National Weather Service San Juan PR
436 AM AST Thu Sep 16 2021
.SYNOPSIS...
Drier and stable conditions aloft are expected during the short
term period. However, a gradual erosion of low level moisture
across the forecast area will promote shower and thunderstorm
development over the interior and west to northwest sections of
Puerto Rico, and also from El Yunque toward the San Juan metro
area today. Long term period looks very uncertain as the local
guidance continue to try solving a few low pressure systems that
could become tropical cyclones.
&&
.SHORT TERM...Today through Saturday...
Tutt low now well west of the region , as a mid to upper level ridge
will gradually build across the region today through Saturday. This
will result in drier and stable conditions aloft during the period.
The broad surface low over the western Atlantic will continue to
lift farther northward while a surface high pressure ridge will
build north and east of the region through Saturday. This will
result in a gradual erosion of low level moisture across the
forecast area. The prevailing southeasterly wind flow will become
more easterly and increase Friday into Saturday as the high
pressure ridge builds across the Northeastern Caribbean. A low to
moderate concentration of Saharan dust particulates will spread
across the region during the next few days. This will also aid in
drying out out the low levels.
However, for the rest of the morning hours, the environmental
conditions will remain favorable enough to generate early showers
and isolated thunderstorms over the coastal waters and parts of the
north and east to southeast coastal sections of the islands. During
the afternoon hours, the available moisture along with daytime
heating and local effects will favor shower and thunderstorm
development over the interior and west to northwest sections of
Puerto Rico, and also from El Yunque toward the San Juan metro area.
Isolated afternoon shower development will also remain possible in
and around the USVI, but the activity should be of short duration.
Urban and small stream flooding will be possible during the
afternoon over Puerto Rico, with ponding of water on roadways and
in poor drainage areas likely with the moderate to locally heavy
rains during the morning hours. As mentioned , Saharan dust will
filter in from the east, resulting in hazy skies.
By Friday, mid-level ridge will strengthen across the region,while
the surface high pressure will generate an easterly wind flow while
slightly increasing. Somewhat drier conditions are expected for
Friday, with a more seasonal weather pattern expected. Passing
trade wind showers can therefore be expected during the morning
hours followed by hazy conditions during the day along with locally
and diurnally induced afternoon shower development. A similar
weather pattern is so far expected for Saturday, however the Saharan
air layer should diminish across the region by then.
.LONG TERM...Sunday through Friday...
The National Hurricane Center, which provides the official
forecast on tropical cyclone activity, continues to monitor
various areas of interest across the Atlantic basin with low to
high formation chance during the next 5 days. The western-most
feature, or Invest 95L, located more than 500 miles south-
southwest of the Cabo Verde Islands may pose a threat to the NE
Caribbean. Since global models continue to disagree on the
evolution of this system, the uncertainty for the long term period
remains higher than normal. Please monitor the Tropical Weather
Outlook and other products from the National Hurricane Center
regarding the tropical waves int he Atlantic. However, current
model guidance indicates that the best possible scenario for the
long term period will be controlled by the typical seasonal
weather pattern. Expect overnight and early morning isolated
showers over parts of eastern portions of Puerto Rico and the U.S.
Virgin Islands. Then, local effects, diurnal heating, ample low
level moisture and favorable dynamics aloft will result in showers
and thunderstorms across the areas of sea breeze convergence in
Puerto Rico. The upper low is expected to move west and meander to
the north of Hispaniola until Thursday. 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.
&&
.AVIATION...
Brief periods of MVFR conds in SHRA/Isold TSRA will
continue across the regional waters and around PR and USVI at least
til 16/14Z. SCT-BKN lyrs nr FL025...FL040...FL100. Prevailing VFR
conds expected at all terminals with mainly VCSH psbl through the
morning hours. Mtn TOP obscr psbl with passing SHRA ovr eastern PR
due to low clds and SHRA. Sfc wnds light and variable to calm...
bcmg fm east to southeast 10-15 kts aft 16/14Z with sea breeze
variations. Aftn convection mainly ovr interior and W PR fm 16/17Z-
16/23Z with VCTS psbl at TJBQ/TJMZ and VCSH psbl at most of
remaining terminals.
&&
.MARINE...
Moderate trade winds are expected to continue for the next several
days. Generally tranquil marine conditions expected across the local
waters due to seas of 4 feet or less with winds between 5 and 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 89 78 88 78 / 40 40 40 40
STT 91 76 91 76 / 50 50 50 30