April 29: Tonight's Thunderstorm Forecast

The thunderstorm forecast for tonight is very low as a weather disturbance comes through the Upper Midwest. The maximum chances for storms will exist mainly on the US/Canada border in North Dakota. The best chances will be up to 70%, so I would expect some spotty storms.
Storms will be exiting Florida at midnight, which is when this image is valid. Offshore, there will be quite a squall line. It will probably produce ample waterspouts, so ships in that area should watch out.
Everywhere else should be clear of storms.

April 27 Aftermath: 4/29 Preliminary Results of Tornadoes (Birmingham, AL NWS Office)

Location(s)	Damage Survey Status
Hackleburg	EF-3+ Tornado. Winds: At least 180mph. Path Length: 25.2 miles, Path Width 3/4 mile. Further Evaluation Underway.
Other portions of Marion, Lamar, Fayette, and Winston Counties	Shotsville: EF-3+ Tornado. Winds: Around 140mph. Path Length: 19.1 miles in Marion County . Path Width: 3/4 mile. Further Evaluation Underway. Haleyville: EF-3+ Tornado. Winds: Around 140mph.Path Length: 31.8 miles, Path Width: 3/4 mile. The track may have continued into the Tennessee Valley.
Pickens County into Northern Tuscaloosa County	Ongoing (Aerial)
Walker and Blount Counties	Ongoing (Aerial)
Tuscaloosa	Ongoing (will likely take several days)
Jefferson County/Birmingham Metro	Ongoing (will likely take several days)
Western portions of Tuscaloosa-Birmingham Supercell (Sumter and Green Counties)	Ongoing Today
Eastern portions of Tuscaloosa-Birmingham Supercell (Saint Clair to Cherokee County)	Ongoing Today
Sumter-Green-Hale-Bibb-Shelby Counties	Being Finalized
Pell City-Talladega-Calhoun-Cleburne Counties	Saturday 4/30
Marengo (continuation from Choctaw County)	Being Finalized
Perry-Dallas-Chilton Counties	Saturday 4/30
Autauga-Elmore-Tallapoosa-Chambers (including Dadeville)	Ongoing Today

April 27 Aftermath: How did the Tornado Outbreak occur?

Such an outbreak like the April 27 one is extremely rare and historic when it happens.
The big question is, how did this happen?

1. JET STREAM

The jet stream was down south, guiding the low pressure center east with the cold front that would produce the tornadoes. The jet stream was unusually strong, and the storm system may have even gotten some of that energy from the jet stream.
The jet stream made for a high wind shear event, favorable for tornadoes in some areas.
Additionally, the jet stream separates warm and cold air. With the cold front bringing the cold air from above the jet stream and the warm front bringing warm air from below the jet stream, it was a very volatile situation to start with.

2. EHI
EHI (Energy Helicity Index) combines wind factors (like helicity, which is a spinning motion) and energy factors (like CAPEs, which produce stronger and more volatile storms) to make a sense of what the potential is for rotating supercells.
The EHI index was incredibly high for the outbreak.
Values may have reached 12.00, which is considered absolutely unbelievable.
This is probably the biggest factor that directly contributed to the tornado outbreak.

3. CAPE
CAPE values indicate potential energy that can be utilized by thunderstorms. The higher the CAPE, the more unstable the atmosphere. CAPE values were witnessed to be up to 4,000 j/kg, possibly even more.
To give you a sense of how unstable 4,000 j/kg is, 2,000 j/kg is considered the threshold for severe thunderstorms for CAPE values.

There were many other things that came into place, but these 3 really stood out from the other factors.
Stay tuned to the Weather Centre for continuing coverage on the aftermath of this horrific incident.