Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 262016
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
416 PM AST Mon Oct 26 2020

.SYNOPSIS...

An easterly disturbance is expected to approach the islands on
Tuesday, resulting in an increase in showers over the region. By mid-
week, conditions are expected to become more favorable for showers
and thunderstorms due to an upper level trough and an approaching
tropical wave. These unsettled weather conditions are expected to
linger through the end of the workweek.

&&

.SHORT TERM...Tonight through Wednesday...

Recent satellite imagery shows groups of showers carried by the
trade winds approaching the area. Under a moderate to strong
northeasterly steering flow generated by a surface high pressure
over the eastern Atlantic, this activity is expected to move fairly
quickly over eastern and northern portions of the forecast area,
without leaving significant rainfall accumulation. Also, local
effects are resulting in shower formation across the southwest
coastal areas, and this activity will last through around sunset.

Later in the night and into tomorrow, an easterly disturbance is
expected to approach the islands, with precipitable water values
approaching 1.9 inches. As a result, an increase in showers is
expected, especially over the U.S. Virgin islands and eastern
Puerto Rico through the day. Additionally, at the upper levels, a
trough will be located to the northeast of the islands. The area
will be located over the subsidence side of this system, which is
not favorable for strong convection to develop. As the easterly
disturbance continues to progress through the islands, winds are
expected to shift from the east to east- southeast, and enough
available moisture should combine with local effects to generate
afternoon showers over the interior and northwestern Puerto Rico.

By Wednesday, as the upper level trough progresses to the north of
the region, with 500 mb temperatures cooling down to near -6 to -7
degree Celsius. With better conditions aloft, and with an
approaching tropical wave supplying low level moisture into the
region, an increase in shower and thunderstorm activity is too be
expected.

.LONG TERM...Thursday through Tuesday.../from previous discussion/

Thursday and Friday is still forecast to remain fairly active and
unstable due to the proximity of an upper trough and associated area
of low pressure, as well as moisture accompanying and trailing the
tropical wave. Therefore expect a fairly wet weather pattern with
good potential for enhanced showers and thunderstorm development
across the islands and coastal waters especially on Thursday, then
gradually improving on Friday. Winds will become more from the ESE,
which will steer the showers and isolated thunderstorms over the
northwestern sections of PR during the afternoon hours. Periods of
Scattered showers will remain possible across the coastal waters
during the overnight and morning hours through Friday. For the
following weekend, so far expect an improvement in the overall
weather conditions as low level moisture erodes and mid to upper
level ridge builds in across the region. Regardless, expect lesser
frequent passing morning showers during the overnight hours,
followed by isolated to scattered locally and diurnally afternoon
convection over the weekend and into the early part of the next work-
week.

&&

.AVIATION...

VFR conds will continue today. VCSH expected across
the local terminals today and winds will be from the east to ENE at
around 10-15kt with occasional gusts and sea breeze variations.
Isolated TSRA is possible around the local flying area overnight,
especially for areas between TJSJ, TIST, and TISX as winds continue
from the east to ENE at 10-15kt.

&&

.MARINE...

Confused seas will continue to produce hazardous marine and
coastal conditions across the local island through much of the
forecast period. Mariners can expect seas up to 8 and winds
around 20 knots with higher gusts. Therefore Small craft
advisories are in effect for much of the local waters. The best
marine conditions or the safest ones are along the southern
beaches and coasts of the islands. However, mariners should
exercise caution due to winds around 20 kts.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  77  89  78  87 /  60  60  30  60
STT  79  87  78  88 /  60  60  40  60

&&

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

     High Surf Advisory until 5 AM AST Tuesday for Culebra-North
     Central-Northeast-Northwest-San Juan and Vicinity-Southeast.

VI...High Rip Current Risk through Tuesday afternoon for St Croix-
     St.Thomas...St. John...and Adjacent Islands.

AM...Small Craft Advisory until 8 AM AST Wednesday for Caribbean
     Waters of Puerto Rico from 10 NM to 17N.

     Small Craft Advisory until 8 PM AST Tuesday for Coastal Waters
     of Northern Puerto Rico out 10 NM.

     Small Craft Advisory until 8 PM AST Wednesday for Anegada
     Passage Southward to 17N-Atlantic Waters of Puerto Rico and
     USVI from 10 NM to 19.5N.

     Small Craft Advisory until 2 AM AST Wednesday for Coastal Waters
     of Northern USVI and Culebra out 10 NM-Coastal Waters of
     Southern USVI Vieques and Eastern Puerto Rico out 10 NM.

     Small Craft Advisory until 8 AM AST Tuesday for Coastal Waters
     of Northwestern Puerto Rico out 10 NM-Mona Passage
     Southward to 17N.

&&

$$

SHORT TERM...CAM
LONG TERM....CS
PUBLIC DESK...GL


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