National Weather Service San Juan PR
416 AM AST Mon Jan 22 2018
.Synopsis...Frontal boundary and associated shear line will remain
north of the region as it dissipates slowly today through Tuesday.
Mid to upper level ridge will hold across the region for the next
several days. Surface high pressure north of the area will maintain
moderate east to northeast trade winds across the forecast areas
through the period. A cold front and associated boundary is forecast
to sink southwards and approach the region by the upcoming weekend.
.SHORT TERM...Today through Wednesday...
Clear to partly cloudy skies prevailed across the local islands
overnight. A dissipating frontal boundary remains across the Atlantic
waters well northeast of the local islands. Terminal Doppler Radar
indicated only few showers across the Atlantic waters as well as across
the surrounding waters of the northern U.S. Virgin Islands. Not precipitation
was detected elsewhere across the region overnight and so far this morning.
A surface high pressure located across the Atlantic waters just to the
north of the region will continue as the main weather feature across
the local islands for the next couple of days. Trade winds will continue
to transport patches of low level moisture across the local islands
from time to time during the next couple of days. Small patches of
low level moisture embedded in the trades will continue to produce
brief passing showers across the U.S. Virgin Islands and eastern Puerto
Rico from time to time, especially overnight and early in the mornings
for the next couple of days. These showers will move quickly westward
with minimal rainfall accumulations. During the afternoons, some cloudiness
with showers are expected to develop across the interior and west sections
of Puerto Rico. The GFS model guidance suggests an increase in low level
moisture by the middle of the week.
.LONG TERM...Thursday through Tuesday of next week.
The upper level high pressure ridge will hold across the region through
Friday. However the prevailing easterlies will continue to transport
sufficient moisture across the region to support periods of passing
late evening and early morning showers, as well as isolated to scattered
afternoon convection. Saturday through Sunday, a polar trough will amplify
and spread eastward across the west and central Atlantic causing a slight
erosion of the upper level ridge. By Monday and Tuesday the mid to upper
level ridge will reestablish over the region and again limit shower
development.
In the meantime, the a cold front is forecast to move across the west
Atlantic and reach the local area by the upcoming weekend. Model guidance
continued to suggest a gradual increase in trade winds moisture during
the latter part of the week with more favorable conditions for moisture
convergence and shower development over the weekend, as the cold front
sinks southwards and stalls just north of the region by Sunday. Improving
weather conditions are however expected by Monday and Tuesday with a
return of more typical passing trade wind showers.
&&
.AVIATION...Mainly VFR conditions are expected across the local
flying area through the forecast period. Little to no shower activity
is expected until 22/15Z. Thereafter, SHRA will begin to develop affecting
mostly TJMZ between 22/18Z and 22/21Z. Easterly winds expected to increase
to about 15 kts with sea breeze variations developing after
22/14Z.
&&
.MARINE...A northerly swell and and moderate to locally fresh trades
winds will affect the local waters today but will continue to slowly
subside and diminish today through Tuesday. Due to the rough and
choppy seas, small craft advisories will remain in effect today for
the Atlantic, local passages and offshore Caribbean waters. Easterly
winds will prevail between 15 to 20 knots. For beach goers, the
high surf advisory has been cancelled as the breaking waves are
now expected to be less than 10 feet. However, high risk of rip
currents will continue for the northwest to north facing beaches
of Puerto Rico, Culebrita beach and also for Saint Thomas.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 74 84 75 / 30 30 30 30
STT 84 75 84 74 / 30 20 40 20