Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 231529

Area Forecast Discussion
National Weather Service San Juan PR
1129 AM AST Fri Mar 23 2018

.UPDATE..Very dry weather conditions prevailed across Puerto
Rico and the U.S. Virgin Islands this morning. Increasing
cloudiness with showers are beginning to develop in El Yunque and
across the Cordillera Central of Puerto Rico. Abundant low level
moisture, combined with daytime heating and local effect would
produce scattered to numerous showers with thunderstorms across
the Cordillera Central of Puerto Rico This activity will spread
across the north central and northwest sections of Puerto Rico.


.AVIATION...VFR conditions will continue to prevail across all TAF
sites through 23/12z. Increasing cloudiness are expected across
TJSJ, TJMZ and TJBQ sites. Showers are expected to increase
across the Cordillera Central of Puerto Rico. Periods of MVFR
conditions are expected across TJBQ and TJMZ and TJSJ from 23/18z
through 23/22z. Low level winds will be mainly southeast at 10 to
15 kts.


.MARINE...No changes from previous discussion.


.PREV DISCUSSION... /issued 522 AM AST Fri Mar 23 2018/

Tropical moisture and atmospheric instability are expected to
increase through the weekend. As a result showers and
thunderstorms are expected to increase in coverage and intensity.
An induce surface trough will combine with the available
moisture, local effect and diurnal heating to result in showers
and thunderstorms over the interior and western sections of Puerto
Rico during the afternoon hours. Then, most of the activity will
move across the local waters through the overnight hours.

SHORT TERM...Today through Sunday...
Low level moisture convergence will continue to increase across
the area today through Sunday as an induced surface trough moving
through the region will increase the pooling of moisture over the
islands and surrounding waters through Saturday. A weakening cold
front/shear line and associated frontal boundary will continue to
sink southwards across the region today through the upcoming
weekend. This feature along with a deepening polar trough will
move across the west Atlantic and quickly erode the mid to upper
level ridge now in place across the region. East to southeast low
level winds will persist today, but is forecast to become more
east northeast by Saturday, then finally northeasterly on Sunday
as a surface high pressure ridge will build north of the region.

Instability aloft along with moisture convergence and local
effects, will increasing afternoon cloudiness and promote a better
potential for showers and isolated thunderstorm development across
the islands and coastal waters. This unfolding pattern will continue
through Saturday when the overall weather conditions are expected to
quickly deteriorate.

At this time, the worst weather conditions across region stills
appear to be beginning later this afternoon and continuing through
Saturday, when an upper level jet segment crosses the region while
supporting divergence aloft along with the good low level moisture
convergence and local effects. Rainfall during the afternoons will
be locally heavy at times especially over parts of the interior,
north central and northwest sections of PR ,as well as in isolated
spots on the USVI particularly on Saturday and Sunday. Isolated
afternoon convection will also be possible along the east interior
of PR where streamers may later form and affect parts of the San
Juan metro area, as well as around the adjacent islands and on the
downwind side of the USVI.

Urban and small stream flooding will therefore be possible over
parts of north,central and western PR today and on Saturday. Some of
the U.S. Virgin islands can also expect periods of isolated to
scattered early morning passing showers, followed by mostly isolated
afternoon shower in the form of streamers during the day. However,
increasing shower activity and isolated thunderstorms is also
possible for some of the USVI during the weekend, as the frontal
boundary sinks farther southward across the islands. As a result,
ponding of water on roadways and in poor drainage areas will remain
possible in isolated areas.

LONG TERM...Monday through Saturday...
The lingering moisture, the proximity of a shear line and a jet
segment aloft would create unsettled weather conditions through
the first part of the upcoming week. GFS Galvez-Davidson index
continues to support the formation of thunderstorms across the
region through this period, especially across the Atlantic
Waters, north and east Puerto Rico, and across the U.S. Virgin
Islands during the night and early morning hours. Then, afternoon
convection is expected to affect the interior and southwestern
sections of mainland PR.

A second frontal boundary is forecast to approach the islands as
a Polar trough amplified late Tuesday into Wednesday. The
proximity of this deep trough will increase instability aloft
and provide good ventilation. The combination of low level
convergence, favorable upper level dynamic and plenty of tropical
moisture will support an unsettled weather pattern through at
least Thursday. When, FROPA was suggested around Thursday,
bringing pleasant temperatures and little or no shower activity.

Friday into the upcoming weekend, model guidance are suggesting a
a gradual improvement as a vertically stacked ridge builds once
again across the region.

Minor changes were done to the long term forecast. Except little
changes in the arrival of the expected activity for Monday through
Wednesday. Because, the latest guidance showed somewhat drier
conditions through this period.

VFR conds expected at all terminals through 23/14Z. SCT ocnl BKN
cld lyrs nr FL022...FL040...FL120 with passing SHRA mainly ovr
coastal waters and btw islands til 23/14z. SHRA/Isold TSRA likely
vcty of TJMZ, TJBQ and TJPS fm 23/17z-23/23z... and ovr ATL
coastal waters. VFR elsewhere. Winds will be lgt/vrb, bcmg fm the
east with sea breeze variations at 10-15kts aft 23/13z.

Tranquil marine conditions expected today, seas will be 5 feet or
less through this evening. Then, seas will slowly increase at 6
feet tonight into Saturday until the arrival of a northerly swell
across the Atlantic Waters late Saturday into Sunday.

Marine conditions will improve Monday and Tuesday, but seas will
increase once again by mid-week of the upcoming week, as a long
period northerly swell is forecast to invade the coastal waters.

The proximity of a frontal boundary and good ventilation aloft
would increase the potential for thunderstorm formation,
especially across the Atlantic waters after this afternoon through
early next week.


SJU  87  74  86  74 /  60  60  60  50
STT  84  73  85  74 /  40  60  60  50




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