Urb Ext Elizabeth, Puerto Rico Weather Forecast Discussion

997
FXCA62 TJSJ 121804
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
204 PM AST Fri Sep 12 2025
.KEY MESSAGES...
* Up to elevated to significant heat risks are expected to
  continue during the weekend.
* Afternoon showers and thunderstorms are expected tomorrow,
  especially across Puerto Ricos interior and northwestern areas,
  and downwind of El Yunque and the local islands. The convective
  activity may bring flooding rainfall and frequent lightning.
* In the U.S. Virgin Islands, expect mainly fair weather with
  warm to hot temperatures and occasional passing showers.
* A tropical wave, currently near 58.5W just east of the Leeward
  Islands, is forecast to increase the chance of showers and
  thunderstorms by Sunday.
* Moderate risks of rip currents are forecast to return for
  beaches of St. Croix, Culebra and northern Puerto Rico on
  Sunday.
&&
.SHORT TERM...Rest of Afternoon through Sunday...
A fairly tranquil morning prevailed across the islands, with
clear to variably cloudy skies and a few very isolated passing
showers over the waters that briefly brushed windward coastal
areas. Elevated heat indices were reported at several weather
stations across coastal Puerto Rico and the smaller islands. Winds
were generally light and calm early this morning due to a weak
surface pressure gradient over the region, with a northeast
component induced by a surface trough north of the islands. The
TJSJ 12/12Z sounding indicated weak steering flow at around 67
kt. By mid- morning into early afternoon, surface heating promoted
the onset of sea breezes across the coastal plains and smaller
islands, with a few gusts up to 20 mph observed, particularly
across southern Puerto Rico.
Convective activity initiated fairly early around midday, as sea-
breeze convergence combined with local effects, strong diurnal
heating, and troughiness aloft to fuel thunderstorm development.
This has resulted in isolated thunderstorms downstream of the
smaller islands and over portions of the southern San Juan metro
area. Expect this activity to continue and expand across the
interior and western Puerto Rico through the afternoon and evening.
The weak steering flow will result in slow-moving showers,
increasing the potential for urban and small stream flooding in
flood-prone areas. Quick river rises and landslides over saturated
soils and steep terrain cannot be ruled out. Activity should
gradually diminish during the evening as diurnal heating wanes.
A similar pattern is expected on Saturday, with a few passing
showers moving across windward coastal areas during the night and
morning hours, although mostly tranquil conditions will prevail.
Another round of afternoon convection is expected across the
interior and western Puerto Rico, with streamer activity developing
downwind of the smaller islands and from El Yunque into the San Juan
metro area. Once again, slow-moving showers could enhance the
flooding threat, though the steering flow is expected to shift more
from the east-southeast, allowing activity to drift more toward the
northwest.
By Sunday, the moisture field associated with a tropical wave,
currently near 58.5W just east of the Leeward Islands, will begin to
affect the region. Precipitable water values are forecast to surge
above normal, potentially exceeding 2.25 inches. Model guidance
suggests an increase in shower activity across the U.S. Virgin
Islands, Vieques, Culebra, and the eastern third of Puerto Rico
during the morning hours. Afternoon convection will strongly depend
on the extent of cloud cover. If cloud breaks occur, another active
afternoon is expected, with deep convection supported by diurnal
heating, orographic lifting, and favorable dynamics associated with
the tropical wave. Troughiness aloft may also support redevelopment
along outflow boundaries. Increasing trade winds with the tropical
wave may help reduce the flooding potential somewhat, as showers
could move more quickly, limiting their residence time over any
given area.
In addition, temperatures at 925 mb are expected to remain above
normal through the short-term period. Combined with very moist air,
this will promote sweltering heat indices, particularly across
coastal and urban areas of Puerto Rico and the U.S. Virgin Islands.
Heat Advisories may be required each day, although the threat on
Sunday will largely depend on cloud coverage associated with the
approaching tropical wave. Should skies clear for extended periods,
dangerous heat indices will once again become a concern. Residents
and visitors are urged to take proper precautions, such as staying
hydrated, limiting outdoor activities during peak heating hours, and
seeking adequate shade or cooling.
