National Weather Service San Juan PR
555 AM AST Tue Sep 11 2018
.SYNOPSIS...Several minor troughs will move through the area ahead
of Tropical Storm Isaac with afternoon showers and thunderstorms
expected in western and interior Puerto Rico, but only isolated
shower in the U.S. Virgin Islands. Considerable uncertainties
continue with regards to the track and intensity of Isaac, but
present forecasts show it passing south of the forecast area as a
Tropical storm, spreading gusty winds, showers and thunderstorms
across the area. Considerable moisture follows Isaac on Saturday
that will keep the weather wet well into next week.
&&
.SHORT TERM...Today through Thursday...
Patches of low level moisture transported by an east to southeast
wind flow will bring light to moderate passing showers over portions
of eastern Puerto Rico and the U.S. Virgin Islands during the
morning hours. Then, the combination of the available moisture with
local and diurnal effects along with favorable upper level
dynamics with the passage of an upper level trough, will favor
the development of showers and thunderstorms, particularly across
interior and western Puerto Rico. Also, streamer-like showers and
thunderstorms are possible over the San Juan metropolitan area.
Urban and small stream flooding is likely with the heaviest
activity. Elsewhere, mostly fair weather conditions expected.
The entrance of an elongated area of a somewhat drier air ahead of
Tropical Cyclone Isaac will result in mostly tranquil weather
conditions across the local area on Wednesday enhanced by the
passage of an upper level ridge. Although limited, the potential
for afternoon convection over portions of the interior and western
Puerto Rico remains. By late Wednesday night into Thursday
morning, Isaac`s forward outer rain band will move across the
local area. This feature will enhance shower and thunderstorms
activity, and will bring gusty conditions across the local islands
and waters. At this time, the intensity and coverage of the
activity, as well as any additional threat or impact this tropical
cyclone could bring to the local area are yet to be determined
given the uncertainties that relates to Isaac`s track and
intensity forecast.
.LONG TERM...Friday through Tuesday...
Although the track and intensity of Tropical Storm Isaac are still
surrounded by uncertainties, the current forecast shows it
passing south of the area Thursday night through Friday night. It
will likely bring seas greater than 8 feet by Wednesday night or
Thursday and seas in the southern part of our Caribbean waters
greater than 15 feet for Friday cannot be ruled out at this time.
Upper level flow between a high to the northwest and a TUTT to
the northeast will be northeast over the area and will contribute
shear that will weaken tropical cyclone Isaac when it arrives on
Friday. After the passage of TC Isaac, moist flow continues
beyond the end of the period with precipitable water forecast to
be above 2 inches after midday on Saturday. During this latter
period many bubbles of divergence aloft pass over the local area
will present an environment favorable to heavy shower activity and
isolated thunderstorms.
Expect that weather will remain wet and that most areas
will have at least some rainfall with local urban and small stream
flooding possible each day. Models do not suggest major/widespread
flooding with the passage of Isaac so far south.
&&
.AVIATION...Mainly VFR conditions expected to prevail across all
terminals during the next 24 hours. However, VCSH expected at TIST,
TISX and TKPK until 11/13Z. Afternoon convection will result in VCTS
across all PR terminals between 11/16Z and 12/00Z with mountain
obscurations and strong sea breeze variations with gusty winds.
This can result in tempo MVFR conditions at TJSJ, TJBQ, TJPS and
TJMZ. Fair weather conditions will return after 12/00Z across all
terminals although some mountain obscurations and scattered
showers will persist overnight in ern PR and around the USVI.
&&
.MARINE...Pulses of swell have been moving through the local outer Atlantic
waters and just beyond from Major Hurricane Florence. This has
mostly resulted in increased rip currents on the north coast of
Puerto Rico. This situation is expected to continue through the
end of the week. Seas from Tropical Storm Isaac will arrive
Wednesday night or Thursday morning should the current forecast
tack and intensity hold. As mentioned above, seas greater than 8
feet are possible by Wednesday night or Thursday and seas in the
southern part of our Caribbean waters greater than 15 feet for
Friday cannot be ruled out at this time. Seas in the Atlantic
will become confused as swell from both Isaac and Florence
combine.
Seas in the Caribbean should fall below 7 feet on
Saturday and seas in the Atlantic should have subsided by Sunday.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 87 79 89 79 / 40 10 40 70
STT 91 80 90 80 / 40 40 70 70