National Weather Service San Juan PR
516 AM AST Sun Mar 19 2017
.SYNOPSIS...Increasing moisture, the presence of a cold front to
our north, a subtropical jetstream that holds just to our north
leaving us under favorable upper level dynamics will all
contribute to an increasingly wet scenario for Puerto Rico and the
U.S. Virgin Islands. Rainfall amounts should be highest on
Saturday, but rain is expected in many areas both early and late
in the week.
.SHORT TERM...Today thru Tuesday...
For the rest of the overnight and early morning hours, passing
showers with brief gusty winds can be expected across the local
waters, as well as along portions of the north and east coastal
sections of the islands. During the afternoon hours, the
prevailing east southeast winds should steer diurnally induced
shower activity across the interior and northern half of Puerto
Rico including portions of the San Juan Metro area. Some areas of
heavy rainfall will also be possible in the northwest. Mainly
isolated showers are expected across the U.S virgin islands during
the afternoon.
The mid to upper level ridge will continue to erode as a broad
polar trough will become amplified and move eastward across the
west and central Atlantic through Tuesday. The subtropical jet
will round the base of this trough Monday through Tuesday placing
the region on the divergent and unstable side of the jet max.
Gradually increasing pooling of moisture and low level convergence
is still forecast beginning later this evening through the early
part of the upcoming week. This is in response to the frontal
boundary northwest of the region and an associated induced trough
which will develop and lift across the region Monday through
Tuesday. This overall pattern will result in the easterly winds
becoming more south southeasterly by Monday, then east southeast
once again on Tuesday. Weather conditions will become fairly moist
and unstable on Monday through Tuesday, with favorable conditions
aloft for increased shower activity and enhanced convective
development across the forecast area.
&&
.LONG TERM...Wednesday through Monday 27 March
On Wednesday a weak high pressure ridge will be found in place
enabling easterly trade winds of moderate to fresh intensity to
continue. That ridge will give way before a strong high pressure
moving out of the eastern United States late Thursday. It will
give another push to the front to our northwest and reinvigorate
the easterly trade winds again through the upcoming weekend. This
will maintain the flow of moisture over the area. An upper level
trough will move from Cuba on Wednesday to just north of Puerto
Rico on Sunday but will flatten out considerably as it does so.
The associated sub-tropical jet just north of us will remain
north of us during the entire period but will destabilize the
atmosphere around us on account of our being in the divergent
right entrance region. Moisture will have peaked on Tuesday but
diminishes only a little on Wednesday and Thursday and will rise
considerably late Thursday and early Friday due to the moisture
flow mentioned earlier. The GFS has been consistent in bringing
precipitable water with values of 1.8 to 2.0 inches early Friday
through beyond Monday of the following week. This will bring a
considerable amount of rain to the area should the forecast hold.
It is too early to determine if conditions are conducive for flash
flooding but localized urban and small stream flooding are
certainly possible with the forecast conditions. We continue with
isolated thunderstorms during the week as the inversion is
expected to be removed by Wednesday. The atmosphere becomes very
moist in nearly the entire column beginning Thursday and cloudy or
mostly cloudy conditions should prevail into early next week.
&&
.AVIATION...Mostly VFR conds at all terminals til 19/15Z. SFC
wnds lgt/vrb becmg E-SE 10-15 kts aft 19/14z...except for local
sea breeze variations. Til 19/15z...Isold to Sct passing SHRA ovr
the coastal waters btwn PR and the nrn Leewards islands...SCT-BKN
lyrs nr FL025...FL050... FL100...with VCSH at most of the local
terminals except TJMZ. Mtn top obscr ovr E PR til 19/14z. Fm
19/17z-19/22z... SHRA dvlpmnt psbl ovr ctrl mtn range and NW PR
with VCSH at TJBQ/TJMZ/TJSJ with MVFR psbl.
&&
.MARINE...Winds and seas are diminishing and all small craft
advisories should be down by 2 AM AST Monday. The system now in
the western Atlantic and its associated cold front will send
northerly swell into the area on Wednesday. Seas may reach 7 feet
in the Atlantic waters and passages. Then the newly refreshed cold
front to our north will begin to send much stronger swell into
the area Friday night. Seas of 10 feet or more are still possible
with this system.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 86 73 85 75 / 30 40 50 50
STT 83 73 82 72 / 50 50 50 50
&&