422
FXCA62 TJSJ 121844
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
244 PM AST Mon May 12 2025

.KEY MESSAGES...

* More stable weather conditions expected in the next few days;
  however, afternoon showers and thunderstorms, driven by local
  effects, are anticipated across western Puerto Rico and
  downwind of the local islands.

* Breezy conditions will persist for the next few days, mainly
  across coastal areas.

* Increasing winds will result in choppy to rough seas and a
  moderate risk of rip currents through the end of the workweek.

* More unstable and wetter weather conditions are expected by
  Friday and continuing into the upcoming weekend.

&&

.SHORT TERM...Tonight through Wednesday...

A surface high-pressure system over the central Atlantic will
maintain a tight pressure gradient across the region, resulting in
breezy conditions with steering flows around 15-20 knots. Weather
stations across the islands have already reported wind gusts ranging
from the mid-20s to low 30s (mph), reflecting the influence of this
pattern.

The weather pattern remains generally on track, with some changes
expected through most of the short-term forecast period. The TJSJ
12/12Z upper-air sounding indicated a slight reduction in moisture
content, with precipitable water at 1.49 inches and mid-level
relative humidity at 45%. Wind direction has also shifted slightly,
with winds from the east-northeast at 14 to 15 knots at the surface
to 2 km and 3 km layers. Radar and satellite data support this
trend, showing streamers developing downwind of the smaller islands
and extending from northeast to southwest. GOES-19 precipitable
water imagery further confirms the drying trend, depicting an
airmass between 1.4 and 1.5 inches moving in from the east.

A ridge building at the mid to upper levels is settling in and will
persist through the short-term forecast period, contributing to a
more stable weather pattern characterized by warming 500 mb
temperatures and decreasing lapse rates. Moisture content is
expected to decrease further on Tuesday before increasing to near or
above normal levels from Wednesday into Wednesday night, as winds
shift from the southeast and pull plenty of tropical moisture across
the CWA. Despite the more stable conditions aloft, sufficient low-
level moisture will remain to support shallow convection each
afternoon, enhanced by diurnal heating and local effects. The
highest flooding risk will be over southwestern Puerto Rico this
afternoon and tomorrow, with Tuesday expected to see the least
shower coverage. On Wednesday, despite the presence of ridging
aloft, the surge in moisture will lead to increased shower activity,
particularly along windward coastal areas during the evening hours,
and across interior and western to northwestern Puerto Rico under
the influence of the southeasterly wind flow.

.LONG TERM...Thursday through Monday...

.PREV DISCUSSION... /issued 527 AM AST Mon May 12 2025/

Increasingly wet and unstable condtions are forecast during most
of the long term forecast period. After mid-week, current model
guidance has precipitable water (PWAT) values close to around 2
inches, above normal values for this time of the week. A decrease
in PWAT values is forecast by late Sunday and Monday, decreasing
to around 1.8 inches. A mid to upper-level ridge present on
Thursday will gradually weaken as a mid to upper level trough
moves eastward from the western Atlantic towards the local
islands, this will promote more favorable conditions for shower
and thunderstorm development late Thursday onwards. A surface
trough will also develop, also providing instability and
promoting, along with a surface high over the central Atlantic,
more east-southeast to southeast steering flow over the region.
Deep tropical moisture will start to move into the region, with
current model guidance showing a completely moist atmospheric
profile, possibly being somewhat confined once again to below
650mb by late Saturday and into early next week. With the
prevailing wind flow, increased temperatures are also forecast in
areas without significant cloudiness, particularly during the late
morning hours. Ultimately this diurnal heating, along with sea
breeze convergence and local orographic effects, can also help
promote shower and thunderstorm development, adding to the already
unstable atmospheric conditions. By Friday and into the weekend,
conditions will be conducive to increased convective activity.
Widespread showers and thunderstorms are forecast, particularly
across the Cordillera Central and western Puerto Rico, with lines
of convection possibly also developing from the local islands and
El Yunque. With already saturated soils and these moist and
unstable conditions, a limited to elevated flooding risk is
forecast during these days, meaning that urban, small stream and
flash flooding is possible. Stay tuned for any updates.

&&

.AVIATION...
(18Z TAFs)

Mainly VFR conditions across most TAF sites. VCSH across TJSJ, TJPS
over the next hours, then tonight across TISX. Main concern will
continue to be the easterly breezy winds, expected around 10 to 15
knots with gusts up to 27 to 30 knots at least through 12/23Z,
decrease to 8 to 12 knots overnight and then increase again tomorrow
by 12 to 14Z.


&&

.MARINE...

A broad surface high pressure system over the central Atlantic will
sink southward over the next few days, leading to increasing winds
and choppy conditions across the offshore waters. A Small Craft
Advisory is now in effect for the Atlantic offshore waters and the
Anegada Passage, with seas expected to build up to 7 feet by this
afternoon. Small craft should exercise caution across most of the
other local waters. Thunderstorm activity is likely across regional
waters, particularly during the afternoon and evening hours, which
may result in locally higher winds and seas.

&&

.BEACH FORECAST...

Tonight, there is a moderate risk of rip currents across all coastal
areas of Puerto Rico, Vieques, Culebra, and the U.S. Virgin Islands.
This moderate risk will persist through most of the workweek in
these same areas, except along the western and central southern
coasts of Puerto Rico, where a low risk is more likely. Life-
threatening rip currents often occur near jetties, reefs, and piers.
A low risk is anticipated by the end of the week and into the
weekend.

&&

.HYDROLOGY...

Forecast models continue to indicate the development of another wet
and unstable weather period in recent updates. By the end of the
workweek, increasing atmospheric instability, combined with abundant
tropical moisture, is expected to result in widespread thunderstorm
activity, with some storms capable of producing heavy rainfall. This
pattern will be driven by the interaction between a deep-layered
trough and unusually high precipitable water values for this time of
year, ranging from approximately 2.0 to 2.25 inches. The wet and
unstable weather pattern is expected to persist through the weekend.

Prolonged periods of moderate to heavy rainfall will increase the
risk of urban and small stream flooding, flash flooding, and
mudslides in areas of steep terrain. Additionally, significant
runoff from excessive rainfall could result in quick river rises,
with some rivers potentially exceeding flood-stage levels. Soils
remain saturated and streamflows elevated due to the extended wet
period experienced in recent weeks. These antecedent conditions will
further exacerbate the aforementioned impacts across the region.
Residents and visitors, especially those in flood-prone areas or
planning outdoor activities, are advised to closely monitor the
latest weather forecasts and updates.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...Small Craft Advisory until 6 AM AST Tuesday for AMZ711-723.

&&

$$

SHORT TERM/HYDROLOGY...CVB
LONG TERM...MRR
AVIATION/MARINE/BEACH FORECAST...YZR