In the morning hours on July 20th, 2021, slow moving thunderstorms developed overnight ,one specific cluster formed over the Northshore that led to significant flooding, with areas of flooding elsewhere from developing clusters of thunderstorms through the morning hours. These storms formed within a rich Gulf, unstable airmass out ahead of an approaching outflow boundary and slow moving cold…
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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 LIX wrote it up.
In the morning hours on July 20th, 2021, slow moving thunderstorms developed overnight ,one specific cluster formed over the Northshore that led to significant flooding, with areas of flooding elsewhere from developing clusters of thunderstorms through the morning hours. These storms formed within a rich Gulf, unstable airmass out ahead of an approaching outflow boundary and slow moving cold front from the west.
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 New Orleans, LA wrote its last area forecast discussion before the first event at Jul 19, 5:34 PM CST, 7 hours 45 minutes earlier.
Area Forecast Discussion National Weather Service New Orleans LA 634 PM CDT Mon Jul 19 2021 .AVIATION UPDATE... Updated for 00z TAF issuance. && .PREV DISCUSSION... /issued 430 PM CDT Mon Jul 19 2021/ SHORT TERM... Stubborn active summertime pattern will continue throughout the work week with a gradual drying trend expected by the end of the week. Upper level troughing over New England in place through the early part of the week with a massive ridge sitting over the western third of the country. PW value will continue to be around 2 inches plus at least through Thursday. A frontal boundary will continue to slowly sag south into northern Louisiana and northern Mississippi through Tuesday. The front will stall and remain over the forecast area through the middle and latter part of the week. The boundary will provide lift along with high moisture contents. POPS will range from 80 to near 100 through at least Thursday. Tuesday looks to be the wettest day with low convective temperatures around 80. Highest PW values expected on Tuesday and Wednesday along a stalled boundary which will also provide the highest lift. Temperatures could struggle to get much above 80 for highs on Tuesday as cloud cover and rainfall are expected throughout much of the day. A gradual return to more typical summertime conditions as 594 upper level ridge tries to slowly build in from the west by the end of the week. Temperatures could climb into the lower and middle 90s by by the end of the weekend with partly to mostly sunny skies and POPS ranging from 40 to 50 mainly during the afternoon and early evening. AVIATION... Mostly VFR conditions are expected for the terminals for most of the forecast period outside of any convection that directly impacts any one terminal. There may be some thunderstorms later than the usual diurnally driven ones during the day later tonight in particular. There also may be a period of some lower MVFR ceilings for the inland terminals. -BL MARINE... Winds will remain out of the south SW at 5 to 10 knots during the day and 10 to 15 knots overnight with 1 to 3ft seas before shifting northwest by late week. Ridging associated with high pressure continues to build in from the east through the coastal waters and should help keep waves and seas at or below 1 to 2 ft. With the heavy rainfall possible each day including over the coastal water and lakes, be cautious that there may be some brief periods of spiked winds and gusts over areas affected by strong showers and storms at that time. Otherwise, expect winds and wave heights to continue to be fair through the forecast period. /TDB/ && .PRELIMINARY POINT TEMPS/POPS... MCB 71 82 71 87 / 70 90 70 90 BTR 74 84 72 87 / 70 90 70 90 ASD 74 85 74 88 / 70 90 70 90 MSY 77 86 77 89 / 70 90 70 90 GPT 75 85 75 87 / 90 90 70 80 PQL 74 85 74 87 / 80 90 60 80 && .LIX WATCHES/WARNINGS/ADVISORIES... LA...None. GM...None. MS...None. GM...None. &&
The national radar mosaic for the moment the first event began.
11 flash flood. Local standard time, as filed.
Water entering several homes. many roads impassable and barricaded.
A social media video showed at least a dozen flooded cars in an apartment complex in Mandeville.
Public reported service road was impassable in Mandeville due to high water.
Flooding outside Cypress Lake apartments in Mandeville. Roads completely impassable.
Public reports ankle deep water entering their apartment.
NWS employed reported Holiday Square Boulevard to be an impassable due to high water.
Social media picture showing water entering gas station.
Tall Timbers has multiple roads impassable water and encroaching on homes.
Roads are beginning to hold water in Monterrey and starting to approaches houses.
Social media photo shows car wash and surrounding road flooded.
Vehicles are pushing flooding water into buildings on street including the fire station.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Louisiana in 2021Every state in 2021This day in weather historyWarnings in Louisiana, July 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: 974449, 974450, 974451, 974452, 974454, 974455, 974456, 974457, 974458, 974459, 974460.