Area Forecast Discussion
National Weather Service San Juan PR
505 AM AST Tue Oct 19 2021
.SYNOPSIS...Slow moving showers and thunderstorms are expected
once again today in a light ENE steering wind flow. Winds will turn
more from the east and increase slightly each day till the end of
the week. Normal to above normal precipitable water (PWAT)
content will continue over the region through at least Thursday.
A mid/upper level ridge pattern is expected to promote drier air
and more stable conditions aloft late in the week into the weekend.
.SHORT TERM...Today through Thursday...
Moisture persists over the region, much as it has for the last
several days, though there is a patch of slightly drier air over the
region this morning. Based on the 0Z sounding and satellite
estimates, PWAT values remain within seasonal normals, generally in
excess of 1.7 inches, a marked decrease compared to previous days.
And yet still more than sufficient to support shower activity. Winds
are increasing over the area; cloud layer mean winds are likely to
be above 5 knots for the first time since Friday. It`s a very
marginal improvement, but an improvement nonetheless. Showers are
still going to be slow-moving, leading to locally high rainfall
accumulations, much like has been seen in prior days, and has
contributed to flooding. The steering flow is east-northeasterly to
northeasterly today, and so rain is expected over the same general
areas again today. Showers will develop during the early to mid
afternoon over the interior of Puerto Rico/Cordillera, then lazily
drift into southern/southwestern Puerto Rico. As such, there remains
an enhanced risk for flooding, mostly urban and small stream
flooding, but flash flooding is possible, as are rapid river rises
Tomorrow, moisture increases somewhat across the area. As the
frontal boundary to the north pulls away, replaced by a ridge
pushing off of the US east coast and building into the central
Atlantic, winds will continue to increase, moreso as we make our way
through midweek into the second half of the week. That said, showers
are still not going to be making any great haste tomorrow, either.
The increase in steering flow will also contribute to moisture
convergence expected over the area. Even with a "dry slot" ahead of
an approaching tropical wave, PWAT values will be at least within
the range of seasonal normals. Additionally, the ridging that has
been in place aloft will erode, making conditions more favorable for
convective activity. The steering flow will become more east-
southeasterly, however, and so the peak in showers and thunderstorms
tomorrow will be over the interior of Puerto Rico into the west and
northwest. The threat for flooding remains, especially for the
A tropical wave is expected to make its way across the region late
Wednesday into Thursday. Widespread shower activity is expected. It
is quickly followed by another, though weaker, tropical wave. There
is a small dry slot between the waves, which could lead to some of
the afternoon convection being inhibited, as it is forecast to be
over the islands in the afternoon. That being said, the dry slot is
likely to be squeezed between these two waves, to the point where it
effectively meets its demise if not over the CWA then not terribly
far to the west. So the impacts of the dry air will be brief, and
the decrease in moisture may not be particularly impressive. As
such, another wet, active day is expected for Thursday. And
therefore also the corresponding enhanced risk for flooding.
.LONG TERM...Friday through Tuesday...
A drying trend is expected during the long term period, as drier
mid-level air filters from the east on Friday and lingers through
early next week. This is in response to a building mid to upper
level ridge over the northeastern Caribbean. A weak Saharan Air
Layer is expected to reach the area during this period. In
addition, more stable conditions are expected, due to warmer 500
mb temperatures. This should inhibit widespread convective
development across the islands, and decrease the potential for
thunderstorm development. At the surface, a broad ridge across the
central Atlantic should promote moderate to locally fresh trades,
with a tropical wave currently forecast to pass well south of the
area on Sunday, having little to no impact in moisture content
across the local area. Another tropical wave, with better moisture
content may reach the area on Tuesday.
Having said that, expect showers to develop in a diurnal pattern
over western PR each afternoon, and across the USVI and eastern
sections of PR during the overnight/early morning hours due to
passing trade wind showers with mostly light rainfall amounts.
Brief isolated thunderstorms could form in the afternoon hours as
the trade wind cap is not forecast to be strong enough at this
.AVIATION...Generally VFR conditions expected across all terminals
next 24 hours. Afternoon convection likely over interior into
southern/southwestern PR. VCSH for TJBQ, and VCTS for TJPS. Cannot
rule out tempo MVFR or worse, mostly for TJPS between 19/18Z and
19/22Z. Passing showers overnight causing VCSH, with SHRA possible,
for TJSJ/TIST/TISX. Winds generally out of the E to near 10 knots
after sunrise, subsiding after sunset.
.MARINE...A small northerly swell will move across the Atlantic
waters later today. However, seas are forecast to remain at 3
feet or less, and the risk of rip currents will continue low
across the islands today. Light winds will prevail once again from
the east-northeast. Winds and seas are forecast to gradually
increase during the second part of the workweek, and another long
period northerly swell is forecast to move across the Atlantic
waters and passages by late Friday into Saturday. The combination
of wind driven seas and the swell will cause choppy seas up to 6
feet and a high risk of rip currents along the northern beaches of
the islands through the weekend.
.PRELIMINARY POINT TEMPS/POPS...
SJU 90 78 88 77 / 20 40 60 60
STT 89 78 88 78 / 50 50 50 50