026
FXCA62 TJSJ 180845
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
445 AM AST Tue Jun 18 2024

.SYNOPSIS...
A broad surface high pressure extends southwest into the central
Atlantic while an elongated area of low pressure extends across
Hispaniola into the eastern Caribbean. A tropical wave with axis
now south of Puerto Rico continues to cross the region. This will
aid in promoting light east southeast winds today. High pressure
is forecast to build north and east of region thereafter and promote
an increase in the easterly winds. The upper trough and area of low
pressure continues to become amplified west of the region through
Wednesday. This along with the tropical wave will maintain moist
and unstable conditions through mid week resulting in periods of
developing showers and isolated thunderstorms each day. A slight
to moderate concentration of Saharan dust is forecast to spread
across the area Wednesday into Thursday.

&&

.SHORT TERM...Today through Thursday...

Isolated to scattered showers were observed all night into the early
morning across the eastern sections of Puerto Rico and the U.S.
Virgin Islands. According to radar estimates, rainfall accumulations
were between 1 to 2 inches over some Humacao, Yabucoa, Ceiba, and
Fajardo coastal areas. Several pockets of showers also affected most
of the local waters, causing localized higher seas and gusty winds.
The islands registered minimum temperatures in the upper 70s to
lower 80s across the coastal areas and in the mid to upper 70s
across the interior sections.

A deteriorated weather pattern will persist at least until late
Wednesday across the region. At the surface, the islands will be
mostly influenced by the interaction between a broad surface high
pressure over the Central Atlantic and an induced surface trough
just northwest, resulting in a light southeast wind flow over the
region. This wind flow will bring plenty of tropical moisture, with
rainfall amounts exceeding 2 inches from the upcoming tropical wave.
In the mid-to-upper levels, a trough will continue to deepen over
Hispaniola, placing the islands under the divergent side of this
TUTT. This instability will enhance colder temperatures at 500 MB
and higher RH values at 750-500 MB. According to the Glvez-Davison
Index (GDI), a good indicator of thunderstorm activity, there is a
high chance for isolated to scattered thunderstorm activity across
the interior. Ample deep-layer moisture will persist through
Wednesday as the tropical wave moves over the region. The available
moisture, saturated soils, and instability aloft will trigger early
afternoon showers, affecting the interior and the San Juan Metro
area. On Thursday, as the induced surface trough moves westward,
building surface high pressure over the North-Central Atlantic will
tighten the pressure gradient and create backing winds. A wet
pattern will continue due to abundant moisture over the CWA, but the
focus of the showers will shift more to the interior and western
sections due to the easterly winds across the area.

Due to the significant rainfall observed over the past two days,
soils are already saturated, leading to urban and small-stream
floods in some areas across the islands. Therefore, it`s crucial for
residents and visitors to be aware of these conditions for safety
and operational reasons and to check weather conditions during the
day for possible advisories or warnings.

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

Conditions are so far forecast to improve by Friday with increasing
stability aloft, as the previously mentioned upper trough west of
the area weakens and high pressure builds across the area from the
central and eastern Caribbean. Low level moisture transport will
also erode as the easterly winds increase resulting in less frequent
passing late evening and early morning shower activity. The
potential for afternoon shower development and isolated
thunderstorms will continue on Friday but mainly due to local
effects and good daytime heating. Shower activity should be more
localized and focused over the interior and west section of Puerto
Rico steered by the moderate trade winds. Overall improving
condtions is expected elsewhere on Friday and into Saturday with
plenty of sunshine and near seasonal temperatures expected.

For the rest of the following weekend prevailing easterly winds are
forecast with periods of passing early morning showers affecting the
windward side of the islands. This will be followed by afternoon
convection limited to mainly parts of the central interior and west
sections of Puerto Rico. Plenty of sunshine and mostly pleasant
summertime weather is expected elsewhere although a weakly induced
easterly perturbation will cross the region late Saturday into
Sunday followed by the next topical wave so far forecast to enter
the eastern Caribbean late Sunday then cross the region Monday
through Tuesday. That said expect lesser showers and thunderstorm
activity across during the early part of the period with the best
potential for increasing instability and convection so far
forecast for the latter part of the period on Monday and Tuesday
due to the approaching tropical wave and associated moisture
field.

&&

.AVIATION...
(06z)

VCTS and -RA will persist through 18/15Z, followed by SHRA & TSRA,
which are forecast from 18/16Z for TJSJ, TJPS, and TJBQ due to
strong showers across the region. These conditions will result in
MVFR and brief IFR conditions across the major TAF sites of the
island. Lower ceilings and limited VIS are expected over the
mountains as well. Winds will continue light and VRB, increasing at
18/15Z from the E-SE at 12 knots with gusty winds near the showers.

&&

.MARINE...
Surface high pressure over the Atlantic and a tropical wave crossing
the region will promote light east to southeast winds today, then
becoming more easterly and increasing to moderate to locally fresh
by Wednesday and through the rest of the week. Passing showers and
isolated thunderstorms will continue to affect the coastal waters
and local passages resulting in localized hazardous marine conditions.

&&

.BEACH AND RIVER FORECAST...

The risk of rip currents is low for all beaches today through
Wednesday morning, however life threatening rip currents may
still occur especially near groins, jetties, reefs, and piers.

Local rivers and streams are at normal levels however expected
afternoon showers and thunderstorm development across the interior
and west sections of Puerto Rico may produce locally heavy rains
at the headwaters and this may cause sudden surges of water along
rivers and streams across theses areas. Please monitor the weather
conditions throughout the day and always take time to look
upstream.

&&

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

$$

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