Areal Flood Advisory 3, NWS JSJ

Area Forecast Discussion
National Weather Service San Juan PR
500 PM AST Thu Mar 11 2021

.SYNOPSIS...Conditions are expected to become more unstable
Friday and Saturday as a mid and upper-level trough sets up just
north of the area. This along with near normal moisture will
result in shower and isolated thunderstorm activity during the
afternoon hours mainly across interior and western portions of
Puerto Rico. Weather conditions improve late in the weekend into
early next week as the trough aloft pulls away and a ridge aloft
builds in, resulting in drier and more stable conditions.

&&

.SHORT TERM...Tonight through Saturday...

Afternoon convection that developed over interior and western
Puerto Rico will diminish during the early evening hours with the
loss of daytime heating. Mostly fair weather conditions are
expected during the overnight hours, however, some passing showers
cannot be ruled over the local waters with some of the activity
affecting portions of the USVI and northeastern Puerto Rico.

Weather conditions Friday and Saturday are expected to become
more unstable as a mid and upper-level trough approaches from the
northwest. This feature will enhance the instability aloft with
500 mb temperatures cooling between -8 and -11 degrees Celsius. In
addition, an upper-level jet will set up over the area, providing
some divergence aloft. The steering flow is also expected to
diminish with low-level winds diminishing to less than 10 knots on
Friday and less than 5 knots on Saturday. The combination of
these features along with near-normal moisture combined with sea
breeze convergence and diurnal heating will result in enhanced
afternoon shower activity, mostly over interior and western Puerto
Rico. Given the cold 500 mb temperatures, isolated thunderstorms
are also possible with the afternoon convection. The light
steering flow will cause the shower activity to move very slowly,
therefore, areas of urban and small stream flooding cannot be
ruled out. Across the USVI, not much in the way of shower activity
is expected as the low-level forcing there will be very weak.

.LONG TERM...Sunday through Thursday...

.PREV DISCUSSION... /issued 401 AM AST Thu Mar 11 2021/

By Sunday and early next week, the upper short wave trough is
expected to weaken as it moves eastward into central Atlantic while
a mid to upper level ridge is forecast to move in from the west and
persists through the long term forecast. At low levels, the
associated surface high pressure over Western Atlantic will move
eastward and settle north of the area through the long term forecast
period. This will re-establish the trade wind cap with improving
weather conditions.

However, as the weather pattern transitions on Sunday, lingering
moisture along with locally induced effects will produce shower
development over the area, especially during afternoon hours. The
surface high pressure to the northwest will continue promoting an
east northeast wind flow over the region focusing shower activity
over the southwestern quadrant of Puerto Rico. Weather conditions
aloft will be less conducive for thunderstorm development than
previous days according to both GFS and ECMWF models, therefore
expect morning isolated to scattered showers over USVI and eastern
Puerto Rico following with scattered to numerous showers over
southwestern Puerto Rico in the afternoon with some showers over
the local waters as evening approaches. Minor urban and small
stream flooding as well as ponding of waters on roadways and in
poor drainage areas will remain possible in isolated areas.

Monday, relatively fair weather conditions can be expected as the
surface high pressure moving eastward north of the region pushes
drier air across much of northeastern Caribbean. GFS suggests
precipitable water values to drop close to an inch or below
throughout the day. However, isolated showers cannot be ruled out.
Patches of moisture from the remnants of the upper short wave trough
well northeast of the region will be advected from time-to-time by
the easterly trades. A strong cap is forecast to remain in place,
thus, the pulses of moisture will be shallow in nature. All in all,
expect mainly fair weather conditions with shallow areas of moisture
embedded in the easterly wind flow reaching the islands from time-to-
time causing light passing showers across PR and USVI.

&&

.AVIATION...VFR conds are expected at all TAF sites thru 12/16Z.
Some SHRA have formed in wrn PR but are not expected to last byd
arnd 11/23Z. Sfc winds diminishing slowly from Thu to Fri being ENE
10-15 kt with gusts up to 25 kt...slightly higher alg the coasts in
sea breezes. Land breezes to commence arnd 11/23Z bcmg less than 8
kt by 12/03Z. Maximum winds W 55-70 kt btwn FL315-470 greatest at
FL430.

&&

.MARINE...Northerly swell will continue to result in choppy to
hazardous seas between 5 and 8 feet across the Atlantic waters and
local passages into Saturday. Across the rest of the local
waters, seas of less than 5 feet are expected. For beachgoers,
a high risk of rip currents continues for the north-facing
beaches of Puerto Rico and Culebra tonight through Friday.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  72  85  72  85 /  30  30  10  20
STT  72  84  71  84 /  40  30  10  30

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late Friday night for North
     Central-Northeast-Northwest-San Juan and Vicinity.

     High Rip Current Risk until 6 PM AST this evening for Mayaguez
     and Vicinity.

     High Rip Current Risk through Friday afternoon for Culebra.

VI...High Rip Current Risk until 6 PM AST this evening for St Croix-
     St.Thomas...St. John...and Adjacent Islands.

AM...Small Craft Advisory until 2 PM AST Friday for Anegada Passage
     Southward to 17N-Coastal Waters of Northern Puerto Rico out
     10 NM.

     Small Craft Advisory until 8 PM AST Saturday for Atlantic Waters
     of Puerto Rico and USVI from 10 NM to 19.5N.

     Small Craft Advisory until 8 AM AST Friday for Mona Passage
     Southward to 17N.

&&

SHORT TERM...CAM
LONG TERM....GL
PUBLIC DESK...FRG