Damaging Wind in Puerto Rico, August 16, 2023

Area Forecast Discussion
National Weather Service San Juan PR
1035 AM AST Wed Aug 16 2023

...New UPDATE, AVIATION, MARINE...

.UPDATE...Minor changes were introduced to forecast package. Dry
and stable weather conditions with hazy skies will continue to
prevail today. Moisture advection is expected Thursday into
Friday, increasing the chance for showers and thunderstorms
across the local islands. Major concern continue to be heat
indices above 108 degrees throughout the day today. Heat warnings
and advisories remain in effect for Puerto Rico and the US Virgin
Islands. Please refer to NPWSJU for details.

&&...//Issued at 947 AM AST Wed Aug 16 2023

.SYNOPSIS...The extensive Saharan Air Layer (SAL) will peak this
morning into the early afternoon, thus maintaining hazy conditions
with limited shower activity. The variable cloud cover and dense
suspended dust particulates during the overnight and early morning
hours, maintained minimum temperatures warmer than normal due to limited
radiative cooling. This warm spell and overall dry atmospheric conditions
will continue across most coastal and urban areas in Puerto Rico and
the U.S. Virgin Islands. Consequently, Excessive Heat Warnings and
Heat Advisories were required today for several areas between 10 am
and 5 pm AST. Breezy northeasterly winds will persist especially along
the coastal areas, then becoming more easterly Thursday into Friday.
Another easterly wind surge is forecast by Saturday. Moisture advection
will be limited across the region, however passing clouds and few tradewind
showers can be expected, especially across the coastal waters and windward
sections in the morning followed by limited afternoon shower development
mainly in west and southwest Puerto Rico.

&&

.SHORT TERM...Today through Friday...
Once again, passing showers over northern and eastern Puerto Rico,
as well as St. Croix overnight. Mainly hazy skies prevailed as a
denser dry air mass with Saharan Dust particles moved over the U.S.
Virgin Islands, Puerto Rico, Vieques, and Culebra. The wind flow was
generally around 5 and 10 mph from the east-northeast. Overnight low
temperatures remain in the low to mid-80s across coastal areas.
While at higher elevations, they stay in the mid to upper 70s. These
warmer low temperatures experienced at night were associated with
the dense haziness that did not allow most of the energy irradiated
by the sun during the day to be released.

Saharan Dust particles will continue to filter across the forecast
area today, decreasing visibility significantly. Dust aerosol
guidances now show optical depths around 0.40 and higher throughout
the day. As mentioned in previous discussions, this can exacerbate
respiratory conditions in immunocompromised, vulnerable, and
sensitive groups. The latest model guidance suggests prevailing
below-normal climatological levels of precipitable water across the
islands today. Any rainfall activity that does develop across the
islands should be minimal/not significant due to the dry air mass
and breezy conditions. The robust high-pressure system over the
Atlantic will continue to enhance east-northeasterly winds and
breezy conditions. This wind flow will push the afternoon rainfall
activity across the western-southwestern quadrant of P.R. Today,
expect daily maximum temperatures to range in the upper 80s to 90s
along the coastal areas, while maximum temperatures should remain in
the upper 70s to 80s along the higher elevations. Therefore, expect
another day with warm temperatures across the tropical islands.
During the day, we anticipate heat indices above 112 degrees
Fahrenheit along the north-central to northwestern, western,
southwestern, and south-central sections of Puerto Rico, as well as
Vieques, Culebra, and the U.S. Virgin Islands. Therefore, an
Excessive Heat Warning was issued for those areas. The expected heat
indices above 108 degrees prompted the issuance of a Heat Advisory
for northwestern, eastern, and southwestern Puerto Rico through 5 PM
AST today.

On Thursday, expect a gradual increase in precipitable water content
as a wind surge approaches the region. Most available moisture will
remain below 700 mb, with drier air aloft. Nonetheless, expect the
Saharan Dust concentrations to remain across the forecast area. At
this time, Thursday looks like the transition day from a dry to a
wet weather pattern.

By the end of the workweek, expect an increase in moisture generated
by a wind surge and a weak tropical wave. These weather features
should enhance showers and thunderstorm activity.

Overall, the Saharan Air Layer (SAL) will continue to affect Puerto
Rico and the U.S. Virgin Islands through the short-term forecast
period. Therefore, dry, hazy, and hot conditions should prevail on
Wednesday, with Thursday being the transition day from a dry to a
wet pattern on Friday.

&&

.LONG TERM...Saturday through Wednesday...
On Saturday into Sunday, fragments of moisture trailing a weak
tropical wave and the moderate east northeast trades will bring
morning showers to the coastal waters and windward side of the
islands from time to time. The activity will be of short duration
with no significant accumulation forecast at this time. lingering
moisture and good daytime heating will favor some enhanced showers
and isolated thunderstorms on Saturday and Sunday. Based on
recent model guidance, a drier airmass will gradually spread
across the region along with low level moisture erosion as the
mid to upper level ridge will build across the region and a strong
surface high pressure ridge will reestablish across the west and
central Atlantic. From late Sunday and during the rest of the
forecast period, weather conditions will continue into a drying
trend, with only periods of a patchy shallow passing low level
clouds and limited trade wind showers likely. The suggested
layered Precipitable Water values (PWAT) will range between 1.7
and 1.95 inches Saturday into early Sunday, then a rapid drying
trend is expected thereafter with values between 1.40-1.60 inches
by late Sunday into Monday and through Wednesday. Surface winds
and steering flow is forecast to become more east northeast by
that time. Regardless of the overall dry pattern expected, early
morning passing showers and isolated to scattered afternoon convection
will remain likely each day, as pockets of shallow moisture will
continue to stream across the region steered by the moderate to
locally strong easterly winds, and combined with good daytime
heating. Overall breezy conditions and excessive heat hazard risks
conditions are so far expected to persist through the long-term
forecast period, with the main impacts focused along the coastal
and urban areas. Significant urban and small stream flooding is
not anticipated during the periods but isolated areas of ponding
water on roads and in poor drainage areas cannot be ruled out
especially during the afternoon hours in western Puerto Rico.

