Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 201522

Area Forecast Discussion
National Weather Service San Juan PR
1122 AM AST Wed Jun 20 2018

Some changes were introduce to the forecast to package to adjust
the expected weather. The strongest activity is forecast over the
northwest quadrant and the southeastern quadrant of Puerto Rico.
However the potential for showers and thunderstorms still in
effect for most of the local islands. A dry air mass will
encompass the islands later tonight. As a result hazy skies with
little or no shower activity is expected overnight and early
Thursday morning.


btwn 20/16-23z. Brief MVFR or even IFR is possible at TJBQ and
across TISX/TIST/TJSJ through this evening. SHRA and TSRA will
dissipate later tonight as a dry air mass reach the islands.
Winds are expected to continue from the ESE-SE at 10 to 15 knots
with sea breeze variations. Higher gusts are expected in/near


A tropical wave south of Dominican Republic will continue to
bring showers and thunderstorms across the region through late
this evening. Mariners can expect seas at 3 to 5 feet with an
east-southeast wind flow between 10 and 15 knots. However,
locally higher seas and winds near showers and thunderstorms are
expected through this period.


.PREV DISCUSSION... /issued 518 AM AST Wed Jun 20 2018/

SYNOPSIS...A tropical wave will move out of the area today with
copious moisture and good instability behind it. Drier air will
move in from the east overnight. Another tropical wave will move
through on Friday. Moisture will linger through at least Sunday
morning with scattered showers. More typical weather will follow
next week in east southeast trade wind flow of modest moisture.

SHORT TERM...Today through Friday...
Lingering moisture from a tropical wave that moved to the south of
Puerto Rico will cause showers and isolated thunderstorms across
most of the area today. A digging TUTT low will continue its
southern progression near Puerto Rico today, and produce upper level
divergence aloft. As a result of the lingering moisture and the
upper level forcing due to the digging TUTT low, and a small jet
streak, isolated thunderstorms are expected across most of Puerto
Rico today. The GFS sounding for San Juan depicts how unstable
the current atmosphere is with southerly wind flow, convective temperature
of 88 degrees, and surface based cape around 2800, and LI index
of minus 9. Recent analysis of the GDI shows values ranging from
35 to 40 which means there is potential for scattered
thunderstorms with some having the capability of producing heavy
rain and frequent lightning similar yesterday. Therefore, any
additional rainfall could cause urban and small stream flooding
and ponding of water on roadways.

Later tonight the moist tropical air gets eroded away by dry saharan
air filtering in from the east, therefore precipitable water values
are expected to decrease until Friday afternoon, but scattered
showers and isolated thunderstorms are possible across the
northwestern Puerto Rico.

Friday, will be a transition day from dry saharan air to a moist
tropical air mass as another tropical wave moves through the
Caribbean waters Friday morning. Showers and isolated thunderstorms
are expected across the outer Caribbean waters and the eastern
islands of Puerto before arriving over Puerto Rico during the

LONG TERM...Saturday through Wednesday

Moisture from the tropical wave that entered the area Friday will
continue through Sunday. Then drier air will move in Monday and
Tuesday. Moisture will gradually increase Tuesday through

High pressure at the surface will shift to the west central
Atlantic over the weekend. This will cause winds at the surface to
become more easterly and will allow some moderation of the warm
highs that we have had on the north coast of Puerto Rico.
This trend will continue into the early part of the week.

At upper levels, a TUTT low will be just north of Puerto Rico
beginning Saturday. This indicates thunderstorms with frequent
lightning will be possible. This low will move northwest and will
be replaced by a weak ridge that moves into place between another
stronger low well northeast of the area. This will also allow
showers and thunderstorms to diminish during the early part of
next week.

AVIATION...VRF/MVFR conds will continue for the remainder
of the TAF period. SHRA/VCTS are expected to affect terminals TIST
and TISX during the morning before moving across terminals TJSJ,
TJPS,TJMZ AND TJBQ by 20/14Z. SHRA/TSRA will persist until 21/00Z
with MVFR VSBYS occurring in +SHRA/VCTS. SHRA/TSRA will diminish
at all terminals by 21/03Z. Sfc winds out of the east south east
at 15 to 20 kts with higher gusts possible within SHRA/TSRA.

MARINE...Seas are relatively low--mostly 5 feet or less until the
weekend. Then seas in the Caribbean will have areas of 6 feet.
Small craft advisories are not expected in the next 7 days.


SJU  90  77  91  78 /  40  40  10  20
STT  87  79  89  79 /  40  20  10  30




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