Urb Guamani Puerto Rico Weather Forecast Discussion

071
FXCA62 TJSJ 140917
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
517 AM AST Tue Oct 14 2025
.KEY MESSAGES...
* Warmer conditions will lead to hazardous heat conditions today.
  A Heat Advisory is in effect for all urban and coastal areas of
  Puerto Rico and the U.S. Virgin Islands from 10 AM through 5 PM
  AST today.
* A deep-layer trough and a frontal boundary will promote wet and
  unstable conditions for the second part of the workweek across
  the islands, increasing flooding and lightning threat.
* An increase in trade wind showers across the U.S. Virgin
  Islands is expected during the nighttime.
* A northerly swell could lead to hazardous marine and coastal
  conditions across the islands by the upcoming weekend into early
  next week.
&&
.SHORT TERM...Today through Thursday...
Isolated to scattered trade-wind showers developed over the
surrounding waters overnight and moved inland or brushed coastal
areas, especially across eastern Puerto Rico, Vieques, Culebra, and
the U.S. Virgin Islands. The highest estimated rainfall totals
peaked around half an inch along coastal areas of the San Juan
metro, with lesser amounts elsewhere. Overnight temperatures showed
little overall change, with most interior and higher elevation
stations falling into the 70s or lower, while a few exposed coastal
sites in eastern Puerto Rico, the USVI, and local islands remained
in the low 80s. Winds were generally light to calm and variable, but
brief gusts occurred near passing showers.
A broad surface high to the north and ridging aloft will keep drier
air and warm mid-level temperatures in place, maintaining a weak
trade wind cap that limits storm growth early and allows more
sunshine and heat to build. Low-level temperatures will remain well
above normal and southeasterly flow will increase moisture,
resulting in extreme heat concerns, and a Heat Advisory is in effect
for all islands, especially urban and coastal areas. Morning trade
wind showers will affect windward and eastern zones, followed by
scattered to locally numerous afternoon showers and isolated
thunderstorms across the western interior and northwest, where brief
heavy rain, localized flooding, and dangerous lightning are possible.
A significant shift toward a wetter and more unstable pattern will
develop as the ridge breaks down and a deep-layer trough with an
approaching frontal and pre-frontal trough influences the regionan
ideal setup for stronger convection and possibly severe
thunderstorms. Winds will veer from southeasterly tonight to
southerly and southwesterly on Wednesday, then become light and
variable by Thursday, allowing heat and moisture to build. Moisture
will increase well above what is typical for this time of year,
while mid-level temperatures cool to normal or slightly below-normal
levelsboth of which strongly favor deep convective development. At
the same time, low-level winds will weaken, limiting storm steering
and allowing slow-moving thunderstorms to persist. Extreme heat will
continue as well-above-normal low-level temperatures and southerly
flow trap hot, humid air. As a result, showers and thunderstorms
will become more widespread and longer-lasting from Wednesday
afternoon through Thursday night, bringing a growing threat of
frequent lightning, gusty winds, and localized flooding. Hazards
will increase each day, peaking late Wednesday into Thursday and
Thursday night.
&&
.LONG TERM...Friday through Tuesday...
No major changes were introduced to the long-term forecast. A wet
and unstable weather pattern is expected for the upcoming weekend,
which may improve for the first part of the workweek. Winds should
remain light on Friday and Saturday, shifting from the southwest
influenced by a surface high pressure system over the central
Atlantic and a broad polar trough, which will pool the remnants
of the frontal boundary and high tropical moisture. From the
latest deterministic guidance of the GFS and the ECMWF,
Precipitable Water (PWAT) values should remain seasonal to above
climatological normal (2.0 - 2.2 inches). Additionally, the polar
trough reflected in the mid to high levels may deepen enough to
cool 500 mb temperatures (-6.5 to -7.5 degrees Celsius),
increasing instability and supporting deep convection. The Galvez-
Davison Index (GDI) also suggests the potential of isolated to
scattered thunderstorms through at least Sunday. Hence, the
frequency of showers and thunderstorms should remain high,
affecting mostly windward sections during the night through early
mornings, with deep convection activity remaining over the
mountain ranges of Puerto Rico in the afternoons. Winds should
increase and turn from the southwest as the surface high pressure
migrates west and approaches the CWA. Due to abundant moisture
content over the area, afternoon convection should concentrate
over northwestern Puerto Rico. Given the expected conditions, the
lightning and flooding threat will remain limited to elevated
during that period.
An improvement in weather conditions is expected by late Monday due
a drier air mass filtering into the region. The latest model
guidance shows a decrease in PWAT values (1.7 - 1.9 inches),
although theres high variability between ensemble members due to a
tropical wave that global models suggest should move south of the
CWA. Nevertheless, the combination of daytime heating, local
effects, and available moisture will enhance shower and thunderstorm
activity, mainly over the western/northwestern portions of Puerto
Rico; however, the flooding threat should remain limited.
The latest model guidance indicates that Sunday may be the
warmest day of the long-term period, followed by a gradual
decrease in 925 mb temperatures for the remainder of the period.
Nevertheless, heat indexes are still very likely to reach 100
degrees and may meet Heat Advisory Criteria. Hence, the heat
threat will remain elevated for the long-term forecast.
&&
.AVIATION...
(06Z TAFs)
Variable weather expected. Trades SHRA early, then SHRA/TSRA shifts
to w-nw this afternoon. SHRA/TSRA likely across all PR terminals
14/16-22Z, with brief MVFR/IFR VIS/CIG in heavier cells closer to
TJSJ and TJBQ. USVI terminals will also see periods of SHRA
throughout the day. Most activity will fade after sunset but may
linger at TJSJ, TJBQ, and USVI terminals overnight. Winds LGT/VRB
(06 kt) overnight/morning, increasing to 1014 kt from ESEE with
sea breeze, then LGT/VRB again after 14/22Z.
&&
.MARINE...
A broad surface ridge north of the region continues to dominate the
local marine area, maintaining moderate east to east-southeast trade
winds today. Then, winds will become light to gentle from the south-
southwest Wednesday onwards. A frontal boundary approaching the
region will induce pre- frontal troughs for the second half of the
workweek, increasing shower and thunderstorm activity across the
regional waters and passages. Pulses of a northerly swell could
deteriorate marine conditions by next weekend into early next
week.
&&
.BEACH FORECAST...
For today and the rest of the workweek, the risk of rip currents
should remain moderate mainly over northern and eastern beaches of
Puerto Rico, Vieques , Culebra, and the U.S. Virgin Islands.
Although the risk is not high, life-threatening rip currents are
still possible along the surf zone. Additionally,showers and
thunderstorms in the afternoon may reach the western/northwestern
coastal areas, increasing the lightning threat. Beachgoers should
exercise caution and stay weather alert. Model guidance continues to
suggest the arrival of a northerly swell by the upcoming weekend,
expected to bring hazardous beach conditions along north- facing
beaches of Puerto Rico, Culebra, and the U.S. Virgin Islands.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>005-007-008-010>013.
VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ001-002.
AM...None.
&&
$$
SHORT TERM/AVIATION...ICP
LONG TERM/MARINE....MNG

NWS SJU Office Area Forecast Discussion