Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 270130
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
930 PM AST Wed Jul 26 2017

.UPDATE...A weak tropical wave and associated trailing moisture
continued to exit the region this evening. Most of the afternoon
showers and thunderstorms moved off shore or dissipated during
the late evening hours, leaving partly cloudy skies over land areas.
A late evening isolated thunderstorm developed just offshore between
Aguadilla and Isabela PR but is expected to dissipated during the
next few hours, while a few passing showers will continue to affect
the coastal waters overnight. Some of these showers may brush the
windward side of the islands by early morning. Otherwise no changes
at this time.

&&

.AVIATION UPDATE...No change to previous AVN discussion. VFR conds
durg entire prd. Few passing SHRA mainly ovr Atl waters and en route
btw local islands. Isold-Sct SHRA/isold TSRA psbl fm 27/17z-27/22z
mainly over N Ctrl and W PR...with VCSH AT TJSJ/TJNR/TJMZ/TJBQ.

&&

.MARINE UPDATE...No change to previous marine discussion. Fairly
tranquil seas during the evening hours except for passing showers
mainly over the local Atlantic waters and an isolated thunderstorm
moving west northwest just off the northwest coast of Puerto Rico.
Thunderstorm activity should however diminish during the overnight
hours.

&&

.PREV DISCUSSION... /issued 320 PM AST Wed Jul 26 2017/

SYNOPSIS...Tropical wave over the eastern Caribbean with axis
extending north across the Mona Passage moving west at 10 to 15
kt. Weak upper level trough extending from an upper low over
central Atlantic southwest to the northern portions of the eastern
Caribbean. Another tropical wave over the Atlantic east of the
Lesser Antilles moving west at 15 kts.

SHORT TERM...Tonight through Friday night...The first tropical
wave mentioned before will continue to move west away from the
local area. As a result shower/thunderstorm activity is expected
to significantly diminish this evening. A slightly drier air mass
should replace this wave...however the proximity of the upper
trough will be enough to result in fairly good upper level
dynamics and unstable conditions Thursday. With this in mind
decided to keep afternoon showers and thunderstorms over the main
island of Puerto Rico with much less activity over the U.S. Virgin
Islands on Thursday. With a mean E-SE steering flow will expect
most of the showers and thunderstorms to concentrate north of the
cordillera central and mostly northwest portion of the big
island. On Friday...The second tropical wave mentioned above will
move across the local islands...However, models are indicating
that ridging will be building eastward across the northeast
Caribbean pushing the upper trough eastward. This could suppress
most convective activity associated with the wave, so decided to
lower the chances for precipitation. Still will expect afternoon
showers/thunderstorms due to local breezes interaction.

LONG TERM...Saturday through Thursday...

An upper level ridge should continue to extend over the local area.
A surface high pressure ridge over the north-central the central
Atlantic north of the area will yield an east to east southeast
wind floe through the early part of next week. Most precipitation
during this period should be as a result of local factors. By
Wednesday models indicate that another tropical wave should
approach the eastern Caribbean which could result in an increase
in the showers and the thunderstorm activity Wednesday through
Thursday.

AVIATION...SHRA are expected to continue across TJSJ/TJMZ/TJBQ
until at least 26/22z. Mostly VFR conditions are expected across the
local flying area after 26/22z with VCSH across the Leeward, USVI
and TJSJ TAF sites. ESE winds of 10 to 15 mph will diminish after
26/23z to light/VRB, increasing again at 10 to 15 kt after 27/14z.

MARINE...Marine conditions should continue on the pleasant side with
seas below 5 feet and winds at or below 15 knots through the end
of the week. The risk of rip currents should also be on the low
side at least through Friday.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  80  90  80  91 /  20  30  20  30
STT  80  89  81  88 /  30  30  30  30

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

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



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