Special Weather Statement, NWS JSJ

Area Forecast Discussion
National Weather Service San Juan PR
415 PM AST Wed Sep 15 2021

.SYNOPSIS...
Conditions are expected to gradually improve across the area
through the end of the workweek. However, seasonal shower activity
is expected to persist. Some Saharan dust will arrive on Thursday,
resulting in hazy skies. The long term forecast has a higher than
normal uncertainty due to the presence of several low pressure
systems. Marine conditions are expected to remain at or below 5
feet through early in the week.

&&

.SHORT TERM...Tonight through Friday...

Active weather continued to affect the local islands. This time,
the bulk of the activity focused across the U.S. Virgin Islands,
where flood advisories and special weather statements were issued.
The high resolution models indicate that the activity will
diminish in the evening, but passing showers will continue to
affect the area, and also eastern sections of Puerto Rico.

The surface low pressure to the north of the Bahamas is pulling
away, and as a result moisture content is expected to decrease in
the area on Thursday. The winds will begin to shift a little more
from the east, but still with a component from the southeast.
However, the environment will continue to be favorable enough to
generate early showers for the east of Puerto Rico and the U.S.
Virgin Islands. Then, in the afternoon, local effects will add to
the mixture, resulting in shower and thunderstorm activity for the
interior and west of Puerto Rico, and also, from El Yunque toward
the San Juan metro area. Urban and small stream flooding will be
possible with this activity. Additionally, some Saharan dust will
filter in from the east, resulting in hazy skies. As the day
progresses, the mid-levels will become drier, hence the widespread
activity that was observed on previous days is not expected.

By Friday, mid-level ridge develops north of the area, while
surface high pressure generates a wind flow from the east, at 10
to 12 knots. The mid level ridge will maintain the moisture
confined to the lower 850 mb. However, the trade winds will drag
some patches of clouds over the islands, with precipitable water
values around 1.7 to 1.8 inches. As a result, a seasonal shower
pattern is expected for this day.

.LONG TERM...Saturday through Thursday.../from prev discussion/

Higher than normal uncertainty in the long term period, as the local
guidance is trying to solve a few low pressure systems that could
become tropical cyclones during this time period. Depending on the
development and eventual trajectory of these systems, the local
forecast will have to change accordingly. At this time, the global
and long range models are suggesting that a surface high pressure
will be over the NE Atlantic, while a SFC low, probably a tropical
cyclone develops across the western Atlantic, and another tropical
cyclone develops across the tropical Atlantic. At this time, the
long range models are not suggesting a direct impact to the local
islands from a tropical cyclone. However, the National Hurricane
Center is closely monitoring these systems and they will provide the
official forecasts on these systems.

Having said that, the best possible forecast at this time is that
essentially every night and early morning, we can expect isolated to
scattered showers, then locally induced showers and thunderstorms
each afternoon over areas of sea breeze convergence in Puerto Rico,
while isolated to scattered brief showers are forecast elsewhere. An
upper low is expected to develop to the NE of the local islands by
Saturday, then move west and meander to the north of Hispaniola
until Thursday. Near to slightly below normal moisture is forecast
this weekend, and the overall moisture thereafter will start to
depend on the development and trajectory of the tropical wave that
is currently a few hundred miles to the south of the Cabo Verde
Islands. We decided to go with the GFS model forecast, having a
tropical cyclone several hundred miles to our east and northeast
early next workweek, causing mainly E-NE winds on Monday into
Tuesday, then more northerly for Wednesday and Thursday. The shower
activity with that scenario will be mainly of scattered showers over
the waters, USVI, and across northern PR.

As far as temperatures, we can expect near normal highs and lows in
the long term period. Highs in the upper 80s to near 90 across the
lower elevations, with lows in the mid 70s to near 80. The higher
elevations could observe highs and lows about 5 to 10 degrees cooler
than the lower elevations, even slightly cooler in a few isolated
areas.

Once again, this forecast has higher than normal uncertainty, so
lower than normal confidence for the long term period. Please
monitor the Tropical Weather Outlook and other products from the
National Hurricane Center regarding the tropical waves int he
Atlantic.

&&

.AVIATION...Brief periods of MVFR conds in SHRA/TSRA will remain
possible in and around all PR and USVI TAF sites through 15/23z. Aft
15/23z...VFR conds are expected to prevail at all PR and USVI TAF
sites with VCSH/VCTS possible. Mostly VFR conds at the Leeward
terminals. ESE winds at 15 knots, becoming less than 10 kts
overnight.

&&

.MARINE...
Generally tranquil marine conditions expected across the local
waters. Nearshore buoys show seas at 1 to 3 feet, with winds at 5
to 15 knots. These conditions are expected to prevail through the
weekend. For the beaches, the rip current risk is low.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  78  90  78  89 /  40  60  30  40
STT  79  90  78  89 /  50  40  30  30

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

SHORT TERM...CAM
LONG TERM....ERG
PUBLIC DESK...CVB