Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 211529
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1129 AM AST Sat Apr 21 2018

.UPDATE...No major changes to the inherited short term forecast package
at this time which had a good handle on present and expected conditions
during the rest of the weekend.Quick surge of low to mid level moisture
accompanying a weak easterly perturbation will continue to cross the
region and provide sufficient low level convergence and instability
to support locally and diurnally induced convection during the rest
of the day.

Recent TJSJ 21/12z upper air sounding showed a slight increase in
layered precipitable water to around 1.70 inches, with low level winds
becoming more easterly in response to a high pressure ridge which will
build north of the area over the rest of the weekend. In addition,
the ridge aloft will slightly erode through Sunday as a short wave
trough will become amplified and move across the southwest Atlantic.
This feature should provide sufficient instability and ventilation
for the development of afternoon showers and thunderstorms mainly
across portions of the interior including parts of the San Juan metro
and the west and northwest sections of Puerto Rico. Locally heavy rainfall
will therefore be possible across these areas. Isolated showers are
forecast elsewhere including the U.S. Virgin Islands where partly cloudy
skies and brief showers and clouds in the form of streamers will occur.

&&

.AVIATION...Prevailing VFR. Sct-Bkn cld lyrs btw FL025-Fl100...
Isold Tops 150-180...except higher with Isold TSRA psbl ovr PR fm
21/17z-21/22z where brief MVFR conds will be likely vcty TJSJ/TJMZ/TJBQ.
Isold SHRA ovr coastal waters and en route btw islands durg prd. Sfc
winds fm E around 15 kts with ocnly higher gusts along coastal areas
and with aftn convection.

&&

.MARINE...Recent data from local buoys suggest gradually subsiding
seas over the local waters with seas no longer at or above small
craft advisory criteria. The small craft advisory for the offshore
Atlantic waters was cancelled. However, the moderate to locally
fresh easterly trade winds will continue to create choppy conditions
and therefore small craft operators should exercise caution over most
of the coastal waters and local passages.

&&

.PREV DISCUSSION... /issued 500 AM AST Sat Apr 21 2018/

SYNOPSIS...At the surface, high pressure system will continue to
dominate the local region through the weekend. A patch of low
level moisture will move across the region today. A sharp increase
in low level moisture is expected early next week. Upper level trough
will dominates the western Atlantic Monday through Wednesday next
week.

SHORT TERM...Short Term...Today through Sunday...

A patch of moisture is expected to move in today, the rainfall has
already made an impact with numerous showers affecting eastern PR
during the overnight hours. This moisture will combine with the
local effects this afternoon and help develop scattered to locally
numerous showers with isolated thunderstorms over the interior into
NW-PR, with showers and thunderstorms possibly streaming off of the
Luquillo Mountain range into the San Juan metro area.  On Sunday,
drier air is expected in and wind flow may shift a bit to the ENE,
which may cause afternoon convection across W and SW-PR with
isolated showers elsewhere. Both Saturday and Sunday, we may observe
isolated to scattered brief showers across the local waters,
USVI, and eastern PR in the nighttime and early morning hours.

Monday may be a bit more active if the guidance were to verify. We
have additional moisture moving in again and the wind flow becomes
easterly. There is also an upper trough that may be digging just
west of the local islands, increasing instability and also
increasing the chances for thunderstorms over the local islands,
especially over several sectors in Puerto Rico when the moisture,
diurnal heating and instability all combine on Monday afternoon.
Isolated to scattered showers can be expected for the USVI during
the day with showers possibly streaming off the islands.

LONG TERM...Monday through Saturday...

For Monday into Tuesday, a sharp increase in low level moisture
is forecast to occur, with best moisture and favorable upper level
atmospheric conditions expected on Tuesday. As a result, an
increase in showers and thunderstorms are expected by early next
week. By late in the forecast period, drier air mass is expected
to encompass the region from the east.

AVIATION...VFR conds in the morning hours across the local
terminals. SHRA/SHRA expected this afternoon may cause moments of
MVFR across TJSJ, TJMZ, and TJBQ after 21/16Z. TEMPO groups were
left out due to exact location uncertainty since guidance suggests
that the activity will be mostly in the vicinity. Winds will be from
the east at around 15kt and gusty.

MARINE....Seas will slowly diminish and most areas will come below
7 feet this morning. However, seas at 7 feet are expected to
continue across the offshore Atlantic waters through this evening.
Elsewhere, small craft should exercise caution.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  87  75  85  75 /  40  20  20  40
STT  86  75  87  75 /  20  20  20  40

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...High Rip Current Risk through late tonight for St Croix.

AM...None.
&&

$$

SHORT TERM...RAM
LONG TERM....RAM


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