FXCA62 TJSJ 180811
Area Forecast Discussion
National Weather Service San Juan PR
411 AM AST Wed Oct 18 2017
A fast-moving tropical wave passed through the Caribbean south of
the U.S. Virgin Islands and Puerto Rico overnight. Moisture will
follow in southeast flow until this late afternoon when drier air
will move in at all but the lowest levels. Although several
patches of moisture move through Saturday and Sunday only minimal
shower activity is expected this tonight through Sunday in the
typical diurnal pattern of east southeast flow.
.SHORT TERM...Today through Friday...
A tropical wave moved rapidly through the Caribbean Sea just
south of the local forecast waters overnight on Tuesday night at
about 23 knots. It was followed by a stream of moisture that
generated a few thunderstorms and north northwest-moving outflow
boundaries during the last 12 hours. This moisture is expected to
impact the U.S. Virgin Islands and Puerto Rico during the morning
hours. An upper level low over the eastern tip of the Dominican
Republic will drop south and reach Venezuela by Thursday and is
forecast to bring divergent flow aloft to enhance convective
activity over southeast Puerto Rico today. During the afternoon
much drier air moves into the area to cut the usual afternoon
convection short. Then, easterly winds ramp up through early
Thursday morning. The GFS shows precipitable water peaking at 12Z
today around 2.23 inches and falling to 1.06 inches as early as
19/18Z (Thursday afternoon). This invasion of drier air will mark
the end of the threat of flooding in the forecast area.
Thunderstorms will then become isolated Thursday and generally
limited to the interior and northwest portion of Puerto Rico. Low
level moisture will bring isolated showers during the overnight
and early morning hours to the windward coasts, but accumulations
will be minimal, if any, Thursday and Friday. The ECMWF is in
general agreement, but holds minor shower activity in the area
through Thursday instead of drying Puerto Rico out late Wednesday
Returning to the current situation, The satellite imagery shows
activity has diminished in intensity and coverage from Monday when
the upper level low was much stronger and more active, but the
MIMIC product suggests that rich moisture is pushing into the
forecast area from the southeast and may trigger thunderstorms
with very heavy rain during the day today before drier air moves
in. Thus the most successful forecast is expected to be a blend in
the timing from the GFS and ECMWF. Because of antecedent
conditions, the Flash Flood Watch will continue for Puerto Rico
through Wednesday evening (at 8 PM AST).
.LONG TERM...Saturday through Tuesday...
Low level moisture, mainly below the 850 mb level will continue
with relative humidities above 90 percent in an almost continual
fashion through the forecast period. This will produce shallow
showers with very little accumulations along the eastern and
southeastern foothills and slopes during the early morning hours
with a little extra help Saturday and Sunday from passing patches
of moisture at 700 mb. Isolated thunderstorms will be possible in
the west northwest portion of Puerto Rico during the afternoons,
but most showers will be find mid-level moisture of less than 10
percent that will be found from Friday through Monday
insurmountable, so any showers that do develop will be short-lived
and unable to deliver much rain. In addition, winds will not have
subsided much from the wind surge expected on Thursday and will
hinder convective development and not allow showers to remain very
long in any one place. Moisture begins to improve on Sunday and
will continue to do so through the following Thursday. An upper
level trough will cross through on Sunday. On Monday and Tuesday
the trough will deepen over the Anegada passage and the GFS is
bringing a 60 knot jet across the area from the northwest. As it
does so, divergence aloft will increase through Tuesday, and
making the best convection of the period Saturday through Tuesday
on Tuesday, even though moisture will still not be optimum or as
rich as it has been the last three days.
SHRA/TSRA are expected to begin early today in the southeast
portion of Puerto Rico and the waters southeast due to moisture
behind a fast moving tropical wave that swept through the
Caribbean Sea overnight. Expect mtn obscurations thru arnd 18/22Z
also ocnl MVFR conds at TJPS/TIST/TISX til arnd 18/18Z. SHRA/TSRA
will also dvlp aft 18/15Z nr TJMZ/TJBQ and psbly TJSJ. Most of
these sites will become VFR as drier air moves into the area
beginning arnd 18/20Z. This dry air and some increased winds has
already entered TNCM/TKPK and conds should remain VFR there thru
byd 19/10Z. Maximum winds will be east southeast 30 knots arnd
FL075 at 19/12Z.
.MARINE...The tropical wave and the moisture behind it will bring
scattered thunderstorms with gusty winds to the local waters
today. Drier air entering the area will be accompanied by a wind
surge which will bring 10 to 20 knot east southeast winds and
areas of seas in the exposed waters of 7 to 8 feet through Sunday.
The Atlantic waters behind Puerto Rico will not be impacted as
.PRELIMINARY POINT TEMPS/POPS...
SJU 90 81 89 81 / 30 20 20 10
STT 84 82 83 82 / 30 20 20 10
PR...Flash Flood Watch through this evening for Central Interior-
Culebra-Eastern Interior-Mayaguez and Vicinity-North
Central-Northeast-Northwest-Ponce and Vicinity-San Juan and
Small Craft Advisory from 2 PM this afternoon to 8 PM AST
Thursday for Southeast.
VI...Small Craft Advisory until 8 PM AST Thursday for St.Thomas...St.
John...and Adjacent Islands.
High Rip Current Risk through this evening for St Croix.
AM...Flash Flood Watch through this evening for Coastal Waters OF
Southwestern Puerto Rico out 10 NM-Coastal Waters of
Southern Puerto Rico out 10 NM.
Small Craft Advisory from 2 PM this afternoon to 8 PM AST
Thursday for Coastal Waters of Southern Puerto Rico out 10
Small Craft Advisory until 8 PM AST Thursday 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 Southern USVI Vieques and
Eastern Puerto Rico out 10 NM-Mona Passage Southward to 17N.