Feeder band moisture and low-level convergence from Tropical Storm Peter, which was located well north of the area, resulted in widespread shower and thunderstorm across the 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.
Feeder band moisture and low-level convergence from Tropical Storm Peter, which was located well north of the area, resulted in widespread shower and thunderstorm across the 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 21, 4:27 PM AST, 2 hours 33 minutes earlier.
Area Forecast Discussion
National Weather Service San Juan PR
427 PM AST Tue Sep 21 2021
.SYNOPSIS...
Decreasing moisture will lead to a gradual decrease in shower
activity tonight through midweek. Still, there will remain
sufficient moisture to support afternoon convection each day, in
northern and interior Puerto Rico tomorrow, and in northwestern and
interior Puerto Rico Thursday into the end of the week. Marine
conditions will improve overnight tonight and tomorrow.
&&
.SHORT TERM...Tonight through Thursday...
Drier air is beginning to make its way into the region from the
south/south-southeast. Based on Total Precipitable Water imagery
from GOES-16, an air mass with PWAT less than 1.4 inches is entering
the CWA. Though occasional patches of moisture will help sustain
some showers, activity will decrease, on the whole, across the
region.
During the afternoon tomorrow, patches of moisture will traverse the
region, supporting afternoon convection, driven by local and diurnal
effects. With winds remaining southerly/south-southeasterly, the
focus of convection will again be northern and interior Puerto Rico.
Warmer conditions are also expected tomorrow, with temperatures into
the 90s, especially for northern parts of Puerto Rico, and this
will likely continue on Thursday.
Winds will back slightly during the day on Thursday, becoming more
southeasterly. With dry air persisting over the area, less shower
activity is expected; local and diurnal effects will place the focus
of the activity that does occur over northwestern and interior
Puerto Rico.
.LONG TERM...Friday through Tuesday...
The dry slot in precipitable water will end on Friday as moisture
lingering from Tropical Storm Peter converges to our east in a
band around an upper level low just north of the area. This
moisture will peak overnight Sunday and then diminish as an area
of subsidence ahead of another tropical system moves over the area
by Tuesday. Low level winds during the period are expected to be
10 to 15 knots. Local effects and good heating are expected to
generate showers and thunderstorms in the west and interior
portions of Puerto Rico each afternoon, with some late night and
early morning showers each day. Models are fairly consistent on
the formation of another tropical system southwest of the Cape
Verde Islands by Sunday and indeed the National Hurricane Center
has given the system an 80 percent chance of formation in the next
5 days. Models are in poor agreement, however, on the track or
time of closest approach of the system next week, but the Tropical
Cyclone will definitely be "a system of interest" late this
month.
&&
.AVIATION...
Mainly VFR conditions expected to prevail across all terminals.
However, SHRA/TSRA over the interior and northern PR, and between
the USVI and Atlantic waters may cause tempo MVFR to brief IFR
conds at TJSJ/TIST thru 21/22z. Winds will continue light and
variable, however, a southerly component should prevail through
Wednesday.
&&
.MARINE...
Hazardous conditions linger for the offshore Atlantic waters, with
seas of up to 7 feet. Elsewhere in the northern waters of the CWA,
seas of up to 5 to 6 feet are possible; in the rest of the local
waters, seas of generally 3 to 4 feet or less are anticipated.
Improving conditions are expected overnight and tomorrow, as
Tropical Storm Peter continues to pull away from the region.
There is a high risk of rip currents for beaches along the north
central and northwest coast of Puerto Rico. Elsewhere, there is a
moderate risk for north-facing beaches for most islands, as well as
beaches of eastern St. Croix and Culebra.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 77 92 78 92 / 50 40 20 30
STT 79 88 79 88 / 50 40 30 30
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR... High Rip Current Risk through Wednesday afternoon for Northwest.
High Rip Current Risk through late tonight for North Central.
VI...None.
AM...Small Craft Advisory until 2 AM AST Wednesday for Atlantic
Waters of Puerto Rico and USVI from 10 NM to 19.5N.
