Low-level moisture combined with daytime heating and local effects to generate shower and thunderstorm activity across interior. Steering flow was very light, therefore, the activity that developed moved slowly.
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.
Low-level moisture combined with daytime heating and local effects to generate shower and thunderstorm activity across interior. Steering flow was very light, therefore, the activity that developed moved slowly.
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 21, 11:01 AM AST, 4 hours 59 minutes earlier.
Area Forecast Discussion
National Weather Service San Juan PR
1101 AM AST Mon Sep 21 2020
.UPDATE...Near normal moisture observed across Puerto Rico this
morning, while slightly higher than normal moisture across the
USVI. There is also an upper low to our northeast, putting us in
the subsident side of the upper low, while there is also an upper
ridge to our north and west. The 12Z sounding indicated that
relatively dry air is present between 850 and 500mb. The local
winds will prevail today from the south to southwest with a strong
sea breeze variation in winds, especially for coastal northern
PR, where winds with a northerly direction are forecast. Because
of the relatively dry air and subsidence aloft, some minor
adjustments were made to the forecast for today. The probability
of significant rainfall was reduced, but scattered showers with
some rainfall accumulation was kept, though flooding rains are not
expected based on the latest guidance and observations. That said,
the available moisture will increase slowly through the day and
into tonight, so there is a better chance of rain across the
Caribbean waters, USVI and south and eastern PR tonight.
&&
.AVIATION...VFR conds expected through the forecast period. Light
southerly winds expected across most terminals, but sea breeze
variations may cause winds at TJBQ to be VRB early, then from the
NW this afternoon. Also, the winds at TJSJ will be light and
variable this morning, possibly becoming NNE in the afternoon.
VCSH possible this afternoon and tonight.
&&
.MARINE...Seas continue to be hazardous into this afternoon due to
a NE swell. Small craft advisories are in effect until 2 PM AST as
seas will be up to 7 feet across the Atlantic and Mona Passage,
but seas will be up to 4 feet elsewhere. There is a high risk of
rip currents for northern PR, Culebra, St Thomas and St Croix.
&&
.PREV DISCUSSION... /issued 450 AM AST Mon Sep 21 2020/
SYNOPSIS...
Though showers are expected over the interior of Puerto Rico and
possibly the San Juan Metro, greater activity is expected for
tomorrow and Wednesday as increased moisture moves in. The biggest
story is that though the most recent swell will weaken throughout
today, another swell associated with Hurricane Teddy will arrive
tomorrow, and likely keep seas hazardous through the workweek. In
the long term, ridging aloft will limit the strength of
thunderstorms, though showers will still be expected.
SHORT TERM...Today through Wednesday...
The islands will be under a light southerly wind flow as a result
of Teddy, which is located well off to the north of the local
islands over the Atlantic Ocean. Maximum temperatures should
remain in the low 90s across the coastal and urban areas with heat
indices in the low to mid 100s degrees Fahrenheit. At upper
levels, a TUTT-low lingering around the Northeast Caribbean will
keep a relatively unstable atmosphere through at least Tuesday.
However, a short wave trough will swing by the region by mid-week,
destabilizing the atmosphere once again.
Mostly clear skies with warm and hot temperatures will dominate
weather conditions this morning. Under a light southwesterly
steering wind-flow, afternoon convection will focus over St Croix
and across and to the northeast of the Cordillera Central,
including the San Juan Metro Area.
Moisture from the Caribbean will move over the islands on Tuesday,
increasing the potential for showers and thunderstorm development.
Then, moisture from an old frontal boundary will move from the
Atlantic Ocean over the islands, and it will interact with a short
wave trough, diurnal heating, and local effects to produce another
round of afternoon convection over the islands. Scattered showers
with possible heavy thunderstorms are possible Tuesday and
Wednesday. However, convection may change its focus on Wednesday,
as winds change more from the east-southeast.
LONG TERM...Thursday through Tuesday...
Heading into the latter-half of the workweek, strong high pressure
will persist to our west at mid to upper levels with a cut-off
area of low pressure to our southeast. Though there will not be
much in the way of upper-level forcing to encourage thunderstorm
development, 500mb temperatures will still be decent, around -6 to
-7 for the whole long-term period. Thus, isolated thunderstorms
will be in the forecast during this period, but they ought not to
be too strong. At low levels, high pressure will primarily
persist over the eastern Atlantic, with weaker pressure gradients
over our region. Thus, fairly light winds are expected over our
region during the long-term period, so slow moving showers and
high rainfall accumulation's could be a threat. There are signs
of a strengthening high over the western to central Atlantic next
week.
Ample moisture from a weakening low will be with us for Wednesday
through Saturday. Precipitable water values will be above average of
around 1.8 to 2.0. With light winds, plenty of shower activity with
isolated thunderstorms can be expected, particularly over the
interior of Puerto Rico. The biggest threat is going to be areas of
isolated flooding under where the most persistent showers occur.
