Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 221600

Area Forecast Discussion
National Weather Service San Juan PR
1200 PM AST Thu Mar 22 2018

.UPDATE...Mid to high clouds streamed across the local area this
morning. Some showers,heavy at times were seen across the U.S.
Virgin Islands. An increase in low level moisture in combination
with daytime heating, sea breeze convergence and local effect,
will enhance the development of showers with isolated
thunderstorms mainly across the western interior and northwest
sections of Puerto Rico.


.AVIATION...Increasing cloudiness are expected across most TAF
sites through the forecast period. Showers are expected to
increase across the western interior and northwest sections of
Puerto Rico. Periods of MVFR conditions are expected across TJBQ
and TJMZ from 22/18z through 22/22z. Low level winds will be
mainly southeast at 10 to 15 kts.


.MARINE...No changes from previous discussion.


.PREV DISCUSSION... /issued 524 AM AST Thu Mar 22 2018/

Showers and clouds will increase in coverage and intensity
especially across the U.S. Virgin Islands and eastern Puerto Rico
as a moist air mass approach from the east through the morning. An
induce surface trough will combine with local effect and diurnal
heating to result in showers and thunderstorms over the interior
and western sections of Puerto Rico during the afternoon hours.
The potential for shower and thunderstorm activity is forecast to
increase late tonight through the weekend, as a mid to upper
level trough breaks the trade wind cap and a surface boundary
move closer to the islands.

SHORT TERM...Today through Saturday...
Moisture is expected to continue to pool across the region as an
induced surface trough moving across the region will increase the
moisture convergence over the islands and surrounding waters
during the rest of today through Friday. A weakening cold front
and associated frontal boundary will continue to sink southwards
across the region Friday and through the upcoming weekend, along
with a deepening polar trough which will enter and move across the
west Atlantic. The trough will quickly erode the mid to upper
level ridge aloft now in place across the region over the next few

Expect increasing cloudiness along a better potential for showers
and isolated thunderstorm development across the islands and coastal
waters beginning this afternoon and continuing through Friday and
Saturday when conditions are expected to deteriorate.

At this time the worst weather conditions appear to be on Friday
afternoon and Saturday as the upper level jet segment rounds the
base of the trough and move overhead allowing good divergence aloft
to combine with the low level moisture convergence and local
effects. Rainfall especially during the afternoons may be locally
heavy at times mainly over parts of the interior and northwest
sections of PR. However for today, some streamer-like afternoon
convection will be possible in and around the San Juan metro area as
well as on the downwind side of the USVI and adjacent islands.

Friday afternoon and Saturday expect increasing showers and isolated
thunderstorm development across the region due to the instability
aloft and good moisture pooling. Urban and small stream flooding
will therefore be possible over parts of north and western PR
especially on Friday and 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 rest of the day. Increasing shower
activity and possibly isolated thunderstorms is also forecast for
parts of the USVI during the weekend, as the frontal boundary sink
farther southward across the islands. Therefore, ponding of water on
roadways and in poor drainage areas will also be possible in
isolated spots across the islands.

LONG TERM...Sunday through Friday.
By Sunday, a frontal boundary will induce an east to northeast
wind flow across the islands. Moisture pooling will combined with
a shortwave trough, which is expected to enhance shower and
thunderstorm activity. GFS Galvez-Davidson index suggested a good
potential for this activity across the Atlantic Waters, north and
east Puerto Rico, and the northern U.S. Virgin Islands during the
night and early morning hours. Then showers and thunderstorms are
expected to move across the interior and southwestern sections
through the afternoon hours. This moist and unstable atmosphere
is expected through Monday. A surface high pressure spreading
from the west to central Atlantic will push the frontal boundary
over the islands until dissipates through around mid week. Based
on the latest model guidance, the islands will move into a wet and
unstable pattern.

A second frontal boundary is forecast to approach the islands as
a Polar trough amplified Tuesday and 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.

By the second part of the upcoming week, model guidance are
suggesting a a gradual improvement as a vertically stacked ridge
builds once again across the region. Under this weather pattern
expect little or no shower activity with pleasant temperatures.

BKN-OVC cld lyrs btw FL200-FL250...SCT ocnl BKN lyrs nr
FL020...FL040...FL100 due to passing low-mid lvl clouds with SHRA.
Few tops nr FL150 ovr coastal waters and en route btw local islandS
til 22/14z. Prevailing VFR at all terminals except for passing SHRA
vcty TNCM/TKPK/TIST/TISX/TJNR/TJSJ til 22/15z/. Aft 22/17z... Aftn
SHRA/isold TSRA mainly ovr E and W interior and NW PR with periods
of MVFR conds VCTY TJMZ/TJBQ til 22/22z DUE TO SHRA/VCTS. SFC wnds
fm E-SE 10-15 kts slightly higher and gusty with sea breeze
variations and passing SHRA.

Overall tranquil marine conditions will prevail across the local
with seas of 3 to 5 feet and east to southeast winds around 15
knots. A north to northwest swell is forecast to reach the islands
late Saturday into Sunday, which could increase seas between 6 and
8 feet. Marine conditions will improve Monday and Tuesday, but
seas will increase once again by mid-week of the upcoming week as
another swell invade the coastal waters.

The potential for thunderstorms is expected to increase late
tonight into at least the first part of the upcoming week across
the Atlantic Waters, especially during the night time and early
morning hours.


SJU  88  75  86  75 /  50  40  70  60
STT  85  75  85  74 /  50  50  60  60




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