Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 191455
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1055 AM AST Thu Apr 19 2018

.UPDATE...Forecast on track, the chance for showers and thunderstorms
this afternoon continues. A drier air mass is expected to encompass
the forecast area late tonight.

&&

.AVIATION...Periods of MVFR conds likely aft 19/16z in and around
JMZ/JBQ and possibly JSJ in SHRA/TSRA. ESE winds around 10-15
knots with some sea breeze variations.

&&

.MARINE...Hazardous seas are expected across the offshore Atlantic
and Caribbean waters, Anegada and Mona Passage due to a long
period northeasterly swell. Seas increasing up to 7 feet
throughout the day.

&&

.PREV DISCUSSION... /issued 531 AM AST Thu Apr 19 2018/

SYNOPSIS...Short wave trough will enhance shower and thunderstorm
development today over portions of Puerto Rico. Drier air will
move quickly later this evening but a surge in moisture returns
once again on Friday and Saturday. Broad surface high pressure
will continue across the eastern Atlantic through early next week
and another surface high will build north of the region by then.
A wind surge will keep moderate to fresh trades through Friday.

SHORT TERM...Today through Saturday...
The weak trough/tropical wave that brought showers and thunderstorms
to the local area Wednesday has moved to south of Haiti. Drier air
is now moving into the Leeward Islands and will pass into the U.S.
Virgin Islands by midday and onto Puerto Rico by mid afternoon.
Surface winds will also increase somewhat today as a wind surge
moves through. An approaching trough at upper levels will bring
divergence to Puerto Rico today and The U.S. Virgin Islands later
this afternoon and evening. This will enhance the convection and
instability in Puerto Rico and another round of thunderstorms with
urban and small stream flooding will be likely. However, the
divergence aloft will be too late to help the U.S. Virgin Islands,
since moisture will already have eroded considerably from the east
there. Drier air will move into the area tonight but will be quickly
followed by another area of moisture--especially at 850 mb, that
will spawn more showers on Friday and Saturday. Upper level support
for these showers will not be found, however, and an area of
convergence will fight convection over the Caribbean Sea on
Saturday. Winds at the surface will become more easterly later
tonight and Friday and gain a little northerly component on
Saturday. This will make high temperatures on the north coast likely
the warmest of the last few days today with sea breezes on the north
coast more effective Friday and Saturday. Temperatures on the south
coast will likely warm several degrees on Friday and Saturday.

LONG TERM...Sunday through Thursday...
Dry mid levels are expected on Sunday and this will limit shower
activity to the west and southwestern portions of PR due to a
northeasterly component on the steering winds. But, mostly sunny
to partly cloudy skies are expected to prevail. From Monday
through Thursday low level moisture will increase to 700mb and
favor the development of afternoon showers and thunderstorms
along the Cordillera Central and western Puerto Rico. Passing
trade wind showers will also increase across the USVI and eastern
portions of PR during the overnight and early morning hours.

AVIATION...Isold TSRA/SHRA will increase later today. VFR conds to
cont til arnd 18/16Z xcp arnd TNCM/TKPK where ocnl MVFR CIGS are
psbl. Aft 16Z areas of +SHRA/iso-sct TSRA will dvlp mainly across
wrn and interior PR with mtn obscurations. Sfc winds ESE 10-15 kt
with sea/land breeze variations. Max winds W 50-60 kt btwn FL350-430
thru 20/06Z.

MARINE...A long period northeast swell will result in life
threatening rip currents across the north, east and southeast
facing beaches of Puerto Rico, Culebra, Vieques and St. Croix
through at least Friday. High surf conditions due to breaking
waves up to 10 feet are expected across the eastern beaches of
St. Croix. Small craft advisories are in effect for the offshore
waters and passages due to seas up to 7 feet. East winds will
continue at 15 to 20 knots across the region.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  87  75  88  75 /  30  20  30  50
STT  86  75  86  76 /  20  20  30  30

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Friday afternoon for Northeast.

     High Rip Current Risk through late tonight for Central Interior-
     Ponce and Vicinity-Southeast.

     High Rip Current Risk through late Friday night for Culebra-
     North Central-Northwest-San Juan and Vicinity-Vieques-
     Western Interior.

VI...High Rip Current Risk through late Friday night for St Croix.

AM...High Rip Current Risk through late tonight for Coastal Waters of
     Southern Puerto Rico out 10 NM.

     Small Craft Advisory from 8 PM this evening to 6 PM AST Friday
     for Caribbean Waters of Puerto Rico from 10 NM to 17N-Mona
     Passage Southward to 17N.

     Small Craft Advisory until 6 PM AST Friday for Anegada Passage
     Southward to 17N-Atlantic Waters of Puerto Rico and USVI
     from 10 NM to 19.5N.

&&

$$

UPDATE...OM
AVIATION....OM


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