Why the forecast changed drastically for our latest winter storm

Published 2 months ago• 4 minute read

Peoria, Ill. (WMBD) — The latest storm system to bring snow to Central Illinois is winding down leaving much of the area with a fresh blanket of snow. For the vast majority of the area snow totals from the system were well below the initial predictions that we made on Monday, but ended up being closer to the adjustments we made on Tuesday and Wednesday morning.

Here are the snowfall reports as of 8:00 pm Wednesday

Towanda – 3.9″
Bloomington (CIRA) – 3.8″
Shirley – 3.7″
Chatsworth – 3.0″
East Peoria – 3.0″
Metamora – 3.0″
Bloomington – 2.9″
Morton – 2.6″
Lincoln – 2.1″
Peoria (PIA) – 2.1″
Eureka – 2.0″
Peoria (WMBD) – 2.0″
Trivoli – 2.0″
Washington – 2.0″

So far it looks like most of the area is going to end up with 2-4 inches of snow, which is where our forecast ended up by Wednesday morning, but it doesn’t change the fact that our initial forecast of 4-7 inches with 3-6 inches along I-55 was still way overdone. It wasn’t as bad as some other sources that called for 6-10 inches and kept that forecast through Tuesday.

So what caused the forecast to change?

The big reduction in snowfall totals was the result of an influx of dry air. The models didn’t really start to pick up on this until Monday night and Tuesday which is really late in the forecast process for such a big change. When it comes to dry air we often know it’s going to be there, and we also know that models tend to not drive that dry air far enough north.

This loop represents the European model’s depiction of the relative humidity around 18,000 feet and is valid at Noon on Wednesday. Each image is from a different run of the Euro going back to Sunday night.

On Sunday night you can see that the model placed the dry air over SE Missouri and gradually lifted it northeast into western Illinois by Monday evening. By Tuesday morning that wedge of dry air had shifted east to the Illinois River Valley.

When we saw this trend it became clear we were going to see less moisture than was previously advertised, so we started lowering totals. However, not all models were advertising this push of dry air and our hi-res guidance disagreed on where it would end up. Without any consistency we don’t want to get too carried away and start cutting totals too aggressively, after all some areas were still getting heavy snow.

You can see the impact this introduction of dry air had on the European ensemble snowfall predictions. Early in the forecast process models actually had this storm impacting southern Illinois but gradually shifted it north through central and northern Illinois. After spending a few days advertising heavier snow across our area there was a sudden reduction in amounts on Monday night and Tuesday. That sudden reduction was the model picking up on the influx of dry air.

So what caused the dry air to push further north?

There are a couple of possible reasons for this. One was the persistent rain that fell across lower Mississippi River Valley on Tuesday which limited a bit of a northward push of moisture from the Gulf. However I think the bigger issue was how the two separate upper-level waves were going to phase with one another.

The initial forecast was for a deeper wave from the northwest to phase with a wave coming out of the southwest over the Central Plains. This would result in a slightly stronger area of low pressure that would do a better job at locking up that dry air to the southwest. Instead, the phasing of the two waves was a bit more sheared and that allowed a wedge of dry air to shoot between the two waves.

You can see this in the loop below with each wave circled in red. The differences between the initial forecast and the latest were subtle, but more than enough to result in a significant reduction in accumulations.

We understand the frustration of a changing forecast, but at the end of the day, they are just predictions. Weather forecasting is extremely complicated with a lot of unknown variables that can greatly impact those predictions. We do our best to make the most accurate predictions with the best data available.

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