National Weather Service San Juan PR
532 AM AST Wed Sep 12 2018
.SYNOPSIS...The proximity of a Tutt low just west of the region
and a plume of moisture in the prevailing east to northeast flow
ahead of Tropical Storm Isaac will support showers and isolated
thunderstorm activity across the coastal waters and parts of the
islands the rest of today. Based on the current forecast track,
Tropical Cyclone Isaac is forecast to move westward and remain
south of Puerto Rico and the U.S. Virgin Islands late Thursday
evening through Friday. A moist and unstable environment will
persist for the next several day and into the upcoming weekend.
&&
.SHORT TERM...Today through Friday...
The entrance of an elongated area of a somewhat drier air ahead of
Tropical Storm Isaac will reach the forecast area early this morning
and will cross the local islands throughout the day. Although this
feature will bring mostly tranquil weather conditions across the
region, streamer-like showers moving over the waters into eastern
Puerto Rico and the U.S. Virgin Islands during the morning hours, as
well as afternoon showers and thunderstorms developing over the
interior and western Puerto Rico remains possible due to the diurnal
effects. Urban and small stream flooding is likely across these areas.
Once this area of drier air continues to move westward and away from
the local islands, weather conditions are expected to deteriorate as
Isaac`s forward outer band streams across the region late Wednesday
night into Thursday. This will feed squally weather conditions along
with gusty winds and frequent lightning activity across the forecast
area as it progress westward. Streamer-like showers and thunderstorms
will move over the waters into land areas early Thursday morning into
Thursday afternoon, then afternoon convection will be triggered over
portions of the interior and western Puerto Rico. A secondary event
is then expected on Friday as Isaac approaches the local area. Although
the storm is expected to remain south of the local islands, showers
and thunderstorms are expected across most of the forecast area. Based
on the current forecast, total rainfall accumulations are forecast
to range between 1 and 3 inches with isolated higher amount`s. Thus,
urban and small stream flooding, rapid rises along rivers, and mudslides
near steep terrain are highly likely during this forecast period.
.LONG TERM...Saturday through Wednesday...
Tropical Storm Isaac is still surrounded by uncertainties as far
as intensity, as it is expected to continue to feel the effects of
westerly and northwesterly shear. However the current forecast
shows it passing south of the U.S.Virgin Islands and Puerto Rico
from Friday through Saturday. It will likely bring seas between 8
to 10 feet at least through Saturday mainly over the southern
portion of the local Caribbean waters and passages. After the
passage of Isaac, a moist southeasterly flow will continue across
the region through most of the upcoming weekend and into early
next week. Recent model guidance continued to suggest layered
precipitable water values in excess of 2 inches into early next
week. This along with the proximity of another Tutt low will
maintain a moist and unstable environment through ahd least Monday
of next week with a gradual improvement by Tuesday. That said,
expect fairly wet weather pattern favorable which will support
periods of moderate to heavy rainfall with showers and isolated
thunderstorm activity most of the period. Local urban and small
stream flooding will remain possible each day with potential for
mudslides in areas of steep terrain. However,model guidance still
do not suggest widespread flooding during the period.
&&
.AVIATION...VFR conditions will prevail across most terminals through
the forecast period. However, MVFR conditions are expected across
the TNCM terminal this afternoon as occasional bands of passing
showers move across the area in the northeasterly wind flow. SHRA/TSRA
are expected to develop across the interior and western portion of
Puerto Rico on Wednesday afternoon for possible impacts across TJMZ.
Surface winds will be from the northeast across all area sites through
the forecast period light up to 8 knots becoming 10 to 15 knots aft
12/14z with sea breeze variations.
&&
.MARINE...Seas up to 6 feet over the Atlantic waters and Caribbean
passages with easterly winds 10 to 20 kts today. Later this evening
and overnight, expect increasing and deteriorating seas up to 8
to 10 feet over the offshore Atlantic and Caribbean waters and
local passages with the approach of Tropical Storm Isaac. Therefore,
a small craft operators are urge to exercise caution today and
small craft advisories will be in effect later this evening.
Please refer to the latest Marine Weather Message (MWWSJU) and
Coastal Waters Forecast (CWFSJU) issued by the National Weather
service WFO San Juan PR.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 89 80 89 79 / 20 60 60 40
STT 92 81 91 80 / 40 50 50 70
&&

