Update at 8:15 a.m. CDT Thursday:
This blog is no longer live, as the severe threat has ended. Detailed reports of the event can be found below.
The major threat on Thursday will be heavy rain and flooding from Upper Plains into the Ohio Valley.
"With all the moisture and humidity across the eastern and northern Plains on Thursday, storms will have plenty of moisture to tap into," AccuWeather Meteorologist Evan Duffey said. "Combine that with the slow-moving nature of the storms, especially in the Midwest and the Plains, and you have a flooding scenario."
Additional severe thunderstorms will continue to develop through the afternoon hours from the Dakotas to western Texas.
The next set of showers and thunderstorms will come this weekend for areas from the Midwest to parts of the southern mid-Atlantic and the South.
8:10 a.m. CDT Thursday: Minneapolis St. Paul International and Chicago O'Hare International airports are delayed due to thunderstorms, according to the FAA.
7:30 a.m. CDT Thursday: An NWS observer reports 24 hour rainfall total for Grant County, Wisconsin, at 3.78 inches.
6:50 a.m. CDT Thursday:
Patchy dense fog w/vis 1/4 mi or less may linger into aftrnoon along #LakeMI shore of Lake IL, Cook, Lake IN & Porter. Use caution on roads!— NWS Chicago (@NWSChicago) June 19, 2014
6:05 a.m. CDT Thursday: More than 4 inches of rain has fallen in Dubuque County, Iowa, since 10 p.m. CDT last night, according to an NWS spotter. Multiple roads are closed across the county this morning due to flooding and a possible mudslide, reports law enforcement.
5:30 a.m. CDT Thursday: Approximately 9,290 Xcel Energy customers without power across Minnesota this morning.
4:43 a.m. CDT Thursday: 225 flights have been canceled early Thursday, 124 of those from O'Hare airport, FlightStats reports.
4:37 a.m. CDT Thursday: 2.94 inches of rain have fallen since midnight Thursday CDT at the Rock River at Dixon, Illinois, according to Mesonet.
4:17 a.m. CDT Thursday: Detroit breaks a 108-year-old June 18 rain record with 1.77 inches. The old record was 1.52 inches, which was set in 1906.
4:00 a.m. CDT Thursday: More than 8,000 Philadelphia-area power customers are without service, utilities report.
3:52 a.m. CDT Thursday: 2.69 inches of rain were recorded since midnight CDT Thursday at the Otter Tail River, Breckenridge, Minnesota, according to Mesonet.
3:35 a.m. CDT Thursday: 1.73 inches of rain fell since midnight CDT Thursday at Yellow River State Forest, Iowa, according to Mesonet.
3:09 a.m. CDT Thursday: About 200 U.S. flights have been canceled and another 150 delayed due to storms, FlightStats reports.
2:55 a.m. CDT Thursday: 2.02 inches of rain in 2.5 hours, 2 miles north of Sterling, Illinois, reports trained spotter.
2:32 a.m. CDT Thursday: Sisseton, South Dakota, shattered its June 18 rain record with 4.27 inches. The old record was 1.97 inches, which was set in 1948.
2:22 a.m. CDT Thursday: A Geauga County, Ohio, school was damaged by storms on Wednesday, according to the Cleveland Plain Dealer.
2:04 a.m. CDT Thursday: More than 19,000 Pennsylvania electric customers are without service at this time, utilities report.
1:50 a.m. CDT Thursday: Campground is being evacuated due to flooding 5 miles southwest of Dubuque, Iowa, emergency management reports.
1:44 a.m. CDT Thursday: 5,500 Indiana power customers are without service at this time, utilities report.
1:21 a.m. CDT Thursday: 1.13 inches of rain fell in one hour at Dubuque, Iowa, reports trained spotter.
1:06 a.m. CDT Thursday: Storms heading toward Philadelphia area:
12:58 a.m. CDT Thursday: Streets are flooding in North Platte, Nebraska.
12:55 a.m. CDT Thursday: More than two-hour delays occurring for arriving flights at O'Hare airport due to thunderstorms, the FAA reports.
