Area Forecast Discussion
National Weather Service San Juan PR
536 AM AST Wed Apr 18 2018
.SYNOPSIS...A surface trough will move across the region today.
Showers with periods of heavy rainfall and thunderstorms are
expected today and on Thursday afternoon. Drier mid levels are
expected Friday through early next week. However, diurnally
induced afternoon showers with possible isolated thunderstorms are
expected each day across western PR. Elsewhere, passing showers
and light rainfall amounts are expected.
.SHORT TERM...Today through Friday...
A weak trough is over Puerto Rico and moisture behind it is spawning
SHRA mainly over Atlantic waters, and, earlier, pockets of TSRA
south southwest of Ponce and east of the Leeward Islands. Showers
are moving northwest in the low level flow. The MIMIC product shows
better moisture crossing the Leeward Islands and is expected to ride
into Puerto Rico and the U.S. Virgin Islands by the time of maximum
heating. Right now these areas have mostly clear skies overhead and
upstream, but clouds and showers are expected to develop during the
day. The best chances for rain will be in the west and interior
portions of Puerto Rico but showers over the local waters are more
likely than yesterday. A weak jet will pass north of Puerto Rico
today and tomorrow ahead of an eastward moving upper level trough
that will pass on Friday. Some general support is therefore expected
from the divergence aloft today. Moisture will also continue to
increase during the day while 500 mb temperatures remain low (less
than minus 8 C) and lifted indices will fall to minus 6. This will
support isolated to scattered thunderstorms and local urban and
small stream flooding in the west, (generally north of the
Cordillera Central) and the interior portions of Puerto Rico. A
streamer off of the high points of the Luquillo range is also
possible and will be the main source for heavy rainfall in the
greater San Juan and Bayamon Metropolitan Areas. Moisture continues
overnight and showers will be slow to move out of the area. However
dry air will be edging into the USVI by 19/18Z and into Puerto Rico
by 20/00Z and showers will diminish dramatically. This drier air
will move through in about 24 hours and by Friday afternoon there
should be enough moisture to bring scattered showers, isolated
thunderstorms and localized urban and small stream flooding to
western and interior Puerto Rico as well as scattered showers to
areas southeast of the main islands, including Saint Croix.
Dynamics will be better on Thursday so areas of urban and small
stream flooding should be greater and more intense Thursday. Friday
will be similar or slightly weaker than Wednesday for much the same
reason as well as the steep decline in moisture at all but the
lowest levels of the atmosphere.
.LONG TERM...Saturday through Wednesday...
A broad surface high pressure will continue across the eastern
Atlantic promoting moderate trades. Drier mid level air is
expected to prevail during the long term period. However, a
typical weather pattern will prevail each day with afternoon
convection developing over the western interior sections of Puerto
Rico and streamers forming off the smaller islands. A short wave
trough will move close on Sunday, enhancing shower and
thunderstorm development during the afternoon and some showers
will linger through the evening hours across portions of western
.AVIATION...Moisture behind a weak trop wave will bring SHRA and
Isold TSRA. VFR conds to cont til arnd 18/15Z xcp arnd TNCM/TKPK
where ocnl MVFR CIGS are psbl. Aft 16Z MVFR CIGs psbl TIST/TISX in
SHRA til aft 19/06Z. Aft 18/15Z 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 by 18/18Z.
.MARINE...A long period northeasterly swell will move across the
local waters on Thursday. This will cause seas to increase around
7 feet mainly across the offshore Atlantic waters and the Anegada
Passage. Therefore, small craft advisories will be in effect.
Also, life threatening rip currents are expected along the
Atlantic beaches of Puerto Rico and portions of the southern coast
of the island as well across St. Croix. Meanwhile seas will
continue to range between 3-6 feet today and winds will prevail
from the east to southeast at 10-15 knots.
.PRELIMINARY POINT TEMPS/POPS...
SJU 86 74 86 75 / 70 70 40 20
STT 84 76 85 75 / 70 70 20 10
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