.LONG TERM...Monday through Friday...
./FROM PREV DISCUSSION...Issued at 359 AM AST Fri Sep 12 2025/
A tropical wave is forecast to move over Puerto Rico and the U.S.
Virgin Islands, bringing increased moisture and a high likelihood of
showers and thunderstorms. The latest precipitable water guidance
indicates normal to above-normal values (2.00 to 2.25 inches),
suggesting a wet pattern will persist through Tuesday. The U.S.
Virgin Islands should anticipate showers and isolated thunderstorms,
particularly from the morning into the afternoon. In Puerto Rico,
showers and isolated thunderstorms are expected over the eastern
municipalities during the morning, with afternoon convective
activity shifting to the western and northwestern municipalities due
to a combination of daytime heating and local effects.
This wet trend is expected to continue from Wednesday through the
rest of the week, as an upper-level trough located northeast of the
region moves southeast. A second tropical wave approaching the area
will combine with the available moisture from the upper-level-
trough, further enhancing the potential for additional showers and
thunderstorms across the islands. Therefore, the risk of excessive
rainfall leading to urban and small stream flooding remains
elevated. There is also a limited risk of frequent lightning
accompanying the strongest thunderstorms.
Despite the expected weather for the long-term period, the 925 mb
temperature is expected to remain above-normal at the beginning of
the week. However, as the week progresses, temperatures will become
slightly cooler due to the increased rainfall. Highs will generally
be in the mid to upper 80s along coastal and urban areas, and in the
upper 70s to low 80s in the higher elevations. Residents and
visitors are advised to monitor weather conditions and plan any
outdoor activities accordingly for the upcoming week.
&&
.AVIATION...
(18Z TAFS)
Prevailing VFR conds thru the aftn, with SHRA/TSRA dvlpg ovr
interior and wrn PR, psbly affecting TJBQ/TJPS with brief MVFR
VIS/CIGs thru 12/23Z. VCTS psbl at TJSJ during the same prd. Sfc
winds E 09-13 kt with sea breeze variations and ocnl hir gusts.  By
Fri aftn, SHRA/TSRA will favor interior and W/NW PR, affecting
mainly TJBQ, with mtn obsc likely. Decreasing winds aft 23Z
across TAF sites, subject to the onset of land breezes across the
islands. Winds picking up aft 13/13z but more ESE 10-13 kt flow
and ocnl hir gusts.
&&
.MARINE...
A surface high pressure across the central Atlantic and a front
over the western Atlantic will promote light to moderate east to
southeast winds through the weekend. Afternoon showers and
thunderstorms are likely each day, particularly over the
northwestern coastal waters of Puerto Rico and over the Mona
Passage. A tropical wave and its moisture field will affect the
local waters on Sunday.
&&
.BEACH FORECAST...
A low risk of rip currents is expected to continue for most
beaches through Saturday. The rip current risk is forecast to
increase to moderate for beaches of north-central to northeastern
Puerto Rico, as well as for Culebra & St. Croix. By Sunday night
and into the start of the workweek, the moderate risk of rip
currents will spread to all northern beaches of Puerto Rico. Under
a Moderate Risk of Rip Currents, life-threatening rip currents
are possible in the surf zone. Even with a low risk, life-
threatening rip currents can still occur near groins, jetties,
reefs, and piers, so beachgoers are advised to exercise caution at
all times. Additional hazards include: isolated afternoon
thunderstorms and an elevated to significant heat risk.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 4 PM AST this afternoon for PRZ001>005-007-
     008-010>013.
VI...Heat Advisory until 4 PM AST this afternoon for VIZ001-002.
AM...None.
&&
$$
SHORT TERM/AVIATION...CVB
LONG TERM...GRS
MARINE/BEACH FORECAST...MRR

NWS SJU Office Area Forecast Discussion