833
FXCA62 TJSJ 252059
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
459 PM AST Wed Sep 25 2024

.SYNOPSIS...

By Thursday, stability will increase somewhat and rain activity
should be less, but local effects and diurnal heating will result
in showers and isolated thunderstorms across the west central
areas of Puerto Rico. 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. Warm to hot conditions are expected to continue over
the next few days. A moderate risk of rip currents will prevail
across the north-facing beaches of the islands through the
weekend.


&&

.SHORT TERM...Tonight through Friday...

Surface weather observations and satellite imagery indicated quiet
weather conditions across the islands during the morning, followed
by increased cloud cover and the development of strong thunderstorms
mainly across the interior and west-central regions of Puerto Rico
after 1 PM AST. According to radar estimates, between 2 to 3 inches
of rain were recorded in those municipalities. Small hail was
reported in Las Marias. Before the rain, it was another hot day,
with heat index values generally exceeding 100 degrees Fahrenheit
in the coastal and lower-elevation areas of Puerto Rico and the
U.S. Virgin Islands.

Rain over the western sectors is already weakening. Additionally,
showers are likely later tonight across the local waters and the
eastern quadrant of Puerto Rico. Tomorrow, we expect some trade
wind showers to filter across the eastern municipalities of Puerto
Rico and the U.S. Virgin Islands during the morning, followed by
the development of shower and thunderstorm activity in the west-
central sections of Puerto Rico. By late Thursday into Friday,
moisture associated with a tropical wave will reach the forecast
area. The latest precipitable water (PWAT) models suggest values
gradually increasing from the normal to above-normal range. This
tropical wave will heighten the potential for rainfall and
thunderstorm activity. The flood threat will range from limited
to elevated each day. Hazards include mudslides in steep terrain,
flooding over roads, and in poorly drained areas. Warm to hot
conditions are expected to continue over the next few days,
maintaining a limited to elevated heat risk across lower and urban
areas of the islands.

&&

.LONG TERM...Saturday through Tuesday...
.PREV DISCUSSION... /issued 538 AM AST Wed Sep 25 2024/

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...
(18Z TAFs)

Mainly VFR conditions expected to prevail. TSRA across western PR
are expected to taper off around 25/22Z. SHRA will then increase
around 06Z in the vicinity of the USVI, potentially reaching TJSJ
at 09-12Z. Periods of reduced VIS and low ceilings are possible in
these areas. Winds will be light and variable from 22-12Z, then
increasing out of the E at 11-15 kts, with stronger gusts.

&&

.MARINE...

A weak high pressure ridging over the region will maintain gentle to
moderate easterly winds over the next few days. Showers and isolated
to scattered thunderstorms will develop from time to time across the
regional waters, particularly over western coastal waters of Puerto
Rico each afternoon. A northerly swell will maintain seas to around
4 to 5 feet through the weekend. Thunderstorms will become more
isolated tonight and Thursday, but the activity will increase again
on Friday as a tropical wave approaches our region.

&&

.BEACH FORECAST...

There is a moderate risk of rip currents along the northern side
of most of the islands tonight and continuing through Tuesday.
The risk may become high on the northwest corner of Puerto Rico on
Friday.

&&

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

$$

SHORT TERM...YZR/MNG
LONG TERM...WS
AVIATION...ERG
PUBLIC...CVB/MNG