Alert archive / Michigan / Severe Thunderstorm Watches in 2021
Severe Thunderstorm Watch 494
The Storm Prediction Center issued this severe thunderstorm watch at Sep 12, 2021, 5:22 PM EDT for 10 counties in Michigan. It was in force for 4 hr.
The area
The outline as issued.
The facts
- Issued
- Sep 12, 2021, 5:22 PM EDT (21:22 UTC)
- Expired
- Sep 12, 2021, 9:49 PM EDT
- In force
- 4 hr
- Issued by
- the Storm Prediction Center
- Largest hail expected
- 1.5 in
- Strongest gust expected
- 60 kt
Counties Genesee County, MI, Lapeer County, MI, Livingston County, MI, Macomb County, MI, Oakland County, MI, Saginaw County, MI, Saint Clair County, MI, Sanilac County, MI, Shiawassee County, MI, Tuscola County, MI
The radar
The Detroit radar (KDTX), 45 km from the middle of the area, began a volume at 21:22 UTC, the scan nearest the moment this went out.
Open the radar archive at this moment
The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.
What SPC expected
The probabilities SPC published with the watch, for the watch area over its life.
- Two or more tornadoes
- 10%
- One or more EF2 to EF5 tornadoes
- 2%
- Ten or more severe wind reports
- 40%
- One or more gusts of 65 knots or more
- 20%
- Ten or more severe hail reports
- 30%
- One or more hailstones of 2 inches or more
- 10%
- Six or more combined severe hail and wind reports
- 70%
Warnings issued inside it
6 tornado, severe thunderstorm and flash flood warnings for its counties while it was in force.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Sep 12, 5:32 PM EDT | Severe Thunderstorm Warning | DTX | Genesee, Shiawassee | 58 min |
| Sep 12, 6:09 PM EDT | Severe Thunderstorm Warning | DTX | Lapeer | 26 min |
| Sep 12, 7:37 PM EDT | Severe Thunderstorm Warning | DTX | Midland, Saginaw | 57 min |
| Sep 12, 8:16 PM EDT | Severe Thunderstorm Warning | DTX | Genesee, Tuscola | 41 min |
| Sep 12, 8:32 PM EDT | Severe Thunderstorm Warning | DTX | Lapeer | 39 min |
| Sep 12, 8:56 PM EDT | Severe Thunderstorm Warning | DTX | Macomb, Saint Clair, Sanilac | 39 min |
Discussions around it
SPC mesoscale discussions over the same counties from four hours before the watch to its end.
The nearest sounding before it
Detroit, MI (KDTX), launched 12:00Z, 9.4 hours before the alert went out and 44 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.
- Mixed-layer CAPE
- 677 J/kg
- Mixed-layer CIN
- -369 J/kg
- Mixed-layer LCL
- 734 m above ground
- 0-6 km bulk shear
- 53 kt
- 0-1 km storm-relative helicity
- 157 m²/s²
- 0-3 km storm-relative helicity
- 299 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.5
The text
As transmitted.
065 WWUS20 KWNS 122120 SEL4 SPC WW 122120 MIZ000-LHZ000-130300- URGENT - IMMEDIATE BROADCAST REQUESTED Severe Thunderstorm Watch Number 494 NWS Storm Prediction Center Norman OK 525 PM EDT Sun Sep 12 2021 The NWS Storm Prediction Center has issued a * Severe Thunderstorm Watch for portions of Southeast Lower Michigan Lake Huron * Effective this Sunday afternoon and evening from 525 PM until 1100 PM EDT. * Primary threats include... Scattered damaging wind gusts to 70 mph possible Isolated large hail events to 1.5 inches in diameter possible SUMMARY...An isolated supercell on the cool side of a surface front will pose a threat for isolated large hail and damaging winds, while other storms farther west could move into southeast Lower Michigan later this evening. The severe thunderstorm watch area is approximately along and 35 statute miles north and south of a line from 30 miles west of Flint MI to 30 miles north northeast of Mount Clemens MI. For a complete depiction of the watch see the associated watch outline update (WOUS64 KWNS WOU4). PRECAUTIONARY/PREPAREDNESS ACTIONS... REMEMBER...A Severe Thunderstorm Watch means conditions are favorable for severe thunderstorms in and close to the watch area. Persons in these areas should be on the lookout for threatening weather conditions and listen for later statements and possible warnings. Severe thunderstorms can and occasionally do produce tornadoes. && OTHER WATCH INFORMATION...CONTINUE...WW 493... AVIATION...A few severe thunderstorms with hail surface and aloft to 1.5 inches. Extreme turbulence and surface wind gusts to 60 knots. A few cumulonimbi with maximum tops to 500. Mean storm motion vector 28040. ...Thompson
SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Sep 12, 2:43 PM EDT | Mesoscale Discussion | SPC | Arenac, Bay, Clare, Genesee, Gladwin, Gratiot and 20 more | 92 min |
| Sep 12, 4:44 PM EDT | Mesoscale Discussion | SPC | Kent, Muskegon, Ottawa | 91 min |
| Sep 12, 5:15 PM EDT | Severe Thunderstorm Watch | SPC | Allegan, Barry, Clinton, Eaton, Gratiot, Ingham and 5 more | 3 hr |
| Sep 12, 6:52 PM EDT | Mesoscale Discussion | SPC | Clinton, Genesee, Gratiot, Ionia, Kent, Lapeer and 7 more | 113 min |
Nearby
Severe Thunderstorm Watches in Michigan, 2021Every severe thunderstorm watch in 2021
Sources
The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.