Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 090135
Area Forecast Discussion
National Weather Service San Juan PR
935 PM AST Thu Dec 8 2016
.UPDATE...Only minor changes made to the inherited forecast
package at this time. Recent TJSJ upper air sounding remained
relatively dry with precipitable water now around 1.30 inches with
mid level ridging still expected to hold during the next 24 hours,
although slightly weakening along with the trade wind cap inversion.
The easterly disturbance wave now crossing the eastern Caribbean
will bring an increase in low to mid level moisture across the
region overnight through Friday. However, recent satellite imagery
and model guidance suggests some southwesterly shear aloft along
the northern periphery of this disturbance. The best moisture
convergence and moisture pooling expected to remain south of the
islands as the easterly disturbance will continue westward overnight
through Friday. Still however expect better chance for showers and
isolated thunderstorm development across the regional waters overnight,
and portions of the islands on Friday. Improving weather conditions
and increasing easterly trade winds thereafter, with a seasonal weather
pattern expected over the upcoming weekend.
.AVIATION UPDATE...Mostly VFR conditions expected across the
local flying area overnight and early Friday morning with VCSH
possible across the Leewards and USVI TAF sites. Winds will be
ENE at 10-15 kt decreasing to around 10 knots overnight.
.MARINE UPDATE...Overall seas will continue to gradually subside
overnight through Friday, due to fading northerly swell action.
However, winds will gradually increase across the regional waters
to maintain a light to moderate chop. Small craft operators should
therefore exercise caution mainly over the offshore Atlantic and
Caribbean waters as well as the local passages.
There is a moderate risk of risk currents for most beaches of the islands.
Please refer to the latest surf zone forecast issued by WFO San Juan
.PREV DISCUSSION... /issued 150 PM AST Thu Dec 8 2016/
SYNOPSIS...Surface high pressure to the north of the local
islands will continue dominating the local wind pattern for the
next few days. Easterly disturbance expected to pass through the
area late tonight into Friday. This will cause an increase in
shower and thunderstorm activity over the area.
DISCUSSION...Variably cloudy skies prevailed across the local
region this morning and early this afternoon. No shower activity
was observed over land areas. Very dry conditions prevailed across
the region with latest 08/12Z TJSJ sounding showing precipitable
water values of only 1.25 inches. Very dry air at mid to upper
levels of the atmosphere will continue to suppress shower
development across the region.
High pressure ridge will remain as the main weather feature
across the region for the next few days. Easterly disturbance is
expected to pass through the area late tonight into Friday. This
system will cause an increase in shower activity over the area. A
more typical December pattern is expected thereafter with
tradewind showers moving in.
AVIATION...Brief periods of SHRA still expected to develop over
western PR and may result in brief MVFR conditions at TJMZ through
08/22z. Mostly VFR conditions expected across the local flying area
overnight and early Friday morning with VCSH possible across the
Leewards and USVI taf sites. Winds will be ENE at 10-15 kt
decreasing to around 10 knots overnight.
MARINE...Seas up to 6 feet are expected across the offshore
Atlantic waters and the local passages. Small craft operators are
urged to exercise caution across the Atlantic waters and local
passages...with winds up to 20 knots across the Caribbean waters.
.PRELIMINARY POINT TEMPS/POPS...
SJU 76 86 77 87 / 10 20 20 20
STT 76 86 76 86 / 30 30 30 30