Special Weather Statement, NWS JSJ

Area Forecast Discussion
National Weather Service San Juan PR
1213 PM AST Tue Sep 14 2021

.UPDATE...Showers have become few in the local area but
thunderstorms persists in the vicinity of Saint Maarten. Cirrus
from earlier activity south of us is just clearing off of eastern
Puerto Rico revealing mostly sunny skies. Maximum temperatures are
also are already no more than 1 or 2 degrees below today's
forecast maximums and actually, Saint Croix has already reached
90 degrees. Latest sounding from San Juan at 14/12Z continues to
indicate very unstable conditions available to fuel showers and
thunderstorms across most of the area today with forecast CAPE
over 4000 J/Kg and a lifted index of minus 7. Will continue with a
forecast with good certainty of scattered to numerous
thunderstorms over western and interior Puerto Rico with some
showers lingering around the U.S. Virgin Islands. Temperatures
look good with heat indices to reach 100-106 degrees soon.

&&

.AVIATION...VFR til arnd 14/17Z when SHRA/TSRA dvlp in wrn and
interior PR with gusty winds to 30 kt and MVFR/IFR conds. Mainly
MVFR at TJMZ/TJBQ however. San Juan only expecting VCTS at this
time. Sfc winds 5-15 kt with gusts to 20kt exc nr TSRA. Max winds
SW at 25-35 kt btwn FL380-440. Aft 14/23Z sfc winds to bcm less
than 10 kts in land breezes. TSRA to linger over lcl waters.

&&

.MARINE...No changes to previous forecast. Seas will remain 5
feet or less next 7 days.

&&

.PREV DISCUSSION... /issued 718 AM AST Tue Sep 14 2021/

SYNOPSIS...
Another wet pattern is expected today with afternoon thunderstorm
activity over the forecast area. As a result, a moderate threat
of flooding is expected today. By Thursday, conditions will
improve as less moisture and some Saharan dust filters into the
region limiting the shower activity. For the long term period,
expect shower and thunderstorm activity due to an upper level
trough moving over the area. There is some uncertainty for the
latter part of the long term period. Tranquil marine conditions
will prevail with seas up to 5 feet or less across the local
waters.

SHORT TERM... Today through Thursday...

There is added uncertainty to the short term forecast, as the
model guidance have begun to diverge a little from each other.
The higher resolution models are a bit more bullish in the
rainfall coverage and amounts than the global models. Moreover,
the rainfall that has occurred over the past few days has caused
some areas to have saturated soils, which will cause quicker
runoff with significant rainfall over the next few days. That
said, the forecast itself has not changed much for today, we still
expect an unsettled and rainy pattern, as a tropical wave moves
in today into early Wednesday, but perhaps with less rainfall and
cloud cover on Thursday. The upper trough that has caused
thunderstorms over the past few days will continue to cause
enhanced thunderstorm potential today, but will have less of an
impact on Wednesday and Thursday as the trough continues moving
west and an upper ridge starts to move in. In terms of moisture,
the above normal precipitable water will continue through the
short term period, with values ranging from around 1.8 to 2.15
inches, probably peaking late tonight into early Wednesday with
the trailing edge of the tropical wave. Also, the mid level
temperatures are slightly colder than normal, and are expected to
range between -7 and -8 today and Wednesday, which would
contribute to enhanced thunderstorm potential. So, overall we
still have the ingredients for showers and thunderstorms over the
area, but the upper level support will be better today than on
Wednesday and Thursday if the models are correct. Therefore, the
forecast for today has showers with isolated to scattered
thunderstorms across several sectors of PR, with a risk of urban
and small stream flooding. The most significant showers and
thunderstorms over Puerto Rico are expected to be across the
northwestern quadrant of the island in the afternoon hours. The
U.S. Virgin Islands could observe moderate showers and nearby
thunderstorms as well, but the rainfall is expected to cause
mainly ponding of water on roadways and poor drainage areas. Most
of the activity near the USVI is expected to be in the morning
hours, with a brief break in the early afternoon, then passing
showers and isolated thunderstorms again in the late afternoon and
in the evening hours.

With the trailing edge of the tropical wave, the shower and
thunderstorm activity on Wednesday morning is expected to be mainly
over the local waters, southern and eastern PR and around the USVI.
Becoming less rainy over PR, although lingering showers persist over
the USVI as Wednesday progresses. However, that's according to
the global models, while the higher res models still favor plenty
of activity over land areas in PR into the afternoon hours.
Because of this uncertainty, we split the difference in the
models, putting showers and thunderstorms in the forecast but with
modest PoPs to account for uncertainty. Thursday looks slightly
drier, with Saharan dust moving in, but possible significant
rainfall over localized areas, while brief scattered showers are
forecast over the local waters and around the USVI.

The daytime high temperatures are forecast to be near 90 degrees
once again today across the lower elevations of PR and the USVI,
with heat index values in the low 100s. However, given the amount of
moisture and possible cloudiness expected today, it would not be
surprising for the daytime high temps to peak in the upper 80s at
the San Juan and Saint Thomas airport, similar to what was
observed yesterday.

LONG TERM...Friday through Tuesday...

On Friday, according to both models (GFS and ECMWF) a high
pressure that was reflected in all the levels will dominate the
weather pattern. Meanwhile, at the surface, several patches of
low-level moisture with precipitable water values above
climatological normals will move into the area. As a result,
afternoon convection over the interior and western sections of
Puerto Rico is expected. Late Saturday into Sunday, a more east-
northeasterly wind flow will dominate the area and will push some
fragments of low level moisture, with precipitable water values
from 1.6 to 1.9 inches over the area. Shower activity is expected
over the local area, as the available moisture combines with a
Tropical Upper Tropospheric Trough (TUTT) moving southwest.
Additionally, mid level temperatures are expected to range from
-6 to -7 Celsius, which is slightly below normal, and could
contribute to enhanced thunderstorms potential. So far, if the
model guidance verifies, the best time for widespread activity is
Sunday and Monday.

