225
FXCA62 TJSJ 160843
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
443 AM AST Sun Jun 16 2024

.SYNOPSIS...

A more east-southeasterly wind flow will relieve the heat indices
today. Although conditions will be warmer, the heat indices will
remain near 108 to 112F for all coastal areas and urban sections
of the vicinity of Caguas. Given the expected conditions, a heat
advisory is in effect for the abovementioned areas, and an
excessive heat watch is in effect for the same areas for Monday.
The available moisture will induce showers across the mountains
and the western interior during the afternoon hours.

&&

.SHORT TERM...Today through Tuesday...

A broad surface trough with axis extending northward across
Hispaniola and the Mona passage and strong High pressure over the
central and northeast Atlantic will maintain a light east southeast
wind flow across the region. Fragments of trailing low level
moisture will continue to affect the region bringing periods of
passing showers to the coastal areas of the islands during the rest
of the morning hours. Recent Doppler weather radar VAD (Velocity
Azimuth Display) wind profile and previous ASCAT Satellite derived
winds both suggest light low level wind from east southeast below
700 millibars. Meanwhile the upper ridge will hold across the region
today into early Monday before eroding, as an upper trough is
forecast to cross the west and southwest Atlantic and settle in just
west of the region by Tuesday afternoon.

This overall local pattern suggest another day of good surface
heating with light winds and local sea breezes. Therefore good
breaks in clouds and plenty of sunshine will give way to afternoon
showers and isolated thunderstorm development over portions of the
central interior and west sections of Puerto Rico. A few late
afternoon showers will be possible in and around the San Juan metro
as well some of the USVI but mostly hot and sunny conditions should
prevail. Only traces of Saharan dust will linger across the region
today through Monday but yet another plume of dust is expected to
filter in and reach the Southeast Caribbean by late Tuesday
accompanying and trailing the next tropical wave. This tropical wave
is so far forecast to enter and cross the southeast Caribbean late
Monday through Tuesday. By then and increase in low level moisture
convergence and the erosion of the aforementioned upper ridge should
aid in enhancing early morning and afternoon convection across the
islands during the latter part of the period.

Daytime high temperatures are forecast to be in the upper 80s to low
90s especially along the north and west coastal areas. This along
with the humid conditions ans southeasterly winds will again favor
maximum heat indices to exceed 100 degrees, resulting in excessive
heat impacts especially along the west, north and east coastal and
urban areas of Puerto Rico as well as the adjacent islands including
the USVI. Please refer to the latest Non-Precipitable Weather
Message(NPWSJU) issued by WFO SJU PR for additional information on
the expected heat impacts.

.LONG TERM...Wednesday through Sunday...

More unstable weather conditions are expected at the beginning of
the long term due to an upper-level trough just west of the
region, with the axis over Hispaniola, leaving the CWA on the
divergent side of the TUTT. Although there are some discrepancies
in the intensity of the unstable conditions between global model
guidance, the islands should expect a deteriorating weather trend
from Wednesday into Thursday. At the surface, moisture from a
tropical wave will combine with this instability, increasing the
potential for shower and thunderstorm development. Isolated
thunderstorms are possible in the afternoon across the mountains
and over the local waters due to colder temperatures at 500 MB,
ranging from -7 to -8 degrees. According to the Glvez-Davison
Index (GDI), there is a potential for isolated to scattered
thunderstorms from Wednesday through Friday. Therefore, the
forecast reflects afternoon showers with a limited flood threat.
Hazy skies might be possible as a big pulse of Saharan Dust Air
Layer invades the local islands.

From Saturday into Sunday, a broad surface high pressure over the
Central Atlantic extending to the Eastern Atlantic will result in
variable weather conditions. Under the east-southeasterly wind
flow, plenty of surface moisture will continue to stream across
the local islands, keeping the potential for convection in the
afternoon, especially across the mountains and the western
interior. Although plenty of moisture will remain in place, more
stable conditions will be present at the upper level, provided by
an upper-level ridge.

A similar temperature pattern is expected across the islands by
the first part of the long term. According to model guidance for
925 MB temperature, the trend calls for warmer conditions,
resulting in heat indices around 108 to 112F. Residents and
visitors are urged to stay informed of possible advisories or
warnings due to excessive heat.

&&

.AVIATION...
(06Z)
Wdly SCT SHRA ovr regional waters and passages between PR and the
USVI til 16/12Z with Isold TSRA ovr Offshore ATL waters. SCT ocnl
BKN lyrs nr FL025...FL050..FL080. VFR conds at all terminals but
VCSH psbl at TJSJ/TIST/TISX til 15/12Z. Brief MVFR with Mtn top
Obscr psbl ovr Eastern PR due to low clds and passing SHRA. SFC calm
to lgt/vrb bcmg mainly fm the SE between 8-12 kts aft 16/14Z. No sig
operational wx impacts attm.

&&

.MARINE...
A broad surface high pressure over the Central Atlantic extending
into the Eastern Atlantic, interacting with an induced surface
trough just north of the CWA, will continue to promote light to
moderate east-southeasterly winds. Under this pattern, the local
waters will experience seas up to 4 feet and winds up to 10 knots.
Isolated to moderate showers with thunderstorm activity could
result in localized hazardous marine conditions. There is a low
risk of rip currents for all the exposed coastal beaches in the
region.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 6 PM AST this evening
     for PRZ001-002-005-008-010-012-013.

     Excessive Heat Watch from Monday morning through Monday
     afternoon for PRZ001-002-005-008-010-012-013.

VI...Heat Advisory from 10 AM this morning to 6 PM AST this evening
     for VIZ001-002.

     Excessive Heat Watch from Monday morning through Monday
     afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM...RAM
LONG TERM....LIS
AVIATION...RAM