The combination of near normal moisture, light steering winds from Hurricane Sam passing well northeast of the area, and local and diurnal effects resulted in active afternoons across portions of interior and western on September 28th and 29th.
Every event with a place to put it. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS SJU wrote it up.
The combination of near normal moisture, light steering winds from Hurricane Sam passing well northeast of the area, and local and diurnal effects resulted in active afternoons across portions of interior and western on September 28th and 29th.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS San Juan, PR wrote its last area forecast discussion before the first event at Sep 28, 12:03 PM AST, 2 hours 57 minutes earlier.
Area Forecast Discussion National Weather Service San Juan PR 1203 PM AST Tue Sep 28 2021 .UPDATE...Fair weather prevailed for most of the morning across the local area, with only isolated showers across the local waters. Late in the morning, cloudiness started increasing across the southern slopes and central Puerto Rico. The shower activity is expected to gradually develop across these areas early in the afternoon, while scattered showers are developing across the Caribbean waters and near the USVI. Showers and thunderstorms are still expected across central and western PR, which may cause urban and small stream flooding. No changes were introduced to the forecast. && .AVIATION...VFR conds expected to prevail across the local terminals. However, developing SHRA/TSRA could affect areas near the local terminals this afternoon. TJBQ has the highest chance of TSRA between 28/17 and 28/21, but it is also possible at the rest of the local terminals for brief periods. Winds will remain light and from the SE with sea breeze variations for the rest of the afternoon. && .MARINE...Tranquil seas expected today, becoming choppy starting late tonight, and then hazardous starting late on Wednesday. This is due to a swell coming in from hurricane Sam, which is expected to remain hundreds of miles NE of the local islands. No changes in the marine forecast. && .PREV DISCUSSION... /issued 415 AM AST Tue Sep 28 2021/ SYNOPSIS...Diurnally induced afternoon convection is expected to develop each day over portions of the interior of PR through the end of the week. An upper level trough west of the area will promote instability on Wednesday. Light steering winds will cause slow moving showers, increasing the potential for urban and small stream flooding. A tropical wave is forecast to increase moisture content through the weekend. Weather conditions are expected to improve early next week, due to a building ridge and drier air intrusion. Swells generated by Hurricane Sam will cause hazardous seas and rough surf conditions late in the workweek. SHORT TERM...Today through Thursday... Patches of moisture continue to filter in over the area from time to time. This morning, there is a patch making its way westward over the southern half of the CWA, and is presently located over/around St. Croix. Based on satellite estimates, this patch has precipitable water values in the neighborhood of 1.8 to 2.0 inches or more. There has been widespread isolated to scattered shower activity through the night, with stronger and more concentrated activity, as well as some thunderstorms, associated with this area of enhanced moisture. Occasional showers pushing onshore will continue this morning, with a typical pattern of shower activity dominating the region throughout the coming days. Steering flow remains light over the area; as such, showers will be generally slow-moving. Portions of interior Puerto Rico could see locally high rainfall amounts from afternoon convection because of this. Aloft, conditions are becoming more favorable for supporting convective development, as the mid-level ridge continues to erode, giving way to some weak troughiness over the area today. Tomorrow, this increase in instability continues. A strong deep-layer trough extending south from over northern Quebec/Labrador this morning will gradually shift southward and then eastward, to be over New England/the Canadian Maritimes by Thursday, when it will begin slowly moving away. This trough will extend to just west of the region on Wednesday, supporting additional increases in instability, providing further support to convection over the region. For Thursday, model guidance suggests decreasing moisture. There is a region of dry air just west to southwest of Hurricane Sam, as is visible on satellite imagery. It is likely that we will see at least some drying effects from this airmass during the storm's passage well to the northeast of the islands. That being said, there will be sufficient moisture in the low levels to support shower activity. The influence of Hurricane Sam will also lead to additional weakening of the steering flow over the area, and the trend continues - slow-moving showers largely affecting the interior of Puerto Rico during the afternoon causing locally significant rainfall, causing an enhanced risk for urban and small stream flooding. Forecast confidence is generally high for today and tomorrow, but is lessened on Thursday, due to the impacts of Hurricane Sam's passage to the northeast. LONG TERM...Friday through Tuesday... As Hurricane Sam lifts further north on Friday, a low-level trough is forecast to develop over the area, and winds are expected to turn from the east-southeast, favoring diurnal convection along the interior and northern slopes of PR. Urban and small stream flooding can be expected with the heaviest showers. The weak southeasterly steering wind flow should promote warmer than normal temperatures. During the weekend, a tropical wave is forecast to cross the region, increasing moisture content and the areal coverage of showers across the islands with isolated thunderstorms expected to develop across the local waters and over portions of the islands each day. Therefore, the flood threat will remain elevated through the weekend. Moisture content is expected to quickly erode by early next week, as a ridge builds from the east, promoting drier air and more stable conditions. However, typical afternoon convection cannot