292
FXCA62 TJSJ 161527
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1127 AM AST Mon Sep 16 2024

.UPDATE...Scattered clouds developed over the eastern quarter of
Puerto Rico this morning as temperatures began warming. No showers
were detected as of 11 AM AST on National Weather Service Doppler
Radar. The morning sounding from San Juan, Puerto Rico at 16/12Z
showed very dry air above the marine layer which had tops of 2500
feet. Only a slight chance of showers is expected at this time
and these will likely be in west northwest Puerto Rico. No
significant changes were made to the forecast.

&&

.AVIATION...VFR conditions. Isold SHRA to dvlp aft 16/17Z--mainly
in wrn PR and ovr the Cordillera Central. East flow of 10-15 kts
with weak sea breeze influences. Maximum winds E-ESE 13-19 kt
blo FL120.

&&

.PREV DISCUSSION... /issued 511 AM AST Mon Sep 16 2024/

SYNOPSIS...

The dry airmass that continues to move across the islands will
limit the shower activity today. Hot and humid conditions will
continue with an elevated to significant heat risk, particularly
across lower elevations of the islands. Easterly winds will
dominate today through mid-week, then winds will become light and
more variable by the second half of the week.

SHORT TERM...Today through Wednesday...

A very quiet night was observed across the islands. A few showers
managed to develop across the area, but mostly stayed over the
waters. The night was on the warm side too, with minimum
temperatures in the low and mid-80s in several areas.

A surface high pressure anchored over the central Atlantic will
drive the trade winds from the east to east-southeast today. At the
mid and upper levels, ridge also dominates. Additionally, the most
recent satellite-derived precipitable water show a dry air
mass pushing in, with values expected to drop below the
climatological value for today. All these factors together translate
into stable conditions across the area. In fact, a trade wind cap
inversion is expected to develop around 850 mb, trapping all the
moisture below these levels. Shower activity will not be favored
today, but strong heating may still lead to isolated thunderstorms
across western Puerto Rico this afternoon.

For Tuesday and Wednesday, the mid level ridge will hold, while the
surface high will migrate westward as Tropical Cyclone Gordon
advances to the north, far away from the islands. The mid to upper
levels will remain dry, with limit the vertical development of the
showers (meaning the showers will not be as strong or widespread),
but small patches of moisture may reach the region from time to
time.

The aerosol models also indicate slight concentrations of Saharan
dust in the area, so skies may look gray and hazy at times today
through Wednesday. In terms of temperatures, above normal values are
expected to continue, with 925 mb temperatures nearly at two
standard deviation above the climatological mean. Therefore,
excessive heat impacts will persist.

LONG TERM...Thursday through Sunday...

A slight increase in trade wind moisture is expected by Thursday
and Friday as winds shift to the east-northeast, bringing bands of
shallow moisture from the central and northeast Atlantic. Although
the available moisture will be confined to lower levels, it will
be sufficient to trigger rain activity due to strong heating and
local effects. Local winds will become light with a northeasterly
component as the surface high pressure ridge weakens north of the
region. Recent guidance suggests that an upper-level trough will
deepen and move southward between Puerto Rico and the northern
Leeward Islands by Friday. This should destabilize the upper
levels and increase the likelihood of early morning and afternoon
convection.

Overall, winds are expected to be light, so any locally and diurnally
induced afternoon activity may produce slow-moving showers and
thunderstorms with locally heavy rainfall, leading to an elevated
flood threat, especially in the central and west to southwest
sections of Puerto Rico.

By the weekend, the latest model guidance suggests that the area of
convergence associated with the trough will be over the region,
which may help to limit rain chances. Daytime heating and local
effects will still enhance afternoon convection in some areas,
particularly in the central interior and western sections of
Puerto Rico. In contrast, the U.S. Virgin Islands is expected to
see mostly isolated showers.

Warm to hot conditions are likely to continue throughout this
period, with temperatures potentially reaching the upper 90s and
heat indices remaining elevated. Residents and visitors should
take precautions to stay hydrated by drinking plenty of water,
wearing light and breathable clothing, and avoiding prolonged
exposure to the sun, especially during peak heat hours.

AVIATION...
(06Z TAFs)

Mainly VFR conditions expected to prevail. Very isolated showers
across the local waters are not expected to have impacts in
operations today. Then, after 18Z, SHRA and isolated TSRA expected
in the vicinity of TJBQ, with brief periods of reduced VIS and low
ceilings possible. Winds are from the ESE at 8-12 kts, with stronger
gusts.

MARINE...

A weak surface high pressure will promote gentle to moderate east to
southeast winds while moving west across the Atlantic waters. By
midweek, winds will turn lighter and more northeasterly. Today,
showers will become isolated over the local waters as drier air moves
in. A few thunderstorms are possible off the western coast of Puerto
Rico each afternoon. Overall, seas will range between 2 to 4 feet,
occasionally up to 5 feet over the offshore Atlantic waters and
passages, and between 1 to 3 feet elsewhere during the next 5 to 7
days.

BEACH FORECAST...

Today, the risk of rip currents will remain low at most area
beaches, with the exception of St. Croix, where there is a
moderate risk. Over the next few days, moderate risk will be
prevalent, particularly at the northern-facing beaches of Puerto
Rico, Vieques, and St. Croix. For a more detailed forecast, please
refer to the Surf Zone Forecast for Puerto Rico and the U.S.
Virgin Islands (SRFSJU).

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001-
     005-008-010-011.

     Heat Advisory until 5 PM AST this afternoon for PRZ002-003-007-
     012-013.

VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002.

     Heat Advisory until 5 PM AST this afternoon for VIZ001.

AM...None.

&&

$$

SHORT TERM/AVIATION/MARINE...WS
UPPER AIR/PUBLIC....MRR/MNG