715
FXCA62 TJSJ 120927
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
527 AM AST Mon May 12 2025

.KEY MESSAGES...

* Afternoon showers and thunderstorms will affect western PR and
  downwind of the local islands.

* More unstable and wet weather conditions are forecast by Friday
  and into the upcoming weekend.

* Breezy conditions will persist for the next few days.

* Increasing winds will lead to choppy to rough seas and a
  moderate risk of rip currents.

&&

.SHORT TERM...Today through Wednesday...

Variable weather conditions persisted across the local islands
during the night and early morning hours. Brief showers were
detected by Doppler radar over portions of the eastern region,
leaving minimal rainfall accumulations. Across the western half of
Puerto Rico, mostly clear skies prevailed. Overnight temperatures
remained in the lower 80s along coastal areas and were noticeably
cooler in the mountainous regions.

From today into early Wednesday, the weather pattern is expected
to become more favorable across the CWA as a mid-to-upper-level
ridge establishes itself over the western Atlantic. This feature
will promote drier conditions aloft and a pronounced trade wind
cap inversion, which will limit the vertical development of
stronger showers. At the surface, a broad high-pressure system
over the central Atlantic extending into the Caribbean will
slightly tighten the pressure gradient, resulting in breezier
conditions across the CWA. These conditions will support a stable
but variable weather pattern during the morning, with extended
periods of sunshine and some brief showers over eastern Puerto
Rico and the U.S. Virgin Islands. Although the potential for
significant rainfall is reduced, afternoon convection due to
diurnal heating is forecast across the western interior and nearby
municipalities. Rainfall totals are expected to range between 1
and 2 inches in areas affected by the bulk of the activity, though
fast-moving showers driven by the prevailing trade winds will
likely result in only localized impacts.

A similar pattern is expected on Tuesday; however, GFS model
guidance suggests a slight decrease in precipitable water values
from 03 km, down to around 1.1 inches. These drier conditions
indicate that Tuesday may be the most stable day across the
region. By Wednesday, the surface high over the western Atlantic
is forecast to sink southward and interact with an induced
surface trough, resulting in veering winds across the region. This
southeasterly wind flow will lead to an increase in atmospheric
moisture, raising the potential for shower development and
contributing to higher heat index values, marking the beginning of
a warming trend.

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

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...

(06Z TAFs)

SFC will will peak at 12/14Z from the E up to 15 knots with gusty
winds up to 30 knots. After 12/18Z localized areas of SHRA/TSRA are
expected with local MVFR conds TJBQ and TJSJ reducing VIS a lowering
flight ceilings. Winds will remain up to 10 k knots after 12/23Z.


&&

.MARINE...

A broad surface high pressure over the Central Atlantic will sink
southward over the next few days, resulting in 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 due to seas building up to 7 feet by this
afternoon. Small Craft Should Exercise Caution over most of the
other waters. Thunderstorm activity is likely across the regional
waters, especially in the afternoons and evenings, resulting in
locally higher winds and seas.


&&

.BEACH FORECAST...

There is currently a moderate risk of rip currents for the
northern to eastern beaches of Puerto Rico, from Aguadilla to
Ceiba, as well as for southwestern Puerto Rico, and most of
Culebra, Vieques, St. Croix, St. John and St. Thomas. The moderate
risk of rip currents will extend tonight to areas of southern and
northwestern Puerto Rico tonight. In general, a low to moderate
risk of rip currents will persist through the workweek. Life-
threatening rip currents often occur in the vicinity of groins,
jetties, reefs, and piers.

If you become caught in a rip current, yell for help. Remain
calm, do not exhaust yourself and stay afloat while waiting for
help. If you have to swim out of a rip current, swim parallel to
shore and back toward the beach when possible. Do not attempt to
swim directly against a rip current as you will tire quickly.

&&

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

&&

$$

SHORT TERM...LIS
LONG TERM....MRR