Urb Medina, Puerto Rico Weather Forecast Discussion

889
FXCA62 TJSJ 271911
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
311 PM AST Sat Sep 27 2025
.KEY MESSAGES...
* Afternoon convection should remain through at least sunset
   across Puerto Rico. We urge people to find shelter if they
   heard a thunder or see lightning because this activity is
   strong and winds are moving slow
* Showers will move across the U.S. Virgin Islands later today
   into the early hours of tomorrow.
* A long period northeasterly swell from Hurricane Humberto will
   continue to increase seas and breaking waves during the
   weekend, and resulting in a high risk of rip currents.
   Additionally, there is a small craft advisory in effect
   across the offshore Atlantic waters.
* An increase in moisture is forecast from Monday
   into Tuesday; however, Saharan dust will also move in,
   resulting in hazy skies in some isolated areas.
&&
.SHORT TERM...Tonight through Monday...
Current satellite derived precipitable water values indicate a band
of moisture with values around 1.9 to around 2 inches over the USVI
and eastern Puerto Rico. This band of moisture is between Major
Hurricane Humberto, over the subtropical central Atlantic around 420
miles NE of St. John, and Tropical Depression Nine, centered between
eastern Cuba and the central Bahamas. Diurnal heating, sea breeze
convergence and light southerly steering winds are promoting
afternoon convective showers and thunderstorms over interior to
northern PR, where a limited to elevated flooding risk continues.
Lines of showers and thunderstorms remain possible downwind of
Vieques, Culebra and the USVI. Light concentrations of Saharan Dust
are also present, increasing somewhat on Monday. PWAT values will
decrease tomorrow as the current band of moisture is pulled away by
the distant TCs. However, trailing moisture from Humberto will reach
the islands late Sunday into Monday. This will increase PWAT values
to around 2 inches during that period. Current southerly steering
flow will gradually turn more east-southeasterly by late Sunday into
Monday while also increasing to around 10 to 15 kts. This moisture
will be accompanied by the above mentioned Saharan Dust. Afternoon
showers and thunderstorms are forecast during the short term period
over the interior and steered towards northern PR tomorrow and more
towards NW PR on Monday. Limited to elevated flooding risks will
persist. Overnight and early morning showers are also forecast over
windward sectors. Temperatures and heat indices have remained well
above normal values. 925 mb temperatures are expected to continue
above normal values during the short term period, with highs in the
low to mid 90s at most coastal areas. Elevated to significant heat
threats will persist for the short term periods, with Extreme Heat
Warnings/Heat Advisories anticipated each day.
.LONG TERM...Tuesday through Saturday...
The long-term forecast remains on track. On Tuesday, the surface
pattern will continue under the influence of Hurricane Humberto
well north of the region and a strengthening surface high pressure
over the central Atlantic, resulting in a weak southeasterly wind
flow. Under this setup, the islands will experience brief
intrusions of Saharan dust with precipitable water values near 1.6
inches, close to the 25th percentile for the year. This pattern
will favor a generally stable day with calm conditions in the
morning and localized afternoon activity. Similar weather will
persist on Wednesday, while on Thursday precipitable water is
forecast to decrease further, with global model guidance
suggesting values nearly two standard deviations below normal.
Relative humidity between 700500 mb also shows a drying trend,
supporting mostly stable mornings with only localized showers
across the interior regions.
A more unsettled pattern is expected from Friday into Saturday as
global model guidance indicates an increase in instability, with
700500 mb lapse rates steepening and 500 mb temperatures slightly
decreasing to around 6 C due to an upper-level trough extending
from the western Caribbean into the islands. At the surface, a
weakening of the pressure gradient is anticipated as a tropical
system moves into the northwestern Atlantic, reducing the
influence of the surface high pressure. This evolving pattern will
induce surface convergence, enhance instability, and result in
very weak steering flow. Consequently, a variable pattern is
forecast, with long-lasting afternoon showers primarily affecting
the interior, but drifting into different sections of the islands
due to weak winds.
&&
.AVIATION...
(18z TAFs)
Mainly VFR conditions. TSRA and VCTS will reach TJBQ/TJSJ, these can
promote brief MVFR conditions through around 27/21Z. VCTS forecast
for TJPS through around 27/21Z. Low level winds remain under a weak
southerly component with sea, and later overnight land, breezes
persisting. Winds will gradually pick up a more southeasterly
component as the period progresses. Overnight to early morning VCSH
forecast over eastern terminals.
&&
.MARINE...
A pulse of a long period northeasterly swell from Major Hurricane
Humberto will continue to increase seas across the local waters
through the rest of the weekend. Buoy 41043 is reporting seas
around 7 feet across the offshore Atlantic waters. As a result, a
Small Craft Advisory was issued. This conditions will continue to
spread into the coastal Atlantic waters and the Anegada Passage
during the weekend. Therefore, the Small Craft Advisory will extend
to the coastal Atlantic water later tonight into Sunday. Seas are
expected to remain between 7 and 8 feet across the Atlantic
waters and up to 5 feet across the Caribbean waters. Improving
marine conditions are forecast from Monday as the energy from the
swell diminish across the area.
&&
.BEACH FORECAST...
A long period northeasterly swell from Major Hurricane Humberto
will continue to increase breaking waves through the weekend. As
a result, deteriorating coastal conditions are expected,
especially across the northern coastal areas of Puerto Rico,
including beaches along the northeast PR. For Culebra, Vieques,
the U.S. Virgin Islands, and northwestern Puerto Rico, the risk
will remain moderate. However, deteriorating conditions are
forecast to spread to more coastal areas, including St. Croix,
Culebra, and the northwestern coast of Puerto Rico, during the
weekend. Improving coastal conditions are expected beginning on
Monday.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Extreme Heat Warning until 5 PM AST this afternoon for
     PRZ001>005-007-008-010-011.
High Rip Current Risk through late Sunday night for PRZ001-002-
     005-008.
Heat Advisory until 5 PM AST this afternoon for PRZ012-013.
VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.
AM...Small Craft Advisory until midnight AST Sunday night for AMZ711.
Small Craft Advisory from 6 AM Sunday to midnight AST Sunday
     night for AMZ712.
Small Craft Advisory from 6 AM to 6 PM AST Sunday for AMZ716.
Small Craft Advisory from noon Sunday to midnight AST Sunday
     night for AMZ741.
&&
$$
SHORT TERM/AVIATION...MRR/MMC
LONG TERM...LIS
MARINE/BEACH FORECAST...MMC

NWS SJU Office Area Forecast Discussion