National Weather Service San Juan PR
538 AM AST Mon Apr 16 2018
.SYNOPSIS...Shallow marine moisture and brief passing showers will
be augmented by better moisture on Wednesday and Thursday with a
weak low-level trough passage late Tuesday. The cold front that
left Florida last night will not be able to reach the local area
before it is forced to retreat on Wednesday.
&&
.SHORT TERM...Today through Wednesday...
An easterly perturbation and associated low level moisture just
east of Guadeloupe will enter the eastern Caribbean during the
next day or so. A cold front entering the western Atlantic is
forecast to remain well to the north of the region as a broad
surface high pressure builds over the east central Atlantic by
midweek. PWAT content is expected to increase from 1.45 inches
today to 1.8 inches by Wednesday evening. East to southeast
steering winds will prevail each day between 10-15 kts. Diurnally
induced afternoon showers and isolated thunderstorms are expected
to develop each day...mainly over the northwest quadrant of Puerto
Rico. Streamers are also expected to develop across the USVI and
over the San Juan metro area from the eastern mountains. Localized
urban and small stream flooding is expected with this activity
each afternoon but mainly in western Puerto Rico. An upper level
trough is expected to enhance convective development over the
region by Wednesday. This trough and the expected increase in
moisture will favor the development of scattered to numerous
showers and thunderstorms across portions of the local waters and
the islands.
.LONG TERM...Thursday through Monday
The conditions bringing the showers and thunderstorms to most
areas of the islands Wednesday will increase slightly and reach a
maximum of Thursday. Most indicators including precipitable water
(PWAT) and the GDFI (Galvez-Davidson Index) will favor the best
chance of thunderstorms and heavier showers on Thursday. This
would also be the day of the most widespread urban and small
stream flooding though, even then, it should be localized to the
most favored areas. On Friday and Saturday the divergence aloft is
no longer favorable and the jet moves east with the passage of a
weak shortwave trough on Saturday. Moisture diminishes after
Saturday as winds at 700 mb turn northeasterly and even surface
flow becomes a little north of east. Precipitable water values
return to those of yesterday or between 1.2 and 1.5 inches for
the rest of the period. This will yield scattered showers
overnight on the windward slopes and coasts and isolated
thunderstorms in the west and interior during the afternoons, but
there should be little more than that until Tuesday of next week.
&&
.AVIATION...Mainly VFR conditions expected to prevail across all
terminals through 16z. However, passing shra could move at times
across TNCM/TKPK/TIST/TISX through the day, causing brief MVFR cigs.
Across the PR terminals, shra/tsra is expected btw 17z-23z across
the NW portions of PR, impacting mainly TJBQ/TJMZ with mtn
obscurations and tempo MVFR conds. ESE winds expected at 10-20
kts blo FL100 with sea breeze variations at the sfc after 14z.
Maximum winds NNW 50-65 kt btwn FL320-470 strongest at FL400 at
16/12Z.
&&
.MARINE...Seas are gradually subsiding with the winds and all
small craft advisories should come down by 2 PM AST this
afternoon. Rip current risk will remain moderate and small craft
will need to exercise caution in some waters. Seven foot seas are
forecast to return to the local outer Atlantic waters by Thursday
afternoon but for no more than 24 hours.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 86 74 87 75 / 50 40 40 40
STT 85 76 86 76 / 40 40 40 50