509
FXCA62 TJSJ 190749
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
349 AM AST Wed Jun 19 2024

.SYNOPSIS...
An elevated risk of flooding, with thunderstorms is expected for
today and Thursday. Conditions will become more seasonal this
weekend, but still with enough moisture to generate showers and
isolated thunderstorms. Hot conditions will persist each
afternoon, with little relief for areas with the lowest cloud
coverage.

&&

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

Isolated to scattered showers were observed all night into the early
morning across some coastal sections of St. Thomas. According to
radar estimates, rainfall accumulations were minimal. Several
pockets of showers with thunderstorms affected most of the local
waters, causing localized higher seas and gusty winds. Due to these
conditions, a marine weather statement was issued for most of the
local waters. The islands registered minimum temperatures in the
upper 70s to lower 80s across the coastal areas and in the mid to
upper 70s across the interior sections.

A similar weather pattern will persist today into Thursday across
the region. At the surface, the islands will continue to be
influenced by the interaction between a broad surface high pressure
over the Central Atlantic and an induced surface trough just
northwest, resulting in a light southeast wind flow over the region.
This wind flow will continue to bring plenty of tropical moisture,
with PWAT values exceeding 2 inches from the previous tropical wave
now located over Hispaniola. A similar pattern will prevail in the
mid-to-upper levels; a deep trough will continue to increase
instability, enhancing colder temperatures at 500 MB and divergence
aloft. These conditions will enrich vertical development, and deep
afternoon convection is forecast again across the interior sections
of the island. According to the Galvez-Davison Index (GDI), a good
indicator of thunderstorm activity, there is a good chance to
observe scattered thunderstorms with the heaviest activity today.
Since the surface winds remain light, resulting from the relaxation
in the pressure gradient, slow-moving showers could result in
significant rainfall accumulations. That is why there is an elevated
risk of flood threats across most of the CWA today. Residents are
urged to check weather conditions for further updates.
The forecast remains on track for Thursday. As the induced surface
trough moves westward, building surface high pressure over the North-
Central Atlantic will tighten the pressure gradient and create
backing winds. This wind flow will continue to drag moisture across
the region, resulting in a persistent wet pattern. The prevailing
winds will focus the showers more on the interior and western
sections. By Friday, the available moisture will diminish, resulting
only in localized showers for the afternoon hours.


Due to the significant rainfall observed over the past few days,
soils are already saturated, leading to urban and small-stream
flooding in some areas across the islands. Therefore, it`s crucial
for residents and visitors to be aware of these conditions for
safety and operational reasons and to check weather conditions
during the day for possible advisories or warnings.

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

A surface high pressure centered over the eastern Atlantic will
drive the trade winds from the east during the weekend, bringing
occasional showers across eastern Puerto Rico and the Virgin
Islands, and followed by afternoon convection across western Puerto
Rico.

Early next week, a mid level ridge will build north of the
region, and the surface high pressure will strengthen as it moves
westward. At this time, an induced surface trough will approach the
area, and a tropical wave will cross the eastern Caribbean. The
moisture from these surface features will bring an increase in
showers to the area, mainly for the Virgin Islands, local waters and
eastern Puerto Rico. Accumulations are not expected to be too
significant because the low level steering flow will be enhanced.
Saharan dust will follow on Tuesday, lasting through Wednesday. Then
on Wednesday, another trough will dig from the northeast, shifting
the winds from this direction. All these days, afternoon convection
is anticipated, mainly for the interior and western Puerto Rico. The
activity could be strong, with possible lightning, and urban and
small stream flooding developing.

The heat risk will be elevated all these days, impacting outdoor
activities, and those without effective cooling or poorly hydrated.


&&

.AVIATION...
(06z)

Briefly, VFR conditions will persist for most of the TAF sites until
SHRA and TSRA develop from 19/Z across the islands. These conditions
will result in MVFR and brief IFR conditions across the major TAF
sites of the island where TSRA ceiling in the FL020 & FL030 are
forecasted. Lower ceilings and limited VIS are expected over the
mountains as well. Winds will continue light and VRB, increasing at
19/15Z from the E-SE at 12 knots with gusty winds near the showers.

&&

.MARINE...
Surface high pressure over the Atlantic will promote moderate to
locally fresh easterly winds through the rest of the week. Passing
showers and isolated thunderstorms accompanying and trailing a
tropical wave will continue to affect the coastal waters and local
passages resulting in localized hazardous marine conditions.

&&

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

$$

SHORT TERM...LIS
LONG TERM....ERG