&&
SHORT TERM...JA
LONG TERM....CRS
PUBLIC DESK...RVTHazardous Weather Outlook...CORRECTED National Weather Service San Juan PR 506 AM AST Tue Sep 21 2021 AMZ710>745-PRZ001>013-220845- San Juan and Vicinity-Northeast-Southeast-Eastern Interior- North Central-Central Interior-Ponce and Vicinity-Northwest- Western Interior-Mayaguez and Vicinity-Southwest-Culebra-Vieques- The nearshore and off shore Atlantic and Caribbean Coastal Waters- 506 AM AST Tue Sep 21 2021 This Hazardous Weather Outlook is for Puerto Rico and the adjacent Atlantic Coastal Waters. .Day One...Today and Tonight .Thunderstorms...There is a chance of thunderstorms with heavy rainfall and brief gusty winds across portions of the area today. .Flooding...Heavy rains could lead to urban and small stream flooding as well as ponding of water on roads and in poor drainage areas. .Waves...Choppy/Rough seas up to 7 feet expected across the offshore Atlantic waters. .Rip Currents...There is a high risk of rip currents for some beaches across north central Puerto Rico. Elsewhere, there is a moderate risk of rip currents for north and east facing beaches in Puerto Rico, including Vieques and Culebra. .Heat...Maximum heat indices between 100 and 106 degrees possible in urban and coastal areas of Puerto Rico this afternoon. .Days Two through Seven...Wednesday through Monday Afternoon thunderstorm activity expected at least through Monday and therefore there will remain a slight threat for urban and small stream flooding mainly over western PR. The slightly elevated heat threat is expected to continue over the forecast area with maximum heat index values in the low to mid 100s at least through Monday. A moderate risk of rip currents will remain for most of the northern and eastern local beaches through the weekend. .Spotter information statement... Spotter activation is not anticipated. Please relay any information about winds, waves, and rip currents to the National Weather Service in San Juan while following all local and state guidelines. AMZ715-725-VIZ001-002-220845- St. Thomas St. John adjacent Islands-St Croix- Nearshore Atlantic and adjacent Caribbean Coastal Waters- 506 AM AST Tue Sep 21 2021 This Hazardous Weather Outlook is for the U.S. Virgin Islands and the adjacent Coastal Waters. .Day One...Today and Tonight .Thunderstorms...There is a chance of thunderstorms with heavy rainfall and brief gusty winds across the local islands today. .Flooding...Heavy rains could lead to urban and small stream flooding as well as ponding of water on roads and in poor drainage areas. .Rip Currents...There is a moderate risk of rip currents for northern and eastern local beaches in St. Croix as well as western beaches in St.Thomas today. .Heat...Maximum heat indices in the low 100s possible across the eastern tip of St. John this afternoon. .Days Two through Seven...Wednesday through Monday The slightly elevated heat threat is expected to continue over the forecast area with maximum heat index values in the low 100s at least through Saturday. All local beaches will have a low risk of rip currents on Wednesday, but will become moderate on Thursday through the weekend. .Spotter information statement... Spotter activation is not anticipated. Please relay any information about winds, waves, and rip currents to the National Weather Service in San Juan while following all local and state guidelines.
The San Juan radar (TJUA), 34 km away, was scanning. The volume nearest the first event began at 23:02 UTC.
7 debris flow, 3 flood, 2 flash flood. Local standard time, as filed.
Flooding reported along Avenida El Comandante.
Flooding reported along Calle Lira.
Flooding reported along Road PR-140 KM 21.
Mudslide reported along Road PR-111 KM 55.9.
Mudslide reported on Road PR-605 Barrio Vivi Arriba, Sector La Cuchilla.
Mudslide reported along Road PR-807 in Barrio Dos Bocas 1.
Mudslide reported on Road PR-159 Sector El Tipico. This caused a tree to fall, affecting powerlines.
Two mudslides reported on Road PR-159 near El Yagrumo Restaurant.
Mudslide reported in Barrio Las Palmas Road PR-140 between KM 19.3 and 20.5.
Mudslide reported along Road PR-111 KM 66.3.
Urban flooding reported along Road PR-2 in the center of the town. People were trapped inside vehicles.
Woman was swept away in her vehicle as she drove the vehicle through a small stream that went out of its banks along Road PR-160 Sector Almirante Norte. Woman was abled to be rescued.
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: 981366, 981368, 981369, 981370, 981371, 981372, 981374, 981376, 981378, 981379, 981380, 981381.