 2024

.SYNOPSIS...

A typical weather pattern is forecasted for the next several
days, featuring afternoon showers across the interior sections of
the island. An increase in shower activity is expected for the
upcoming workweek due to the arrival of abundant tropical moisture
and instability facilitated by a mid to upper-level trough.
Residents and visitors are urged to stay tuned for further updates
for the upcoming workweek.

&&

.SHORT TERM...Today through Friday...

Skies were mostly clear to partly cloudy overnight with isolated
passing showers noted over the offshore coastal waters. Some showers
reached the north and east coastal areas but so far rainfall
accumulations were less than a tenth of an inch. Overnight low
temperatures were in the upper 70s to low 80s along the coastal
areas and in the mid 60s to low 70s in higher elevations and
valleys. Surface winds were calm to light and variable.

During the rest of the morning, sunny skies will prevail with
easterly winds around 5 to 10 mph. Local sea breezes can be expected
during the afternoon with winds increasing to 10 to 15 mph with
locally higher gusts along the coastal areas. Some diurnally induced
shower development may occur across the central and west sections of
Puerto Rico and mainly downwind of the U.S. Virgin Islands.

For the rest of the period, a mid-to upper level high-pressure
system will continue spread eastwards across the region from the
west and southwest Atlantic, while a surface high pressure builds
across the west and central Atlantic. This will increase the trade
wind cap inversion while eroding low level moisture through at least
Thursday. Thereafter, another cold front will enter and cross the
west Atlantic by Friday as the surface ridge shifts farther east
into the central and northeast Atlantic. Recent analysis and the
aerosol optical thickness products suggest light suspended Saharan
dust particulates will continue to diminish today. Recent model
guidance and satellite derived analysis initialized well and
continue to suggest PWAT values ranging between 1.35 and 1.7 inches
today, then slightly increase to between 1.5-1.8 inches Thursday
through Friday as winds become more east to southeast and an
easterly perturbation approaches the forecast areas along with an
upper trough which will cross the west Atlantic by Friday. This will
result in more unstable conditions aloft and better chance for
afternoon showers and isolated thunderstorm development.

The expected synoptic conditions are forecast to be less conducive
for widespread or significant convective development,however shower
development and isolated thunderstorm will remain possible each
afternoon especially over parts of the central interior and west
sections of Puerto Rico. The activity will be locally steered by the
east to northeast low level wind flow. Overall expect mostly fair
weather skies in most areas each day with isolated showers possible
mainly across the central and west sections of Puerto Rico during
the afternoon hours due to local effects and the intense heating.

.Expected Impacts...
Warm temperatures will continue to be one of the main factors for
the period, with daytime temperatures peaking in the upper 80s to
low 90s along the coastal and urban areas and mid to upper 80s at
higher elevations. Maximum heat indices may range between 100-107
degrees Fahrenheit during the afternoon hours. Residents and
visitors to the islands are urged to stay hydrated and limit direct
sun exposure when possible.
There will be also be slight risk for minor urban and small stream
flooding in isolated areas each day but locally heavy rains with the
afternoon convection will lead to ponding of water on roads and poor
drainage areas.

.LONG TERM... Saturday through Wednesday...

A surface high pressure system extending from the central
Atlantic will maintain southeasterly winds across the region in
the initial phase of the long-term forecast. These winds will
carry patches of tropical moisture to the local islands, enhancing
the likelihood of shower formation, particularly in the
afternoon. Most showers are anticipated to develop across the
interior, the northwestern quadrant, and the San Juan metro area.
Some sections of the islands may experience clear skies in the
mornings, coupled with available moisture, which could elevate
heat indices, posing a limited to elevated heat threat for the
northern coastal area of Puerto Rico.

Instability is expected to increase by late Sunday and persist
throughout the forecast period as a mid to upper-level trough
moves from the western Atlantic. According to guidance from global
models such as the GFS and ECMWF, the strongest and most
favorable side of the TUTT (Tropical Upper Tropospheric Trough)
and its associated jet stream will cover the local islands from
Monday through Wednesday. With aloft instability and colder
temperatures at mid-levels, abundant tropical moisture will be
present at the surface, with precipitable water (PWAT) values
exceeding climatological norms by over 2 inches. Consequently, a
wet period is forecasted for all the islands, characterized by
morning shower activity followed by widespread showers and
thunderstorms in the afternoon. Residents and visitors should
anticipate periods of heavy showers and an increased risk of
flooding from Tuesday through Wednesday.

&&

.AVIATION...
(06Z TAFS)
VFR conds durg fcst prd. Isold SHRA fcst over the regional waters
and en route btw islands. SCT OCNL BKN lyrs nr FL025...FL040.
VCSH at TJSJ/TISX/TIST til 15/12Z. Aftn convection with Isold TSRA
psbl ovr central and west interior sections of PR with mountain
obscuration fm 15/17Z-23Z. VCSH expected at the TJSJ/TJBQ/TJPS
15/18Z-15/23Z. Sfc wnds calm to lgt/vrb bcmg mainly fm E 10-12 kts
with sea breeze variations and ocnly hir gusts aft 15/14Z.

&&

.MARINE...
A surface high pressure extending from the western Atlantic into
the Central Atlantic will continue to promote light to moderate
easterly winds across the region. With this pattern, seas will
remain up to 4 feet across all the local waters and passages.
These conditions will prevail across the local waters during the
period. Similar tranquil marine conditions will prevail for the
coastal areas where the risk for rip currents will remain low for
the next five days or so.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

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