&&

.AVIATION...VFR conds are expected to prevail at all TAF sites
with reduced visibilities due to Saharan Dust. Nevertheless,
visibilities are expected to remain P6SM. ENE winds at 15 to 20
knots with higher wind gusts, becoming at 12 knots or less
overnight.

&&

.MARINE...A surface high pressure in the central Atlantic will maintain
moderate to fresh easterly winds, causing choppy marine conditions.
A wind surge is expected by the end of the workweek, and may
generate hazardous seas across portions of the regional waters
with seas up to 7 feet and wind gusts near 30 knots.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>004.

     Excessive Heat Warning until 5 PM AST this afternoon for PRZ005-
     007-008-010>013.

VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ001-
     002.

AM...Small Craft Advisory from 8 AM Thursday to 8 PM AST Friday for
     AMZ711.

&&

UPDATE/AVIATION/MARINE...OMS

Perspectivas sobre las Condiciones Peligrosas del Tiempo
Nacional de Meteorologia en San Juan PR
443 AM AST miercoles 16 de agosto de 2023

AMZ711>745-PRZ001>013-170845-
San Juan y Vecindad-Noreste-Sureste-Este Interior-Norte Central-
Interior Central-Ponce y Vecindad-Noroeste-Oeste Interior-Mayaguez
y Vecindad-Suroeste-Culebra-Vieques-Aguas cercanas y lejanas a la
costa del Atlantico y del Caribe-
443 AM AST miercoles 16 de agosto de 2023

Esta perspectiva sobre las condiciones peligrosas del tiempo es para
Puerto Rico.

.DIA UNO...Hoy y esta noche

.Rayos...Es posible el desarrollo de tormentas electricas aisladas,
a traves de los municipios del oeste y suroeste de Puerto Rico. Este
atento a las tormentas electricas en su area y este preparado para
refugiarse.

.Calor Excesivo...Indices de calor en exceso a 112 grados para los
municipios del norte central, noroeste, oeste, suroeste y sur central
de Puerto Rico, Vieques y Culebra. Agotamiento por calor es probable
con una exposicion prolongada. Indices de Calor en encima de los 108
grados sobre el noreste, este y suroeste.

.Vientos...Condiciones de brisa fuerte. Los articulos no asegurados
podrian salir volando.

.Condiciones Maritimas...Las embarcaciones pequenas deben ejercer
precaucion a traves de las aguas mar afuera del Atlantico y el
pasaje de Anegada.

.Corrientes Marinas...Las corrientes marinas amenazantes a la vida
son posibles en la zona de olas rompientes, particularmente en las
playas del norte, este y sureste de Puerto Rico, Vieques y
Culebra.

.DIAS DOS A SIETE...jueves a martes

Se espera que condiciones brumosas prevalezcan para la mayoria del
periodo. Condiciones ventosas, oleaje picado y riesgo moderado de
corrientes marinas prevalecera. El riesgo de inundaciones aumenta
para el viernes.

.Comunicado sobre informacion reportada por observadores...

Se recomienda la activacion de los observadores. Transmita cualquier
informacion sobre inundaciones, vientos, oleaje y corrientes marinas
al Servicio Nacional de Meteorologia en San Juan.

AMZ716-726-VIZ001-002-170845-
San Tomas, San John y las Islas Adyacentes-Santa Cruz-Aguas cercanas
a la Costa del Atlantico y aguas costeras adyacentes del Caribe-
443 AM AST miercoles 16 de agosto de 2023

Esta perspectiva sobre las condiciones peligrosas del tiempo es para
las Islas Virgenes Estadounidenses.

.DIA UNO...Hoy y esta noche

.Calor Excesivo...Indices de calor de 112-114 grados a traves de las
Islas Virgenes. Es probable el agotamiento por calor con una exposicion
prolongada. El golpe de calor es posible.

.Vientos...Condiciones de brisa fuerte. Los articulos no asegurados
podrian salir volando.

.Condiciones Maritimas...Las embarcaciones pequenas deben ejercer
precaucion a traves de las aguas mar afuera del Atlantico y el
pasaje de Anegada.

.Corrientes Marinas...Las corrientes marinas amenazantes a la vida
son posibles en la zona de olas rompientes de St. Thomas y St. Croix.

.DIAS DOS A SIETE...jueves a martes

Se espera que condiciones brumosas prevalezcan para la mayoria del
periodo. Condiciones ventosas, oleaje picado y riesgo moderado de
corrientes marinas prevalecera. El riesgo de inundaciones aumenta
para el viernes.

.Comunicado sobre informacion reportada por observadores...

Se recomienda la activacion de los observadores. Transmita cualquier
informacion sobre inundaciones, vientos, oleaje y corrientes marinas
al Servicio Nacional de Meteorologia en San Juan.