Areal Flood Advisory 101, NWS JSJ

Area Forecast Discussion
National Weather Service San Juan PR
1013 AM AST Fri May 28 2021

.UPDATE...A surge of low-level moisture is currently moving
between the U.S. Virgin Islands and eastern PR. Cloudiness and
showers were noted through the morning hours across the USVI,
Vieques, Culebra and the eastern half of Puerto Rico. For this
afternoon, this surge in moisture in combination with the sea
breeze convergence and daytime heating will result in shower and
thunderstorm development once again across portions of the
interior and western PR, and as well as downwind from el Yunque
into San Juan and vicinity. The main threat will be urban and
small stream flooding, and localized flash flooding can not be
ruled out due to slow-moving showers(steering winds are 5-10 kts
from the east). No change needed from the previous forecast.

&&

.AVIATION...Mainly VFR conditions are expected to prevail across
all the terminals through much of the forecast period. However,
SHRA/TSRA developing this afternoon over mainland PR may result in
tempo MVFR conditions across the PR terminals fm 16z-22z. The 12z
SJU sounding indicated east winds up to 14 kt blo 5000 ft.

&&

.MARINE...A small northerly swell will continue across the
Atlantic waters through at least tonight. This will cause a
moderate risk of rip currents along the northern beaches of
Puerto Rico and Culebra. Light to moderate southeast trades will
prevail today. Seas should range up to 5 feet across the offshore
waters and up to 4 feet across the nearshore waters of the
islands.

&&

.PREV DISCUSSION... /issued 430 AM AST Fri May 28 2021/

SYNOPSIS...A seasonal weather pattern, dominated by overnight
and early morning showers moving over windward areas from the
waters followed by enhanced afternoon showers and isolated
thunderstorms clustering along the interior, will prevail during
the next few days. Since soils are saturated, this could result
in urban and small stream flooding and possible mudslides near
areas of steep terrain. A slightly wetter pattern is expected for
the next work week following a similar shower pattern. Tranquil
marine conditions will prevail through early next week.

SHORT TERM...Today through Sunday...

Showers continued over the nearshore Atlantic waters around
central Puerto Rico and also around Saint Croix in the Caribbean.
A few of these showers were momentarily heavy, but were brief.
Fredericksted reported only traces.

Today, upper levels will see the approach of a weakening ridge
between several cut-off lows. The first low was the one that passed
on Thursday, the next is over the Bahama Islands. That low is
weakening and is expected to pass well north of the area on Sunday
morning. At mid levels, high pressure will come to dominate the west
central Atlantic over the next several days. At the surface the
pressure pattern changes very little with high pressure maintaining
a presence just west of the Azores. This high will drift further
west by the end of the weekend. Low level flow will alternate
between east and east southeast to maintain normal to slightly above
normal temperatures area wide, and slightly warmer on Puerto Rico's
northern coast. Although precipitable water values will remain
fairly steady, patches of mostly low level moisture will cause
fluctuations between 1.4 and 1.75 inches. This in turn will cause
showers move onshore each day after midnight on the east coast, and
showers and thunderstorms in the west and central portions of Puerto
Rico. Passing showers will occur in the U.S. Virgin Islands but
accumulations will be light and generally brief. Heavy amounts will
occur over Puerto Rico during the afternoon periods but amounts will
be light on the south coast and near the coast on the northern
coasts. 500 mb temperatures will cool about a degree and a half
today through Sunday which could lead to somewhat more vigorous
thunderstorms, but convective activity will still have moisture as
the limiting factor. Nevertheless, a weaker wind flow will allow
heavy rains to linger longer in some areas that would produce urban
and small stream flooding.

A slight improvement in visibility values is expected now through
Tuesday as Atlantic air begins to replace Saharan Air over the local
area and the amount of dust in the air decreases.

LONG TERM...Monday through Saturday...

Model guidance continues to suggest a near to above normal
moisture supporting a generally seasonal shower pattern. That is,
periods of overnight and morning showers moving from the waters
over eastern Puerto Rico and the US Virgin Islands, followed by
locally and diurnally induced shower development across the
interior and western sections of Puerto Rico and portions of the
San Juan metropolitan area. Nevertheless, slight variations in the
intensity, spread, and frequency of the activity can be expected
due to fluctuations in moisture content and the evolution of mid
to upper-level conditions, as well as the timing of key driving
features streaming across the region.

