276
FXCA62 TJSJ 250938
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
538 AM AST Wed Sep 25 2024

.SYNOPSIS...Increased instability and very warm to hot
temperatures with high relative humidity will bring energetic
showers and thunderstorms to the area today with frequent
lightning as well. On Thursday and Thursday night stability will
increase and this activity will be reduced, but a tropical wave
will move through on Friday and Friday night. Moisture will still
be abundant over the weekend and Monday and Tuesday, but less
shower activity will occur then.

&&

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

Overnight, some showers developed over Saint Croix and the eastern
municipalities of Puerto Rico (Humacao, Naguabo, Fajardo, and
Luquillo), as well as the island of Vieques. The rest of the islands
experienced fair weather conditions. Like previous days,
lightning activity lit the sky, particularly along the eastern,
northern, and southern coastal areas of Puerto Rico and the U.S.
Virgin Islands, as some showers and isolated to scattered
thunderstorms developed over the Atlantic and Caribbean waters as
well as the Anegada Passage. Tonight, the low temperatures
remained in the upper 60s to lower 70s in the mountains and in the
upper 70s to lower 80s in the urban/coastal areas. At the Luis
Munoz Marin International Airport, the minimum temperature stayed
at 82 degrees Fahrenheit as of 5 AM AST which will be a record
warm minimum if we can keep it through sunrise. The winds were
relatively light and variable across Puerto Rico and the U.S.
Virgin Islands.

Today, the citizens and visitors of Puerto Rico should experience
mostly fair weather through the morning hours, with some trade wind
showers filtering in from the east, particularly across the
eastern municipalities of Puerto Rico and the U.S. Virgin Islands.
However, these showers should move quickly as the winds are
slightly stronger, around 10 to 15 knots today. Each afternoon,
we anticipate the development of shower and thunderstorm activity
across the interior and western sections of Puerto Rico. In fact,
the latest guidance shows that 500 mb temperatures will be around
minus 8 degrees Celsius, meaning that there is an increased
potential for more thunderstorm development, strong wind gusts,
small hail and locally heavy rainfall that would likely cause
urban and small stream flooding and possibly flash flooding.

By late Thursday into Friday, moisture associated with a tropical
wave that will move mostly south of the islands will reach Puerto
Rico and the U.S. Virgin Islands. The latest precipitable water
(PW) models suggest values gradually increasing from the 50th
percentile (around normal PW content) to the 75th percentile
(above normal PW content). This tropical wave will increase the
potential for rainfall and thunderstorm activity across the
islands through the weekend. Some impacts associated with both this
and today`s rainfall will be mudslides in steep terrains,
flooding over roads, and poorly drained areas.

Temperatures will be in the upper 70s to lower 80s over the higher
terrain, and the upper 80s to lower 90s will persist across the
lower elevations of the islands during the morning and early
afternoon hours. Therefore, excessive heat conditions can be
anticipated throughout the short-term period. Advisories and
warnings for heat have been issued again today.

.LONG TERM...Friday through Tuesday...

A tropical wave will begin pushing against a weak high pressure
north of the area around 30 north and the flow behind the wave
will turn briefly east southeast after it passes. However, high
pressure at 700 mb over the Bahama Islands and a tropical cyclone
developing between 40 and 50 west and moving north through 20
north will bring flow at lower levels back to the east northeast
by Saturday morning. On Sunday a tropical wave approaches the area
but it is highly tilted and not very strong, making flow chaotic
and generally less than 12 knots Monday and Tuesday.

At upper levels on Friday high pressure is building over the
waters north of the western Bahama Islands and allowing a jet to
strengthen to 60 knots at 250 mb northwest of Puerto Rico and the
area in general. This will increase divergence over the area on
Friday enhancing activity with the wave passage. A TUTT low near
19 north and 58 west will drop rapidly southwest to tighten the
gradient even more across the area Saturday and Saturday night
adding to the ventilation across the area. High pressure at mid
levels to the north during the period will keep temperatures at
500 mb relatively high (warmer than -4 degrees C) and so
convection will not reach its full potential. Precipitable water
will be mostly below 2 inches Sunday through Tuesday with some
drying in the lower levels as well.


&&

.AVIATION...
(06Z)

VFR conditions at all Taf sites. TJSJ, TJPS, and TJBQ will
experience brief MVFR conds, particularly after 25/18Z. Winds less
than 15 kt with a few higher gusts along the coasts and near TSRA
till 25/23Z. TSRA will still be found offshore and near TJSJ, TIST,
and TISX aft 25/22Z.

&&

.MARINE...Seas will be generally benign, but the passing tropical
wave with increasing winds and a north northeast swell that will
pass through the area Friday and Saturday will cause seas to reach
4.5 to 5.5 feet then. Nevertheless, small craft advisory
conditions are not expected through the first few days of October.

&&

.BEACH FORECAST...
There will be a moderate risk of rip currents along the northern
side of the islands beginning today and continuing through
Tuesday. The risk may become high on the northwest corner of
Puerto Rico on Friday.

&&



.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>004-012-
     013.

     Excessive Heat Warning until 5 PM AST this afternoon for PRZ005-
     007-008-010-011.

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

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

AM...None.

&&

$$

SHORT TERM/AVIATION...MMC
LONG TERM/MARINE....WS