Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 211746

Area Forecast Discussion
National Weather Service San Juan PR
146 PM AST Tue Jan 21 2020

.SYNOPSIS...The remnants of an old frontal boundary will continue
to affect the area tonight and continue on Wednesday under a
generally southeasterly steering flow, favoring eastern Puerto
Rico and the USVI. By late Wednesday night, a frontal system and
associated features will result in an increase in moisture that
along with favorable conditions aloft will enhance shower and
thunderstorm activity across the forecast area through the weekend
and potentially into early next week. A potential for flooding
can be expected with this activity.


.SHORT TERM...Tonight through Thursday...

Model guidance, as well as recent satellite imagery, suggest that
an extended area of low level moisture, associated to a nearly
stationary weak frontal boundary, will continue to move in from
the southeast and stall over the area through at least Wednesday
evening. As a result, a slight increase in shower activity is
expected, focused over the USVI and the eastern half of Puerto
Rico. Elsewhere, limited shower activity can be expected until a
pre-frontal through and associated plume of moisture begins to
move in from the southwest. As this feature continues to invade
the area and the frontal boundary approaches the region from the
northwest, winds are expected to favor a more southerly component
through the rest of the short-term period. By late Wednesday night
and continuing on Thursday, the combination of a strong low-level
convergence, an increase in low-level moisture (GFS suggests
precipitable water vapor up to 1.80 inches by Thursday
afternoon), and favorable conditions aloft (upper-level polar
trough with 500 mb temperatures dropping around -8C) will support
a significant increase in shower activity and the potential for
isolated thunderstorms across the forecast area. This activity
could lead minor flooding and ponding of water on roadways.

.LONG TERM...Friday through Tuesday...

.PREV DISCUSSION... /issued 504 AM AST Tue Jan 21 2020/

The surface ridge across the Atlantic is forecast to lift farther
northeast into the north central Atlantic, as a cold front will
move across the western Atlantic and sink southwards across the
region. An amplifying mid to upper- level polar trough will also
weaken the ridge aloft resulting in a weakening of the trade wind
inversion leading to unstable conditions aloft through at least
early Friday. The shift in the weather pattern will result good
low level moisture pooling and convergence as the frontal boundary
sink southwards across the region through Friday. Model guidance
remain persistent and suggests a vigorous mid to upper level jet
max crossing the region through early Friday. The combination of
the instability aloft and good low level moisture convergence
will therefore favor the development of scattered to numerous to
showers and possibly isolated thunderstorms with periods of
locally heavy rains across the region until early Friday.

By late Friday and through the weekend, model guidance suggest
lingering low level moisture but gradually improving and stable
conditions aloft with a much drier airmass. Therefore expect less
frequent early morning shower activity across the islands and
coastal waters. This will be followed by isolated to scattered
showers of short duration during the afternoon hours. Improving
conditions are forecast by Sunday and through Tuesday, as the
surface high pressure will once again build and spread across the
Atlantic, resulting in a more east northeast wind flow by Monday
then a more southeasterly on Tuesday.


.AVIATION...Mainly VFR conditions expected across all terminals
during the next 24 hours. However, SHRA over NW Puerto Rico may
cause tempo MVFR conditions at TJBQ/TJMZ. ESE winds will continue at
10-15 knots.


.MARINE...Although the northerly swell and winds continue to
subside, hazardous marine conditions will continue through this
evening. Thus, Small Craft Advisories continue in effect for most
waters except for coastal waters of western and southern Puerto
Rico, as well as the Mona Passage. Thereafter, small craft
operators are urged to exercise caution due to seas up to 6 feet.
Relatively tranquil marine condition will persist through at least
late Thursday night, when the arrival of another northerly swell
will generate hazardous seas. There is a high risk of rip currents
for beaches along the north coast of Puerto Rico, including
Vieques and Culebra, as well as Saint Thomas and Saint Croix. The
high risk will drop to moderate later tonight.


SJU  73  87  74  84 /  40  40  40  50
STT  73  81  75  81 /  40  40  40  50


PR...High Rip Current Risk through late tonight for Culebra-North
     Central-Northeast-Northwest-San Juan and Vicinity.

     High Rip Current Risk until 6 PM AST this evening for Mayaguez
     and Vicinity-Vieques.

VI...High Rip Current Risk through late tonight for St.Thomas...St.
     John...and Adjacent Islands.

     High Rip Current Risk until 6 PM AST this evening for St Croix.

AM...Small Craft Advisory until 8 PM AST this evening for Anegada
     Passage Southward to 17N-Atlantic Waters of Puerto Rico and
     USVI from 10 NM to 19.5N-Caribbean Waters of Puerto Rico
     from 10 NM to 17N-Coastal Waters of Northern Puerto Rico
     out 10 NM-Coastal Waters of Northern USVI and Culebra out
     10 NM.



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