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March 31st – April 1st, 2023 Super-Outbreak, 1 year later

This weekend will mark the 1-years anniversary of the March 31st – April 1st Severe Weather Super-Outbreak sequence, which impacted many parts of the Great Plains, Mississippi Valley, the Great Lakes region, and the East Coast.

Atmospheric Setup:
A few days ahead of the event, on March 26th, multiple models were pushing out the potential for a hazardous situation, and the Storm Prediction center issued a rare slight risk 5 days before the event. The following day introduced an enhanced risk for severe thunderstorms, with the potential for significant severe storms being realized very far out ahead of March 31st.

All of this was warranted from the ejection of a deep upper level trough from the West Coast. This trough of lower pressure would push east, allowing for surface cyclogenesis off of the Rockies. As this surface cyclone would push off of the rockies, warm, moist air was expected to surge north from an attendant warm front from the system as the low pressure system strengthens. A cold front would then sweep through, and storms along and ahead of the cold front were expected to be very potent. Plenty of speed shear and directional shear was expected for this event, as winds veer (turning clockwise with height) with height, which favors rotating updrafts. Any cells that form ahead of the cold front would be favorable for rotating updrafts. Environmental instability was expected to be favorable for surface-based storms, and tying into the rotating updrafts, ample lift, and shear, you have a particularly dangerous situation looking to unfold. Two distinct dangerously favorable modes, especially for supercells, were noted to be possible near the surface cyclone track for central and eastern Iowa, northern Illinois, and another mode in central Arkansas, northwestern Mississippi, Tennessee, Missouri bootheel, southwestern Kentucky, and northern Alabama.

2 Days ahead of the event, the Storm Prediction Center had a large area of an Enhanced Risk (Level 3 of 5), with the entire area of said risk in a 30% chance for severe weather across much of the Mississippi Valley, with a hatched risk indicating the potential for significant severe weather.

The day before the event, the Storm Prediction Center issued two separate Moderate Risk areas (Level 4 of 5!), which followed suit to the two distinct modes for supercell potential discussed above. Parts in between were still expected to see severe weather, but not as much of a tornado risk. Large hail and damaging winds were also a big risk in this setup, especially within the moderate risks as well, where the risk for violent thunderstorms was expected.

On the morning of March 31st at 2AM EDT, the Storm Prediction Center increased tornado potential to a 10% chance within a 50 miles radius, along with a 10% chance for a significant tornado, for eastern Missouri and western Illinois. This tornado potential across the majority Mississippi Valley would only increase as the morning went on, with the 8 AM outlook highlighting a Moderate risk newly introduced for eastern MO and western IL, and was based on the increasing potential for violent, long-track tornadoes in that area, along with the still standing moderate risks elsewhere. The violent tornado potential would not stop there.

Looking at the above image, take note of how large of an area the hatched, black lines are. This would be a regional outbreak if it verifies, impacting multiple metropolitan areas, causing significant damage across the mid-Mississippi Valley, and the Great Lakes region as the system pushes east.

The surface map below (this was at 2PM on March 31st) shows the surface cyclone, as expected, in western Iowa, pushing east, and it is a strong low, pushing in warm moist air to areas east of the low. A cold front was draped across western MO, eastern OK, and eastern to central Texas. Along with this cyclone, all of the ingredients have come together for a dangerous regional severe weather event. Lift is provided from this large weather system, instability is provided by little cloud cover and high dewpoints, and shear is plentiful as winds change speed and direction with height.

Moderate risks are very serious. Not many are issues every year, and they often mean a severe weather outbreak is expected in that area. Most often, in moderate risks, you see tornado outbreaks, with tornadoes being strong to violent possible, or derechos. A moderate risk is issued when criteria for very large hail, damaging winds, or tornadoes is large enough to become a severe concern to life and property. Moderate risks are not the highest level, but they are never anything to scoff at, as again, tornado outbreaks are quite common within a tornado-driven moderate risk. For March 31st, the moderate risk areas were all tornado-driven, which means that the tornado potential is at a particularly dangerous level, and multiple tornado are expected across that risk area.