12:41 a.m. CDT Thursday: Heavy rains cause road and property flooding in Sisseton, South Dakota, trained spotter reports.
12:37 a.m. CDT Thursday: Climbing power outages: More than 30,500 Maryland power customers are without service at this time, Maryland Emergency Management Agency reported.
12:33 a.m. CDT Thursday: More than 16,500 Pennsylvania customers are without electricity, utilities report.
12:20 a.m. CDT Thursday: Michigan rain records for June 18:
Grand Rapids and Muskegon both set daily rainfall records on Wednesday. Muskegon received 3.93 inches, and GR received 2.08 inches. #wmiwx— NWS Grand Rapids (@NWSGrandRapids) June 19, 2014
12:11 a.m. CDT Thursday: 1.24 inches of rain in one hour with many roads flooded at Maynard, Minnesota, trained spotter reports.
12:07 a.m. CDT Thursday: More than 20,000 Maryland power customers are without service at this time, Maryland Emergency Management Agency reported.
12:04 a.m. CDT Thursday: A house basement wall collapses from flooding in Belview, Minnesota, trained spotter reports.
11:45 p.m. CDT Wednesday: More than 20,000 Dominion Electric customers in Virginia are without service, the utility reports.
11:38 p.m. CDT Wednesday: 62 mph gust in Jefferson Manor, south of Washington, D.C., according to Mesonet:
11;33 p.m. CDT Wednesday: Relief efforts continue in Wessington Springs, South Dakota.
A trailer with more supplies and volunteers are en route from Sioux Falls to support the shelter in Wessington Springs, SD. #Tornado— Red Cross Dakotas/MN (@RedCrossDakotas) June 19, 2014
11:25 p.m. CDT Wednesday: More than 4,700 Indiana power customers are without service, utilities report.
11:19 p.m. CDT Wednesday: Numerous roads are flooded and closed in Stark and Summit counties in Ohio, law enforcement reports.
11:11 p.m. CDT Wednesday: Another round of storms is headed for Baltimore:
10:55 p.m. CDT Wednesday: 64 mph gust and flash flooding at Kokomo, Indiana, reports trained spotter.
10:48 p.m. CDT Wednesday: More than 2,000 Maryland electric customers were without service at this time, the Maryland Emergency Management Agency reported on its website.
10:34 p.m. CDT Wednesday: No fatalities reported at Wessington Springs, South Dakota:
10:29 p.m. CDT Wednesday: Tornado on the ground 3 miles northeast of Marshall, Minnesota, reports trained spotter.
10:24 p.m. CDT Wednesday: More than 28,000 Pennsylvania electric customers are without service as line of storms plows across the state, utilities report.
10:11 p.m. CDT Wednesday:
I have deployed 100 @SD_Guard soldiers and equipment to Wessington Springs and the Red Cross is also on the way.— Gov. Dennis Daugaard (@SDGovDaugaard) June 19, 2014
10:02 p.m. CDT Wednesday: The Associated Press is reporting that one person was hurt after a tornado struck near Wessington Springs, South Dakota.
9:51 p.m. CDT Wednesday: About 35,000 DTE Energy customers in the Detroit area are without power, the utility reports.
9:41 p.m. CDT Wednesday: South Dakota tornado damage:
9:34 p.m. CDT Wednesday: A tornado on the ground 10 miles north of Platner, Colorado, report trained spotters.
9:28 p.m. CDT Wednesday: Colorado tornado reported:
Storm Chasers report Tornado on the ground 1N Burdett in Washington County. #COwx— NWS Boulder (@NWSBoulder) June 19, 2014
9:24 p.m. CDT Wednesday: Storm near North Platte, Nebraska:
9:22 p.m. CDT Wednesday: About 62,000 Ohio power customers without power, utilities report.
9:06 p.m. CDT Wednesday: 70 mph gust 4 miles south of Long Island, Kansas, reports trained spotter.
9:02 p.m. CDT Wednesday: Numerous trees downed by a storm in DuBois, Pennsylvania, reports trained spotter.
