National Weather Service San Juan PR
440 AM AST Tue Nov 30 2021
.SYNOPSIS...A seasonable weather will continue to prevail during the
next several days with overnight and early morning isolated to
scattered showers over the local waters affecting portions of
eastern Puerto Rico and the USVI followed by locally induced
afternoon showers over western Puerto Rico. A mid-level ridge
will continue to hold, hindering the development of deep,
organized convective activity and thus widespread, significant
rainfall activity is unlikely through the period. Choppy marine
conditions due to a northeasterly swell will continue across
portions of the Atlantic waters through mid-week.
&&
.SHORT TERM...Today through Thursday...
Mid level clouds continue to fade over Puerto Rico overnight as the
weak trough that brought showers and some thunderstorms to the
forecast area--mainly the local outer Atlantic waters--is now being
subsumed by a trough at 700 mb that is near 60 west and moving west.
This trough is expected to lose strength, but will move onshore over
the U.S. Virgin Islands and eastern Puerto Rico around midnight
Wednesday morning. Moisture from the top of a tropical wave moving
through the southern Caribbean will then reach the area on Wednesday
where it will spawn showers and isolated thunderstorms during the
day. Bands of moisture, none too distinct, will continue moving
through the area on the easterly flow. Low level flow at the surface
will be mainly driven by lower pressure over South America and the
higher pressure across the central Atlantic through the period.
Although a polar trough will travel east over the Atlantic several
hundred miles away, it is going to be too far north to have any
significant impacts over the local area. A mention of isolated
thunderstorms has been kept in the forecast for today and Wednesday
despite decreasing overall moisture due to 500 mb temperatures
lowering from less than minus 4 this morning to almost minus 6 on
Thursday afternoon according to the GFS. These may not occur if
local circulations to not render any support, however. Moisture
returns on Thursday afternoon and will give convection a boost over
western Puerto Rico.
The U.S. Virgin Islands will generally see fair conditions with only
isolated passing showers and limited rainfall, as synoptic features
remain weak.
.LONG TERM...Friday through Wednesday...
Not much has changed in the overall synoptic pattern for the long-
term period. A mid-level ridge will continue to remain well
entrenched across the northeast Caribbean through the period. This
will hinder the development of deep, organized convective activity
and thus the potential for significant, widespread rainfall
activity. Meanwhile, at lower-levels, a broad surface high
pressure is forecast to build across the eastern and central
Atlantic. This will tighten the local pressure gradient during the
upcoming weekend and thus the low-level winds will increase
somewhat, resulting an advective weather pattern with patches of
low-level moisture moving across the area. This will feature
overnight and early morning passing showers across the local
waters, with some of the activity moving into portions of eastern
Puerto Rico and the USVI followed by locally induced activity
during the afternoon hours across western Puerto Rico. By early
next week, there are hints amongst the model guidance that a frontal
boundary will move into the western Atlantic due to a
extratropical low pressure system moving off the northeastern
United States and into the northwestern Atlantic. This frontal
system, at this time, is expected to remain far enough north to
not have any direct impacts to our area with the only indirect
effect being weakening the local pressure gradient and thus
resulting in lighter winds.
&&
.AVIATION...VFR conds expected across all local terminals thru 30/20Z.
Brief passing SHRA ern PR aft 30/10Z, then aft 30/16Z SHRA ovr
wrn and interior PR with lcl MVFR and mtn obscurations till
30/23Z. Land breezes less than 10 kt thru 30/12Z, increasing to
10-15 kts from the E with sea breeze variations aft 30/14Z. Max
winds W 35-45 kts btwn FL400-510.
&&
.MARINE...A northeasterly swell between 11 and 14 seconds will
continue to affect the regional waters through mid-week. Seas are
expected to range between 3 and 5 feet with occasional seas of up
to 6 feet, with the highest seas expected mostly across the
offshore Atlantic waters. A high risk of rip currents continues
for the northwest to northeast beaches of Puerto Rico as well as
for the beaches of Culebra, Vieques and the USVI.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 87 75 86 76 / 30 50 20 30
STT 86 76 86 77 / 40 40 20 30