Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
228 PM AST Monday Sep 1 2014
Synopsis...mid-upper ridge will dominate the eastern Caribbean
region through Thursday. A series of tropical waves will move
across the islands this week. The first wave reaches the local
islands late Tuesday and the second wave is forecast to cross the
region late Thursday and Friday. Cloud and shower activity will
increase with both waves...however the second wave is better
Discussion...trade winds have transported patches of low level
moisture across the coastal waters...U.S. Virgin Islands...Vieques...
Culebra and the eastern third of Puerto Rico early today. Low level
winds shifted to the southeast this afternoon allowing cumulus lines to
form downwind of the USVI...Culebra and Vieques. Some of these cumulus
lines/streamers have moved over eastern PR producing brief periods
of moderate to locally heavy rainfall over El Yunque and portions
of the San Juan metropolitan area. In addition...sea breeze convergence
over western PR have resulted in the typical afternoon convection.
Satellite loops suggest a dry slot upstream of the USVI/PR ahead
of the tropical wave. This will result in limited shower activity
tonight and overnight. Moisture and instability associated with
the next tropical wave now located over the Lesser Antilles will
arrive to the forecast area early Tuesday morning. Therefore...cloud
and shower coverage will increase as the wave moves through the
islands. However...no significant accumulations are expected with
this wave due to the presence of the aforementioned middle-upper
ridge aloft. Drier air mass is expected on Wednesday in the wake
of the wave. Things will change again late Thursday through
Friday when the second wave approaches from the east. At this
time...it appears that this wave will bring organized convection
and better rainfall amounts to the islands...especially on Friday
when the axis of the wave passes and the winds veer to the southeast.
Aviation...shra/tsra this after mainly near tjmz/tjbq/tjsj but
streaming rain showers may barely bypass tjsj toward SW. Areas obscured mountains
rain showers/thunderstorms and rain currently beginning to arrive at tncm/tkpk will continue into
night. Otw VFR all taf sites except brief MVFR in isolated/scattered rain showers. Wind below
fl200 east-east-southeast increase 13-22 knots tonite/Tue.
Marine...seas of 3 to 4 feet and winds of 13-18 knots can be expected
over the next several days. Small craft advisories are not expected
through at least the end of the week.
Preliminary point temps/pops...
sju 78 90 78 90 / 30 40 30 30
stt 79 89 80 89 / 40 40 40 30