00:00Here on the forecast feed we want to take a look at some of the drivers behind the evening and
00:05overnight severe weather outbreak that will continue farther east into the day on Wednesday.
00:11We have big concerns out there, tornadoes, damaging winds, widespread areas with hail, and also again a lot of flash
00:19flood concerns here too among the issues that we're facing.
00:22Now we're going to look at the big picture and then we'll jump into some computer models, but I just
00:27want to be clear about some of the things that we're up against here with this setup.
00:30So here we are and you can see our severe risk areas that we've highlighted.
00:36This is from our team here at AccuWeather and we have a four-tiered severe risk system here and our
00:43high risk is highlighting two areas.
00:47The highly populated area around the southern suburbs of Chicago, Peoria, Illinois, other areas into northwest Indiana, and then also
00:55into west Texas.
00:56Abilene and San Angelo are in on this.
00:58Abilene here along Interstate 20, a big point of reference as well.
01:02So those are the areas with the most heightened risk of tornadoes, hail, damaging winds, and there is a flash
01:09flood risk there as well.
01:10And you can see this is level three on the AccuWeather Severe Threat Index here.
01:16There's potential that somebody out there, the high-end potential for this storm setup could bring somebody a gust of
01:22wind up to 90 miles per hour.
01:23But frankly, I'm most concerned about tornadoes here this evening.
01:26That's a big issue here this evening into the area around Illinois especially.
01:31So if we were to highlight, again, these areas, west central Texas, and then also from northeast Missouri through central
01:38Illinois into northwest Indiana, damaging winds, large hail, and multiple tornadoes.
01:43Some cities at risk here.
01:44Again, Chicago's on the edge of the high-slash-moderate risk for us here at AccuWeather.
01:49But Kankakee, south of town Peoria, Illinois, right in the middle of our high risk.
01:53Hannibal, Missouri along the river there.
01:54And Abilene, Texas, again, in the southern zone, facing a significant risk of damaging winds, large hail, and also tornadoes.
02:04So here's the big picture.
02:06We have, sometimes when we talk about winter storms, we often talk about the idea of phasing.
02:11A disturbance in the northern branch of the jet stream and a disturbance in the southern branch of the jet
02:16stream.
02:16And, again, some of the driving principles that lead to atmospheric strengthening and strong storms here are also in play
02:23when it comes to severe thunderstorms.
02:25It looks different, but the general idea here is the same.
02:28We are dealing with a disturbance in the northern branch of the jet stream that earlier today was up in
02:32northern Wyoming,
02:33and then a southern branch disturbance south of the boot heel of New Mexico.
02:37And look at this.
02:38They are in phase with one another, reaching the Mississippi Valley around the same time, and they are in phase.
02:45Now, your eye might be drawn to these bright colors, and you might be saying, well, that's 2 o'clock
02:48Wednesday.
02:49What's the deal?
02:49You're talking Wednesday about Pittsburgh and Ohio and northeast Kentucky.
02:53Keep in mind, this is the trough axis, and the action is in advance of this.
02:58The action is in advance of it.
03:00So once the base of the trough rolls through your area, the show is over at that point.
03:04The severe risk is off to the east.
03:05So it's really about what's coming your way as opposed to what's in your area right now when it comes
03:10to the driving factors of loft.
03:11And this is a measure of vorticity or spin in the atmosphere.
03:14It encourages lift, cloud production, and again, we got the broad-scale lift in the atmosphere.
03:20But with thunderstorms, you have small-scale on the scale of maybe a few counties lift as well, sometimes smaller
03:27than a county.
03:29So this rolls east here, and you can see it becomes a fairly sharp trough here, consolidating into one northern
03:35branch and southern branch all working together.
03:37Big picture, here is the precipitation forecast.
03:41There's a snowy side to this as well, but Tuesday night into Wednesday morning.
03:45Again, with this kind of a depiction, rain looks like rain.
03:49So you could have an EF3 tornado, and it may look like half an inch of rain or an inch
03:53or two of rain.
03:53But just giving you an idea, the big picture, the timing of this.
03:57This is through 8 a.m. Wednesday with heavy thunderstorms plowing through Illinois and into northwest Arkansas through north Texas.
04:05And then in the morning hours Wednesday, and I know it says 2 p.m., but look at it this
04:10way.
04:10This is the six-hour precipitation ending at that time.
04:13So it's like what is in the rain gauge for the six hours leading up to 2 p.m. Eastern,
04:171 central.
04:18Wednesday morning, numerous drenching thunderstorms, probably maybe a little less severe, but drenching thunderstorms in Indiana and Ohio.
04:25There will be some severe risk that continues through the morning, but we're going to see things flare back up
04:29here.
04:30And here's the front, cold front, a sharp cold front, bringing an end to the big warmth here for the
04:34east coast.
04:35And again, in advance of that and along that, you'll have the heavy thunderstorms.
04:40Again, you can see the precip plotted behind it because it's the six hours leading up to that moment.
04:45So we want you to pick up on that here.
04:47And we're really concerned about northern Illinois here overnight tonight, especially with the tornado risk into northern Indiana, perhaps extreme
04:55southwest Michigan.
04:56So this map here has CAPE, convective available potential energy.
05:00This is driven by warmth and humidity at the surface where we all live near the ground versus much colder
05:06air aloft.
05:07And when you have that warm, humid air under much colder air upstairs, you get buoyancy.
05:12And you're going to have these parcels of air, these big bubbles of air that start rising.
05:16And suddenly as they rise, they're warmer than the neighboring air, so they continue to rise.
05:20That's an unstable environment.
05:21So there's a lot of instability out there that we're very concerned about across this area.
05:27So there's going to be plenty of lift, plenty of thunderstorm development, no doubt about that.
05:30And you can see the instability wanes a bit into Wednesday, but still the dynamics, the support aloft will be
05:37in conjunction with some moderate instability to still bring strong and severe thunderstorms east through Ohio and Pennsylvania.
05:44So this is the low-level jet.
05:46We're looking at winds one mile up into the atmosphere, and this is a big driver here.
05:50Look at what happens during the evening and overnight.
05:53The winds increase, sharply increasing, sharply, sharply increasing in contrast with the front here.
05:59Behind the front is the wind from the north.
06:00So that's going to be high-octane fuel, very warm, humid air driving, funneling, strong support for these aggressive thunderstorms
06:09to continue to roll through Illinois and Indiana.
06:11Here's the energy helicity index.
06:15So helicity, think about maybe a spiraling football, the idea of spin in the atmosphere.
06:21If you get that kind of motion in the atmosphere, and then you're able to tilt that in the vertical
06:26because of an updraft, you can begin to get rotating thunderstorms.
06:29And just, I know we're tight on time, but just wanted to show you a lot of strong signals here
06:33this evening.
06:34Illinois, northern Indiana, that's a big risk of tornadoes, and that's what we're most concerned about, along with the hail
06:39and other issues.
06:40Stay with us here on AccuWeather.
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