Ext Roseville, Puerto Rico Weather Forecast Discussion

963
FXCA62 TJSJ 071815
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
215 PM AST Sun Sep 7 2025
.KEY MESSAGES...
* A trade wind perturbation will continue to promote showers and
  thunderstorm activity today, leading to an elevated flooding and
  lightning risk across much of Puerto Rico.
* For the U.S. Virgin Islands, passing showers and isolated
  thunderstorms may lead to ponding of water on roadways this
  afternoon. A similar weather pattern is expected during the
  overnight hours into early Monday.
* Warm temperatures will continue across Puerto Rico and the U.S.
  Virgin Islands during the week. Heat indices values are expected
  to reach or exceed 108 degrees Fahrenheit each day along the
  coastal and urban areas of the islands.
* A tropical wave is expected to move into the local area by mid-week,
  increasing the potential for showers and thunderstorms.
&&
.SHORT TERM...Rest of Afternoon through Tuesday...
The forecast remains largely on track with the easterly perturbation
driving scattered showers and isolated thunderstorms across the
regional waters and portions of the US Virgin Islands and eastern
Puerto Rico during the morning hours. As expected, most of this
activity weakened by mid to late morning, leaving behind residual
cloud cover, particularly over the eastern half of the forecast
area. This lingering cloudiness, along with a shift to more easterly
surface winds, limited solar heating and resulted in lower 925 mb
temperatures compared to yesterday. Consequently, while muggy
conditions were observed, maximum heat indices did not reach the
levels observed yesterday. By midday, diurnal heating had promoted
renewed convection, with isolated thunderstorms developing across
the interior and western parts of Puerto Rico. This activity is
expected to persist and gradually expand in coverage through the
rest of the afternoon, maintaining the risk of localized flooding in
vulnerable areas.
Above-normal tropical moisture will continue to influence conditions
through Monday, maintaining the potential for a similar pattern of
daytime convection with scattered showers and thunderstorms.
However, satellite imagery and model guidance show the upper-level
trough gradually shifting westward and weakening, leading to a more
stable environment aloft. This is reflected in rising 1000-500 mb
thicknesses, warming 500 mb temperatures, and increasing 250 mb
heights. As a result, while afternoon convection is still expected,
it may be somewhat less intense and less widespread than in previous
days.
A notable change arrives late Monday as a narrow dry slot moves into
the region. If this feature arrives earlier than currently forecast,
it could quickly suppress convective activity. Regardless of timing,
precipitable water values are forecast to plummet to well below
normal levels by late Monday night, even dipping below the 10th
percentile of the climatological normal. Mid-level relative humidity
will also drop significantly, at times below 10%, signaling strong
subsidence ahead of the next tropical wave (formerly Invest 91L).
Depending on the exact timing of this dry intrusion and subsequent
moisture rebound, Tuesday could evolve in one of two ways: either a
relatively quiet day with only isolated showers, or another active
afternoon if the tropical wave arrives sooner than expected. Current
guidance suggests rapid recovery of moisture levels following the
dry slot, but confidence in timing remains low.
Heat risks will continue through the period, with maximum heat
indices in the 100s, particularly in areas with limited cloud cover
or rainfall. Although todays values were tempered by cloudiness and
wind shifts, and similar conditions are expected on Monday,
residents of Puerto Rico and the USVI should remain alert for
excessive heat conditions, particularly Monday afternoon, when
convection may be less widespread. Hotter temperatures are
anticipated on Tuesday; however, with much lower moisture levels,
heat index values may not be as high, reducing the overall heat
threat across the region.
.LONG TERM...Wednesday through Sunday...
(from previous discussion)
A weakening in the mid-level ridge pattern is expected early in
the cycle as troughiness develops northeast of the local islands.
However, by the end of the cycle, the mid-level ridging will
return. At lower levels, a surface high pressure will prevail
across the tropical Atlantic, promoting east- southeast winds.
Based on the latest grand ensemble, precipitable water is expected
to remain above 2.0 inches from Wednesday through Friday, with a
sharp decrease anticipated over the upcoming weekend as the
developing mid-level ridge erodes the available moisture. The
highest values of precipitable water early in the forecast cycle
coincide with the passage of a tropical wave (former Invest 91L)
currently located near the 40W.
Under this evolving pattern, the highest chance of showers and
thunderstorms is expected on Wednesday and Thursday, with a
transition on Friday and into the weekend. Having said this, the
intensity and coverage of showers and thunderstorms are expected
to decrease by the end of the workweek, resulting in a more
seasonal pattern with afternoon showers and thunderstorms
primarily focused on the western areas of Puerto Rico. As a
result, the flooding risk remains elevated throughout the forecast
period, decreasing somewhat by the upcoming weekend.
Maximum temperatures will continue to range in the upper 80s to
mid 90s with heat indices exceeding 100 degrees F under the
prevailing east southeast wind flow. Therefore, the heat risk
will remain elevated for much of the forecast cycle. Some
haziness is anticipated Wednesday and Thursday.
&&
.AVIATION...
(18Z TAFS)
Prevailing VFR conds thru the aftn, however SHRA/TSRA dvlpg ovr
interior and wrn PR, psbly affecting TJBQ/TJPS with brief MVFR
VIS/CIGs thru 07/23Z. VCTS psbl at TJSJ during the same prd. Sfc
winds E 10-15 kt with sea breeze variations and ocnl hir gusts. Aft
08/03Z, SHRA/ISOLD TSRA ovr the waters may impact ern PR and USVI
terminals (TJSJ/TIST/TISX). Aft 08/13z, similar wind pattern to
persist but more ESE 10-15 kt flow.
&&
.MARINE...
A surface high pressure across the Central Atlantic will promote
moderate to locally fresh easterly winds, resulting in moderate
chops through the middle of the week. An easterly perturbation will
continue to promote showers and thunderstorms across the regional
waters through tomorrow. Then, a tropical wave is expected to move
near the islands by Wednesday, increasing the local winds and
the potential for showers and thunderstorms.
&&
.BEACH FORECAST...
A low risk of rip currents will continue to prevail across all
shorelines for the next few days. Nonetheless, isolated stronger rip
currents may occur in the vicinity of groins, jetties, reefs, and
piers. Beachgoers must also stay aware of the weather conditions
across the area, particularly in the afternoons and evenings, due to
thunderstorm activity that may approach the surf zone. Beachgoers
must leave the water and seek shelter if thunderstorms are close by.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&
$$
CVB/GRS

NWS SJU Office Area Forecast Discussion