HIGH IMPACT SEVERE WEATHER

SEVERE STORMS LIKELY TODAY WITH AN ELEVATED TORNADO THREAT...

After a robust day of severe weather Wednesday, the final stages of a complex late spring storm are coming together early Thursday for what is expected to be another enhanced day of severe weather. Much like Wednesday, multiple rounds of strong convection are expected. Two waves of energy on the nose of an unseasonably strong 80-100kt jet will open the door for two periods of severe weather. Each will will have the ability to produce hail up to 2 inches, winds in excess of 75 mph, and potentially tornadoes, some strong. The enhanced tornado threat is expected later in the afternoon, especially between 2 and 6:00pm when instability and deep layer shear is maximized.

AGAIN, MORNING STORMS TO KICK OF THE DAY.

The first wave of storms (fast movers) arrives early Thursday morning, likely to cluster into a strong MCS with storms capable of damaging wind gusts and some hail. Depending on the position of an outflow boundary and the amount of instability available, this round may have ample wind shear to work with which could again make this episode a potent wind producer with gusts of 65-75 mph possible. This likely will move into central Iowa as early as 6 a.m. with the latest guidance taking them into Eastern Iowa and western Illinois between 9 and 11:00 a.m.

AFTERNOON TORNADO THREAT THURSDAY...

Following the morning storms, the local environment is shown rapidly warming and destabilizing ahead of the next, more potent wave of storms in the late afternoon. Rapid development of strong convection is expected ahead of a surface low and advancing cold front approaching eastern Iowa which is expected to generate strong low level shear for rotating thunderstorms. All modes of severe weather are on the table, including several strong tornadoes. The significant tornado parameter of the HRRR shows elevated values up to 10 which is quite substantial over many parts of the area.

Looking at CAPE, a measure of potential instability, most of my region is shown reaching big levels of 3,500 to 4,500 j/kg in the afternoon before the second round of convection explodes. The extreme instability combined with deep layer shear in the lower levels at 0-3 km, would support rotating updrafts enhancing tornado development. Depending on how the mesoscale details unfold Thursday afternoon, the greatest tornado threat may end up close to EC Iowa and NW Illinois on the north fringe of the CAPE boundary in purple below. It's there that the flow will be highly backed on a sharp warm front attached to the advancing surface low.

Most convective allowing models are showing strong helicity tracks (paths of rotating thunderstorms) over much of the area, indicating the tornado threat is not just confined to the area near and north of I-80.

I would not be surprised to see tornado watches issued for all of the region at some time Thursday afternoon. It is also possible that SPC could upgrade to a moderate risk (level 4 of 5), but that is conditional on how the set-up evolves in the afternoon. As it stands now, the entire area is under an enhanced level 3 of 4 risk indicating widespread severe storms are expected.

Extremely high levels of moisture will also lead to intense rainfall rates and potential areas of flash flooding where the stronger storms track. Again, Thursday looks to be a day of high weather awareness. Pay attention to warnings and have a plan what to do if one is issued for your area.

However Thursday ends, a cold front will put a stop to the severe weather potential in all areas by mid evening, bringing a quiet day of weather Friday. Roll weather...TS