Bo Cascajo, Puerto Rico Weather Forecast Discussion

889
FXCA62 TJSJ 200925
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
525 AM AST Sat Sep 20 2025
.KEY MESSAGES...
* The available moisture combined with the todays sunshine will
  drive warmer temperatures and heat indices today.
* Extreme Heat Warning issued for Puerto Rico and a Heat Advisory for
  Vieques, Culebra, Saint Thomas, Saint John, and Saint Croix.
* A variable weather pattern is expected for the US Virgin
  Islands during the weekend, with shower activity during the
  morning and early evening hours.
* Afternoon shower activity is expected each day across the
  norther interior to northwest sections of the island, inducing
  an elevated flood threat and a limited lightning threat.
&&
.SHORT TERM...Today through Sunday...
Overnight, showers and thunderstorms developed over the U.S. Virgin
Islands, Vieques, and Culebra, while mainland Puerto Rico stayed
mostly dry, aside from light rainfall after 8 PM from lingering
afternoon activity. Rainfall estimates reached up to 1 inch in St.
Thomas and around a quarter of an inch in Vieques and Culebra, with
only trace to light amounts observed across Puerto Rico.
Temperatures cooled into the 70s across most areas, dipping into the
lower 70s or below in higher elevations, with all observing stations
reporting lows under 80F. Winds remained light to gentle from the
south over the waters, while over land they were light, calm, and
variable.
Expect a dangerous combination of hazards. With even less cloud
cover than yesterday, more sunshine will drive warmer temperatures.
A generally light to gentle southerly trade wind will further
support this warming trend while steering added tropical moisture
into the region. With precipitable water values remaining well above
normal (>2.1 inches), these factors are expected to lead to extreme
heat conditions. An Extreme Heat Warning is in effect for Puerto
Rico, with a Heat Advisory for the U.S. Virgin Islands, Vieques, and
Culebra. A lingering mid-to-upper level trough north of the region
remains the dominant feature, helping to maintain marginally
unstable conditions. As a result, the combination of extreme heat,
abundant moisture, and weak steering flow will favor the development
of slow-moving afternoon showers and thunderstorms, especially
across northern Puerto Rico, where the light southerly flow will
concentrate activity. These storms could be strong and will likely
bring the risk of frequent lightning, gusty winds, and localized
flooding from prolonged heavy downpours.
Dangerously hot conditions will persist through the period. On
Sunday, with less available moisture (lower PWAT), afternoon storms
will likely be fewer and less intense, though still capable of
producing frequent lightning and localized flooding. By Monday, a
more patchy wet/dry pattern is expected as subsidence from an
approaching tropical wave interacts with a weaker mid-level trough.
At the same time, cooling mid-level temperatures will create a more
favorable environment for deep convective development. If the timing
of these features aligns, conditions could support another round of
strong to severe thunderstorms Monday afternoon, despite only modest
moisture. Overnight periods will remain mostly quiet, with only
isolated showers and a stray thunderstorm over coastal waters and
eastern Puerto Rico.
.LONG TERM...Tuesday through Saturday...
The long-term forecast remains consistent with previous discussions.
Beginning Tuesday, conditions are forecast to transition into a
wetter and more unstable pattern. An approaching wind surge will
bring an increase in both wind speeds and moisture levels,
allowing for widespread rainfall. From Wednesday to Thursday, a
tropical wave currently being tracked by the National Hurricane
Center is forecast to move near the region, further enhancing the
potential for rainfall and thunderstorm activity across Puerto
Rico and the U.S. Virgin Islands. Currently, the wave is located
over the eastern Atlantic, with a low probability of development:
near 0% within the next 48 hours and 20% over the next 7 days.
A weak upper level troughing combined with low-level
southeasterly flow around the Atlantic high pressure will maintain
a steady influx of tropical moisture into the area through
Friday. This setup will support a prolonged period of unsettled
weather, with daily episodes of heavy rainfall. The latest model
guidance suggests precipitable water values of around 2.0 to 2.2
inches and mid- level temperatures near -7 to -8 C, indicating a
high potential for deep convection with thunderstorms.
Given saturated soils from recent rains, the primary hazards include
landslides in vulnerable terrain, rapid river rises, and an
elevated risk of flash flooding. While current model trends
suggest any tropical development is more likely after the system
passes west of the area, the evolving situation will be monitored
closely. Updates will be provided as new data becomes available
from official resources such as the National Hurricane Center and
your local National Weather Service.
&&
.AVIATION...
(06Z TAF)
VFR will prevail at most Puerto Rico terminals this morning. Across
the U.S. Virgin Islands, slow-moving SHRA/TSRA will bring brief
periods of MVFR. After 20/16Z, SHRA/TSRA will shift into central and
northern Puerto Rico, with MVFR to brief IFR expected at TJSJ after
20/16Z and at TJBQ after 20/17Z. Thunderstorms will also spread into
the vicinity of TJPS. Winds will remain light, driven by sea-breeze
circulations at 810 kt, before becoming calm and variable after
20/22Z. Gusty, erratic winds are likely in and near SHRA/TSRA.
&&
.MARINE...
A surface high pressure, in combination with Tropical Storm
Gabrielle located about 550 miles east-northeast of the northern
Leeward Islands, will continue to result in a gentle to light
southeasterly wind flow across the local waters through the rest of
today into the weekend. As a result, seas will range from 2 to 6
feet across most of the waters. Localized hazardous marine
conditions are possible due to showers and thunderstorms that could
produce locally higher seas and gusty winds. A long-period
northeasterly swell generated by Tropical Storm Gabrielle, currently
moving over the central Atlantic, will reach the local waters and
passages during the weekend. However, seas are forecast to remain
with or below Small Craft Advisory criteria.
&&
.BEACH FORECAST...
A moderate rip current risk persists today along north and eastern
Puerto Rico, as well as Culebra and Saint Croix, while the rest
remain with a low risk, though dangerous currents remain possible
near piers, reefs, and jetties. This risk will increase again
from today into early next week as swells from Tropical Storm
Gabrielle arrive. Strong afternoon thunderstorms would trigger the
lightning threat along the northern coastal side.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Extreme Heat Warning from 10 AM this morning to 4 PM AST this
     afternoon for PRZ001>005-007-008-010-011.
Heat Advisory from 10 AM this morning to 4 PM AST this afternoon
     for PRZ012-013.
VI...Heat Advisory from 10 AM this morning to 4 PM AST this afternoon
     for VIZ001-002.
AM...None.
&&
$$
SHORT TERM/AVIATION...ICP
LONG TERM/MARINE/BEACH FORECAST...MMC

NWS SJU Office Area Forecast Discussion