8:57 p.m. CDT Wednesday: About 15,000 First Energy customers in Pennsylvania are without electricity, the utility reported.
8:36 p.m. CDT Wednesday:
828 pm: New tornado reported 3 miles northwest of Forestburg, SD. #sdwx— NWS Sioux Falls (@NWSSiouxFalls) June 19, 2014
8:35 p.m. CDT Wednesday:
Roping out slowly pic.twitter.com/bEqaHcXky2— Dick McGowan (@stormpics) June 19, 2014
8:33 p.m. CDT Wednesday:
Fat stovepipe doing damage n of woonsocket sd pic.twitter.com/UV8t3ZIqmi— Dick McGowan (@stormpics) June 19, 2014
8:05 p.m. CDT Wednesday:
Very very bad situation here IN SD pic.twitter.com/KhgnYyTrYr— Ethan Mulnix (@Tornadochaser91) June 19, 2014
8:04 p.m. CDT Wednesday:
Massive damage with tornado in SD.. SEARCH ONGOING pic.twitter.com/ZJ079pFkp3— Ethan Mulnix (@Tornadochaser91) June 19, 2014
7:46 p.m. CDT Wednesday:Wind gusts up to 77 mph reported near Aurora, Illinois, according to Mesonet.
7:45 p.m. CDT Wednesday:
Official Release via @SouthDakotaDOT: State officials have re-opened Highway 85 in western South Dakota.— SD Emergency Mgmt (@SDemergencyMgmt) June 18, 2014
7:33 p.m. CDT Wednesday:
Tornado-warned storm in SE SD: pic.twitter.com/1Lw7yzHj3R— Jeff Frame (@VORTEXJeff) June 18, 2014
7:19 p.m. CDT Wednesday: A large rain-wrapped tornado was spotted near Wessington Springs and remained on the ground for 10 minutes, reports NWS spotter.
7:02 p.m. CDT Wednesday: Tornado reported on the ground 1 mile east of Gann Valley, South Dakota, reports NWS spotter at 6:54 p.m. CDT.
6:15 p.m.CDT Wednesday:Canton, Ohio:
6:00 p.m. CDT Wednesday:Due to wind, multiple trees and power lines downed near New Castle, Indiana, reports law enforcement.
5:44 p.m. CDT Wednesday: Wind gusts up to 65 mph reported near Sheffield, Ohio, according to emergency management.
4:51 p.m. CDT Wednesday:
4:45 p.m. CDT Wednesday:
4:36 p.m. CDT Wedensday: Approximately 15,000 DTE Energy customers without power, the utility reports.
4:00 p.m. CDT Wednesday:Flooding across highway 66 near Mankato, Minnesota, reports deptarment of highways.
2:30 p.m. CDT Wednesday: Wind gusts up to 81 mph reported near Adrian, Michigan, according to an NWS observation.
2:22 p.m. CDT Wednesday: Wind gusts up to 56 mph reported near Shambo Ranch, South Dakota, reports Mesonet.
1:02 p.m. CDT Wednesday: A 15 minute rain total of .53" fell was recorded near East Grand Rapids, Michigan, reports NWS spotter.
12:49 p.m. CDT Wednesday: A co-op observer reported a 53 mph wind gust in East Grand Rapids Michigan.
12:05 p.m. CDT Wednesday:
Classes are cancelled, and campus is closed to the public on Wed., June 18, as assessment and cleanup after Monday night's tornado continues— UW-Platteville (@uwplatteville) June 17, 2014
11:26 a.m. CDT Wednesday: A co-op observer in Buffalo County, Wisconsin, reports 2.31 inches of rain.
10:55 a.m. CDT Wednesday: Watch Meteorologists Andrew Baglini and Heather Waldman provide the latest updates on the severe weather threat on this edition of 'AccuWeather LIVE.'
10:29 a.m. CDT Wednesday: The FAA is listing ground delays at Chicago O'Hare due to the thunderstorms.
9:52 a.m. CDT Wednesday: An NWS-trained spotter in Clarion County, Pennsylvania, is reporting multipel flooded roadways.
