Scientific Forecaster Discussion
fxca62 tjsj 221521
Area forecast discussion
National Weather Service San Juan PR
1121 am AST Sat Jul 22 2017
Update...cloudiness has increased across the local islands due to
orographic lifting. So there is clouds developing across the
higher elevations and also downwind from it into southwestern and
also northwestern PR. A few light showers were observed across
the local waters and moderate showers across portions of northern
PR, but the accumulations were mainly under a quarter of an inch
of rain as the showers moved west.
Easterly winds are expected to continue today and showers are
expected to continue developing early in the afternoon as a
tropical wave approaches from the east, increasing the available
moisture. For that reason, scattered to numerous showers with
isolated thunderstorms were kept in the forecast with the more
persistent showers occurring over the western half of PR and
passing showers elsewhere.
Aviation...VFR conds expected for most of the local terminals
with vcsh expected this afternoon and tonight as moisture
increases. Thunderstorms in the vicinity likely at tjmz and tjps after 22/17z with possible
tempo thunderstorms and rain at tjmz. Winds continue from the east at about 10-15kt
with higher gusts this afternoon.
Marine...seas will continue at generally 3-5 feet with winds at
around 10-15kt for today and tonight.
Previous discussion... /issued 445 am AST Sat Jul 22 2017/
Synopsis...tropical wave will move across the region this
afternoon into Sunday. TUTT low will develop and moves across the
southwest Atlantic early next week.
Short term...Saturday through Monday...
Although a mid to upper level ridge will prevail throughout the
forecast period; there is a chance for showers and thunderstorms
across the local islands this afternoon into Sunday as a tropical
wave moves across the northeast Caribbean.
Mostly fair weather conditions will continue to prevail this morning
across Puerto Rico and the U.S. Virgin Islands. However, as a
tropical wave which is now located over the Lesser Antilles moves
closer to the forecast area, precipitable water is expected to
increase above the normal range. This will assure locally induced
showers and thunderstorms across the interior and western areas of
Puerto Rico this afternoon followed by passing showers with isolated
thunderstorms across the USVI and east PR this evening throughout
Lingering moisture combined with local effect will result in another
round of showers and thunderstorms over and north of the Cordillera
Central Sunday afternoon. As the wave moves away, a drier air mass
will encompass the forecast area Sunday night into Monday. Urban and
small stream flooding is likely with the heaviest showers.
Long term...Tuesday through Sunday...
At upper levels...TUTT low will dominates the northeast Caribbean
through Sunday. At low level, a tropical wave will move across
the region Thursday. The combination of these features, with
daytime heating and local effects will result in high chances for
showers and thunderstorms across the local islands during most of
Aviation...VFR conds are expected to prevail at all taf sites
through the early morning hours. However, as a tropical wave moves
over the Leeward Islands, brief MVFR conds are possible at tkpk/tncm
late this morning as well as into the afternoon hours. Aft 22/16z
rain showers/thunderstorms and rain expected near jmz/jbq with MVFR conds possible through
22/22z. Easterly winds 10-15 knots aft 22/13z with some sea breeze
Marine...seas up to 6 feet and winds up to 20 knots are expected
to prevail across the local waters. Small craft should exercise
caution across the offshore Atlantic waters.
Preliminary point temps/pops...
sju 89 79 90 81 / 30 40 50 20
stt 90 80 89 81 / 50 60 60 20