National Weather Service San Juan PR
524 AM AST Sun Apr 16 2017
.SHORT TERM...A rich plume of moisture has been moving north over
Hispaniola and turning toward the east over the local area around
a ridge at 700 mb that extends from the Windward Islands to
Hispaniola. Yesterday most of the moisture from this flow stayed
over our local Caribbean waters and just south of there. Today it
is forecast to move over Puerto Rico. The center of a jet of west
winds at 22 degrees north latitude has been moving east and is
currently located northeast of the Leeward Islands. Although
convergence aloft was over the area yesterday at 15/18Z, today
good divergence aloft is expected in the right entrance of the jet
over the local area. This should bring better dynamics to the
area and enhance the local rainfall mainly over the interior of
Puerto Rico. Areas of moisture at mid levels can already be seen
in the latest GOES-IR clean channel moving into Puerto Rico out of
the southwest. Although this moisture may impede initial convection
as it shields the island from early heating, when convection does
fire off later this afternoon we should have ample moisture to keep
it going. The models did not end convection over the area during
the evening hours this evening, but rather kept some showers active
right through Monday. Expect most showers in the interior to end
before midnight though. With lower level winds increasing as high
pressure sags south in the western Atlantic onshore winds should
keep showers off the northern coast as flow from the north is not
particularly moist, but there is still a slight chance of one hundredth
of an inch.
Moisture fades insignificantly on Monday and Tuesday, and upper level
dynamics, that were not so favorable in previous runs, now appear to
increase and may cause another several rounds of heavy showers and
isolated thunderstorms. These will fight the approach of the mid
and upper level ridge to the west and the dry air that is encroaching
from the northeast and north, but the driest air remains northeast
of the forecast area as it is pulled into the low that will cut off
in the central Atlantic today. Therefore we will see better than usual
precipitation both of those days.
.LONG TERM...Weather conditions are forecast to significantly improve
across the region by Wednesday, as a surface high pressure ridge
will continue to spread eastward into the central Atlantic inducing
a dominant northeast wind flow. This expected pattern will tighten
the local pressure gradient consequently increase the east to northeast
low level winds. In addition, with the building and spreading of the
mid to upper level ridge just west of the region, expect increasing
subsidence aloft and a much drier airmass to filter in across the region.
Although the overall moisture transport will continue to decrease by
Wednesday and during the latter part of the upcoming work week, still
expect the moderate trade winds to advect occasional patches of shallow
low level moisture across the region from time to time. This will
be sufficient to combine with local and diurnal effect to produce
early morning showers and afternoon convection mainly over parts
of the interior and west to southwest sections of Puerto Rico.
Meanwhile, the remainder of the islands should experience partly
cloudy to mostly sunny skies with only brief early morning passing
showers followed by isolated areas of streamer like afternoon shower
activity mainly downwind of the islands each day.
&&
.AVIATION...Currently VFR. The GFS forecasts the best moisture of
next 7 days today with enhanced upper air dynamics begg aft 16/12z.
Therefore, aft 16/16Z expect areas of SHRA/isold TSRA with MVFR-LIFR
and hir trrn obscured to dvlp ovr much of interior PR. Brief pds of
MVFR are psbl at TJMZ, TNCM, TKPK otherwise VFR will prevail at TAF
sites. Some VCSH TJSJ, TJMZ, TJPS and in USVI and Leeward Islands
psbl. LLVL flow NNE-ENE but sea breezes dvlp aft 15/14Z arnd 10 to
15 kt. Max winds W up to 80 kt FL350-400 diminishing during the day
at the lower altitude.
&&
.MARINE...Mariners can expect increasing easterly winds of 10 to 15
knots and seas increasing up to 7 feet across the regional waters
today. Winds and seas are expected to continue to increase through
at least Wednesday. Therefore small craft advisories will be in effect
for the Atlantic Waters and portions of the local passages. Please refer
to the latest Coastal Waters Forecast and Marine Weather Message issued
by the WFO National Weather Service San Juan PR for the latest update
and information.
For the beach goers, the risk of rip current will increase to high
along the Atlantic Coastline of Puerto Rico and Culebra today, and
to moderate along most of the U.S. Virgin Islands.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 75 85 74 / 20 30 40 40
STT 85 75 85 75 / 20 20 20 20