673
FXCA62 TJSJ 180900
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
500 AM AST Wed Sep 18 2024

.SYNOPSIS...

The combination of moisture and local effects will result in
showers along the interior, east and west sectors of Puerto Rico,
as well across the U.S. Virgin Islands. A typical shower pattern
is expected over the next few days, heightened by the end of the
week due to increased instability and available moisture. Northeast
winds will help keep temperatures somewhat cooler today resulting
in a limited heat risk, but excessive heat levels in the coming
days will pose a risk to vulnerable individuals, especially those
outdoors without proper cooling or hydration.

&&

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

The most recent satellite-derived precipitable water imagery show a
patch of moisture approaching from the north. This has resulted in
showers and isolated thunderstorms brushing the northern coast of
Puerto Rico, Saint Thomas, Saint John, and Saint Croix.

A surface high pressure north of the Caribbean and a low pressure
system well northeast of the islands will maintain a northeast wind
flow today at around 10 knots. There is another trough leaving the
eastern coast of the United States that will work to weaken the wind
flow for the rest of the week, with speeds around 3-6 knots on
Thursday and Friday. Today, the patch of moisture will combine with
local effects to generate showers along the interior, east and west
of Puerto Rico, and over the Virgin Islands. This activity is
expected to taper off in the evening hours as a small area of dry
air filters in from the northeast.

Similar conditions are expected to repeat on Thursday, with another
patch of moisture reaching from the northeast. Thus, shower activity
will increase again in the night hours over the northern coast of
Puerto Rico and the Virgin Islands. The main difference will be that
with lighter winds anticipated, the afternoon activity could linger
longer over the Cordillera Central, then spreading toward the
coastal areas in the afternoon hours. Some minor urban flooding and
water surges along rivers can be anticipated.

On Friday, moisture lingers, but under light winds coming from the
south-southeast. Under this flow, temperatures will warm up once
again, with 925 mb values over two standard deviation above
climatology. Excessive heat conditions will be likely.

&&

.LONG TERM...Saturday through Tuesday...

Latest satellite-derived precipitable water values indicate a
patch of drier air moving into the area around next Saturday, with
PWAT values expected to range from 1.4 to 1.6 inches. Heights will
begin to increase as the axis of an upper-level trough moves away,
leaving the subsiding portion over our region, which will help
limit rain development a little. While there wont be enough
moisture for widespread rainfall, local effects and diurnal
heating will still promote afternoon convection in some areas,
particularly in the interior, northwestern, and north-central
Puerto Rico. In contrast, the U.S. Virgin Islands are expected to
experience mostly isolated showers. Similar conditions are
anticipated to persist into Sunday.

By Monday and Tuesday, moisture will increase from the south,
resulting in better rain chances across the islands. During this
period, winds are expected to remain very light and somewhat
variable. As a result, urban and small stream flooding may occur
due to locally induced slow-moving showers and thunderstorms with
heavy rainfall, especially in the central and western sections of
Puerto Rico.

Hot conditions will likely dominate over the long-term period,
with potentially heat indices remaining elevated. Over the
weekend, guidance are suggesting that temperatures at 925 mb will
remain two standard deviations above normal. The combination of
high temperatures and humidity could create hazardous heat
conditions, particularly in urban and low-elevation areas of the
islands. Therefore, it is likely that Heat Advisories and Excessive
Heat Warnings will be issued. Residents and visitors are encouraged
to monitor the forecast, take precautions to stay hydrated by
drinking plenty of water, wearing light and breathable clothing,
and avoiding prolonged sun exposure, especially during peak heat
hours.

&&

.AVIATION...
(06Z TAFs)

Passing SHRA across TJSJ and the USVI terminals is expected to
continue through the forecast period, with brief periods of reduced
VIS and low ceilings. After 17Z, additional activity is expected to
develop in the Cordillera Central, causing mountain obscuration and
likely reaching TJPS. Winds will be from the ENE at 10-12 kts, with
stronger gusts from 14-22Z. Winds will slow down after 22z, out of
the ENE at 9 kts or less.

&&

.MARINE...

East-northeast winds will dominate, gradually becoming variable
and weakening from Thursday throughout the weekend. Showers are
expected to persist over the local Atlantic waters and Caribbean
passages this morning, with a chance of isolated thunderstorms off
the western coast of Puerto Rico each afternoon. Overall, seas
will range from 3 to 4 feet, occasionally reaching up to 6 feet in
some areas, while southern coastal regions will experience lower
seas in the coming days.

&&

.BEACH FORECAST...

Today, the risk of rip currents will remain moderate along the
northern beaches of Puerto Rico, while elsewhere it will be low.
Over the coming days, a moderate risk will persist along the
northern-facing beaches of Puerto Rico, with a return to moderate
risk for the rest of the islands by the end of the week into the
weekend. For a more detailed forecast, please consult the Surf
Zone Forecast for Puerto Rico and the U.S. Virgin Islands
(SRFSJU).

&&

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

$$

SHORT TERM/AVIATION...ERG
LONG TERM/MARINE....YZR