037
FXCA62 TJSJ 211901
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
301 PM AST Wed May 21 2025

.KEY MESSAGES...

* Afternoon showers and thunderstorms are expected daily across
  the Cordillera Central of Puerto Rico, creating a limited to
  elevated risk of flooding rains.

* The warm to hot period will extend into tomorrow, Thursday, but
  will linger, in less magnitude, throughout much of the forecast
  period.

* An unstable weather pattern may return by early next week.

&&

.SHORT TERM...Tonight through Saturday...

Skies remained mostly clear today across Puerto Rico and the U.S.
Virgin Islands, allowing for strong diurnal heating. As a result,
maximum heat index values reached between 100 and 110F,
particularly across urban and coastal areas. Winds prevailed from
the east-southeast at 5 to 15 mph, with locally higher gusts
observed near passing showers. Doppler radar detected isolated
thunderstorm development downwind of La Sierra de Luquillo during
the early afternoon, followed by additional convective activity
along the Cordillera Central later in the day, particularly from
Villalba to Las Maras.

Today`s atmospheric sounding reveals a favorable setup conducive
to convective development across Puerto Rico, particularly in the
interior and northwestern sectors of the island due to the east-
southeast wind flow. One of the most noteworthy features is the
presence of steep lapse rates at both low and mid-levels of the
atmosphere. This stability parameter, combined with the wind
flow, the island`s topography, diurnal heating, and sea breeze,
will act as a lifting mechanism enhancing convective initiation
during the afternoon hours. Given the instability, moisture, and
favorable wind flow, there is an elevated risk for localized
flooding, especially in areas prone to poor drainage or where
soils are already saturated from recent rainfall events.

As the afternoon progresses into the evening hours, rain activity
will slowly dissipate. By sunset, most of the shower and
thunderstorm activity is expected to wane, allowing for clearing
skies and relatively calm weather conditions overnight. These
clear and calm conditions will promote nighttime radiational
cooling, with low temperatures dropping into the low 70s along the
coastal zones, while the mountainous interior could see
temperatures dip into the mid-60s, providing a cooler and more
comfortable night.

Although the warming/hot trend is forecast to wane by Friday,
there is a moderate to high confidence that we will observe
another Heat Advisory due to above normal values tomorrow,
Thursday, between 10 am and 4 pm AST. Generally, the typical May
rain pattern will hold through the short-term period, with calm
conditions during the mornings and overnight hours, without ruling
out one or two passing showers. Then, afternoon convection will
return mainly due to the diurnal heating, local effects, and sea
breeze.

.LONG TERM...Saturday through Wednesday...

The weekend will begin with a typical seasonal weather pattern,
as near-normal to slightly below-normal moisture levels combine
with marginal instability aloft, associated with weak ridging over
the northeastern Caribbean. Shower activity will follow the
characteristic diurnal pattern, with passing showers affecting
windward coastal areas of the islands during the night and early
morning hours. This will be followed by afternoon convective
activity, driven by surface heating, sea breeze convergence, and
orographic lifting over the interior and western portions of
Puerto Rico.

A few isolated thunderstorms may develop at times during this
period, but they are expected to be brief. Additionally, breezy
conditions will result in fast-moving showers, limiting rainfall
accumulations in any specific area. At most, a limited flooding
risk is expected, potentially leading to ponding on roadways and
poor drainage areas, with very localized urban and small stream
flooding. A limited excessive heat risk will also persist through
the weekend across urban and coastal areas, primarily affecting
heat-sensitive individuals, especially those outdoors without
effective cooling or adequate hydration.

Uncertainty persists by early next week due to the interaction
between a deepening upper-level trough to the west of the forecast
area and an approaching tropical wave from the east, currently
located near 28.5W. The timing and placement of these synoptic
features remain fairly consistent across model guidance, with the
latest runs positioning the trough axis over Hispaniola between
Tuesday and Wednesday. This places our area on the favorable
(eastern) side of the trough. At the same time, the tropical wave
is expected to begin lifting northward, moving over or near our
region. However, the most active portion of the wave now appears
to be slightly farther west compared to previous model runs.
Despite this shift, well above-normal moisture and increased
instability aloft will be present across the forecast area,
increasing the potential for numerous showers and thunderstorms.
This could lead to an increased flooding threat for Puerto Rico
and the U.S. Virgin Islands.

However, the Caribbean may be under the influence of the
suppressing phase of the MJO, which is indicative of reduced
upper-level divergence and decreased favorability for deep
convective activity. In fact, the latest CPC Global Tropics
Hazards Outlook assigns a 50% to 65% probability of below-average
rainfall across the northeastern Caribbean. This introduces
additional complexity to the forecast. Nevertheless, it is
important to continue monitoring the situation, as there remains a
chance for increased moisture convergence and instability to
result in a heightened flooding risk by early next week.

&&

.AVIATION...
VFR conditions will continue across all TAF sites. However,
SHRA/TSRA will develop along the Cordillera Central through
21/23z, creating mountain obscuration and possibly affecting JBQ,
where TEMPOs are in place. Winds will continue from the E-ESE at
10-15 kt but locally higher near TSRA/SHRA, and with sea breeze
variations, winds will become calm to light and variable after
21/23z. Winds will turn more from the E at around 15 kt tomorrow
after 22/13z. We expect a similar weather pattern tomorrow.

&&

.MARINE...
Light to gentle easterly winds continue today across most local
waters. However, a building surface high pressure over the central
Atlantic will bring moderate to locally fresh easterly trades tonight
through the weekend. Isolated thunderstorms will continue to develop
across the regional waters, particularly across the northern and
western coastal waters of Puerto Rico each afternoon.

&&

.BEACH FORECAST...

Beachgoers, the risk for rip currents is low tonight and
throughout the rest of the week. However, please exercise caution
as life-threatening rip currents often occur in the vicinity of
groins, jetties, reefs, and piers.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 4 PM AST this afternoon for PRZ001-003-005-
     007-008-010-011.

VI...None.
AM...None.
&&

$$

Day shift...CAM/GRS
Evening crew...LIS/MRR