Area Forecast Discussion
Issued by NWS San Juan, PR

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462
FXCA62 TJSJ 211555
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1155 AM AST Sun Apr 21 2019

.UPDATE...
The easterly trades brought a patch of moisture across the
region today. As a result, passing showers will continue to move
across the local waters and inland the rest of the day. The
heaviest activity is expected to affect the western sections of
the islands this afternoon.

&&

.AVIATION...
Passing SHRA are expected across the local flying area today. The
strongest activity will be in the VCTY of Mayaguez thru this
evening (around 21/23z), and possibly affecting TJMZ/TJBQ. Passing
SHRA will move across/vicinity of the local terminals thru this
evening. Activity will diminish overnight, but brief passing SHRA
shouldn`t be ruled out across E-PR/USVI and Leeward Islands
terminals overnight. Winds will be from the E at 15 to 20 knots
with higher gusts and sea breeze variations.

&&

.MARINE...
Small craft advisory is in effect for the Atlantic Offshore Waters
due to seas up to 7 feet. Elsewhere, mariners should exercise
caution due to choppy seas up to 6 feet and winds up to 20 knots.

Beachgoers, there is a high risk of rip currents for beaches
along the northwest to north central coast of Puerto Rico.
Elsewhere, a moderate risk of rip current is expected.

&&

.PREV DISCUSSION... /issued 543 AM AST Sun Apr 21 2019/

SYNOPSIS...A weak cold front riding in on northeast flow
generated by high pressure north of the area and a developing low
in the mid central Atlantic will bring better showers on Monday
followed by another period of drying. Long term models suggest a
moisture plume from a tropical wave passing well south of the area
may bring more showers next Sunday. Winds will begin to diminish
Monday and be rather quiet by Wednesday.

SHORT TERM...Today through Tuesday...

A surface high pressure spreading across the west and central
Atlantic will continue to promote a moderate to fresh and briefly
strong easterly flow across the region today. Under this flow, a
patches of low-level moisture will continue to move across the
forecast area today, enhancing cloud cover and shower activity.
Overnight and early morning showers will continue to stream across
the local waters into portions of eastern Puerto Rico and the
U.S. Virgin Islands. Given the intensity of the flow, some of
these showers may move further inland over portions of the
interior. This will be followed by afternoon convection clustering
over the interior and western sectors of Puerto Rico supported by
diurnal heating and local effects. Streamer-like showers are
possible over the San Juan metro area and eastern Puerto Rico
downstream from El Yunque and the local islands, respectively.
Model estimated PW values between 1.35-1.44 inches around
21/12-18Z could support significant shower activity with the most
intense showers in the afternoon.

A weak mid-level trough will continue to drift over the northeastern
Caribbean until it weakens by Monday. In the meantime, a surface
induced trough will continue to migrate south to southwest over the
central Atlantic and linger to the northeast of the region later
today through early next week, promoting a northeast wind flow
across the region. This combination will result in the weakening
of the trade wind inversion and enhance mid-level moisture
transport while supporting deeper convective instability. Although
the showers will favor a seasonal distribution, favorable
conditions aloft will result in enhanced shower coverage and
intensity with moderate to locally heavy rainfall. The forecast
continues to call for the most intense activity on Monday
afternoon. Precipitable water values are estimated to be 1.53
inches. Showers will be scattered to numerous showers and will
favor eastern Puerto Rico and the U.S. Virgin Islands in the
morning, and portions of the interior and southwestern Puerto
Rico in the afternoon. Considerable drying will occur on Tuesday
and shower activity will diminish.

LONG TERM...Wednesday through Sunday...

After drying on Tuesday behind the cold front, moisture will
return from the northwest at 700 mb on Wednesday. Flow becomes
very light on Wednesday so that most showers will be induced by
local effects of sea breeze convergence and inland heating. The
surface low to our northeast will fade Wednesday and Thursday and
withdraw on Saturday, to be replaced by a weak high just north of
the forecast area. This will bring another period of drier
conditions. At the surface, high pressure will build south into
the tropical Atlantic Saturday night and Sunday, and a weak low
will move from the Gulf of Mexico on Saturday to north of the
local area on Sunday, producing moderate southeasterly flow.

We continue to follow a tropical wave between 30 and 40 west--
mostly south of 10 north. It is forecast to move into the eastern
Caribbean on Thursday and join with moisture streaming into the
southern Caribbean from the southwest. As high pressure at 700 mb
moves into the eastern Caribbean on Saturday, these combined plumes
of moisture will be swept into Puerto Rico and the U.S. Virgin
Islands on Sunday, while the wave continues through the central
Caribbean. Currently, the GFS has been favoring this development
and has been forecasting precipitable water values of 1.8 to 2.0
inches in several of the last runs, if only for a brief period of
24-36 hours. This would bring another round of showers to the area
if the axis of the high continues to be south of Puerto Rico.

AVIATION...Mainly VFR conditions expected. However, SHRA will
result in briefly MVFR conditions at TSJU between 21/08-10Z and VCSH
conditions across E-PR/USVI and Leeward Islands terminals through
21/14Z. Afternoon convection will bring VCSH in the vicinity of San
Juan and Mayaguez between 21/16-22Z. Winds will continue from the
E at 10-15 knots with higher gusts across E-PR/USVI and Leeward
Islands terminals through 21/13Z, but light and variable
elsewhere. Winds will increase at 15-20 knots with higher gusts
after 21/13Z.

MARINE...A weak swell of almost 3 feet from the northeast and
winds of 15 to 20 knots are combining to bring seas of 5 to 7 feet
to the north central and northeastern local outer Atlantic waters.
After Monday, sea conditions will improve, and after Wednesday
some of the quietest seas in several months will be seen for a few
days. Seas south of Puerto Rico and the U.S. Virgin Islands may be
as low as 1 to 3 feet Thursday and Friday.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  85  75  85  73 /  40  70  70  40
STT  86  76  86  74 /  40  70  20  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk from Monday morning through Monday
     afternoon for Northeast-San Juan and Vicinity-Southeast.

     High Rip Current Risk through Monday afternoon for North Central-
     Northwest.

VI...None.
AM...Small Craft Advisory until 8 AM AST Monday for Atlantic Waters
     of Puerto Rico and USVI from 10 NM to 19.5N.

&&

$$

SHORT TERM...CAM
AVIATION...CAM



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