Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 280141
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
941 PM AST Mon Mar 27 2017

.UPDATE...Forecast on track...Environmental conditions will
remain favorable for shower and thunder development tomorrow
afternoon over and north of the Cordillera Central, particularly
the San Juan Metro Area. A fair weather pattern with limited
shower activity is expected by midweek.

.AVIATION...VFR conds expected overnight at all TAF sites.
TSRA/SHRA with MVFR/IFR conds expected at JSJ/IST/ISX after
28/16z. Light and variable winds expected overnight.

.MARINE UPDATE...Seas 3 to 5 feet will continue to prevail across
the Atlantic waters, with seas 2 to 4 feet across the Caribbean
waters. A moderate risk of rip current will continue across the
Atlantic shoreline of Puerto Rico during the next few days. A
northerly swell will arrive across the local waters Wednesday
into Thursday with seas increasing up to 6 feet.

&&

.PREV DISCUSSION... /issued 248 PM AST Mon Mar 27 2017/

SYNOPSIS...
A mid-low level trough is forecast to swing by the islands the
rest of today into late Tuesday night. This system will be
accompanied by plenty of tropical moisture through at least mid
week. This will combine with a light southerly wind flow to result
in afternoon convection along and to the north of the Cordillera
Central, including the San Juan metropolitan area and Northeast
PR through at least Tuesday. This pattern is expected to change
after Wednesday night, as winds are forecast to become from the
North-Northeast with the formation of a trade wind cap over the
region.

SHORT TERM...Today through Wednesday...
A surface low is about 380 miles north of Punta Cana and is
moving northeast at about 15 mph. This motion will continue and
perhaps accelerate during the next 48 hours. Bands of convection
will form in the southwest flow around the islands today and
tomorrow, but moisture has diminished somewhat and convection has
been weaker in strength and coverage. There is little consensus in
the models as to where a band might form in the flow tonight, the
GFS wants to have convection form northwest of Puerto Rico after
28/00z and shift to southeast of Puerto Rico aft 28/06z. HiRes
models have scattered showers on both sides of the island. It is
likely that a band will form parallel to the flow however--and
over water--leaving better than one quarter to one half inch of
rain over smaller islands-- if any are in its path and nearby
coasts.

On Tuesday a plume of moisture at 700 mb will form an arc that
travels from Colombia north to the local Caribbean waters from the
west southwest. This plume will enhance the moisture over the area.
Then, during the day Tuesday, with moisture higher and the upper
level jet and trough a little closer conditions will be a little
better even though the GFS is still not putting much divergence
aloft over the forecast area. With strong directional shear near the
surface and good speed shear above 700 mb--albeit decreasing with
time--small showers could develop with land or water spout
potential, once diurnal heating gets underway.

Currently the model trend has been to peak the moisture on Tuesday
but the GFS has been raising its moisture forecast for Wednesday.
Therefore cannot expect too much improvement until Thursday when
drier air will move in from the north. Temperatures on Tuesday will
continue above normal for the north coast despite afternoon
cloudiness, but on Wednesday winds should shift to more northerly as
the low pressure moves east across our longitude and temperatures on
the north coast will return to normal and temperatures on the south
coast will rise by even as much as 5 degrees.

LONG TERM...Thursday through Monday... The surface low will
continue to move north into the north central Atlantic on Thursday
and the ridge across the western Atlantic builds north of the
islands through the end of the week. This will generate a
northerly wind flow from Wednesday through at least Saturday. A
much drier airmass will prevail over the region from Thursday
through late Friday...this will result fair weather conditions
through this period. East to southeast winds return from early
Saturday through Sunday with moisture increasing from the
southeast...associated with remnants of the surface low/frontal
boundary.

AVIATION...
SHRA are forming along a sea breeze front in nrn PR and
will generate very localized brief MVFR conds in CIGS/VSBYS. SHRA
will diminish aft 28/00z. Some areas of hir terrain will be
obscured. Aft 28/06z areas of moisture will move thru the USVI and
Leeward islands from the SE to bring brief pds of MVFR in SHRA to
TISX/TNCM/TKPK. Sfc winds SE-S 10 to 15 kt. Maximum winds WSW 55
knots btwn FL290-420, but winds will incrs to SW 30 kt as low as
FL120 by 28/06z.

MARINE...
Buoy network continues to indicate subsiding seas across the
local waters. Mariners can expect seas at 2 to 5 feet across the
nearshore waters and up to 6 feet across the Atlantic Offshore
waters. Models are bringing a short period northerly swell by mid
week.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  75  86  77  85 /   0  60  30  20
STT  75  84  75  84 /  30  40  30  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

UPDATES...OM
AVIATION..OM


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