Strong storms/heavy rains expected this weekend

The National Storm Prediction Center has outlooked the northern halves of Illinois and Indiana including the Chicago area, as well as southern Iowa and northern Missouri for a slight risk of severe thunderstorms Saturday, Saturday night into Sunday (see yellow-shaded area on outlook map below).

As low pressure develops in the central plains and slowly approaches our area from the west, southerly flow will strengthen and persist – steering warm moist air from the southern plains and Gulf of Mexico up the Mississippi Valley into northern Illinois, southeast Wisconsin and northwest Indiana – beginning Friday night and continuing through much of the coming weekend. In addition to the potential for damaging winds and large hail, frequent downpours from strong storms could give weekend rainfall amounts well over two inches across the Chicago area.

The National Weather Prediction Center has northern Illinois and northwest Indiana in areas of potential 1 to 2-inch rainfall Friday evening through Saturday evening (second map below) and again the 24-hour period from Saturday evening until Sunday evening (third map below). The one-inch areas are enclosed in a blue line, while brown lines outline 1.5 inches or more).

Map of Slight Risk of Severe thunderstorms Saturday into Sunday

Map of projected heavy rainfall 6PM CDT Friday – 6PM CDT Saturday

24-Hr Day 2 QPF

Map of projected heavy rainfall 6PM CDT Saturday – 6PM CDT Sunday

24-Hr Day 3 QPF

Thursday marks the first time in 4 weeks back to back days have remained precipitation-free

By Meteorologist Tom Skilling

It’s been a while—four weeks to be precise—since back to back days have remained completely rain free. But, it may happen Thursday. If no rain falls between now and midnight Thursday, Chicago will record its longest string of rain-free days since a four-day stretch this past June 13-16.

This underscores just how wet the meteorological summer 2014 period, which began back on June 1, has been. And it’s not just Chicago which is running wetter than normal. We highlight a series of Midwest cities which have logged large summer rain tallies—many of them WELL above normal—as part of our graphic feature Thursday.

Chicago’s rainfall since June 1 this year has reached 9.17”—twice normal and nearly 5” above the most recent 30-year July average. In only two of the 143 years since official weather records began here in 1871 has more July rain occurred by the 10th.

July’s opening 9 days here have been the coolest in 5 years

Not only has the metro area been wet, it’s been cool too! The first 9 days of this month have averaged nearly three degrees below normal and have generated an average temperature of 71.0-degrees—a reading which makes this the coolest July to date in 5 years and the 46th coolest of the past 143 years.

Lake Michigan’s waters running much cooler than a year ago

Not only has the air been cool, so too is Lake Michigan. The Chicago shoreline temp of 64-degrees is 8-degrees below last July 10’s 72-degree reading.

Abundant sunshine Thursday before warm-up

By Meteorologist Paul Dailey

As it slowly moves east, high pressure will dominate into Friday. Abundant sunshine and a light lake breeze come Thursday, then a warming trend sets in Friday as southerly winds take hold and humidity increases. The first chance of thunderstorms here will be Friday night. Then, warm, humid conditions with periods of showers or thunderstorms are likely as low pressure develops in the Plains and moves east. The center of low pressure, along with its associated cold front, will pass through Chicago early Monday.

A building high-pressure ridge over western Canada will cause a buckling jet stream to surge south of our area early next week. This shift will bring a cool, northwesterly flow aloft that will persist much of the coming workweek, with daytime highs 6 to 10 degrees below normal.