The combination of an upper-level low west of the area and a tropical wave that provided deep moisture resulted in enhanced afternoon thunderstorm activity across the western half of Puerto Rico.
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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 an upper-level low west of the area and a tropical wave that provided deep moisture resulted in enhanced afternoon thunderstorm activity across the western half of Puerto Rico.
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 14, 12:13 PM AST, 3 hours 32 minutes earlier.
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.
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
CAMComunicado Especial sobre las Condiciones del Tiempo
Servicio Nacional de Meteorologia San Juan PR
227 PM AST martes 14 de septiembre de 2021
PRZ006-141945-
Interior Central-
227 PM AST martes 14 de septiembre de 2021
Una tronada fuerte impactara sectores de Orocovis, Coamo, Villalba,
Ciales y vecindad hasta las 346 PM AST...
A las 224 PM AST, el radar Doppler detecto una tronada fuerte cerca
de Orocovis. Esta tronada estaba casi estacionaria.
PELIGROS...Vientos en exceso de 30 mph y granizo pequeno.
RECURSO...Radar.
IMPACTOS...Rafagas de vientos pudieran derrumbar objetos sueltos.
Danos menores a objetos en el exterior son posibles.
MEDIDAS DE PRECAUCION/PREPARATIVOS...
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...0.25 PULGADAS
RAFAGA DE VIENTO MAXIMA...30 MPH
CAM/CVBSpecial Weather Statement
National Weather Service San Juan PR
247 PM AST Tue Sep 14 2021
PRZ008>010-142000-
Northwest PR-Mayaguez and Vicinity PR-Western Interior PR-
247 PM AST Tue Sep 14 2021
...A cluster of thunderstorms will impact portions of the western
interior, northwest and north central municipalities, especially
Utuado, Lares, San Sebastian Rincon and Aguadilla through 400 PM
AST...
At 245 PM AST, Doppler radar was tracking a strong thunderstorm near
Lares, moving north at 10 mph.
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 1848 6692 1849 6677 1833 6677 1831 6671
1833 6659 1830 6657 1828 6660 1824 6661
1825 6663 1816 6666 1822 6670 1830 6723
1837 6724 1841 6716 1845 6716 1852 6708
TIME...MOT...LOC 1845Z 167DEG 7KT 1836 6692
MAX HAIL SIZE...0.25 IN
MAX WIND GUST...40 MPH
CAMComunicado Especial sobre las Condiciones del Tiempo
Servicio Nacional de Meteorologia San Juan PR
247 PM AST martes 14 de septiembre de 2021
...Un grupo de tronadas impactara sectores del oeste interior, noroeste
y norte central, especialmente los municipios de Utuado, Lares, San
Sebastian, Rincon y Aguadilla hasta las 400 PM AST...
A las 245 PM AST, el radar Doppler detecto una tronada fuerte cerca
de Lares, moviendose hacia el norte a 10 mph.
PELIGROS...Vientos en exceso de 30 mph y granizo pequeno.
RECURSO...Radar.
IMPACTOS...Rafagas de vientos pudieran derrumbar objetos sueltos.
Danos menores a objetos en el exterior son posibles.
MEDIDAS DE PRECAUCION/PREPARATIVOS...
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...0.25 PULGADAS
RAFAGA DE VIENTO MAXIMA...40 MPH
CAM/ERGSpecial Weather Statement
National Weather Service San Juan PR
304 PM AST Tue Sep 14 2021
PRZ005-008-141945-
Northwest PR-North Central PR-
304 PM AST Tue Sep 14 2021
...A strong thunderstorm will impact portions of Arecibo, Hatillo,
and Manati Municipalities through 345 PM AST...
At 303 PM AST, Doppler radar was tracking a strong thunderstorm near
Utuado, moving north at 10 mph.
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...
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 1848 6659 1849 6656 1847 6645 1838 6644
1832 6669 1832 6671 1835 6673 1833 6677
1848 6677 1847 6670 1850 6663
TIME...MOT...LOC 1903Z 185DEG 7KT 1834 6671
MAX HAIL SIZE...0.00 IN
MAX WIND GUST...30 MPH
CAMComunicado Especial sobre las Condiciones del Tiempo
Servicio Nacional de Meteorologia San Juan PR
304 PM AST martes 14 de septiembre de 2021
...Una tronada fuerte impactara sectores en los municipios de
Arecibo, Hatillo y Manati hasta las 345 PM AST...
A las 303 PM AST, radar Doppler detecto una tronada fuerte cerca de
utuado, moviendose hacia el norte a 10 mph.
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...
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 MPHThe San Juan radar (TJUA), 70 km away, was scanning. The volume nearest the first event began at 19:42 UTC.
4 thunderstorm wind, 1 hail. Local standard time, as filed.
Emergency managers reported a fallen electrical post in Rio Abajo.
Emergency managers reported a fallen electrical post in Barrio Caonillas.
Fallen tree obstructing a road was reported in El Jobo, Lares.
Fallen tree was obstructing Ramal 111.
Small hail was reported in Barrio Caguana, Utuado.
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: 981349, 981350, 981351, 981353, 981352.