Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 231958
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
358 PM AST Sun Apr 23 2017

.SYNOPSIS...
Instability is expected to increase somewhat today, as a short
wave trough swing near the region this afternoon. TJSJ, 23/12z,
RAOB showed a weakening trade wind inversion. A subtropical jet
will hold aloft through at least Monday. A polar trough is
forecast to move over the Western Atlantic Ocean late tonight into
early Monday morning, then will reach the islands by mid-week.
This trough is forecast to amplifies over the region, increasing
the local instability each day through at least Wednesday. Under
a southeasterly wind flow tropical moisture will pull over the
region increasing the chance for shower and thunderstorm
development each day trough at least mid-week.

&&

.SHORT TERM...
The combination of a short wave trough, low level convergence,
tropical moisture and local effects produced mostly cloudy skies
and showers over Puerto Rico and the U.S. Virgin Islands during
the afternoon. Shower activity is expected to continue through
sunset. Thunderstorm activity can be expected particularly over
the interior and northwest quadrant of PR. The model guidance
continues to indicate a southerly wind flow by Monday and
Tuesday. This will enhance moisture advection from the Caribbean
waters over the local islands. While surface moisture convergence
occurs, a mid to upper level trough will increase local
instability, resulting in a better chance of showers and even
thunderstorms development through midweek.

Residents in PR and in the USVI should monitor the progress of
the weather conditions as periods of frequent rainfall can result
in urban and small stream flooding as well as localized flash
flooding with the heaviest showers.

.LONG TERM...from the morning discussion...
An upper level trough is expected to race over the northeast
Caribbean as an associated surface low moves northeastward across
the Canadian Maritime provinces. As this happens weather
conditions will remain unstable across the local islands Wednesday
but PWAT values are expected to decrease below 2.0 inches by that
time. Weather conditions will improve rapidly by Thursday and
thereafter as an upper level ridge build across the region. A
surface high pressure across the CentraL will shift the local
winds more from the east late Wednesday and Thursday. Although the
instability is expected to decrease by the middle of the week,
this change in wind direction is expected to induce the
development of some showers and possible a couple of thunderstorms
each afternoon across interior and western sections of Puerto
Rico due to the combination of daytime heating and local effects.
A weak surface trough will increase slightly the chances of
showers across the region by the upcoming weekend.

&&

.AVIATION...
Prevailing VFR at all TAF sites...Incr SHRA with isold TSRA psbl
mainly ovr interior and W-NW PR thru 22z. Brief MVFR conds psbl
due to SHRA and Low CIG vcty TJBQ,TJMZ, TJNR,TJSJ. Sfc wnd fm SE
10 to 15 kts ocnl hir gusts. L/lvl wnds fm SE veering w/hr and
bcmg fm W abv FL200. In general, easterly winds at 4-8 kt aft
24/02z.

&&

.MARINE...
San Juan buoy observed seas between 6 and 7 feet with 13 seconds,
which produce breakers between 8 and 10 feet. Winds between 15 and
20 knots were observed around the coastal areas. Marine guidance
continues to indicate waves between 5 and 8 feet. Therefore, small
Craft advisories continues in effect for most of the local waters
through at least Monday night. Marine conditions should improves
by Tuesday with seas below 6 feet and winds of 18 knots or less.

A high Risk of Rip Currents will continue for the northwest to
northeast beaches of Puerto Rico, Culebra and Vieques and for the
U.S. Virgin Islands.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  75  87  76  88 /  40  50  40  60
STT  75  83  76  84 /  60  60  50  50

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Monday afternoon for Culebra-North
     Central-Northeast-Northwest-San Juan and Vicinity-Southeast-
     Western Interior.

     High Surf Advisory until 6 AM AST Monday for Culebra-North
     Central-Northeast-Northwest-San Juan and Vicinity-Southeast-
     Western Interior.

     High Rip Current Risk until 6 PM AST this evening for Vieques.

     High Rip Current Risk through late tonight for Mayaguez and
     Vicinity.

VI...High Rip Current Risk through Monday afternoon for
     St.Thomas...St. John...and Adjacent Islands.

     High Rip Current Risk through late tonight for St Croix.

AM...High Rip Current Risk through Monday afternoon for Coastal
     Waters of Northern USVI and Culebra out 10 NM-Coastal
     Waters of Southern USVI Vieques and Eastern Puerto Rico out
     10 NM.

     High Surf Advisory until 6 AM AST Monday for Coastal Waters of
     Southern USVI Vieques and Eastern Puerto Rico out 10 NM.

     Small Craft Advisory until 8 PM AST Monday for Anegada Passage
     Southward to 17N-Atlantic Waters of Puerto Rico and USVI
     from 10 NM to 19.5N-Caribbean Waters of Puerto Rico from 10
     NM to 17N-Coastal Waters of Northern Puerto Rico out 10 NM-
     Coastal Waters of Northern USVI and Culebra out 10 NM-Mona
     Passage Southward to 17N.

     Small Craft Advisory until 8 AM AST Monday for Coastal Waters OF
     Northwestern Puerto Rico out 10 NM-Coastal Waters of
     Southern USVI Vieques and Eastern Puerto Rico out 10 NM.

&&

$$

SHORT TERM...CAM
LONG TERM....Morning Discussion
AVIATION...RAM


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