Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 231544

Area Forecast Discussion
National Weather Service San Juan PR
1144 AM AST Wed Aug 23 2017

.UPDATE...A mid-level shortwave ridge will continue to promote
stable conditions across the area the rest of the morning. However...
diurnal heating will destabilize somewhat the atmosphere this
afternoon...inducing the development of showers and thunderstorms
over the Central Interior and Western Puerto Rico this afternoon.
Cloud lines downwind of EL Yunque could also produce scattered
showers across portions of the San Juan Metro area later today.
Most of the afternoon convection will taper off just before
sunset. Minor changes were introduced to the forecast package
this morning as it remains in line with the latest thinking.


.AVIATION...VFR conditions expected across most of the TAF sites.
TJBQ and TJMZ may experience MVFR/IFR conds at times with the
afternoon TSRA. East winds of 10-15 knots will prevail below


.MARINE...Tranquil marine conditions will continue through the
forecast period. Mariners can expected seas of 3-5 feet and winds
around 10 kts across the local waters.


.PREV DISCUSSION... /issued 501 AM AST Wed Aug 23 2017/

SYNOPSIS...Upper level trough will sink southwest across the
Mona Passage Thursday. At the surface, High pressure across the
Central Atlantic will continue to produce a light to moderate
trade wind flow across the region.

SHORT TERM...Today through Friday...

Generally a typical pattern was observed during the overnight hours.
Brief isolated showers affected the local waters overnight and could
continue into the morning hours with very little rainfall
accumulation over land areas. There is decrease in moisture expected
today but there will still be enough moisture to combine with the
local effects and cause the development of showers and thunderstorms
across the interior into western and northwestern Puerto Rico. Some
of the showers could be heavy at times and could cause some urban
and small stream flooding. There is also a chance for the famous San
Juan streamer to develop and affect some of the municipalities in
the San Juan Metro Area. The rest of the forecast area is expected
to have isolated and brief showers, causing little if any
accumulations. The max daytime temps are expected to be in the 90s
across the lower elevations and in the low 80s across the higher
elevations today, apparent temperatures could reach 100 degrees in
the lower elevations.

For Thursday, there may be an increase in moisture that could
combine with an upper trough north of the local area, to cause
showers and thunderstorms over the local area. The latest guidance is
favoring the showers and thunderstorms to affect the northern half
of PR with the heaviest showers across NW-PR. At this time it looks
like the USVI will observe mainly isolated showers. Then for Friday,
a thin line of moisture is forecast to move through the local area
through the day starting in the early morning hours, which will
cause scattered showers across the local waters, USVI, and eastern
PR in the morning, then across the western half of PR and the
western half of the local forecast area for the afternoon hours.
Thunderstorms are possible across the interior and western sections
of PR.

LONG TERM...Saturday through Wednesday...

Upper level low will retrograde westward as upper level ridge
builds across the region. Moisture will increase somewhat by
Sunday as a tropical wave passes mainly south of the region.
Patches of low level moisture embedded in the trade winds will
affect the region Monday and Tuesday. A much drier air with
possible Saharan dust is forecast by computer models to approach
the region Wednesday next week.

AVIATION...ISOLD ad brief -SHRA will continue across the local
waters this morning with VFR conditions prevailing across the local
terminals. SHRA/TSRA expected to develop across PR after 23/16Z,
which will cause at least VCSH/VCTS at TJSJ, TJBQ, and TJMZ. Winds
will have sea breeze variations but will be generally from the east
at around 10-15KT after 23/14Z.

MARINE...Seas up to 5 feet and winds up to 15 knots are expected
to prevail across the coastal waters through the weekend.


SJU  89  80  92  80 /  20  20  40  40
STT  89  81  89  81 /  20  20  30  30




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