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  • Tim McGill

April Showers & Trending Towards A Cooler Week

Updated: Apr 12, 2021

Looking Back At The Palm Sunday Outbreak

This weekend marks the 56 anniversary of one of the most deadly tornado outbreaks in history. The Palm Sunday Outbreak produced 55 tornadoes across six states. 271 people lost their lives and another 3400 were injured. The damage toll reached nearly $200 million in 1967 or more than $1.5 billion in today's dollars.

There were a total of four confirmed tornadoes in northern Illinois on April 11, 1965. The most devastating tornado here formed near Crystal Lake and eventually widened to nearly a quarter of a mile. It moved quickly northeast crossing the Fox River and then tore through Island Lake. Five people were killed in Crystal Lake and it later claimed another fatality in Island Lake. The tornado was rated an F4 on the Fujita scale with estimated winds between 207 to 260 mph.

Mostly Quiet But Cooler

More April showers coming today and Sunday to help put a dent in the rainfall deficit. Only a little over a third of an inch of rain has fallen in the past fifteen days. O'Hare is 1.85 inches of rain below average since March 1st. As I mentioned yesterday, this weekend's rain could come just in time to prevent a drought in some spots from developing further.

The combination of showers and a brisk breeze off the lake will keep highs in the middle 50s or just about average for this time of the year.

Expect 50s for highs today and tomorrow. The biggest threat of rain is late this afternoon and evening. There could be a few thunderstorms south of I-88 that produce gusty winds. The showers should be more scattered on Sunday.

Keep The Umbrella Handy For Now

The GFS model shows the best chances for rain late today into early tonight. It also shows a period of showers Sunday afternoon into early Monday.

The GFS model squeezes out a little over a half inch of rain this weekend south of the city to nearly an inch and a half north of Chicago.

April To Trend Cooler

The hourly temperature meteogram compares many different forecast models. Even though Friday's high of 62° was 20° cooler than Wednesday's high of 82° it was still a bit above average. We will fall to about average today and tomorrow with highs in the middle 50s and then a bit below average for Tuesday and Wednesday next week. Highs Wednesday and Thursday may fail to get out of the 40s along the lakefront.

The GFS model's temperature anomaly forecast has northern Illinois dropping to around 5° to 10° below average by Tuesday afternoon. That means highs here in the lower 50s.

Consistent Cooler Pattern Signal

The month of April started off mild to warm. We've already seen a few days with highs in the 70s and a couple with highs in the 80s. The longer range forecasts suggest we shouldn't get used to it. The latest 6-10 day temperature probability forecast has us outlooked for below average temperatures overall from April 15th through April 19th. The 8-14 day temperature probability forecast has us favored for below average temperatures overall from April 17th through April 23rd too. The forecast signal for a cooler pattern has only gotten stronger in these most recent forecasts with higher probabilities of being below average. Average highs in Chicago move from the upper 50s to near 60° during this period.

The even longer range Subseasonal Experiment (SubX) forecast is signaling a warmer than average end to April. The temperature anomaly forecast for the week ending April 23rd has us outlooked to be above average for the period. The forecast for the following week ending on April 23rd has us outlooked to stay above average overall. Average highs for the end of April are in the middle 60s.

Drier Long Range Forecast

The longer range precipitation probability forecasts have been signaling a drier than average pattern the past few days. Hopefully this weekend's rainfall will help hold of any further drought development. The latest 6-10 day forecast has Chicago favored for below average precipitation from April 15th through April 19th. The longer range 8-14 day forecast has us outlooked for below average precipitation overall from April 17th through April 23rd too. The probabilities of this drier pattern have gone up in these most recent forecasts. The risk for spring flooding is above average for many area rivers. More on that below.

Spring Flood Risk Forecast

The National Weather Service considers many risk factors for spring flooding. They include snow cover, soil moisture, and current river conditions.

"A significant snow cover with high water content can increase the chances of flooding once warmer weather melts the snow. Elevated soil moisture conditions reduce the amount of rainfall that is soaked up by the ground and increase the amount of water that then runs off into area streams. Above average river levels reduce the river rise required to reach flood stage, while below average river levels would require an increased amount of river rise to reach flood stage."

The map shows the locations where there is a greater than 50% chance of moderate or minor flooding during March through May 2021.

Their latest forecast suggests the flood risk is above average for many of our area rivers. The Des Plaines and Fox rivers in particular need to be watched closely.

Here is my 7 day forecast:

Today: Cloudy, showers mainly late (t-storm possible south), breezy & cooler High: 56

Sunday: Fog early, mostly cloudy, scattered showers Low: 46 High: 57

Monday: Partly to mostly sunny, breezy Low: 45 High: 62 (cooler lakeside)

Tuesday: Partly to mostly sunny, breezy Low: 40 High: 53 (cooler lakeside)

Wednesday: Partly to mostly sunny Low: 38 High: 55 (cooler lakeside)

Thursday: Partly to mostly sunny Low: 40 High: 58 (cooler lakeside)

Friday: Partly to mostly sunny Low: 42 High: 59 (cooler lakeside)



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