075
FXCA62 TJSJ 170900
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
500 AM AST Tue Sep 17 2024

.SYNOPSIS...

A dry air mass continues to dominate the region and will allow
mostly stable conditions today. Hot conditions will prevail; an
Excessive Heat Warning has been issued for the urban and low-
elevation areas of northern and western Puerto Rico, as well as
for St. Croix. For the rest of the coastal areas, including
Vieques, Culebra, St. Thomas, and St. John, a Heat Advisory is in
effect, both from 10 AM to 5 PM AST. Easterly winds will dominate
from today through tomorrow, then winds will become light and more
variable in the latter half of the week. An upper level trough is
expected to bring favorable conditions for the development of
showers and isolated thunderstorms by the end of the week.

&&

.SHORT TERM...Today through Thursday...

A line of showers developed along the coast of northeast Puerto
Rico, reaching portions of the San Juan metro area during the
overnight hours. Other than that, only a few showers were observed
in the offshore Atlantic and Caribbean waters.

The islands are still covered by a pocket of dry air today, as is
evident in satellite imagery. There is a patch of moisture just to
the northeast of the region, but the moisture should stay away for
today. A surface to upper level ridge continues to linger today,
with a trade wind cap inversion around 850 mb. Above these levels,
the air mass is dry, and the available moisture is trapped closer to
the surface. In fact, the dewpoint depression for today is around 20
degree Celsius, an indicative of dry conditions. Rainfall activity
should be very limited, but showers should manage to form over
western Puerto Rico in the afternoon. This activity will be short-
lived, and accumulations are not expected to be too significant.

The main story for today, again, is the scorching heat across the
U.S. Caribbean territories. Temperatures at the 925 mb are above
normal, and with lows only dropping to the low 80s, and lots of
sunshine, it will feet very hot during noon and in the afternoon
hours.

On Wednesday, a small area of moisture will cross the islands from
north to south. Most of the activity should arrive in the overnight
or in the early morning hours, but the increase in moisture should
lead to stronger and more widespread showers over the interior and
western Puerto Rico in the afternoon. Isolated to scattered showers
are also expected to reach the Virgin Islands and eastern Puerto
Rico during this period.

On Thursday, the surface high weakens, and so will the winds. In
fact, the low level steering flow is expected to be from the east at
5 knots or less. Moisture levels are not going to be too high, but
with enough heating, showers are again expected for the interior and
west, drifting toward the south and southwestern Puerto Rico.

&&

.LONG TERM...Friday through Monday...

Recent model guidance indicates that an upper-level trough will
deepen and move southward between Puerto Rico and the northern
Leeward Islands by Friday. This will help destabilize the upper
levels and increase the likelihood of early morning and afternoon
convection across the interior and western sectors of Puerto Rico.
Additionally, the latest satellite-derived precipitable water
values show an increase from mostly below normal to normal levels.
Therefore, while there wont be enough moisture for widespread
rain, there will be enough to develop showers. Throughout 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 and diurnally induced slow-moving showers and
thunderstorms with locally heavy rainfall, especially in the
central and western sections of Puerto Rico.

By the weekend, surface winds are expected to shift to a more south-
southeasterly direction and remain very light. The latest models
suggest that the area of convergence associated with the upper-
level trough will be over the region, which may help to limit rain
chances. However, daytime heating and local effects will continue
to enhance afternoon convection in some areas, particularly in
the interior, northwestern, and north-central Puerto Rico. In
contrast, the U.S. Virgin Islands is expected to experience mostly
isolated showers.

Hot conditions are likely to persist throughout this period, with
temperatures potentially reaching the upper 90s and heat indices
remaining elevated. Temperatures at 925 mb will remain well above
normal through the weekend. The combination of high temperatures
and moisture could create hazardous heat conditions, particularly
in urban and low-elevation areas. Therefore, it is very likely
that Heat Advisories and possibly Excessive Heat Warnings will be
issued daily for some areas. 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.

&&

.AVIATION...
(06Z TAFs)

Mainly VFR conditions expected to prevail through the forecast
period. Isolated SHRA will be possible in the vicinity of USVI and
TJSJ, but without significant impacts to operations. Winds will be
from the E at 12-16 kts, with stronger gusts.

&&

.MARINE...

A weak surface high-pressure system will promote gentle to moderate
easterly winds. By midweek, the winds are expected to become lighter
and more northeasterly. Today, very isolated showers are anticipated
over the local waters as drier air continues to settle in. A few
thunderstorms are possible off the western coast of Puerto Rico each
afternoon. Overall, seas will range from 3 to 5 feet, occasionally
reaching up to 6 feet across most local waters, with seas between 1
to 3 feet across most coastal waters over the next few days.

&&

.BEACH FORECAST...



&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning from 10 AM this morning to 5 PM AST this
     afternoon for PRZ001-005-008-010.

     Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ002-003-007-011>013.

VI...Excessive Heat Warning from 10 AM this morning to 5 PM AST this
     afternoon for VIZ002.

     Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ001.

AM...None.

&&

$$

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