Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 180117

Area Forecast Discussion
National Weather Service San Juan PR
917 PM AST Fri Nov 17 2017

.UPDATE...Recent TJSJ upper air sounding data and satellite
imagery continued to suggest a fairly moist and unstable
environment under a dominant south to southwest low level wind
flow. Proximity of the elongated upper trough across the west
Atlantic will continue to erode the ridge aloft and provide good
ventilation and instability to maintain potential for shower and
thunderstorms development across the region through most of the
weekend.A gradual improvement expected by Sunday into Monday. Only
minor changes made to the inherited short term forecast for now.

The flash flood watch will continue for communities downstream
from the Guajataca Dam, especially the low lying areas along the
river through Saturday, and probably will be extended as long as
the potential exists for dam failure due to flash flooding.


.AVIATION UPDATE...Multiple BKN-OVC clds lyrs nr FL100...FL150...FL200
and SCT ocnl Bkn nr FL030-FL060. Prevailing VFR, but passing SHRA/-SHRA
over islands, with Isold TSRA ovr Atl waters and en route btw PR and
the USVI. Brief Mtn top obscr ovr ctrl and E PR due to low clds and
passing -SHRA til 18/12z. Wnds veering fm S-SW 15-25 kts blo FL250,
then bcmg fm W and incr w/ht abv.


.MARINE...No change to previous marine forecast discussion. Seas
of 3-5 feet most of the local waters...except 1-3 feet near shore
waters of western PR. Winds 10 to 15 kts. Offshore Atlantic buoy
suggests a 12-14 sec nrly swell arriving across the local waters
and expected to continue through Saturday night.


.PREV DISCUSSION... /issued 439 PM AST Fri Nov 17 2017/

A mid to upper level trough will amplify over the western
Caribbean Sea into the weekend. A shear line will continue near
the islands through next week. The islands are forecast to
continue under a southerly wind flow.

Showers and thunderstorms are possible across the across the U.S.
Virgin Islands and the eastern, interior and southern portions of
Puerto Rico through the overnight hours due to the combination of
surface convergence and diffluence aloft. In addition, a shear
line will aid in the formation of showers and thunderstorms across
the Mona Passage and Atlantic Waters of Puerto Rico. The southerly
wind flow continues to push a moist air mass over the islands.

Today we tied the record high temperature at the Luis Munoz Marin
Airport. This tied the old record of 91 Fahrenheit set in 1983.

The combination of a weak frontal boundary, an induced surface
trough, a southerly wind flow and good ventilation aloft will
enhance showers and thunderstorm convection on Saturday. This
activity could increase the risk for flooding as well as mudslides
along steep terrains mainly across the U.S. Virgin Islands and the
interior and eastern sections of Puerto Rico.

A mid to upper level ridge is expected to build across the forecast
area early next week and hold much of the forecast period. As this
feature strengthens Mon-Tue, available low level moisture is
expected to quickly erode with precipitable water values remaining
below the normal range Tue-Fri. At lower levels, east to east
southeast winds will prevail across the local isles as a surface
high dominates the central Atlantic. Under the aforementioned
pattern, expect mainly fair and stable weather condition much of
the next week with trade wind showers across USVI and E PR as well
as some locally induced afternoon showers across west areas of
Puerto Rico.

VCSH/VCTS across the local terminals today with MVFR CIGS
possible across Puerto Rico eastern terminal sites. SFC winds out
of the S to SW at 10 to 15 kts but FL340 - FL400 winds are out of
the SW to W at 20 to 30 kts. VCSH/VCTS should diminish across the
western terminals by 19/00Z then continue to the eastern terminals
by 18/03Z.

Although seas and winds are expected below 5 feet and at less than
18 knots, mariners should exercise caution due to the possibility
of shower and thunderstorm development mainly across Caribbean
Passages and Atlantic Waters. A northerly swell is forecast to
move across the local waters of the Atlantic Waters by Saturday.


SJU  77  86  78  87 /  40  60  60  60
STT  78  85  79  87 /  50  60  60  40


PR...Flash Flood Watch through Saturday afternoon for Northwest-
     Western Interior.



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