Thereafter, somewhat drier air is expected to move in, with low-
level wind speeds picking up a bit into the next week. Thus, the
next week is not expected to be as impactful given that the showers
should at least move along a bit more quickly.
AVIATION...
VFR conditions will prevail thru the forecast period. Expect
mostly clear skies with just a few passing clouds. Some
clouds/SHRA may develop during the afternoon and may impact JSJ
between 21/15-20z. Winds will remain calm to light/VRB. However,
gusty winds in/near showers, as well as sea breeze variations, are
possible.
MARINE...
The first swell associated with Hurricane Teddy will be
dissipating throughout the day. However, another swell will be
impacting the region tomorrow from the north, and this one is also
associated with Teddy. Small Craft Advisory criteria will once
again be met beginning tomorrow and this will last likely into
Friday. A High Surf Advisory may be needed as well.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 91 80 92 78 / 30 20 60 30
STT 89 81 87 80 / 20 50 30 20
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Tuesday afternoon for Culebra-
Mayaguez and Vicinity-North Central-Northeast-Northwest-San
Juan and Vicinity-Southeast.
VI...High Rip Current Risk until 6 PM AST this evening for St Croix-
St.Thomas...St. John...and Adjacent Islands.
AM...Small Craft Advisory until 2 PM AST this afternoon for Atlantic
Waters of Puerto Rico and USVI from 10 NM to 19.5N-Coastal
Waters of Northern Puerto Rico out 10 NM-Coastal Waters of
Northern USVI and Culebra out 10 NM-Coastal Waters of
Northwestern Puerto Rico out 10 NM-Mona Passage Southward
to 17N.
Small Craft Advisory from 8 AM Tuesday to 8 PM AST Thursday for
Atlantic Waters of Puerto Rico and USVI from 10 NM to 19.5N.
&&
SHORT TERM...JA
LONG TERM....CS
PUBLIC DESK...GLPerspectivas sobre Condiciones Peligrosas del Tiempo Servicio Nacional de Meteorologia San Juan PR 755 AM AST lunes 21 de septiembre de 2020 AMZ710>745-PRZ001>013-221200- 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 mar afuera del Atlantico y Caribe- 755 AM AST lunes 21 de septiembre de 2020 Esta perspectiva sobre las condiciones peligrosas del tiempo es para Puerto Rico y las aguas costeras del Atlantico. .Dia Uno...Hoy y esta noche .Oleaje...Oleaje picado de hasta 7 pies se espera a traves de las aguas del Atlantico y pasajes del Caribe. .Corrientes Marinas...Existe un riesgo alto de corrientes marinas para las playas desde el noreste al noroeste de Puerto Rico y Culebra. .Calor...Vientos del sur resultaran en indice de calor sobre lo normal en las localidades urbanas y costeras. .Dias dos hasta el siete...martes hasta el domingo Una marejada del noreste, mantendra alto el riesgo de corrientes marinas a lo largo de la costa del Atlantico hasta finales de la semana laboral. El oleaje aumentara nuevamente el martes y hasta el final de la semana laboral. Tronadas se desarollaran a traves de la zona Metropolitana de San Juan el martes. Tambien, tronadas podrian ser posibles cada tarde sobre el interior de la isla. Esto podria causar inundaciones urbanas y de riachuelos. Temperaturas maximas estaran alcanzando los bajos y medios 90s cada tarde. .Comunicado sobre informacion reportada por observadores... No se anticipa activacion 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-VIZ001-002-221200- San Tomas, San John e Islas Adyacentes-Santa Cruz-Aguas cercanas del Atlantico y aguas costeras adyacentes del Caribe- 755 AM AST lunes 21 de septiembre de 2020 Esta perspectiva sobre las condiciones peligrosas del tiempo es para Islas Virgenes Estadounidenses y las aguas costeras adyacentes. .Dia Uno...Hoy y esta noche .Oleaje...Oleaje picado de hasta 7 pies se espera a traves de las aguas del Atlantico y pasajes del Caribe. .Corrientes Marinas...Existe un riesgo alto de corrientes marinas para las playas expuestas al noreste de las Islas Virgenes. .Dias dos hasta el siete...martes hasta el domingo Una marejada del noreste, mantendra alto el riesgo de corrientes marinas a lo largo de la costa del Atlantico hasta finales de la semana laboral. El oleaje aumentara nuevamente el martes y hasta el final de la semana laboral. Temperaturas maximas estaran alcanzando los bajos y medios 90s cada tarde. .Comunicado sobre informacion reportada por observadores... No se anticipa activacion 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.
The national radar mosaic for the moment the first event began.
1 flood. Local standard time, as filed.
Emergency manager reported flooding in Calle Cibuco in the Urbanization of Brisas del Rio. Some water was getting into the houses nearby.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Puerto Rico in 2020Every state in 2020This day in weather historyWarnings in Puerto Rico, September 2020
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: 921116.