Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 250201 CCA

Area Forecast Discussion...CORRECTED
National Weather Service San Juan PR
1001 PM AST Tue Apr 24 2018

.UPDATE...Only the lightest showers are seen on the local radars.
The 25/00Z sounding showed 1.47 inches of precipitable water
which is down almost one half inch from yesterday at this time.
The dry slot that has caused all the precipitation to dissipate is
now working its way through the forecast area. Saint Martin
(TNCM) is also seeing the back edge of that dry slot with showers
and ceilings below FL035. This area of cloudiness is expected to
dissipate before reaching Saint Croix 25/08Z or Puerto Rico at
25/11Z as indicated by the GFS however moisture at 850 mb does
come through the area and there may be enough moisture for
isolated showers as in the current forecast.


.AVIATION...Lower CIGS are now encroaching on TNCM/TKPK with
SHRA, but so far CIGS are above FL030. Some brief MVFR is
possible thru 26/00z. Isold SHRA are psbl in ENE flow onto PR and
arnd the USVI otherwise VFR is indicated thru 25/16Z. Btwn
25/16-18Z areas of CIGS AOB FL050 will dvlp over wrn and interior
PR with SHRA and mtn obscurations. Brief MVFR psbl in TJMZ. Sfc
winds E 5-10 kt with land / sea breezes bcmg 10 to 15 kt aft
25/14Z. Max winds W 60 kt at FL470.


.MARINE...Showers will increase beginning Wednesday night, but
small craft advisories are not expected during the next 7 days.


.PREV DISCUSSION... /issued 336 PM AST Tue Apr 24 2018/

Although satellite imagery detected dry air filtering from the
east over the islands, the lingering moisture combined with
diurnal and local effect will result in shower activity to the
west of the Cordillera Central through the evening hours. A fair
weather pattern with an easterly wind flow and little or no shower
activity is expected overnight and early Wednesday morning.

SHORT TERM...Tonight through Thursday...
Showers were detected across the western municipalities of Puerto
Rico, and the eastern section of Puerto Rico and the U.S. Virgin
Islands have mostly clear skies with no shower activity. As the
short wave trough axis exit the region an upper level ridge will
build aloft. A dry air mass already filtered over the islands,
which will limit shower activity through late Wednesday night.
However, the lingering moisture will interact with diurnal and
local effect to produce a few showers across the northwest
quadrant of Puerto Rico.

A ridge aloft will continues in place through at least Friday,
but a surge of moisture is expected to bring shallow passing
showers across the U.S. Virgin Islands and southeast Puerto Rico
early Thursday morning into mid-day followed by afternoon
convection across the northern half of PR on Thursday.

LONG TERM...Friday through Wednesday...

Near normal moisture and upper ridge will cause a weather pattern
of some isolated to scattered brief showers across the local
waters, USVI and eastern PR during the overnight and early morning
hours. Afternoon shower and possible thunderstorm development will
be mainly due to local effects over PR. Therefore, the available
moisture will combine with the diurnal heating and sea breeze
convergence to cause some showers and possible thunderstorms. This
weather pattern can be expected through the weekend and into
early next week.

The very long range models started hinting at a significant
increase in moisture and in shower activity by next Wednesday as
a frontal boundary settles north of the local islands. This is way
too far into the forecast period and the confidence is very low.
We will continue to monitor but it would not be surprising if the
model guidance varies widely from today until that date arrives.

SHRA/TSRA over western PR should end by 22z, impacting
the flying area of TJMZ/TJPS/TJBQ. However, mainly VFR conditions
are expected to prevail across all the terminals through the
forecast period. Low level winds will continue from the ENE at 10-20
kt...turning ESE after 25/08z.

Mariners can expect tranquil marine conditions with seas between 2
and 4 feet across the coastal waters of the islands and up to 6
feet across the Atlantic offshore waters through tonight. Winds
are forecast to continue from the east at 10 to 20 knots but are
expected to veer from the east-southeast after Wednesday.


SJU  75  89  76  88 /  20  20  40  50
STT  75  86  77  86 /  20  40  40  40




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