National Weather Service San Juan PR
456 AM AST Tue Nov 17 2020
.SYNOPSIS...
An increase in shower and thunderstorms activity is
forecast for today. Passing showers are expected to continue to
move from the waters to eastern and northern sections of the local
islands in the morning hours. Then, in the afternoon hours
afternoon convection is expected over northwestern Puerto Rico.
Ponding of waters over roadways could not be ruled out with the
heaviest showers. Tranquil marine conditions will prevail for
today with seas between 3 to 5 feet and winds up to 15 knots.
&&
.SHORT TERM...Today through Thursday
Tutt Low and associated deep layered trough east of the region
will continue to sink southwards across the Lesser Antilles while
gradually filling over the next few days. The upper level ridge
has shifted slightly west of the region but will maintain a
northerly wind flow aloft while keeping the forecast area on the
subsident and stable side the aforementioned upper trough. A
surface high pressure ridge has shifted eastward into the north
central Atlantic while a broad induced deep layered trough
continues across the northeastern Caribbean. This pattern will
maintain an east southeast low level wind flow across the region
through the period. An easterly disturbance will continue to cross
the region today accompanying a surge of low level moisture. This
will be followed by a weak tropical wave also forecast to move
across the region Wednesday into Thursday.
For the rest of the early morning hours, patches of low level clouds
with embedded showers will continue to affect the coastal waters
while brushing parts of the islands from time to time. Some showers
will produce periods of moderate to locally heavy rains that may
cause ponding of water on roadways and in poor drainage areas as well
as brief hazardous driving conditions. Significant rainfall
accumulations are however not expected as the shower activity will be
fast moving.
Skies will become mostly sunny to partly cloudy the rest of today.
However, the available pooling of low level moisture along with
daytime heating and local effects will give way to afternoon shower
development and possibly isolated thunderstorms mainly over parts of
the central interior and west section of Puerto Rico. Shower
activity over the U.S. Virgin Islands should be limited to mainly
portions of the west end of the islands or just downwind over the
coastal waters. Some afternoon showers may again affect parts of the
San Juan metro but the activity will also be of short duration.
Minor urban and small stream flooding as well as ponding of waters
on roadways and in poor drainage areas will be possible in isolated
areas with the heaviest rains mainly over the west sections of
Puerto Rico.
Recent model guidance initialized will and is in phase with the
present and expected weather pattern. A prevailing east southeast
wind flow is forecast to persist through the period with model
guidance suggesting increasing layered precipitable water to between
1.70 to 2.0 inches today through Thursday. This along with the
amplified upper trough and cooler 500 millibar temperatures east of
the region should enhance shower development and consequently induce
isolated thunderstorm development over the coastal waters between
eastern Puerto rico and the U.S. Virgin Islands during the overnight
and early morning hours. Therefore, still expect good potential for
early morning showers and daytime convection for the rest of the
period. That said, the potential for minor urban and small stream
flooding as well as ponding of waters on roadways and in poor
drainage areas will remain possible each day in isolated areas with
the heaviest rains. Near seasonal temperatures should persist across
the region through Thursday.
.LONG TERM... Friday through Wednesday
At the surface, abundant tropical moisture from the last tropical
wave are forecast to linger over the region as a surface low
pressure build over the western Atlantic. According to the GFS
model guidance plenty of moisture will prevail at 850 mb
increasing the potential for more widespread shower activity for
the weekend. In the upper levels, a building upper level trough
(TUTT) will sink into the south inducing a surface trough at our
north. Generally speaking, a unsettle weather pattern is expected
on Friday until through at least Sunday. Therefore, urban and
minor flooding is expected in the areas with the heaviest showers.
In terms of temperatures, a more warmer daytime temperatures are
expected on Sunday as a more southeasterly wind flow dominate the
region.
On Tuesday, the TUTT will be in a better position for the
instability aloft over the area. This will enhance the potential
for shower and thunderstorms activity over the areas. Especially
on Tuesday when a more cooler temperatures are forecast at 500 mb
around -8 Celsius. The limiting factor for a more widespread
showers will be the lack of moisture over the area. As a building high
pressure move over the area with a more somewhat drier air filter
into the region on late Tuesday into Wednesday. That said, less
showers activity with mostly partly cloudy skies were prevail for
the last day of the long term period.
&&
.AVIATION...
VFR conds will prevail durg prd. VCSH at most terminals
with SHRA psbl at TJSJ/TISX/TIST/TNCM and VCTS at TKPK til 17/14Z.
SCT-BKN lyrs nr FL025...FL050 en route btw islands and ovr coastal
waters. Fm 17/17Z-17/22Z VCSH/Isold TSRA psbl at TJBQ/TJMZ/TJSJ. Sfc
wnds lgt/vrb bcmg fm E at 10-15 KT with sea breeze variation and hir
gusts btw 20-25 KT aft 17/14Z. L/lvl wnd fm ESE 15-25 kts BLO FL200
then backing and incr w/ht ABV.
&&
.MARINE...
Relative tranquil marine conditions are expected for today with moderate
easterly winds across the regional waters. Seas are expected to
be generally between 3 and 5 feet over all the regional waters. A
increase in shower and thunderstorms activity is expected this
afternoon as a tropical wave move across the northeastern
Caribbean. Seas will become hazardous on Friday when a northeast
swell enters the area.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 86 76 87 77 / 50 40 50 20
STT 85 76 85 75 / 50 60 50 40