Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 240137

Area Forecast Discussion
National Weather Service San Juan PR
937 PM AST Wed May 23 2018

.UPDATE...While most of the cloudiness and shower activity diminished
over land areas , a few shower lingered along the north coastal sections
of Puerto Rico into the late evening hours. Expect cloudiness and shower
activity to diminish and dissipate during the next hour or so, leaving
mostly clear to partly cloudy skies. A few passing showers will continue
to develop and move across the coastal waters, with some brushing the
east coastal sections of the islands from time to time. On Thursday
expect similar weather pattern with mostly sunny skies and near normal
high temperatures, followed by locally and diurnally induced afternoon
convection, mainly over parts of the interior and northern half of
Puerto Rico. Only isolated shower activity of short duration is forecast
elsewhere. No changes made to the inherited forecast package and reasoning
at this time.


.AVIATION UPDATE...VFR conds at all terminals and across the flying
areas durg prd. SCT ocnl BKn lyrs nr FL030...FL050. Few -SHRA/SHRA
en route btw islands and ovr coastal waters til 24/14z. L/Lvl wnds
fm ESE 5-15 kts blo FL100...bcmg fm N-NW abv and incr w/ht.


.MARINE UPDATE...Recent data from the surrounding buoys suggest a slight
decrease in seas offshore, ranging around 6 feet and nearshore less
than 5 feet. Trade winds continued between 15-20 knots with occasionally
higher gusts. Small craft operators should therefore continue to exercise
caution at least through Thursday afternoon.


.PREV DISCUSSION... /issued 444 PM AST Wed May 23 2018/

A short wave trough aloft is forecast to swing north of the
islands as a TUTT low moves northeastward from the Leeward
Islands the next few days. The islands will be wedge by a ridge
to the northwest and a TUTT low to the northeast through the
upcoming weekend. Tropical wave will approach the Lesser Islands
around Sunday.

SHORT TERM...Tonight through Friday...
The Azores-Bermuda High brought sufficient surface moisture which
enhanced by low level convergence produced periods of moderate to
strong rainfall across the west half of the U.S. Virgin Islands
and streamed westward into the the local waters. Also, strong
showers were observed across portions of the San Juan Metro Area,
the interior, north and west Puerto Rico. Urban flooding
associated with this activity were reported by local Emergency
Managers and local observers across Toa Baja and Bayamon. Showers
are expected to continue mainly across the north central, the
interior and western sections of Puerto Rico through this evening.
Then the west and interior sections can expect fair weather
conditions with no shower activity. Although the U.S. Virgin
Islands and the eastern half portion of PR will observe fair
weather conditions, low level convergence will bring passing
showers at time overnight.

Even though no strong upper level dynamic is expected at this time,
low level moisture, surface convergence, diurnal heating and local
effects will result in shower activity across the region during
each day. During the morning hours, shower activity is expected
across the U.S. Virgin Islands and the eastern half portion of
PR. Under a east to southeast wind flow, the afternoon convection
should be focused across the northwest quadrant of PR as well as
across portions of the San Juan metro area downwind from El
Yunque. Some of these showers during the afternoon could be heavy
enough to result in ponding of water on roadways and in low-lying
areas as well as some localized minor urban and small stream
flooding. Thunderstorm activity if any, is expected to remain very
isolated as the mid levels of the atmosphere remain very dry.

On Friday, as a retrogressive TUTT low amplifies into the
northeast Caribbean, deeper moisture is forecast to pool over the
forecast area. This will produce an increase in the coverage and
intensity of showers as well as in the thunderstorm potential.

LONG TERM...Saturday through Thursday (Previous Discussion)...

A moist southeast wind flow will continue to dominates the region
on Saturday, maintaining good chances for showers and
thunderstorms across the area.

For Sunday and Monday, the combination of a tropical wave with an
upper level trough promise to bring an increase in low level
moisture both days. As a result, a wet pattern is anticipated for
early next week. Then, a drier air mass is forecast to encompass
the region by later in the forecast period. But, occasional
patches of low level moisture will result in a typical weather
pattern each day.

AVIATION...Mainly VFR conds expected across the local terminals
through the forecast period except TJMZ and TJBQ which could observe
MVFR conds until 23/23Z. SHRA/TSRA developing across the western and
northwestern sections of PR, so TEMPO TSRA with MVFR conds on TJMZ
and TJBQ TAF, VCSH possible elsewhere. Winds from the east at 10-15
KT with a few gusts and sea breeze variations. Winds to decrease
after 23/23Z and weather is also expected to subside by this evening.

Choppy and rough seas are forecast to continue through Thursday
afternoon, then seas will slowly diminish into the upcoming
weekend. Mariner can expect seas up to 6 feet across local
passages and offshore waters and below 5 feet at protected waters.
East to east-southeast winds at 10 to 20 knots are expected
through much of the week.


SJU  76  87  77  88 /  40  30  30  40
STT  77  87  77  87 /  40  30  30  30




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