585
FXCA62 TJSJ 152027
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
427 PM AST Sun Sep 15 2024

.SYNOPSIS...

The dry airmass that continues to move across the islands will
limit the shower activity through the overnight hours. Hot and
humid conditions will continue into early next week with an
elevated to significant heat risk possible across portions of the
islands. The prevailing southeasterly winds will become more
easterly by Monday as a surface high pressure ridge will build
north of the region.

&&

.SHORT TERM...Tonight through Tuesday...

Sunshine and southerly winds have dominated the weather picture so
far. San Juan has already broken its daily temperature record with
95 degrees and many stations along the north central have reached
heat indices between 117 to 122 degrees Fahrenheit. The heat
advisory along the north central was upgraded to an excessive
heat warning. Temperatures elsewhere in the lower elevations
ranged from around 90 to the mid 90s. Temperatures in the mid to
upper elevations ranged from the mid 80s to the upper 70s
respectively.

High pressure at the surface continues in the northeast Atlantic
with only a tenuous ridge extending southwest into the northeast
Caribbean. Tropical Storm Gordon will continue its westward travel
through Monday and has split the ridge so that on Monday a weak high
pressure will be found only 400 miles north northeast of San Juan.
This will turn the low level flow more easterly and give a few
degrees of relief to the persistent heat we have been experiencing.
Unfortunately, it will likely not be enough, with the high dewpoints
we have been having, to end the daily occurrence of heat advisories
in the lower elevations. Overall moisture is expected to increase on
Tuesday with a slight increase in shower activity. Additionally,
instability will reach a bottom on Monday evening with only a slow
recovery on Tuesday to limit convective activity in the area.

At upper levels, a ridge over San Juan, Puerto Rico extends about
1000 miles east northeast to its associated high pressure. A lobe of
this high pressure will pass north of Puerto Rico to the Dominican
Republic by Tuesday morning leaving the local area under subsidence
and a drying influence. The high dewpoints represent only a shallow
layer and convection tonight through Tuesday will be minimal,
although a few afternoon thunderstorms cannot be ruled out over
Puerto Rico. A few minor showers will also continue over the local
waters.

&&

.LONG TERM...Wednesday through Sunday...
//from previous discussion//

An overall dry and stable weather pattern is forecast at least
through Wednesday with winds becoming more easterly and gradually
diminish. A slight increase in trade wind moisture is forecast by
Thursday and rest of the workweek as winds become more east
northeast and bands of shallow moisture is brought in across the
area from the central and northeast Atlantic. Thereafter the local
winds become light and variable with a northeasterly component as
the surface high pressure ridge relaxes north of the region. Recent
guidance continue to suggest a retrograding Tutt will deepen and
sinks southwards between Puerto Rico and the northern Leeward
islands by Friday into the weekend. This in turn will destabilize
the upper levels and favor better chance for early morning and
afternoon convection across the region over the weekend. Activity
however should be more localized and not widespread. However, good
daytime heating and local effects will support enhanced afternoon
convection in some areas across portions of central interior and
west sections of Puerto Rico each afternoon. Lesser activity is so
far forecast around the U.S.Virgin islands where mostly isolated
showers will be possible for the most part.

The prevailing surface features will be a building surface high
pressure over the west and southwest Atlantic, interacting with
what is now Tropical Storm Gordon. This is still forecast to move
well northeast of the region over the Central Atlantic and have
no direct impacts on the islands. Under this expected pattern, a
mix of sunshine and clouds and variable weather conditions are
forecast, with an overall drier airmass to remain in place, along
with some traces of suspended dust particulates. The occasional
passage of bands and shallow pockets of moisture from time to time
can be expected across the region. This will be sufficient to aid
in the development of nocturnal and diurnal convection each day.
This scenario along with the intense daytime heating will lead to
periods of enhanced afternoon convection in isolated areas especially
across portions of Puerto Rico each day.

For the most part, the winds are expected to be fairly light and
therefore any locally and diurnally induced afternoon activity will
will be sufficient to produce slow-moving showers and thunderstorm
with locally heavy rains. This may lead to periods of elevated flood
threat especially across the central and west to southwest sections
of Puerto Rico.

&&

.AVIATION...
(18Z TAFs)

Expect mostly VFR except vcty SHRA/TSRA where mtn obscurations will
be likely. Most TAF sites will be unaffected, but TJBQ may have
brief interludes of MVFR conds. Sfc winds SE 10-15 kts with sea
breeze intrusions along the northwest and west coasts and where they
would occur due to the onshore nature of the general flow. Aft
15/22Z land breezes will develop and SHRA activity will greatly
diminish. Winds will shift to more easterly before 16/12Z and sea
breezes will recommence arnd 16/12Z at 10-15 kts with gusts to 22 kt
psbl. Outlook: Conds impvg with much less mstr Monday and Tuesday.

&&

.MARINE...

A weak surface high pressure, now northeast of the area, will move
west across the Atlantic waters about 400 miles north of Puerto Rico
Monday night. East southeast winds of 10 to 15 knots will prevail,
turning more easterly tonight and Tuesday. Showers will become
isolated over the local waters as drier air moves in. A few
thunderstorms are possible off of the western coast of Puerto Rico.
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...

Tonight, beachgoers can expect a low risk of rip currents along
most of the local beaches of the USVI and PR. Winds along the
coastal areas will be between 10 to 15 mph with sea breeze
variations and occasionally higher gusts during the afternoon
hours.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>003-007-
     008-010>013.

     Excessive Heat Warning until 5 PM AST this afternoon for PRZ005.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM...WS/ICP/MMC
LONG TERM...RAM
AVIATION...WS
PUBLIC DESK...ICP