Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
1110 am AST Tuesday Sep 23 2014
Update...satellite imagery shows high cirrus clouds advecting
over PR and USVI this morning from the north. These cirrus clouds
have maintained the skies variably cloudy across most of the local
region. In contrast...Doppler radar detected scattered showers
across the Caribbean waters just south of Puerto Rico moving west-northwest
around 15 knots. Tjsj 12z sounding came in pretty wet and unstable...
with precipitable water of 2.2 inches and ki/cape of 36/1900
respectively. In general...environmental conditions across the
region support the development of heavy rain across portions of
the islands later today...however the limiting factor could be the
high clouds. The focus of the shower and thunderstorm activity
will be mainly across the interior...north and west portions of PR
this afternoon. The steering flow is from the southeast...so the strongest
storms are expected over northwest PR during afternoon hours. We decreased
the probability of precipitation by a 10%-20% mainly across the south and eastern PR for
this afternoon. The rest of the forecast package remains very
similar to this morning.
Aviation...MVFR/IFR conds and mountain obscurations likely in and
around tjmz and tjbq between 17z-21z. Vcsh/thunderstorms in the vicinity also possible near
tjsj and tjps in the afternoon. Low level southeast winds of 10-20 knots can be
expected through the forecast period. Gusty winds likely near thunderstorms and rain.
Marine...seas of 3-5 feet expected today...subsiding to 2-4 feet
tonight. However...strong gusty winds...locally higher seas and
dangerous lightning can be possible near thunderstorms. Therefore...
boaters should avoid thunderstorms.
Previous discussion... /issued 508 am AST Tuesday Sep 23 2014/
Update...TUTT low over the central Atlantic northeast of the
region...will amplify into the northeast Caribbean during the
next few days. Tropical wave across the Mona Passage will move
westward and away from the local islands tonight.
Discussion...the Doppler weather radar detected scattered showers
and isolated thunderstorms across the surrounding coastal waters.
Under an east southeast wind flow...showers and thunderstorms moved
over southeast and south sections of Puerto Rico as well as across
Vieques...Culebra and the U.S. Virgin Islands. However...not
significant or widespread precipitation is expected. For the
afternoon hours...this moisture will combine with daytime heating
and local effects to induce the development of showers and
thunderstorms over western...interior and northwest sections of
Latest satellite images and surface analysis this morning depicted a
tropical wave over the Mona Passage. This wave will continue to move
westward and away from the region today and tonight. The moisture
associated with the wave will continue to affect the local region
today...inducing an increase in shower and thunderstorm activity
over the region. The GFS computer guidance suggests precipitable water values
slightly above 2.00 inches into the middle of week. Late this
afternoon...tonight and into Tuesday...frequent passing showers
with thunderstorms could be expected across USVI...eastern and
southern sections of Puerto Rico.
Aviation...tropical wave bringing rain showers/thunderstorms and rain activity across the
local area overnight and Tuesday. Conds should prevail VFR until
about 23/08z. Thereafter...shra/tsra activity is expected to start
affecting the tkpk and tncm terminals...then tist and tisx a few
hours after that and so on as the wave continues to move west. MVFR
conditions are possible across the local terminals but likely across
western PR...in and around tjbq and tjmz after 23/16z where thunderstorms and rain are
expected to develop.
Marine...seas of 3-5 feet expected over the next several days.
However...strong gusty winds...locally higher seas and dangerous
lightning are likely near thunderstorms today. Therefore...boaters
should avoid thunderstorms.
Preliminary point temps/pops...
sju 87 78 88 78 / 40 10 10 20
stt 88 78 88 79 / 30 20 20 30