9:17 a.m. CDT Wednesday:
House surrounded by mud from heavy rains on County Road 33 in Mankato. pic.twitter.com/IQOAhV0f34— KEYC News 12 (@keyc) June 18, 2014
9:06 a.m. CDT Wednesday: "There is a squall line moving eastward across northern Illinois and southeastern Wisconsin that will hit Chicagoland during the midday hours with blinding rain, gusty winds and frequent lightning strikes," said AccuWeather.com Senior Meteorologist Alex Sosnowski.
8:30 a.m. CDT Wednesday: Hail the size of ping-pong balls reports in Kent County, Michigan, by an NWS-trained spotter.
8:11 a.m. CDT Wednesday: Quarter-sized hail reported by NWS-trained spotter in Jamestown, Michigan.
7:44 a.m. CDT Wednesday: FlightStats lists excessive delays amid thunderstorms in Detroit:
7:16 a.m. CDT Wednesday: Quarter-sized hail reported by an NWS-trained spotter in Grant County, Wisconsin.
7:00 a.m. CDT Wednesday: A co-op observer in Iduna, Wisconsin, reports 4.72 inches of heavy rain.
6:27 a.m. CDT Wednesday: Law enforcement reports flash flooding across Trempealeau County, Wisconsin, closing many roads.
6:10 a.m. CDT Wednesday: Early morning lightning flashes in Iowa:
5:55 a.m. CDT Wednesday: Hundreds are reportedly without power across portions of Wisconsin, Iowa and Michigan. Numbers are expected to climb as people wake up later this morning.
5:28 a.m. CDT Wednesday: States of emergency have been declared for parts of Wisconsin, South Dakota and Alberta, Canada.
5:03 a.m. CDT Wednesday: Penny to nickel-sized hail reported in Mower County, Minnesota, by local law enforcement.
4:25 a.m. CDT Wednesday: Law enforcement in Algona, Iowa, report multiple flash floods overtaking roadways.
3:10 a.m. CDT Wednesday: A wind gust of 54 mph was measured in Waseca, Minnesota.
1:58 a.m. CDT Wednesday: The NWS via law enforcement reported damage to farm outbuildings near Royal, Iowa, southwest of Spencer. It is not known at this time if damage was the result of a tornado.
1:22 a.m. CDT Wednesday: A wind gust of 66 mph was measured in Spencer, Iowa, with this thunderstorm. Doppler radar indicating rotation, which means a tornado is possible.
12:45 a.m. CDT Wednesday: A nearly stationary thunderstorm is causing flooding in Mankato, Minnesota, southwest of Minneapolis. Law enforcement is reporting several roads are impassable in the city. Water is reported to be up to car bumpers in Rapidan, Minnesota, to the southwest of Mankato. Doppler radar estimates that rainfall rates of 2-3 inches an hour are occurring.
11:31 p.m. CDT Tuesday: Thunderstorms rapidly developing in the Milwaukee, Wisconsin metro area. They can produce high winds, hail and downpours as they move through the area.
11:22 p.m. CDT Tuesday: Record flooding is occurring on the Big Sioux River along the South Dakota and Iowa border. Below is the river gage data from the USGS at Akron, Iowa, courtesy of NOAA.
10:18 p.m. CDT Tuesday: The Storm Prediction Center has issued a severe thunderstorm watch until 4 a.m. CDT Wednesday for parts of Iowa, Wisconsin, Illinois and Indiana. This includes Milwaukee, Wisconsin, and the entire Chicago metro area.
10:08 p.m. CDT Tuesday: An NWS employee reports the tornado near Laurel, Nebraska, is still on the ground and nearly stationary.
10:00 p.m. CDT Tuesday: Squall line with damaging wind gusts of 60 mph or higher moving through New York State. Albany, New York, could be impacted later tonight.
8:45 p.m. CDT Tuesday:
Still getting reports of a rain wrapped tornado on the ground northeast of Coleridge, NE.— NWS Omaha (@NWSOmaha) June 18, 2014
8:38 p.m. CDT Tuesday: A tornado was spotted 1 miles northwest of Coleridge, NE, reports emergency management.
