Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 230112

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

.UPDATE...The showers that affected western and north central PR
dissipated right around sunset. Now only a few clouds remain and
fair weather over the local forecast area. Isolated to scattered
showers are expected overnight but these are expected to be brief.
Moisture will gradually increase over the next couple of days,
which will allow for more coverage in the shower and possible
isolated thunderstorms across the forecast area from Friday
through the weekend.

No significant changes from the previous forecast, except a slight
reduction in probability of rain for tonight and early Friday. A
very minor modification as the latest observations and guidance
were drier than the inherited short term forecast.


.AVIATION...Mainly VFR conds expected through 23/16Z across the
local terminals. SHRA and possible TSRA may affect TJMZ, TJBQ and
TJPS on Friday afternoon, but the rest of the local terminals
should observe mainly VFR conds through the period. Winds will be
from the east at around 5 to 10kt overnight, then after 23/13Z
from the east with sea breeze variations at around 10 to 15kts.


.MARINE...No changes from the previous discussion. Seas will be
5 feet or less through Saturday morning, then the Atlantic waters
will gradually observe seas of 6 feet in the afternoon and 7 by
Saturday night.


.PREV DISCUSSION... /issued 514 PM AST Thu Mar 22 2018/

Mid to high clouds moved across the islands with the
heaviest shower activity along portions of northwest and northern
Puerto Rico. Streamers did develop today but quickly collapsed
over northeast Puerto and the U.S. Virgin Islands. Low level
moisture will continue to increase over the region through the
weekend and moisten the dry layers, which will result in showers
and isolated thunderstorms through early Sunday.

SHORT TERM...Tonight through Saturday...
The short term period will be quite active with rain and isolated
thunderstorms across portions of western and northern Puerto Rico
through Saturday. Urban and small stream flooding will be be
possible over parts of north and western Puerto Rico 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

At the low levels east southeast winds will continue to pull
deep tropical moisture over the region through Sunday. The
combination of a weak surface trough which will provide lift and,
deep tropical moisture and will result in scattered to numerous
showers across northern, central and northwest Puerto Rico through
the weekend. Saturday, the aforementioned trough will phase out
and become a weak shear line across Puerto Rico, dissipating on

At the mid to upper levels, the 21/12Z TJSJ sounding still shows
drier air from 750 to 300 mb. This has somewhat limited
thunderstorm growth today due to westerly winds shearing higher
clouds tops off.

Friday, the drier air begins to moisten with the only drier layer
remaining at between 600 to 300 mb. This is why showers will
become more concentrated over portions of northern, central and
northwest Puerto Rico Friday afternoon.

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

VFR conditions will continue to prevail through the
23/00 TAF period. SHRA will decrease across northwest sections of
Puerto Rico by sunset. SFC east southeast winds 5 to 10 kt across
all TAF until 23/12Z.

Tranquil marine conditions across the region with seas
ranging between 3 to 5 feet, and an east southeast winds of 10 to
15 kts. A northern swell is forecast to reach the region on late
Saturday into Sunday. The northern swell will cause seas to
increase to 6 to 8 feet across most of the local water ways
starting on late Saturday.


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




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