Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 240155

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

.UPDATE...Scattered to numerous showers have developed across the
southeast and eastern interior sections of Puerto Rico tonight.
Rainfall amounts of 2 to 3 inches has fallen across San Lorenzo,
Yabucoa and Maunabo. Latest 24/00Z TJSJ sounding revealed
a very moist atmosphere across the area. Showers will continue to
affect these sections of Puerto Rico overnight. minor changes
were made to short term grids to reflect this scenario.


.AVIATION...Periods of MVFR conditions can be expected across TIST
through at least 24/04Z due to a line of showers moving across the
island. Elsewhere, mainly VFR conditions are expected to prevail
across all TAF sites overnight. Surface winds will be light and


.MARINE...No changes to previous discussion.


.PREV DISCUSSION... /issued 358 PM AST Sun Apr 23 2017/

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.

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.

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

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.


SJU  75  87  76  88 /  40  50  40  60
STT  75  83  76  84 /  60  60  50  50


PR...High Rip Current Risk through late Monday night 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 AM AST Monday for Mayaguez and

     High Rip Current Risk from Monday evening through late Monday
     night for Mayaguez and Vicinity.

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

     High Rip Current Risk until 6 AM AST Monday for St Croix.

AM...High Rip Current Risk through late Monday night for 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-Caribbean Waters of Puerto Rico from 10 NM
     to 17N-Coastal Waters OF Northwestern Puerto Rico out 10 NM-
     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 2 AM AST Tuesday for Atlantic Waters
     of Puerto Rico and USVI from 10 NM to 19.5N.

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

     High Rip Current Risk through Monday afternoon for Coastal
     Waters of Northern USVI and Culebra out 10 NM.



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