597
FXCA62 TJSJ 160917
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
517 AM AST Mon Jun 16 2025

.KEY MESSAGES...

* An upper-level low meandering north of the region today through
  Wednesday and abundant moisture content will support the
  development of showers and thunderstorms in the afternoons.

* Hazy skies are expected through most of the workweek due to a
  weak Saharan Air Layer, reducing visibility and affecting air
  quality.

* Increasing winds will result in breezy to locally windy
  conditions across coastal areas. Choppy seas are likely
  especially in exposed waters, small craft should exercise
  caution.

&&

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

Overnight radar and satellite imagery indicated partly to variably
cloudy skies over the local islands, primarily due to a persistent
layer of mid- to upper-level clouds that has lingered since
yesterday. A few light showers brushed windward coastal areas, while
more active showers and isolated thunderstorms developed over the
Mona Channel and along the eastern coast of Hispaniola. These
conditions are associated with a TUTT-low currently located north of
Puerto Rico. Additional showers are expected to develop over
regional waters and move across the eastern portions of the islands
during the morning hours. Wind speeds are gradually increasing, with
some coastal weather stations reporting maximum gusts in the low to
mid-20s mph overnight, slightly higher than those observed the
previous night.

The GEOS-5 model indicates the presence of suspended Saharan dust
over the region, which is expected to persist through at least
Wednesday. This will result in hazy skies, particularly in areas
experiencing limited shower activity. Breezy to locally windy
conditions are forecast to return today and persist through the end
of the forecast period, as the pressure gradient over the
northeastern Caribbean strengthens.

As the TUTT-low and its associated surface trough continue to move
westward, another cut-off low is forecast to develop to the
northeast. This will place the local area between the two systems in
an upper-level omega-like pattern, characterized by some ridging
aloft. Although the ridge is not expected to be particularly strong,
it will contribute to increasing upper-level stability, especially
by Wednesday. Significant moisture variability is anticipated
throughout the period, with precipitable water (PWAT) values
decreasing from approximately 2.10 inches today to around 1.40
inches by Wednesday. This reduction is primarily due to mid-level
drying, as relative humidity in the 700-500 mb layer is forecast to
drop from 70-80% today to 30-40% by midweek, in response to the
evolving upper-level pattern.

As a result, today is expected to be the wettest day of the forecast
period, driven by the combination of the TUTT-low and above-normal
moisture levels. Partial cloud cover may somewhat limit surface
heating, which could inhibit the development of afternoon
convection. However, sufficient dynamical forcing is still expected
to support some convective activity later this afternoon. For the
remainder of the short-term forecast, shower activity will generally
follow the typical diurnal pattern, with reduced areal coverage due
to decreasing moisture availability. Afternoon thunderstorms will
continue to develop but are expected to be more localized. A limited
to elevated flooding threat may accompany the strongest activity,
with the highest risk occurring during the afternoon hours.

&&

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

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. The latest TWD puts
the axis of the tropical wave southeast of the CWA early on
Thursday, however, model solutions locate the highest PWATs south
of the region. 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...

(06Z TAFS)

Mostly VFR at all TAF sites durg prd but VCSH at TJSJ and USVI
terminals til 16/14Z. Mstly isold passing SHRA ovr regional waters
en route btw islands. Aftn VCSH/Isold TsrA psbl ovr ctrl interior
and west PR til 16/23Z...some may briefly impact areas in and around
TJBQ. SFC wnd fm E-SE 12-18 knots with local sea breezes accompanied
by higher gusts. Wnds bcmg less than 10 kts aft 16/23Z.

&&

.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...

Increasing winds will maintain the moderate risk of rip currents
for most beaches of Puerto Rico and the U.S. Virgin Islands.
Beachgoers must exercise caution due possible life-threatening
rip currents along these beaches. They should also stay weather
alert due possible shower and thunderstorm activity in the
afternoon that may move over the coastal areas particularly over
western/northwestern Puerto Rico.

&&

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

$$

SHORT TERM...CVB
LONG TERM....MNG
AVIATION...CVB