Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 220136 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
936 PM AST Sat May 21 2022

.UPDATE...The nose of the inversion in the latest sounding from
San Juan, Puerto Rico at 22/00Z was less pronounced. Hence, when
heating comes in the afternoon tomorrow for western Puerto Rico,
there will be even less resistance then, than there was today to
convective activity. Nevertheless, despite better 850 mb
moisture, overall moisture may be just a little less than today,
so will keep POPs lower over the local waters, but good over west
central Puerto Rico and include isolated thunderstorms as we had
today. Fortunately, accumulations under the thunderstorms will be
significant even if not widespread. Only minor changes to grids
tonight.

&&

.AVIATION...VFR conds at all sites tonight. Isol SHRA dvlpg aft
22/10Z ern PR. Then aft 22/17Z SHRA/Isol TSRA dvlp wrn central PR
with lcl mtn obscurations and MVFR/IFR conds. Altho CIGS psbl blo
030 TAF sites are expected to remain VFR thru 22/21Z. Sfc winds
ENE-E less than 10 kt in land breezes bcmg aft 22/13Z E 10-20 kt
with hir gusts in sea breezes alg north and south coasts. Max
winds NNW 60-70 kt btwn FL420-470 dcrsg aft 22/12Z.

&&

.MARINE...No changes to marine grids or forecast discussion.

&&

.PREV DISCUSSION... /issued 420 PM AST Sat May 21 2022/

SYNOPSIS...
A Saharan Air Layer continues across the local islands, but dust
concentrations will begin to diminish by tomorrow. Afternoon
activity is expected to increase in coverage tomorrow for the
interior and western Puerto Rico. A small northerly swell will
cause high rip current risk along the beaches of the north-central
coast of Puerto Rico.

SHORT TERM...Tonight through Monday...

Diurnal heating was strong enough to break the cap inversion and
resulted in isolated thunderstorms along the central and western
interior. This activity is expected to taper off before sunset as
temperatures cool down. In the big picture, a surface high
pressure over the western Atlantic continues to generate a wind
flow out of the east, with a low level steering flow around 15
knots. A Saharan Air Layer is embedded in these winds, causing
hazy skies. However, the aerosol models show the dust should begin
to diminish by late tomorrow.

At the mid-levels, a ridge holds over the western Atlantic,
with very dry air between 850 to 400 mb. This ridge will begin to
erode in response to a mid to upper level trough approaching from
the northeast. As this happens, conditions aloft could become a
little more favorable for showers and isolated thunderstorm
formation, but focused in the afternoon hours and across the
interior and western Puerto Rico. Moisture will continue to be
limited however, with precipitable water values of 1.3 inches on
Sunday and 1.4 on Monday (slightly below the climatological
value). In terms of temperatures, the thicknesses from 1000-500mb,
as well as the 925 mb temperatures will drop a little by early in
the workweek, which could lead to temperatures just a tad lower
for Monday, although heat indices will still approach or surpass
the 100 degrees mark mainly for the northern coast of Puerto
Rico.

LONG TERM...Tuesday through Saturday.../from prev discussion/

A lingering TUTT-low will promote a long wave trough pattern from
the central Atlantic into the northeastern Caribbean through much
of the long-term period. A surface low pressure over the Central
Atlantic will relax the local pressure gradient, weakening the
local winds. A surface high pressure moving from North America
into the Western Atlantic will tight the local pressure gradient
increasing the winds after Wednesday onward.

A seasonal weather pattern will dominate the local weather
conditions next week. The seasonal weather pattern consists of
passing showers with periods of moderate to locally heavy rains
overnight and early in the morning across the Virgin Islands and
eastern Puerto Rico. This activity will be followed by afternoon
convection across the interior and western sections during the
afternoon, where isolated thunderstorms could develop each day.
Urban and small stream flooding will remain possible with the
heaviest activity.

AVIATION...VFR conditions will prevail through the forecast period.
Easterly winds will continue through 21/22Z at around 10-15 KT with
occasional gusts and sea breeze variations. Haze, associated with
Saharan dust particles, will keep the visibilities across the flying
area between 6-9 SM through the day, improving tonight. Winds will
be between 5-10 KT overnight, BUT WILL INCREASE ONCE AGAIN AFTER
22/13Z.

MARINE...
The nearshore buoys show increasing swell period along the north
coast of Puerto Rico due to an arriving small northerly swell.
These swell should continue to affect the area through late
tomorrow, although with seas remaining 5 feet or less. Winds will
be out of the east at 10 to 15 knots, with stronger gusts.

There is a high rip current risk for many of the north facing
beaches of Puerto Rico from Arecibo to Carolina.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  89  76  88  77 /  10  20  20  30
STT  86  77  86  76 /  10  10  10  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk until 6 AM AST Sunday for San Juan and
     Vicinity.

     High Rip Current Risk through Sunday afternoon for North Central.

VI...None.
AM...None.
&&

$$

SHORT TERM...WS
LONG TERM....ERG
PUBLIC DESK...GRS


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