8:15 p.m. CDT Tuesday:
8:15 p.m. CDT Tuesday: Tornado reported near Hartington, Nebraska, reports NWS spotter.
8:01 p.m. CDT Tuesday:
7:02 p.m. CDT Tuesday: Multiple tornadoes reported in Cherry County, Nebraska, reports NWS. Tornado spotted at 6:43 p.m. near Cody, Nebraska, NWS reports.
6:55 p.m. CDT Tuesday:
648p CT - hail nearly SOFTBALL size reported with the Cherry Co. storm #NEwx— NWS North Platte (@NWSNorthPlatte) June 17, 2014
5:46 p.m. CDT Tuesday:
544p CT - CORRECTION: tornado is 12 miles northwest of Merriman, nearly stationary #NEwx— NWS North Platte (@NWSNorthPlatte) June 17, 2014
5:39 p.m. CDT Tuesday: Hail 1.5 inches in diameter reported near Merriman, Nebraska, reports a NWS spotter.
4:26 p.m. CDT Tuesday:
4:20 p.m. CDT Tuesday: Tornado reported 9 miles northwest of Capitol, Montana, reports NWS trained spotter.
3:41 p.m. CDT Tuesday:
A tornado was reported 4 miles west of Capitol at 225pm pic.twitter.com/ggGL3laYPW— NWS Billings (@NWSBillings) June 17, 2014
3:28 p.m. CDT Tuesday: Quarter sized hail reported near Ismay, Montana, reports NWS spotter.
2:36 p.m. CDT Tuesday: Multiple reports wind damage and downed power lines in Penfield, New York, reports law enforcement.
1:54 p.m. CDT Tuesday:
Damage survey complete for Tor 1 near Stanton. Damage survey ongoing for the 2 tornadoes that impacted Pilger, with EF4 damage in Pilger.— NWS Omaha (@NWSOmaha) June 17, 2014
1:02 p.m. CDT Tuesday:
12:14 p.m. CDT Tuesday:
Very preliminary, damage survey teams have found at least EF3 damage west and north of Stanton, and also in Pilger. Surveying continues.— NWS Omaha (@NWSOmaha) June 17, 2014
11:46 a.m. CDT Tuesday:
10:38 a.m. CDT Tuesday:
Pilger damage video is up on NEMA YouTube page: https://t.co/AdzdRljuEH— NEMA (@NEMAtweets) June 17, 2014
9:33 a.m. CDT Tuesday: Tree branches went down in Wisconsin when storms tore through the area last night into this morning:
9:09 a.m. CDT Tuesday: An NWS-trained spotter reports half-dollar sized hail covering the ground in Brockway, Montana.
8:51 a.m. CDT Tuesday: According to FlightStats, Chicago O'Hare International Airport has the most delayed incoming flights of any domestic hub.
8:29 a.m. CDT Tuesday: CNN is reporting that the death toll from the twin tornadoes has risen to two.
8:06 a.m. CDT Tuesday: Nearly 20,000 MidAmerican Energy customers still without power.
As July draws to a close, a storm system swinging up from the Deep South will bring downpours that will break the back of the heat wave in much of the northeastern United States.
Heavy downpours will raise the concern for flash flooding along the Gulf Coast and lower Mississippi Valley through midweek.
A stifling heat wave will remain entrenched across the Northeast much of this week, despite a brief reprieve in humidity for some.
Highs will run between 10 and 15 degrees Fahrenheit above average across much of the western United States into the upcoming weekend.
Severe thunderstorms rumbled through the Northeast on Monday, lashing the region with damaging winds while also unleashing heavy downpours that triggered flash flooding.
Here are five easy ways to stay cool in sweltering summer heat.
Barrow, AK (1992)
Pace, FL (1992)
Four persons struck by lightning at Quail Run Trailer Park; two critically injured.
Waldrof, MN (1995)
81 mph wind gust (near Mankato).