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What Happened Yesterday? Recapping Saturday’s Severe Weather Event

Peoria, Ill. (HOI ABC) - Let's start with the big question. Was yesterday's severe weather event a bust? Well, yes and no.

We had a few tornado warnings, including one confirmed EF-0 tornado in Ipava and one either EF-0 or EF-1 tornado near the Peoria Airport. NWS in Lincoln will have a full report on the Peoria tornado later today. However yesterday's severe weather did not live up to what it could have been.

I am thankful that it did not. As forecasters we obviously always want to be right, but anytime we could avoid a chance for significant severe weather and strong tornadoes, that is a good thing and I'd be happy to be wrong every time.

So let's examine what happened yesterday...

The Storm Prediction Center issued a PDS Tornado Watch for all of our area yesterday afternoon and evening. PDS mean "particularly dangerous situation" and is a tag reserved for the potentially strongest and most violent tornadoes. The Washington Tornado had a PDS Tornado Warning with it.

They also had our area in a Moderate Risk. On the SPC's scale, moderate is 4th highest out of 5. This means that the SPC expected widespread severe weather, including the risk of strong tornadoes, large hail, and damaging winds. We also echoed this in our forecasts.

The setup was textbook for spring severe weather in Illinois. We were south of a warm front with an approaching cold front from the west, and near a triple point (where the cold, warm, and occluded front meet). This area is where severe weather and tornado ingredients usually come together.

The setup was there, but clearly things went wrong (or right depending on which way you look at it). So what happened?

I think a few things interfered with our storm development. First, the rain that we saw around lunchtime helped to overturn the atmosphere and delay the onset of storms later in the day.

There was a lot of instability in the upper levels of the atmosphere yesterday, but at the lower levels there was a strong cap. Think of the cap as a lid. The best analog I can think of is a lid over boiling water. If you have a lid over the pot, the steam can not rise, but once you remove the lid there is a rapid upward motion of all of that trapped, hot air.

Here is a look at upper air soundings from Lincoln at 1, 4, and 7 PM yesterday. All you need to look at is the red line on the graph to the left (the one underneath observed). Notice how toward the bottom it initially moves toward the right from the bottom before moving to the left. This is the cap. The movement to the right means that temperatures were rising as you went up in the atmosphere, which created a stable layer of air that we could not break through and produce storms. Notice how the cap weakens as we went throughout the evening.

1 PM
4 PM
7 PM

Here is a another view of our cap. Notice the area in blue over Illinois. This was the stable layer of air that initially prevented thunderstorm formation.

When the cap finally broken yesterday evening, we were able to produce more widespread storms. Additional lift from the passing cold front was able to break through any remaining cap that may be been in place.

One of the severe weather ingredients that we talk about is wind shear, which is defined as the change of wind speed and/or direction with height. Wind shear is important for severe thunderstorm development because it separates the updraft (rising air in the storm which helps to create a thunderstorm) from the downdraft (falling air).

To put it another way, if these two are not separated, storms can rain themselves out and either weaken the storm or not be able to produce a new one. Separating the updraft from the downdraft allows for sustained, strong updraft development.

Yesterday, we likely had too much wind shear. Here is a look at a map of wind shear from the surface to 6 kilometers. Notice over Illinois the area of 90 knot shear. That is a very high amount of shear.

The amount of instability that we had was not able to keep up with this amount of shear. To put it another way, we had too much shear for the amount of instability. This meant that storms could not sustain strong updrafts because they were tilted too far forward.

Think of this analog. Have you ever seen smoke rising from a chimney during a strong breeze? Strong winds blow the smoke away as soon as it leaves the chimney. Our storms yesterday were able to produce some strong updrafts at times, but overall they struggled to consistently organize.

The ones that were succesfully able to tap into our severe weather ingredients did pack a punch. We had two confirmed tornadoes yesterday in Ipava and Peoria.

Here is the first storm in Ipava. This didn't look like a classic supercell, but you can see the red/green couplet that we look for on our velocity radar mode. This is a sign of rotation within the storm.

NWS confirmed this as a EF-0 tornado with minor damage to 3 homes, as well as damage to a garage and machine shed.

The second tornado was near the Peoria Airport. As of writing this, NWS is describing this as an EF-0 or weak EF-1. They will issue their final report later today, but they are reporting roofs damage and downed trees in that area. There were also reports of power flashes around 9 PM southwest of downtown Peoria.

So again to recap, yesterday could have been much worse. We did have some severe weather with a few damage reports and confirmed tornadoes, but this event definitely did not live up to it's potential.

But if you ask me, I'm ok with that.

Brian Walder

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