072 
FXCA62 TJSJ 230854
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
454 AM AST Fri Apr 23 2021

.SYNOPSIS...

An easterly perturbation will move across the islands today,
providing enough low-level moisture promoting passing showers
across the windward portions of PR/USVI in the morning, followed
by afternoon convection during the afternoon across the western
sections. A dry air mass will arrive on Saturday, and a ridge will
build aloft, resulting in fair weather conditions. In the long
term, a moist and unstable weather pattern could be the weather
story between Tuesday and Thursday next week. 

&&

.SHORT TERM...Today through Sunday...

An easterly perturbation will promote somewhat unstable weather 
conditions for today. Passing showers will continue to affect 
portions of eastern Puerto Rico and the U.S. Virgin islands during 
the morning hours. This activity will be followed by afternoon 
convection over parts of interior and northwestern quadrant of 
Puerto Rico, as well over the San Juan metro area. This activity 
could lead to at least ponding of water on roadways and poor 
drainage areas. Showers will diminish by sunset. 

For tomorrow, mainly fair weather conditions with limited rainfall 
activity is expected across the forecast area. Model guidance now 
suggest a drier air mass coupled with a surface high pressure that 
will yield to less available moisture content at the low levels. 
Aloft, stable conditions expected due to a building upper-level 
ridge. Precipitable water values are forecast to be below normal. 
Hence, Saturday seems to be the driest day of the short term 
period. Incursion of African dust particles is suggested by
observations, thus somewhat hazy skies could be notice.

Patches of moisture will reach the islands by Sunday afternoon 
due to the influence of a polar trough, located in the western 
Atlantic. However, these showers are not expected to be impactful
as they will not lead to significant rainfall accumulations. The 
most active part of the day will be in the afternoon, as local 
effects and diurnal heating can spawn showers over parts of 
northwestern Puerto Rico.

.LONG TERM...Monday through Friday...

A polar trough will move from the western to the eastern Caribbean
Monday through Tuesday. This mid to upper-level trough will then
amplify over PR/USVI Tuesday through Thursday, to be then
displaced by a ridge building from the west on Friday. Meanwhile,
a surface-high pressure will push disturbance in the easterly
winds over the islands early next week. Then, a cold front will
approach the islands from the northwest Tuesday through Thursday,
enhancing tropical moisture over us. Another surface-high will
build over the Western Atlantic Thursday and onward. 

That said, expect the advective pattern to continue Monday and
early Tuesday, which could bring passing showers across the
windward sections in PR/USVI during the overnight and morning
hours, followed by afternoon convection across the northwest
quadrant and interior portions of Puerto Rico. The weather pattern
should be transitioning to a more unstable one Tuesday through
Thursday as the polar trough amplifies and tropical moisture
increase across the islands. Therefore, the potential to have
inclement weather due to thunderstorms, urban and small stream
flooding could increase, especially Tuesday through Thursday, as
model guidance continues to suggest. A more stable pattern should
return by Friday.

&&

.AVIATION...

Mostly VFR conditions expected to prevail over the next 24 hours 
across all terminals. VCSH and brief gusty winds with and around 
SHRA expected for TJSJ and TJPS through 23/10. Afternoon convection 
will yield in SHRA for TJBQ/TJSJ til 23/22. Winds out of east-
southeast at 10 to 15 knots, with sea breeze variations with 
occasionally higher gusts during the afternoon.

&&

.MARINE...

Mariners can expect tranquil marine conditions during the next 
several days. Seas will range between 3 and 5 feet most of the
time. Light to moderate east to southeast winds will prevail 
through early next week. 

Beachgoers, a moderate risk of rip currents is expected to 
continue across some of the east, northern and southern beaches 
of the islands.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  88  75  88  74 /  40  30  10  20 
STT  87  75  87  75 /  50  20  20  30 

&&

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

$$

SHORT TERM...FRG
LONG TERM....CAM