Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 280906
Area Forecast Discussion
National Weather Service San Juan PR
506 AM AST Tue Feb 28 2017
.SYNOPSIS...Bands of moisture bearing scattered showers will pass
through the area bringing the most rain to the windward sides of
the islands. Showers diminish a little on Thursday but return over
the weekend. Cold fronts, remain well north but will be of some
effect in the local area.
At upper levels...High pressure extends from the southwest
Caribbean across Hispaniola, but will slowly weaken as a trough
moves into the area Wednesday and Wednesday night. Weak high
pressure approaches from the west, but a much broader trough to
the north forces it to the east over the weekend. A branch of the
subtropical jet will pass by just north of the area early next
At mid levels...High pressure extending out of the northwest
Caribbean will remain just north of the local area until Saturday
when a broad long wave trough passes north of the area from the
west. The trough will dig weakly into the western tropical
Atlantic over the weekend and early next week to leave us in
northwesterly flow. Mid levels remain mostly dry.
At lower levels...High pressure sinking east southeast into the
central Atlantic will cause gradients to tighten over the area
today and Wednesday, resulting in moderate to strong winds across
the local area. Bands of shallow showers moving through will ride
these winds to bring brief modest amounts of rain to the area today
and Wednesday as well as over the weekend.
.DISCUSSION...Fast moving showers in several bands of moisture
moving through the area have brought light amounts of rain to
portions of Culebra, Vieques and the U.S. Virgin Islands and the
northeast half of the island of Puerto Rico. The heaviest amounts
fell over the north coastal area of Puerto Rico and the adjacent
waters with amounts of up to four tenths of an inch since midnight
as measured by National Weather Service Doppler Radar. These
showers will continue coming in through Wednesday. East northeast
trade wind flow is relatively moist in the lower levels and
precipitable water varies only a little between now and Monday,
reaching a minimum Thursday morning and then increasing through
Friday and again Monday through the following Wednesday. A
passing trough at upper levels will accentuate the rain early next
week but, over all, amounts will be under one half inch everywhere
even during the wettest episodes. The approach of cold fronts
will assist some of the shallow shower formation, but all fronts
are currently forecast to remain well north of the forecast area
through at least the next 10 to 12 days.
.AVIATION...VFR conds are expected to prevail at all TAF sites with
brief MVFR conds possible early this morning in and around
TJBQ/TJSJ/TIST/TISX and the Leeward terminals in -SHRA. SHRA
development likely aft 28/16z across SW PR, and this may result in
VCSH at TJMZ through 28/22z. Easterly winds 15-20 knots with sea
.MARINE...Sea have begun to increase across all but the most
protected waters and seas at buoy 41043 have risen above 7 feet.
Model solutions have tended to increase from run to run and now
show 9 foot seas in the northeast corner of the forecast area
early Wednesday through Thursday. Have not yet introduced these
heights to our forecast since they will depend on winds stronger
than forecast to verify, but will monitor in case winds of 23 to
25 knots--sustained--are found. Seas in the outer waters are
expected to continue at or above 7 feet through at least Saturday.
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 75 84 75 / 40 40 40 30
STT 85 74 86 75 / 30 40 40 30
PR... Small Craft Advisory from 2 AM Wednesday to 8 PM AST Thursday
VI...Small Craft Advisory until 8 PM AST Friday for St.Thomas...St.
John...and Adjacent Islands.
AM...Small Craft Advisory until 8 PM AST Friday 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 USVI and Culebra out
10 NM-Coastal Waters of Southern USVI Vieques and Eastern
Puerto Rico out 10 NM-Mona Passage Southward to 17N.
Small Craft Advisory from 2 AM Wednesday to 8 PM AST Thursday
for Coastal Waters of Southern Puerto Rico out 10 NM.
Small Craft Advisory from 2 PM this afternoon to 8 PM AST Friday
for Coastal Waters of Northern Puerto Rico out 10 NM.