Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 290120

Area Forecast Discussion
National Weather Service San Juan PR
920 PM AST Tue Mar 28 2017

.UPDATE...Forecast on track...partly cloudy skies with a few
passing showers possible overnight; mainly across the local
waters as well as the east and southeast coastal areas.
Environmental conditions will remain favorable for shower and
thunder development. Therefore expect another round of showers
with isolated thunderstorms possible late in the morning and into
the afternoon hours. Low level winds are expected to become more
northerly; therefore afternoon showers and thunderstorms will be
focused over and south of the Cordillera Central. A fair weather
pattern with limited shower activity is expected Thursday and
Friday as a drier air mass moves across the forecast area.

.AVIATION...VFR conds expected overnight at all TAF sites through
the forecast period. TSRA/SHRA with MVFR/IFR conds expected at
JPS after 29/16z. Light and variable winds expected overnight.

.MARINE...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 late Wednesday
into Thursday with seas increasing up to 6 feet.


.PREV DISCUSSION... /issued 341 PM AST Tue Mar 28 2017/

SYNOPSIS...SFC low to the north of the local area will continue
to promote a southerly wind flow across the local area, bringing
moisture, which is helping the development of showers across the
local islands, especially across central and northern PR when the
moisture combines with the local effects and favorable upper level
conditions. North swell is expected to affect the local waters
late Wednesday and Thursday.

SHORT TERM...This afternoon 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

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...The latest guidance is now suggesting that Wednesday
will not be as dry as it had suggested in the past few days. So
even though the winds are expected to shift to a northerly
direction, this means that the interior and southern sections of
PR have the better chance for afternoon showers. The rest of the
the forecast area should have isolated to briefly scattered
showers. Then fair weather overnight Wednesday into Thursday with
afternoon showers on Thursday afternoon developing across the SW
quadrant of PR, and once again isolated showers elsewhere,
including the USVI. However for Thursday, the latest guidance
still suggests drier air over the local area as a possible frontal
passage pushes through. Fair weather mainly on Friday, then the
front may retrograde on Saturday, increasing moisture
significantly and so will the shower activity over the forecast
area over the weekend.

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...Seas generally of 3 to 5 feet are to be expected across
the Atlantic waters and local passages, 2 to 4 feet across
southern PR and south of the USVI. The northerly swell expected
late Wednesday into Thursday may cause seas to go up to 6 feet
across the Atlantic waters and local passages.


SJU  77  85  75  83 /  40  20  20  10
STT  74  85  73  83 /  40  40  20  20




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