© 2019 Terry Swails

STRONG TO SEVERE STORMS A THREAT IN THE SOUTH...

March 19, 2020

If there is one thing I've learned about severe weather in the Midwest it's the fact that averages can be deceiving. If you can get warm abnormally moist air in place, you can get strong storms any time of the year. Obviously it's much harder to do in January but it can and has been done. Take January 24, 1967 for example. Temperatures and dew points in the 60s combined with a strong triple point set up to create numerous strong tornadoes with significant damage and loss of life over SE Iowa.

These are the tracks and intensity ratings from the 1967 tornado outbreak. In northwest Iowa temperatures were in the teens as the tornadoes raced through the southeast part of the state.

A summary of the outbreak from the NWS Quad Cities.

I guess the point is that even in mid-March it's rare for tornadoes to occur in Iowa and northern Illinois but the set-up Thursday appears to have some potential for those of you in far southern Iowa (especially south of I-80).

 

There's two really key ingredients to watch as the day unfolds. First is the warm front and how far north it gets. That determines hows far moisture and warm air can penetrate into my area. The EURO is one of the warmer solutions with 63 to Cedar Rapids, 66 to Moline, and 71 in Keokuk! The warm front gets close to HWY 20.

The 3k NAM though is having none of that and stops the warm front south of I-80...closer to HWY 34. The high only reaches 52 in Cedar Rapids, 57 in the Quad Cities, but...does reach 72 in Keokuk.

Ultimately the final position of the warm front will make or break the severe weather potential. Nothing surface based is likely north of the front so everything severe (most certainly anything tornadic) would be right on or south of the front.

 

My feeling is the southern play of the 3k NAM is more likely. The reason I say that is the warm front will have difficulty pushing north due to east southeast winds coming off the cold waters of Lake Michigan. That's a common occurrence this time of year that often spares much of my area from early season severe weather outbreaks. More often than not the warm front gets hung up over far southern Iowa and WC Illinois, about what the 3k NAM is showing. There's also the issue of morning precipitation and clouds which adds another challenge for the warm air to overcome. 

 

There's no way to know for sure how it plays out but let's say the 3k NAM is correct. That means the instability which is measured in CAPE is again near or south of I-80.

That's also where the best potential for rotating storms known as supercells would be. You can see that in the supercell composite index Thursday evening.

It's no surprise then that the significant tornado parameter also ends up in that area.

This will be a fluid situation that needs to watched as the day develops. However, based on the 3k NAM I think the greatest potential for strong to severe storms will be over far southern Iowa, Missouri, and perhaps WC Illinois, especially south of I-80. Storms will likely go linear by sunset meaning strong winds would take over after the initial threat of a few tornadoes as first develop. The latest outlook from the Storm Prediction Center early Thursday still shows the threat further north and west than I'm thinking but they are the dudes and that's their job and that's the official outlook. I'm sure there will be some tweaks as the day progresses and parameters become more clear.

 

After the storms kick out of the south in the evening, all areas will see occasional rain along with sharply colder temperatures Thursday night. Lows Friday morning will range from the 20s in my northern counties to near freezing in the south.

Winds will also get very frisky out of the northwest and gusts of 35-45 mph will bring a sting. These are the wind chills projected Friday morning. After highs near 70 around Keokuk Thursday, wind chills Friday will be in the low 20s

Actual temperature late Friday will be 35-40 degrees colder than 24 hours earlier.

I should also mention there could be some snow showers or flurries around late Thursday night and Friday morning, especially in my northern counties. A dusting is possible mainly north of HWY 30. Higher amounts are possible from HWY 20 north. Something else to keep an eye on. The EURO, 3k NAM, and 12k NAM show this for snow totals,

That's where things stand late very Wednesday night. Will let you know how things are trending later Thursday. Until then, roll weather...TS 

 

 

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