Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 210055

Area Forecast Discussion
National Weather Service San Juan PR
855 PM AST Fri Apr 20 2018

.UPDATE...The San Juan sounding was wetter overall compared to 12
and 24 hours ago with precipitable water coming in at 1.67 inches.
Nevertheless radar shows that skies are mostly rain-free and an
inversion is setting up over nearby land areas, making any
showers--at least for the next 3 to 4 hours or so--unlikely. With
flow continuing to be east southeast and a weak cap at around 8600
feet only scattered clouds are expected overnight. Some showers
may develop over the local waters and move inland over eastern
Puerto Rico later tonight and so have left POPs there at 40
percent. By mid afternoon, sea breezes should also limit afternoon
temperatures, which would otherwise be susceptible to being above
normal, owing to the east southeast flow, and will also keep
significant precipitation and cloudiness inland from the coast
near the Greater San Juan Metropolitan Area Saturday afternoon.

The GFS seemed to be very wet at 850 mb until the mass of dry air
moves in from 21/18-22/00Z, but satellite pictures suggest that
this relative humidity--also confirmed in the sounding--is not
producing as much cloudiness or showers as might be expected.
Therefore, have lowered expectations overnight somewhat.

Minor tweaks were made to near-term grids, but overall
expectations are as below.


.AVIATION...VFR conds are expected at all TAF sites except as
noted: Aft 21/16Z SHRA and MVFR CIGS with mtn obscurations will
devlp ovr western and interior PR and down stream from higher
trrn. Isold TSRA are expected--mainly in the west half of PR.
Conds clrg aft 21/23Z. Sfc winds E 5-10 kt bcmg 10-20 kt with
land/sea breeze influences. Max winds W 50-60 kt btwn FL370-450
bcmg up to 80 kt aft 22/00Z at FL410.


.MARINE...Seas will slowly diminish and most areas will come below
7 feet before 6 AM AST Saturday morning. Small craft advisories
are not expected again until aft 28 Apr 2018. Thunderstorms over
local waters are only expected immediately downstream from Puerto
Rico during the late afternoons and early evenings.


.PREV DISCUSSION... /issued 426 PM AST Fri Apr 20 2018/

SYNOPSIS...Broad surface high pressure will continue across the
eastern Atlantic through early next week. Patches of moisture
embedded in the trades will move occasionally across the region
for the next several days, inducing the development of showers and
thunderstorms each afternoon across interior and western Puerto
Rico with isolated to scattered showers across the U.S. Virgin
Islands and eastern Puerto Rico during the nights and early
morning hours.

SHORT TERM...Tonight through Sunday...
A patch of moisture embedded in the trades moved across the U.S.
Virgin Islands, Vieques, Culebra as well as across the eastern
sections of Puerto Rico this morning inducing the development of
scattered to numerous passing showers over those sectors. This
moisture in combination with daytime heating and local effects
will continue to produce showers and thunderstorms over western
and interior sections of Puerto Rico until late this afternoon.

Drier air will continue to move across the region tonight and
Saturday. However...patches of moisture embedded in the trades
will move occasionally across the region during the next several
days. This moisture will induce the development of isolated to
scattered showers across the U.S. Virgin Islands, Vieques, Culebra
as well as across the eastern sections of Puerto Rico during the
night and early morning hours followed by scattered showers and
isolated thunderstorms across western and interior sections of
Puerto Rico each afternoon. Latest satellites images depicted
another patch of moisture to the east of the region. This feature
is expected to produce scattered showers tonight and early
Saturday morning across the local waters, USVI and eastern PR.
By Sunday, drier air is expected to dominate the region. Not
significant changes are expected in the present weather pattern
during the forecast period.

LONG TERM...Monday through Friday...From previous reasoning...
Moisture peaks on Monday and Tuesday and an upper level trough
will support thunderstorm development each day. But overall the
same weather pattern is expected to prevail each day. Shower and
thunderstorm development is expected to develop each afternoon
over portions of western Puerto Rico. Trade wind showers will
persist across the USVI and eastern portions of PR during the
overnight and early morning hours. Drier air is expected to move
once again from the east by Friday. Day time maximum temperatures
are expected to reach the high 80s to low 90s across the lower
elevations and overnight minimum temperatures are expected to
range from the low 60s across the higher elevations to low 70s
along the lower elevations.

AVIATION...Periods of MVFR conds likely in and around JMZ/JBQ and
possibly JSJ in SHRA/TSRA through 20/22z. Expect VCSH at the Leeward
and USVI terminals this afternoon and early tonight. ESE winds
around 10-15 knots with some sea breeze variations, becoming at 10
knots or less overnight.

MARINE...A long period northeast swell will continue to result
in life threatening rip currents across the north, east and
southeast facing beaches of Puerto Rico, Culebra, Vieques and St.
Croix. Small craft advisories are in effect for the offshore
waters and passages due to seas up to 7 feet. East winds will
continue at 15 to 20 knots across the region through the weekend.


SJU  75  86  75  85 /  40  40  20  10
STT  76  86  75  87 /  30  20  20  20


PR...High Rip Current Risk until 6 AM AST Saturday for Central
     Interior-Culebra-North Central-Northeast-Northwest-San Juan
     and Vicinity-Southeast-Vieques-Western Interior.

     High Rip Current Risk until 6 AM AST Saturday for Coastal
     Waters of Southern Puerto Rico out 10 NM.

VI...High Rip Current Risk through late Saturday night for St Croix.

AM...Small Craft Advisory until 6 PM AST Saturday for Atlantic
     Waters of Puerto Rico and USVI from 10 NM to 19.5N.



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