At the surface, high pressure over the north-central Atlantic will
maintain a generally moderate easterly flow through the period.
Under this flow, the passage of various weak easterly waves along
with a gradual increase in the available moisture is expected. For
Monday, mid to upper-level ridging will maintain a trade wind
inversion over the region. This may be strong enough to inhibit
organized deep convection, but not strong enough to suppress
shallow development. Even so, intense diurnal heating may aid in
the development of a few isolated thunderstorms in the afternoon.
On Tuesday and continuing through the rest of the workweek into
the weekend, an amplifying trough to the west will push the ridge
eastward and away from the local area. As a result, expect the
erosion of the trade wind inversion along with a significant
increase in tropical moisture as high as 2.10 inches by Friday
afternoon into Saturday. This scenario will support enhanced
shower and isolated thunderstorm development, resulting in the
potential for urban and small stream flooding and possible
mudslides near areas of steep terrain, especially with the
strongest activity each afternoon. Although global models have
favored a wetter pattern during the last few days and the forecast
follows this trend, there is still low to medium confidence on
whether it will fully materialize to the extent suggested by the
models.

AVIATION...Mainly VFR conditions are expected to prevail across all
terminals in the USVI but only until 28/16Z in PR. Aft 28/16Z
SHRA/TSRA will devlp ovr wrn and interior PR and downstream from El
Yunque causing areas of mtn obscurations and periods of MVFR conds.
This will most affect TJMZ/TJBQ with VCSH/VCTS at TJSJ/TJPS. Low
level winds will continue from the east up to 15 kts, with gusts to
25 kts in sea breezes, then less than 8 knots aft 29/01Z. Maximum
winds W 50-65 knots btwn FL380-480 strongest at FL450.

MARINE...Tranquil marine conditions with seas at 5 feet or below
and east winds up to 15 knots will prevail during the next few
days. There is a moderate risk of rip currents for north-facing
beaches of Puerto Rico and Culebra. Elsewhere, a low risk will
continue.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  88  78  90  77 /  40  40  30  30
STT  88  76  88  77 /  30  30  30  40

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

DSR/OMS/CRS

Perspectivas sobre Condiciones Peligrosas del Tiempo
Servicio Nacional de Meteorologia en San Juan PR
343 AM AST viernes 28 de mayo de 2021

AMZ710>745-PRZ001>013-280745-
San Juan y Vecindad-Noreste-Sureste-Este Interior-Norte Central-
Interior Central-Ponce y Vecindad-Noroeste-Oeste Interior-
Mayaguez y Vecindad-Suroeste-Culebra-Vieques-Las aguas costeras
cercanas y lejanas a la costa del Atlantico y Caribe-
343 AM AST viernes 28 de mayo de 2021

Esta perspectiva sobre las condiciones peligrosas del tiempo es para
Puerto Rico y las aguas costeras del Atlantico.

.Dia Uno...Hoy y esta noche

.Tronadas...Existe el potencial de tronadas con lluvias fuertes y
vientos en rafaga breves esta tarde a traves de sectores del oeste e
interior de Puerto Rico.

.Inundaciones...Lluvias fuertes pudieran resultar en inundaciones
urbanas y de riachuelos a favor del viento desde El Yunque y en el
oeste e interior de Puerto Rico.

.Corrientes Marinas...Existe un riesgo moderado de corrientes
marinas para las playas del norte de Puerto Rico desde Rincon hasta
Ceiba y Culebra.

.Dias dos hasta el siete...sabado a jueves

Con un aumento en la humedad durante la proxima semana, existe el
potencial de inundaciones urbanas y de riachuelos cada tarde en el
oeste e interior de Puerto Rico y a favor del viento desde El
Yunque. El riesgo moderado de corrientes marinas para las playas del
norte de Puerto Rico desde Rincon hasta Ceiba continuara durante los
proximos 7 dias. El riesgo de corrientes marinas en la costa sur de
Puerto Rico aumentara el jueves.

.Comunicado sobre informacion reportada por observadores...

No se espera activacion amplia de observadores. Favor de reportar
cualquier informacion sobre vientos, oleaje y corrientes marinas al
Servicio Nacional de Meteorologia en San Juan mientras siguen las
indicaciones de las autoridades locales y estatales.

AMZ715-725-VIZ001-002-280745-
San Tomas, San John e Islas Adyacentes-Santa Cruz-
Aguas cercanas del Atlantico y aguas costeras adyacentes del Caribe-
343 AM AST viernes 28 de mayo de 2021

Esta perspectiva sobre las condiciones peligrosas del tiempo es para
Islas Virgenes Estadounidenses y las aguas costeras adyacentes.

.Dia Uno...Hoy y esta noche

.Corrientes Marinas...Existe un riesgo moderado de corrientes
marinas para el extremo este de Santa Cruz esta noche.

.Dias dos hasta el siete...sabado a jueves

El riesgo moderado de corrientes marinas a lo largo de las costas de
Santa Cruz aumentaran a moderado temprano la proxima semana.

.Comunicado sobre informacion reportada por observadores...

No se espera activacion amplia de observadores. Favor de reportar
cualquier informacion sobre vientos, oleaje y corrientes marinas al
Servicio Nacional de Meteorologia en San Juan mientras siguen las
indicaciones de las autoridades locales y estatales.