265
FXCA62 TJSJ 161930
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
330 PM AST Mon Jun 16 2025

.KEY MESSAGES...

*Unstable and favorable weather conditions are expected to persist
 across the Northeast Caribbean due to an upper-level low that
 will meander north of the region through at least midweek.

*Haziness will continue due to suspended dust particles
 associated with a Saharan Air Layer.

*A surface high pressure across the Atlantic Basin will increase
 the local pressure gradient, creating choppy wind-driven seas
 across the regional waters.

&&

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

Once again, the presence of a thick upper-level cloud deck over
the islands delayed the onset of afternoon showers. However, the
Doppler radar detected a streamer downwind from the northern USVI
and Culebra, spreading into the northeast PR and affecting the San
Juan Metro area. Sea breezes developed, producing convection
across western Puerto Rico. Maximum temperatures ranged from the
upper 80s to the low 90s, with heat indices reaching around 103
degrees Fahrenheit due to the available moisture, primarily at
urban and coastal sites.

Showers and thunderstorms may result in flooding rains through
this evening, mainly across the west municipalities of PR. The
lingering Saharan Air Layer will produce hazy skies through at
least the middle of the week. This air mass will limit nighttime
cooling, promoting warmer-than-normal minimum temperatures across
Puerto Rico and the US Virgin Islands.

No significant changes were introduced to this afternoon`s
forecast as it continues on track. Thus, from Monday night through
Wednesday, the local area will be positioned between a westward-
moving upper-level low and a developing cut-off low to the
northeast, resulting in an upper-level omega-like pattern with
weak ridging aloft. This setup will enhance upper-level stability
by Wednesday and lead to a notable drying trend, with precipitable
water values dropping from around 2.10 inches to 1.40 inches and
mid-level relative humidity decreasing significantly. As moisture
availability declines, shower activity will follow a typical
diurnal pattern with reduced coverage. While localized afternoon
thunderstorms may still develop, the overall flooding threat will
be limited to elevated levels during peak heating hours.

.LONG TERM...Thursday through Monday...from prev discussion...

Variable conditions are expected for the long-term forecast.
Easterly-southeasterly winds will dominate most of the period due to
a surface high pressure over the western Atlantic. Based on the
latest global guidance, Precipitable Water values will gradually
increase by the end of the workweek due to a tropical wave
currently located over the eastern Atlantic. By Friday into
Saturday, moisture associated to the tropical wave is expected to
pool over the region, but suspended Saharan dust lingering over
the region may suppress significant shower activity over the local
area. The presence of a mid- level ridge over the CWA should
limit deeper convection activity during that period as well, with
warmer 500 mb temperatures (around -5 Celsius). Sunday may also be
the "driest" day of the long- term, based on recent guidance,
with PWAT value decreasing to below normals (around 1.4 inches),
with RH values in the low to mid levels plummeting to 10%.
Although deeper convection activity may be limited, the
combination of diurnal heating, sea breeze convergence, and local
effects could enhance shallow convection over western/northwestern
PR each afternoon. Another tropical wave should approach the
local islands by Sunday night into Monday, pooling well above
normal moisture (2.0 - 2.2 inches). Temperatures in the 500 mb
layer should cool to seasonal to near below normals (- 7 to - 8
Celsius), supporting deep convection activity. The frequency of
showers and thunderstorms should increase across the CWA, but
there`s uncertainty due variations in previous model cycles.

Under an E-SE wind flow with abundant moisture, heat indexes are
likely to exceed 100 degrees Fahrenheit each day, mainly over urban
and coastal areas of Puerto Rico. Hence, the risk will remain
limited, affecting individuals sensitive to heat.

&&

.AVIATION...
Expect mostly VFR conditions at all TAF sites during the period,
although hazy skies will persist due to SAL. VCTS will develop
near JBQ, and some SHRA/+SHRA will impact JSJ through 16/23z.
SHRA/TSRA will expand across the surrounding waters and may impact
local terminals, especially JSJ/IST/ISX, after 16/22z into
tomorrow. E-SE 15-20 knots with local sea breezes accompanied by
higher gusts. Winds bcmg less than 10 kts aft 16/23Z, and
returning to similar wind conditions as today after 13/13z
tomorrow.

&&

.MARINE...

The surface high pressure northeast of the region will continue to
gradually shift east over the coming days. While some fluctuations in
the wind pattern are expected, moderate to locally fresh winds from
the east to east-southeast will promote choppy conditions mainly
over exposed areas, small craft should exercise caution. A plume
of suspended Saharan dust is expected to linger through midweek,
potentially causing a slight reduction in visibility. Meanwhile,
increasing moisture and instability associated with an upper-level
trough will likely lead to a rise in thunderstorm activity into
early in the week.

&&

.BEACH FORECAST...

Breezy to locally windy easterlies will maintain a moderate risk
of rip currents for most beaches of Puerto Rico and the US Virgin
Islands. Beachgoers must exercise caution due to life-threatening
rip currents along exposed unprotected beaches. Additionally,
please be advised of the possibility of seeing afternoon
thunderstorms, particularly over western and northwestern Puerto
Rico.

&&

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

$$

SHORT TERM...99
LONG TERM....99
AVIATION...99