Ext Sanchez Puerto Rico Weather Forecast Discussion

729
FXCA62 TJSJ 070924
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
524 AM AST Tue Oct 7 2025
.KEY MESSAGES...
* A tropical wave will continue to move over region today,
  enhancing the shower and thunderstorm activity across the local
  islands. There is a limited to elevated flood risk, particularly
  across central, northwestern, and northeastern (including the
  San Juan metro area) Puerto Rico.
* Travel disruptions, frequent lightning, and urban or small-
  stream flooding possible with isolated flash flooding cannot
  ruled out.
* Increased risk of heat-related illness especially for vulnerable
  groups and outdoor workers. Heat impacts will rise on Wednesday
  and persist through the weekend with greatest effects in
  coastal and urban areas.
* For the US Virgin Islands, minor flooding and lightning possible
  today. Elevated heat will affect outdoor activities every day.
&&
.SHORT TERM...Today through Thursday...
Moisture from a tropical wave has increased precipitable water
values (PWAT) to high end normal to above normal values over the
islands. Current satellite derived PWAT values indicate around 1.90
to 2.30 inches over the islands. Since midnight, showers and
isolated t-storms have reached windward sectors of the islands under
a northeasterly steering flow. Radar estimated accumulations (as of
4 AM) were observed over eastern and northern Puerto Rico (highest
estimated accumulation was 0.45 in over Loiza), as well as Culebra.
Showers and t-storms have mostly remained in the vicinity of the
USVI, with very low accumulations over isolated coastal areas. Lows
were in the mid to upper 70s over coastal and urban areas of Puerto
Rico and in the low 60s over interior Puerto Rico. Lows were in the
upper 70s to low 80s over the USVI, Culebra and Vieques.
An upper level low is centered southwest of Puerto Rico and a
tropical wave will continue to move over the region today.
Northeasterly steering flow is forecast to veer and become
southeasterly by this morning as the axis of the wave moves over the
area. Southeasterly steering flow will dominate for the rest of the
period. 500 mb temperatures are forecast to remain at normal values
at around -7.1 to -6.5 degrees Celsius, before becoming even colder
during the long term period. 700 mb to 500 mb lapse rates are
forecast at normal to above normal values 5.7 to 6.4 degrees C/km
(peaking today). PWAT values will also remain generally above 2
inches today and Thursday, with the lowest values forecast for
Wednesday. The upper level low will gradually move northward today
and tomorrow, as an upper level ridge is forecast to develop. With
the above mentioned features and values, as well as with diurnal
heating, sea breeze convergence and orographic effects, an active
afternoon is expected today. A limited to elevated flooding risk
will persist with convective showers and thunderstorms forecast for
interior to western, northern and northwestern Puerto Rico today,
with lines of convection also possible downwind of the local islands
and El Yunque. Advective shower and isolated t-storm activity is
also forecast to affect windward sectors of the islands during the
morning and overnight hours as the nearby above mentioned features
provide enough instability and moisture. Hazards include frequent
lightning, ponding of water on roadways, urban and small- stream
flooding and possible isolated flash floods. 925 mb temperatures
will be normal to above normal during the period, possibly tampered
by expected weather today but southeasterly flow will make this
hazard more pronounced for the rest of the period under
southeasterly flow. Lower PWAT values and ridging aloft can limit
afternoon convection on Wednesday but diurnal heating, sea breeze
convergence and local effects will promote convection over mainly
the northwestern quadrant of PR. Moisture is forecast to increase
once again on Thursday, aiding in afternoon convection under
southeasterly flow once again, normal to above normal 500 mb
temperatures, and normal 700mb to 500mb lapse rates. This late short
term period forecast will start to be influenced by the now AL95,
located about 1500 miles east of the Windward Islands, and with a 80
to 90% chance of cyclonic formation in 2 to 7 days (according to the
NHC). The latest tropical weather outlook informs that a tropical
depression or storm is likely to form within the next day or so and
is expected to be near or north of the northern Leeward Islands on
Thursday and Friday. Residents and visitors should monitor updates
regarding this system.
.LONG TERM...Friday through Tuesday...
The forecast remains on track with the arrival of increased moisture
associated with a tropical wave (Invest 95L) into the region. The
National Hurricane Center (NHC) is currently monitoring an area of
low pressure over the central Atlantic with a high formation chance
of becoming a tropical cyclone within the next 48 hours (80%) and in
the next seven days (90%). This system is expected to gradually
become better organized over the next day and is likely to form into
a Tropical Depression or Tropical Storm soon. The latest model
guidance continues to suggest this system will be near or north of
the Lesser Antilles by Friday.
On Friday, moisture associated with the system, in combination with
daytime heating and local orographic effects, will enhance the
potential for convective activity (showers and thunderstorms) across
Puerto Rico and the U.S. Virgin Islands. By Sunday, as the system
moves north of the region, winds will veer from the south. This wind
shift, along with the arrival of trailing moisture from the system`s
tail, will promote scattered showers and thunderstorms across
portions of central and northern Puerto Rico and the U.S. Virgin
Islands. Unstable conditions are expected to persist across the area
through at least Monday.
Conditions are expected to improve quickly by Tuesday as a high-
pressure system at all levels settles over the central Atlantic.
This pattern will promote drier and more stable atmospheric
conditions aloft through the end of the workweek.
Across the local area, uncertainty remains high in terms of precise
rainfall amounts and any potential local threats related to the
Invest 95L. Residents and visitors are therefore urged to monitor
the progress of this system closely over the coming days and stay
updated with the latest forecasts.
&&
.AVIATION...
(06Z TAFs)
Mainly VFR conditions expected to prevail. During the period, SHRA
and TSRA are forecast to continue reaching or moving close to
TJSJ/TIST/TISX, under ENE steering flow. Steering flow is expected
to gradually veer today and become more ESE. After around 17Z,
SHRA/TSRA over the Cordillera will spread mainly to
western/north/northwestern PR with TSRA affecting PRs terminals or
their VCTY, mainly TJSJ/TJBQ. These can produce brief periods of
reduced visibilities and lower ceilings. Winds up to around 10 kts
with sea breeze variations and local effects, decreasing after 7/23Z.
&&
.MARINE...
A long-period northeasterly swell will gradually continue to fade
across the local Atlantic waters and passages today. A tropical wave
will influence the region through Wednesday, increasing the
frequency of showers and thunderstorms over the regional waters,
especially during the afternoon hours, with activity further enhanced
near coastal areas by diurnally driven convection.
&&
.BEACH FORECAST...
Although dissipating, a long-period northeasterly swell will
continue to produce life-threatening beach conditions through
later this afternoon, mainly along Atlantic-exposed beaches. A
moderate risk of rip currents is expected to return by Wednesday
through the upcoming weekend.
Please remember that rip currents can sweep even the best
swimmers away from shore into deeper water, where it becomes
difficult to return to safety. Residents and visitors are strongly
urged to monitor the latest beach forecast, follow the beach flag
warning system, and stay out of the water.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through this afternoon for PRZ001-002-005-
     008-010-012-013.
VI...High Rip Current Risk through this afternoon for VIZ001-002.
AM...Small Craft Advisory until 6 AM AST early this morning for
     AMZ711-741.
&&
$$
MIDNIGHT SHIFT: MRR/GRS

NWS SJU Office Area Forecast Discussion