be ruled out over the western interior of PR, and from streamers downwind of the USVI. Moderate to fresh trade winds are expected to return during this period in response to the building ridge to our northeast. AVIATION...Generally VFR conds are expected across all terminals for the next 24 hours. SHRA/TSRA activity near TISX has diminished, MVFR conds no longer likely. VCSH after 28/18Z for TJBQ; cannot entirely rule out VCTS, but most activity will likely be S/SE of terminal, outside of vicinity. Winds generally east to 10 to 12 knots, with sea breeze variations, after 28/14Z. Winds subside after sunset, becoming light. MARINE...Seas up to 4 feet are expected to prevail today, gradually building between 6-8 feet by Thursday across the northern and eastern waters of the islands. Light trade winds will continue to prevail around 10 knots or less through much of the workweek. There is a low risk of rip currents across all the islands today, as the primary wave direction is from the east to southeast with the 4 ft seas across the Atlantic waters. However, deteriorating and hazardous seas are expected by late Wednesday night through Friday, due to swells generated by Hurricane Sam. These swells will also cause rough surf conditions and life-threatening rip currents along the east and north facing beaches of the islands. && .PRELIMINARY POINT TEMPS/POPS... SJU 90 78 89 78 / 30 10 30 40 STT 88 78 88 77 / 20 30 30 40 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && SHORT TERM...JA LONG TERM....CVB PUBLIC DESK...ERG
Perspectivas sobre las Condiciones Peligrosas del Tiempo Servicio Nacional de Meteorologia en San Juan PR 449 AM AST martes 28 de septiembre de 2021 AMZ710>745-PRZ001>013-280900- 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- 449 AM AST martes 28 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 aisladas con lluvia fuerte y rafagas de vientos breves esta tarde a traves de sectores del interior de Puerto Rico. .Inundaciones...Lluvias fuertes podrian resultar en inundaciones urbanas y de riachuelos. .Calor...Se esperan indices de calor entre 102-107 grados a traves del norte central de Puerto Rico y sectores del este y sur de Puerto Rico. .Dias dos hasta el siete...miercoles a lunes Se esperan aguaceros y tronadas en las tardes durante la semana causando un riesgo elevado de inundaciones urbanas y de riachuelos mayormente para el interior de Puerto Rico. Se espera que el deterioro en las condiciones maritimas comience manana, asociado a marejadas generadas por el Huracan Sam a medida que pase muy al noreste de la region. Se anticipan condiciones maritimas y costeras peligrosas para la segunda mitad de la semana. .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-VIZ001-002-290900- San Tomas, San John e Islas Adyacentes- Santa Cruz-Aguas cercanas del Atlantico y aguas costeras adyacentes del Caribe- 449 AM AST martes 28 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 .Inundaciones...Lluvias fuertes pudieran resultar en acumulacion de agua en carreteras y en areas de pobre drenaje. .Corrientes Marinas...Existe un riesgo moderado de corrientes marinas para las playas del este de Santa Cruz esta noche. .Dias dos hasta el siete...miercoles a lunes Se espera que el deterioro en las condiciones maritimas comience manana, asociado a marejadas generadas por el Huracan Sam a medida que pase muy al noreste de la region. Se anticipan condiciones maritimas y costeras peligrosas para la segunda mitad de la semana. .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.
Special Weather Statement
National Weather Service San Juan PR
225 PM AST Tue Sep 28 2021
PRZ008>010-281915-
Northwest PR-Mayaguez and Vicinity PR-Western Interior PR-
225 PM AST Tue Sep 28 2021
...A strong thunderstorm will impact portions of Lares, San
Sebastian, Utuado, Adjuntas and Moca Municipalities through 315 PM
AST...
At 225 PM AST, Doppler radar was tracking a strong thunderstorm over
Lares. This storm was nearly stationary.
HAZARD...Winds in excess of 30 mph.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
If outdoors, consider seeking shelter inside a building.
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 1818 6675 1833 6713 1841 6709 1830 6670
TIME...MOT...LOC 1825Z 210DEG 1KT 1827 6687
MAX HAIL SIZE...0.00 IN
MAX WIND GUST...30 MPH
GLComunicado Especial sobre las Condiciones del Tiempo
Servicio Nacional de Meteorologia San Juan PR
225 PM AST martes 28 de septiembre de 2021
...Una tronada fuerte afectara sectores de los municipios de Lares,
San Sebastian, Utuado, Adjuntas y Moca hasta las 315 PM AST...
A las 225 PM AST, el radar Doppler estaba siguiendo una tronada
fuerte sobre Lares. Esta tormenta estaba casi estacionaria.
PELIGROS...Vientos en exceso de 30 mph.
RECURSO...Radar.
IMPACTOS...Rafagas de vientos pudieran derrumbar ramas de arboles y
objetos sueltos.
MEDIDAS DE PRECAUCION/PREPARATIVOS...
Si se encuentra en el exterior, considere buscar refugio dentro de
un edificio.
Lluvias torrenciales estan ocurriendo con esta tronada y pudieran
causar inundaciones localizadas. No conduzca su vehiculo por
carreteras inundadas.
Rayos frecuentes de nube a suelo estan ocurriendo con estas tronadas.
Los rayos pueden afectar hasta 10 millas lejos de la tronada. Busque
lugar seguro dentro de un edificio o vehiculo. Vientos en rafagas
hasta 10 mph son posibles con estas tormentas.
&&
TAMANO MAXIMO DE GRANIZO...0.00 PULGADAS
RAFAGA DE VIENTO MAXIMA...30 MPH
GL/ICPThe national radar mosaic for the moment the first event began.
3 flood, 3 debris flow. Local standard time, as filed.
Flooding due to poor-drainage reported along Road PR-159 near Urbanization Brisas del Rio.
Mudslide reported on Road PR-111 near San Sebastian.
Flooding reported along Road PR-114 towards Road PR-102.
Urban flooding reported along Road PR-111 in the town of Utuado.
Several mudslides reported along Road PR-611 between KM 1 and KM 5.8.
Several mudslides reported along Road PR-124 in Barrios Buena Vista and Bucarabones.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Puerto Rico in 2021Every state in 2021This day in weather historyWarnings in Puerto Rico, September 2021
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 981421, 981423, 981424, 981427, 981430, 981435.