Fajardo, Puerto Rico Weather Forecast Discussion

762
FXCA62 TJSJ 081839
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
239 PM AST Mon Sep 8 2025
.KEY MESSAGES...
* Shower and thunderstorm activity over the waters and across land
  will persist through the evening, associated with a combination of
  a trade wind perturbation, diurnal heating, and local effects.
* Shower activity will abruptly diminish this evening as a dry slot
  and subsidence set in ahead of a surface trough and a tropical
  wave, which will move over the region on Tuesday afternoon and
  Wednesday, respectively.
* Southeasterly winds will bring above-normal temperatures and
  abundant tropical moisture for much of the period, resulting in a
  return of elevated, or even significant, heat threats over urban
  and coastal areas of Puerto Rico by midweek onwards.
* For the U.S. Virgin Islands, drier air is already entering the
  area, resulting in reduced shower activity. The overall weather
  pattern and heat threat will be similar to Puerto Rico, with
  elevated heat concerns expected from midweek onward.
&&
.SHORT TERM...Rest of today through Wednesday...
Partly cloudy to cloudy skies persisted since the morning, with
some additional showers across northeastern, southeastern, and
south-central Puerto Rico, as well as across the USVI and
Caribbean waters, with no significant impacts. The 12Z sounding
showed 2.28 inches of precipitable water (PWAT), which is
considered above normal. After 11 AM AST, as anticipated, the
frequency of showers started to increase in coverage across Puerto
Rico as a trade wind perturbation continued to swing by the
region. For the rest of the afternoon, these showers are expected
to continue and prevail across the northwest quadrant through at
least the late afternoon hours, with an elevated flood threat
across northern and western Puerto Rico, including flooding in
urban areas, roads, small streams, and washes. Daytime
temperatures have been in the mid to low 90s across urban and
coastal areas and the USVI, and in the upper 70s across the
mountains. Heat indices remained mainly below 105 degrees
Fahrenheit.
By the early evening hours into the overnight hours, rain will
diminish, and calmer weather conditions should prevail across the
forecast region as a somewhat drier air mass, with PWAT of 1.20
to 1.40 inches, dominates over the northeast Caribbean while a
brief mid- level ridge pattern is in place. By Tuesday, an induced
surface trough will increase moisture content up to 2 inches,
resulting in variable and showery weather early in the morning
across the USVI and from mid-morning onward across Puerto Rico,
particularly across the eastern half. Then, during the afternoon,
showers and thunderstorms are once again expected across the
northwest quadrant, resulting in a limited flood threat.
Late Tuesday into Wednesday, a wetter pattern should evolve as a
tropical wave (formerly Invest 91L and currently located near
50W) combined with a trough pattern aloft, will approach the
region and maintain moisture content in the above-normal range.
Moderate concentrations of Saharan dust are expected to be
embedded within the wave, resulting in slightly hazy skies and
elevated heat risk, especially where rainfall remains limited. The
flood risk will likely be elevated by midweek due to this
tropical wave, particularly across the interior, northern, and
western sectors of Puerto Rico.
.LONG TERM...Thursday through Monday...
.PREV DISCUSSION... /issued 436 AM AST Mon Sep 8 2025/
A mid-to-upper-level trough will linger off to the northeast of
the USVI, near the Leeward Islands, inducing occasional easterly
perturbations that will arrive across our region from Thursday to
Saturday. Then, a mid-to-upper-level high-pressure system will
build from the northeast into our islands between Sunday and early
next week, strengthening the trade wind cap and promoting dry air
with subsidence aloft. What this weather pattern means to us,
residents and visitors of the USVI and PR, we can expect the
arrival of more frequent surges of moisture in the second part of
the week, followed by a more stable weather pattern on Sunday and
early next week. However, due to the excessive heating that has
been prevailing locally, strong afternoon convection will be
likely each day, especially across the western locations of PR,
and downwind from the USVI. Additionally, nighttime cool air
advection due to above-normal sea surface temperatures will result
in scattered to numerous showers, especially during the second
part of the week.
Additionally, model guidance suggests the arrival of a Saharan
Air Layer with low concentrations of suspended particles on
Wednesday and Thursday, and possibly Friday. This air mass will
play an essential role in terms of the amount of rain that we
might observe across Puerto Rico and the US Virgin Islands. On the
other hand, if concentrations are not high enough to inhibit rain
activity, we might observe periods with enhanced thunderstorm
activity due to the presence of suspended particles.
Maximum temperatures are projected to range from the upper 80s to
the mid-90s, with heat indices potentially exceeding 100 degrees
Fahrenheit, attributed to the existing east-southeast wind
patterns. Consequently, the risk associated with heat will remain
elevated throughout the forecast period, particularly between
Wednesday and Thursday, when warmer-than-normal overnight low
temperatures due to the haziness attributed to the SAL are
anticipated.
&&
.AVIATION...
(18Z TAFS)
TSRA/SHRA resulting in brief periods of MVFR conditions at TJSJ
and TJBQ through 08/22-23Z. Mainly VFR conditions across the USVI
TAF sites the rest of the afternoon, with VCSH or -SHRA increasing
after 09/09Z and then across TJSJ and TJPS after 09/12Z. Surface
winds E-SE 10-15 kt with sea breeze variations becoming lighter
tonight.
&&
.MARINE...
A surface high pressure across the Central Atlantic will promote
moderate to locally fresh east to southeast winds, resulting in
moderate chops through the middle of the week. An easterly
perturbation will continue to promote showers and thunderstorms
across the regional waters through this evening. Then, a tropical
wave is expected to move near the islands by Wednesday, increasing
the local winds and the potential for showers and thunderstorms.
Induced surface troughs will swing by the islands, pooling
additional moisture in the second part of the week.
&&
.BEACH FORECAST...
Beachgoers, expect shower and thunderstorm activity to continue
developing through the afternoon across Puerto Rico, with brief
heavy downpours and gusty winds possible.  Conditions are expected
to improve by this evening as the drier air settles in. Over the
U.S. Virgin Islands, drier air is already filtering in, leading to
reduced shower activity through this evening.
Looking ahead, southeasterly winds will bring above-normal
temperatures and plenty of tropical moisture through the period. As
a result, elevated to locally significant heat threats are
anticipated over urban and coastal areas of Puerto Rico, while the
U.S. Virgin Islands will also experience elevated heat concerns
beginning midweek.
For mariners and beachgoers, a moderate risk of rip currents will
persist along north-facing beaches of Puerto Rico and St. Croix
through at least Thursday. Always use caution near piers, jetties,
and reefs, where rip currents can still become life-threatening.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&
$$
SHORT TERM/AVIATION...YZR
MARINE/BEACH FORECAST...CVB
LONG TERM...CAM

NWS SJU Office Area Forecast Discussion