Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 040834

Area Forecast Discussion
National Weather Service San Juan PR
434 AM AST Sat Apr 4 2020

.SYNOPSIS...A cold front will drift into the area Saturday night
and Sunday bringing some additional shower activity and then dissolve
on Monday. A band of better moisture southeast of the area Sunday
will move toward Puerto Rico and the U.S. Virgin Islands, but
weaken. This band will reform over the area late Thursday and


.SHORT TERM...Today through Monday...

Clear skies prevailed across the local islands during the overnight
hours with a few sprinkles noted over the Atlantic offshore waters.
The overnight low temperatures ranged from the upper 70s to around
80 degrees at the lower elevations. Winds were light and variable.

As a frontal boundary approaches the forecast area from the north-
northwest, low level moisture is expected to increase with
precipitable water values ranging between 1.40 and 1.60 through at
least Monday. At upper levels, a gradual intensification of the
subtropical jet maxima north of Puerto Rico is expected, which may
enhance shower development across the forecast area throughout the
day today. Nevertheless by Sunday, the convergent/subsident side of
the upper jet will dominate the region. Therefore, under available
moisture near the normal range, fair upper level dynamics and light
surface winds, expect shower development over central, north and
east Puerto Rico this afternoon. By the evening and early morning
hours on Sunday, shower activity will be then focus over the north
and east coastal areas as well as over the US Virgin Islands.
Locally induced afternoon showers are expected once again Sunday
afternoon, although intensity and areal coverage may decrease due
to the lack of upper level dynamics. Somewhat drier conditions are
expected on Monday.

Slow moving showers are expected under light winds today, therefore
there is an increasing potential for ponding of water on roadways
and urban and small stream flooding.

.LONG TERM...Tuesday through Monday...

On Tuesday the drier air behind the weakening frontal boundary
will filter into the forecast area. Nevertheless there will be
enough moisture for early morning showers along the windward coast
in the northeast of Puerto Rico during the overnight and early
morning period. This moisture will play into local effects and
also produce showers over southwest Puerto Rico Tuesday afternoon.
Moisture then increases Tuesday night through Thursday evening as
a band of moisture that was pulled out of South America Sunday
and formed southeast of the area, slowly moves over Puerto Rico
and the U.S. Virgin Islands. Since surface flow Tuesday through
Thursday will remain east northeast this moisture will help
generate showers for the eastern coasts of Puerto Rico with some
isolated showers over the U.S. Virgin Islands.

High pressure at the surface over the western tropical Atlantic
will move into the Caribbean over the next weekend and dissipate.
Surface winds become southeasterly Friday and saturday and the
northeast coastal portion of Puerto Rico will warm into the upper
80s to lower 90s again. The high pressure will be closely
followed by a trough in the lower levels that will drag some of
the best moisture in several weeks across the area late in that
weekend, Sunday and Monday.


.AVIATION...VFR conds are expected to prevail at all TAF sites
through the forecast period. Aft 04/16z, however, shower development
is expected over central PR with VCSH/SHRA remaining possible in and
around JSJ. Mountain obscurations. Winds are expected to be light
and variable with sea breeze variations developing after 04/14z.
Max winds WSW 75-90 kts btwn FL330-465. Strongest winds at FL400.


.MARINE...Northerly swell is increasing and will peak today. Seas
will subside only slightly before swell increase again on Monday
morning. The risk of rip currents will be high for coasts with a
northern exposure on the Atlantic side through at least Tuesday,
but seas should begin to subside after that through the end of the


SJU  88  77  84  75 /  50  40  40  40
STT  86  75  79  74 /  30  40  40  40


PR...High Rip Current Risk through Tuesday afternoon for Mayaguez and
     Vicinity-North Central-Northeast-Northwest-San Juan and

     High Rip Current Risk through Tuesday afternoon for Culebra.

VI...High Rip Current Risk through Tuesday afternoon for
     St.Thomas...St. John...and Adjacent Islands.

AM...Small Craft Advisory from 2 PM this afternoon to 8 PM AST Monday
     for Coastal Waters of Northern Puerto Rico out 10 NM.

     Small Craft Advisory until 8 PM AST Monday for Atlantic Waters
     of Puerto Rico and USVI from 10 NM to 19.5N.

     Small Craft Advisory from 8 AM Sunday to 8 PM AST Monday for
     Mona Passage Southward to 17N.

     Small Craft Advisory from 8 AM Sunday to 8 AM AST Monday for
     Coastal Waters of Northwestern Puerto Rico out 10 NM.



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