For Tuesday, there is still uncertainty with this forecast, as
the different model guidance is not in agreement with the
position of the upper level trough. Nevertheless, an increase in
tropical moisture is expected with a wind surge from an
approaching tropical wave, currently being monitored by the
National Hurricane Center with a medium chance of formation (50%)
in the next 48 hours and a high chance (80%) of formation in the
next 5 days.

AVIATION...SHRA over the local area is causing VCSH or VCTS across
the USVI terminals early this morning, while VCTS is forecast at
TJBQ after 14/16Z. Winds will be light and VRB through 14/13Z, but
gusty near the SHRA/TSRA, otherwise winds will increase to around 10-
15KT from the east. Sea breeze variations in wind direction expected
at TJPS after 14/13Z. Overall, passing SHRA/TSRA are expected across
the local flying area today and tonight.

MARINE...Relatively tranquil marine conditions will prevail for
the next several days across all of the local waters and passages
with seas up to 5 feet. Wind flow will be mostly from east up to
15 knots. For today, isolated showers and thunderstorm clusters
are forecast over the local waters. There is a moderate risk of
rip currents for the north exposed beaches of Puerto Rico, as well
as the beaches located over the eastern tip of Culebra, Vieques,
and St. Croix.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  90  78  88  78 /  50  60  50  40
STT  89  78  87  79 /  40  50  60  40

&&

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

SHORT TERM...OM
LONG TERM....WS
PUBLIC DESK...TW

Perspectivas sobre las Condiciones Peligrosas del Tiempo
Servicio Nacional de Meteorologia en San Juan PR
352 AM AST martes 14 de septiembre de 2021

AMZ710>745-PRZ001>013-150800-
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-
352 AM AST martes 14 de septiembre de 2021

Esta Perspectiva sobre las Condiciones Peligrosas del Tiempo es para
Puerto Rico y las aguas costeras adyacentes del Atlantico.

.Dia Uno...Hoy y esta noche

.Tronadas...Existe el potencial de tronadas con lluvia fuerte y
rafagas de vientos breves a traves de porciones del area hoy.

.Inundaciones...Lluvias fuertes podrian resultar en inundaciones
urbanas y de riachuelos al igual que deslizamientos en areas de
terreno empinado.

.Corrientes marinas...Existe un riesgo moderado de corrientes
marinas para algunas de las playas del norte de Puerto Rico al igual
que las playas del este en Vieques y Culebra.

.Calor...Se esperan indices de calor elevados entre 100 y 106 grados
a traves de la mayoria de las areas bajas, con los indices mas altos
a traves de las areas costeras del norte de Puerto Rico.

.Dias dos hasta el siete...miercoles a lunes

Humedad tropical profunda y una vaguada en los niveles altos al
oeste del area continuara resultando en aguaceros y tronadas
generalizadas el miercoles. Inundaciones urbanas y de riachuelos al
igual que deslizamientos en areas de terreno empinado seran posibles
con la actividad de lluvia mas fuerte y persistente. Indices de
calor elevados seran probables durante los proximos dias.

.Comunicado sobre informacion reportada por observadores...

No se espera activacion amplia de observadores. Favor de reportar
cualquier informacion sobre inundaciones, 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-VIZ001-002-150800-
San Tomas, San John e Islas Adyacentes-Santa Cruz-Aguas cercanas
del Atlantico y aguas costeras adyacentes del Caribe-
352 AM AST martes 14 de septiembre 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.

.Tronadas...Existe el potencial de tronadas con lluvia fuerte y
rafagas de vientos breves a traves del area, especialmente esta manana.

.Inundaciones...Lluvias fuertes podrian resultar en acumulacion de
agua en carreteras y en areas bajas.

.Corrientes marinas...Existe un riesgo moderado de corrientes marinas
para las playas del este en Santa Cruz.

.Dias dos hasta el siete...miercoles a lunes

Humedad tropical profunda y una vaguada en los niveles altos al
oeste del area continuara resultando en actividad de aguaceros
dispersos y tronadas aisladas hasta el miercoles. Acumulacion de
agua en carreteras y en areas bajas seran posibles. Indices de calor
elevados seran probables durante los proximos dias.

.Comunicado sobre informacion reportada por observadores...

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

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Special Weather Statement
National Weather Service San Juan PR
227 PM AST Tue Sep 14 2021

PRZ006-141945-
Central Interior PR-
227 PM AST Tue Sep 14 2021

...A strong thunderstorm will impact portions of Orocovis, Coamo,
Villalba Ciales and the Vicinity Municipalities through 345 PM AST...

At 224 PM AST, Doppler radar was tracking a strong thunderstorm near
Orocovis. This storm was nearly stationary.

HAZARD...Winds in excess of 30 mph and pea size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 1833 6636 1837 6631 1833 6628 1832 6621
      1828 6621 1824 6627 1819 6626 1815 6622
      1810 6622 1802 6633 1804 6643 1808 6644
      1808 6651 1815 6653 1815 6664 1817 6665
      1830 6657 1833 6659 1834 6658 1838 6642
TIME...MOT...LOC 1824Z 134DEG 4KT 1819 6643

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...30 MPH

CAM