Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 230831
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
431 AM AST Sun Apr 23 2017

.SYNOPSIS...A ridge aloft will erode rapidly today as a short
wave trough swing across the region. A subtropical jet will hold
aloft through at least Monday. A polar trough is forecast to move
over the Western Atlantic Ocean today, then will reach the
islands into mid-week. This trough will increase the instability
across the region today through at least Wednesday. The low level
southeast wind flow, will pull tropical moisture from the
Caribbean waters over the region increasing the chance of showers
and thunderstorms today through at least mid week.

&&

.SHORT TERM...Today through Tuesday...
Surface high pressure northeast of the area moves further into
the Central Atlantic today, winds will shift from the east-
southeast and trof pattern establishes across the Western Atlantic
into the Central Caribbean. PWAT values are forecast to increase
above 2 inches across much of the forecast area and trof pattern
will provide good divergence aloft to enhance the development of
thunderstorm with periods of heavy rainfall, mainly over northwest
PR today and spreading across the rest of the forecast area
through at least late Tuesday night.

Currently global models indicate high moisture content and favorable
conditions for scattered to numerous showers and thunderstorms
across the Atlantic waters and over the northern USVI on Monday.
High res WRF model indicates best rainfall amounts this afternoon
across the northwestern coastal areas of PR. Other high res models
are indicating more widespread shower activity across the rest of
the interior and southeastern sections of PR later this evening and
increasing through Monday morning. Residents in PR and in the USVI
should monitor the progress of the weather conditions as periods of
frequent rainfall can result in urban and small stream flooding as
well as localized flash flooding with the heaviest showers.

.LONG TERM...Wednesday through Tuesday...
An upper level trough is expected to race over the northeast
Caribbean as an associated surface low moves northeastward across
the Canadian Maritime provinces. As this happens weather
conditions will remain unstable across the local islands Wednesday
but PWAT values are expected to decrease below 2.0 inches by that
time. Weather conditions will improve rapidly by Thursday and
thereafter as an upper level ridge build across the region. A
surface high pressure across the CentraL will shift the local
winds more from the east late Wednesday and Thursday. Although the
instability is expected to decrease by the middle of the week,
this change in wind direction is expected to induce the
development of some showers and possible a couple of thunderstorms
each afternoon across interior and western sections of Puerto
Rico due to the combination of daytime heating and local effects.
A weak surface trough will increase slightly the chances of
showers across the region by the upcoming weekend.

&&

.AVIATION...Mainly VFR conditions are expected to prevail
early in the forecast period across all terminals. BKN-OVC cigs abv
FL050 spreading from the west through the day. Afternoon SHRA/TSRA
across the northwestern quadrant of PR can result in MVFR conds at
TJBQ/TJMZ. -RA/SHRA possible near TJSJ due to SHRA streaming from
the eastern mountains. ESE winds below FL050 at 12-18 knots.
Cloudiness and SHRA increasing late in the forecast period into
Monday morning across the forecast area.

&&

.MARINE...Seas will continue to deteriorate for the next couple
of days as a northerly swell continue to invade the local waters.
Seas will increase up to 8 feet with breaking waves reaching 10
feet of more across the Atlantic waters. For this reason a High
Surf Advisory is in effect for the exposed Atlantic coastline of
Puerto Rico. Small Craft Advisories will remain in effect for most
of the coastal waters. A high Risk of Rip Currents potential
continues for the northwest to northeast beaches of Puerto Rico,
Culebra and Vieques and for the U.S. Virgin Islands beaches.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  86  75  87  76 /  20  40  50  40
STT  84  75  83  76 /  20  60  60  50

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Monday afternoon for Culebra-North
     Central-Northeast-Northwest-San Juan and Vicinity-Southeast-
     Western Interior.

     High Surf Advisory until 6 AM AST Monday for Culebra-North
     Central-Northeast-Northwest-San Juan and Vicinity-Southeast-
     Western Interior.

     High Rip Current Risk through this afternoon for Vieques.

     High Rip Current Risk through late tonight for Mayaguez and
     Vicinity.

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

     High Rip Current Risk through late tonight for St Croix.

AM...High Rip Current Risk through Monday afternoon for Coastal
     Waters of Northern USVI and Culebra out 10 NM-Coastal
     Waters of Southern USVI Vieques and Eastern Puerto Rico out
     10 NM.

     High Surf Advisory until 6 AM AST Monday for Coastal Waters of
     Southern USVI Vieques and Eastern Puerto Rico out 10 NM.

     Small Craft Advisory until 8 PM AST Monday for Anegada Passage
     Southward to 17N-Atlantic Waters of Puerto Rico and USVI
     from 10 NM to 19.5N-Caribbean Waters of Puerto Rico from 10
     NM to 17N-Coastal Waters of Northern Puerto Rico out 10 NM-
     Coastal Waters of Northern USVI and Culebra out 10 NM-Mona
     Passage Southward to 17N.

     Small Craft Advisory until 8 AM AST Monday for Coastal Waters OF
     Northwestern Puerto Rico out 10 NM-Coastal Waters of
     Southern USVI Vieques and Eastern Puerto Rico out 10 NM.

&&

$$

SHORT TERM...DS
LONG TERM....JF


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