319
FXCA62 TJSJ 190750
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
350 AM AST Thu Sep 19 2024

.SYNOPSIS...
Winds will weaken over the islands in the upcoming days,
with slow moving showers and thunderstorms developing each
afternoon. The risk for flooding will be elevated almost each
afternoon. Hot temperatures are also expected to persist across
the islands.

&&

.SHORT TERM...Today through Saturday...

Currently, radar and satellite imagery show calm, fair-weather
conditions across the islands, with light and variable winds.
Isolated showers across the local waters. Temperatures in coastal
and lower-elevation areas of Puerto Rico range from the upper 70s to
lower 80s, while higher elevations experience cooler temperatures in
the mid to upper 60s. In the U.S. Virgin Islands, temperatures are
currently in the upper 70s to lower 80s.

Today, a patch of moisture moving in from the northeast will rapidly
increase precipitable water values (PWAT) to above-normal levels,
reaching between 2.0 and 2.2 inches. Winds at the surface will have
a northeasterly component while becoming lighter, around 3 to 6
knots. This moisture will combine with local effects to generate
showers across the islands this afternoon. As a result, minor urban
and small stream flooding may occur due to locally and diurnally
induced slow-moving showers with heavy rainfall and thunderstorms,
especially across the Cordillera Central and western sections of
Puerto Rico. Similar conditions are expected to prevail on Friday.

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 1.7 inches. Heights will begin
to increase as the axis of an upper-level trough moves away, leaving
a subsiding portion over our region. However, local effects and
diurnal heating will still promote afternoon convection in some
areas, particularly in the interior, northwestern, and north-central
Puerto Rico. The U.S. Virgin Islands are expected to experience
mostly isolated showers.

Northeast winds will help keep temperatures somewhat cooler today,
but warm to hot conditions will likely continue in the coming days,
with heat indices remaining elevated. Guidance shows temperatures at
925 mb two standard deviations above normal. Residents and visitors
should 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.

As a summary, the increase in moisture and favorable conditions
aloft will lead to more showers and isolated thunderstorms today and
Friday, with a broader area likely experiencing limited to elevated
flooding and lightning risks. Better conditions are anticipated on
Saturday, but local effects and diurnal heating will still promote
afternoon convection in some areas. Warm to hot conditions will
likely continue in the coming days.

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

The pressure gradient will remain weaken late on the weekend and
early next week, with a low level steering flow coming from the
south-southwest of 1 to 4 knots. These light winds are due to a
series of trough over the western Atlantic. At the surface a small
pocket of moisture is expected to reach the islands. This will
act to fuel afternoon showers and thunderstorms along the
Cordillera Central and then spreading toward the coastal areas of
Puerto Rico. Some of this shower activity, although at a lesser
scale, are also expected for the Virgin Islands.

On Tuesday, the combination of a low pressure system developing
over the western Caribbean, and an upper level trough near the
Bahamas will escort an area of mid to upper level clouds into the
region. Conditions aloft will be favorable for showers and
thunderstorms too, so if enough sunshine filters in, another
active afternoon will be likely for the Cordillera Central, and
with the showers then drifting toward northeastern Puerto Rico and
the San Juan metro area. These showers will later move into the
vicinity of the Virgin Islands.

Late Tuesday and Wednesday, the trade wind will recover, gaining
an easterly component, and at 7 to 11 knots. The unsettle weather
pattern is expected to persist, as an upper level low develops
near the islands. The risk for flooding will be elevated all these
days. Temperatures will be high too, but the excessive risk will
be dependent on how deep and widespread the cloud coverage finally
materialize.

&&

.AVIATION...
(06Z TAFs)

Mainly VFR conditions are expected to prevail through the morning
forecast period, with VCSH likely across TJSJ and U.S. Virgin
Islands TAF sites. After 19/1700Z, additional activity is
anticipated to develop in the Cordillera Central, causing mountain
obscuration and likely affecting TJPS. Winds will be from the NE at
10 knots or less, becoming calm or light/variable after 19/2300Z.

&&

.MARINE...
East-northeast winds will dominate, gradually becoming variable and
weakening throughout the weekend. Showers are expected over the
Atlantic waters today, with a chance of isolated thunderstorms off
the south and west coast today.


&&

.BEACH FORECAST...
Low rip current risk is expected today for most of the local
beaches, but the risk will increase to moderate tonight and will
stay like that until Sunday.

&&

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

$$

SHORT TERM...YZR
LONG TERM....ERG