The two areas of the initial moderate risks have become a seriously dangerous environment for severe weather, especially for strong to violent tornadoes (EF2+), more-so than the rest of the newly introduced moderate risk. To account for this, the SPC issued the highest risk possible for severe thunderstorms. This high risk was issued for an increased tornado potential in the two areas of SE Iowa and NW IL, and another in eastern AR and western MS and TN. These high risks entailed a 30% chance for tornadoes within a 50-mile radius, along with the threat for large, violent tornadoes, which is a very serious threat.

Getting into the afternoon, storms were expected to form along/ahead of the cold front, being favorable for all hazards of severe weather, especially tornadoes for storms ahead of the cold front. This cold front would push through Iowa and NW Illinois through the afternoon, with the cold front reaching western Indiana in the evening. The southern high/moderate risk mode would contain multiple long-track supercells as the front would sit out to the west, allowing for several supercells to move across the same area throughout the day. Memphis, TN was on high alert for severe potential.

As thunderstorms fired off in Missouri and Iowa, tornado watches were issued across most of Iowa, western Illinois, and northern Missouri. This tornado watch was highlighted as a “Particularly Dangerous Situation” (PDS) watch, denoting a 90% chance of 1 or more strong tornadoes (EF2-EF5). Another PDS watch was issued with supercell thunderstorms starting to form off in western AR, with another 90% chance of 1 or more strong tornadoes, and it covered the entire region of the high risk and moderate risks across AR, northwestern MS, western TN, western KY, southeastern MO, and southwestern IL.

In the afternoon, multiple storms fired up and quickly gained organization and rotating updrafts in central IL and eastern IA. Many tornado warnings were issued in the afternoon, along with PDS (particularly dangerous situation) warnings. More supercell thunderstorms with tornadic activity would continue across Iowa and Illinois throughout the evening. The most notable tornado in this mode is the Keota–Wellman, Iowa EF-4 tornado, with maximum winds of EF-4, with many other other damaging tornadoes across the region.

In the South, multiple supercells were pushing east through Arkansas, and one storm produced a strong tornado through Little Rock, AR, prompting a Tornado Emergency with the tag for catastrophic damage applied to the tornado. The tornado did not cause any fatalities, however it injured 54 and totaled many homes and businesses. Another storm produced a tornado through Wynne, AR, causing EF-3 damage and killing 4. These storms would continue to push east through Arkansas and across the Mississippi, causing more destruction. More supercells would fire off through the area, continuing into the overnight hours, with multiple tornadoes causing major damage and just barely missing Memphis multiple times.

More tornadoes (some particularly strong), large hail, and damaging winds continued eastward past the Mississippi Valley and into Great Lakes/Ohio River Valley for the overnight hours. This includes Indiana. The storms as they approached Indiana were more linear than anything, but contained multiple areas of rotation along the line. The one exception to this was a rogue supercell that pushed through central IL and into Indiana, staying ahead of the main line of storms and the cold front. This tornado caused catastrophic damage, prompting multiple tornado emergencies, causing EF3 damage to Robinson, IL, and most notably the EF3 damage in Sullivan, IN. Multiple more EF3 tornadoes would touch down across Indiana, ones notably in McCormick’s Creek State Park, Whiteland, and Gas City, IN. A total of 23 tornadoes touched down in Indiana from this event. 5 fatalities occurred across Indiana a result. A 100 mph non-tornado wind gust would be recorded in Harlan, Indiana as well.

As tornadoes created intense damage across the Mississippi Valley and Great Lakes regions, multiple mass casualty events were issued, with multiple disaster areas and state of emergencies were issued in Kentucky and Missouri ahead of the storms, with Arkansas declaring one after-the-fact.

Did this event verify? Well as you can assume by the name of the article, yes, it did verify. This event would continue on for another day, spawning more tornadoes along the east coast. This system also gave snow to areas in Minnesota, Wisconsin, and Michigan, with 22″ recorded near Caspian, Michigan. This storm system brought everything.

In total, this super-outbreak of damaging tornadoes, large hail, and damaging winds, caused at least $5.44 Billion in damages, knocking out power to at least 1 million customers, killing 26 (10 in TN, 1 in AL, 1 in DE, 5 in AR, 4 in IL, 5 in IN, 1 in MS) and injuring over 200 people. March 31st, 2023 will be a day long remembered across the Midwest.

–CWS Forecast Lance Huffman

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