HUGE NORTH/SOUTH CHICAGO AREA TEMP SPREAD–FROM 40s NORTH LAKESHORE TO LOW 80S SOUTH SUBURBS (59 AT O’HARE)–BUT WARM FRONT TO SWING THROUGH AND PUMP WARM AIR INTO THE AREA–T-STORM CLUSTERS SPORADICALLY IN THU EVENING WITH 70s THURSDAY–BUT MARKEDLY COOLER PATTERN FOR THE WEEKEND AND BELOW NORMAL TEMP REGIME TO DOMINATE NEXT 2 WEEKS

WEDNESDAY AFTERNOON SATELLITE VIEW OF THE LATEST SPRING STORM SYSTEM which is sending warmer air into Chicago overnight and Thursday:

A CLUSTER OF HAIL GENERATING T-STORMS PORTIONS OF THE METRO AREA—generating reports of hailstones ranging in size of 0.75″ to as much as 2″ in diameter.

The storms have flared as two very different air masses clash. Chicago area is bisected by a northbound warm front–with mid afternoon temps ranging from 41 at Waukegan Harbor, 42 at Kenosha, WI, 43 Lake Forest, 48 at the offshore Harrison-Dever Crib, 50 Waukegan and 54 at Belmont Harbor—–to 82 at Dwight, Kankakee, Peotone and Roselawn, IN. The near 80 deg temps had swept as close to Chicago as Gary and Ogden Dunes in northwest Indiana.

As the warm front swings north Wednesday night, the southeast winds off chilly Lake Michigan (with water temps averaging 44°) winds shift “SSW” and the warm air downstate will spread over the Chicago area. Temps downstate at mid afternoon Wednesday include 84° on the University of Illinois campus in Champaign/Urbana, 88° in Terre Haute, IN and 80° in both Bloomington and Springfield. By the time that air reaches Chicago, 60s will sweep into the Chicago area and surge into the 70s for a day Thursday.

But the warm, unstable air downstate has led to some hail producing t-storms. They’ve occurred in clusters–with sizeable rain-free areas between them. They’ve also been hail producers with ping pong to golf ball size hail in a few of the more active storms downstate–and healthy 1″ diameter hailstones showering down on northwest Illinois near Dubuque and Galena north into southern Wisconsin–including the Madison area.

HOURLY PRECIP FORECASTS Thursday afternoon and evening—NOON CDT Thursday through 12. a.m. FRIDAY:

That’s the way it appears any t-storms are to occur in and around Chicago once the air mass moves in tonight. They’re likely to be scattered, impacting 20 to 30% of the area at one point or another ovenight and Thursday morning.

An afternoon cold front may consolidate and increase t-storm coverage Thursday afternoon and evening. While a shower or t-storm could pop at any time, modeling suggests PARTS OF THE AREA may see increased coverage toward and an hour or two beyond lunchtime–and even better coverage appears possible in roughly to 4 or 5 pm to 9 ot 10 pm period Thursday evening.

I’ve posted a succession of hourly precip forecasts off the NWS/Storm Prediction Center’s HREF model so you can see the modeled evolution of showers and t-storms Thursday afternoon and evening.

THERE CURRENTLY MODEST LOOKING CHANCE A FEW STORMS MAY BE SEVERE—the risk level currently assessed a “1” on the 5-level STORM PREDICTION CENTER “risk scale”–but mentionable nonethless.

SLIDESHOW: Spring Storm Precip, Temps & Winds

  • STRONG SOUTH WINDS indicate Wed afternoon just south of Chicago will sweep much warmer air into the area overnight and into Thursday
  • 24 hour T-STORM CHANCES for the period ending 7am CDT Thursday
  • 24 HOUR T-STORM CHANCES Thursday and Thursday night
  • 24 HOUR T-STORM CHANCES Thursday and Thursday night
  • MOISTURE LATER THURSDAY as the cold front arrives fueling potential t-storm development
  • ESTIMATED RAINFALL from the incoming SPRING STORM
  • JET STREAM FORECAST Thursday afternoon/evening as cold frontal t-storms reach the area. T-storms can reach into these high altitude wind fields have divert wind energy to the surface in these storm’s wind gusts
  • NWS HREF MODEL estimate of average rainfall through Thu night
  • WIND GUSTS Wed afternoon
  • WED AFTERNOON wind gusts
  • FORECAST THURSDAY HIGH TEMPS
  • The storm system web of isobars (lines of equal pressure) offer an insight into the scale of the wind field it’s producing
  • OBSERVED TEMPS WED AFTERNOON
  • 24 HOUR TEMP CHANGE WED AFTERNOON indicating the impact of the northbound warm air from the Gulf of Mexico
  • OBSERVED WIND GUSTS Wed afternoon
  • TEMPS AROUND MIDDAY WED
  • CHICAGO AREA’S WIDELY VARIED TEMPS WED AFTERNOON—ranging from the 40s along Lake Michigan north of Chicago and the 80s to the south which extended north into northwest Indiana
  • SNOW IS TO ACCUMULATE in the northern Plains and far northern Midwest
  • ESTIMATED PEAK WIND GUSTS BETWEEN NOW AND FRIDAY EVENING WITH THE INCOMING SPRING STORM. NOTE: T-storm gusts can exceed the gusts indicated here
  • SEVERE WEATHER RISK DAY #1 (Wed & Wed night)
  • SEVERE WEATHER RISK DAY #1–Wed & Wed night
  • Here’s the probability of severe weather taking the form indicated here
  • SEVERE WEATHER RISK DAY #2–Thu & Thu night per the NWS Storm Prediction Center
  • THE NWS HREF model’s estimate of peak wind gusts with Thursday t-storms

FRIDAY QUIETS DOWN–precip chances ebb. Indications of a second chillier band of rain has eased Friday night into Saturday—but isn’t “ZERO”. Models have shifted a secondary low pressure farther east–but worthy of keeping an eye on just in case the western edge of the precip shield reaches into the Chicago.

THE BIG STORY NEXT WEEK INTO THE WEEK THAT FOLLOWS is the COOLER THAN NORMAL TEMP REGIME which appears to be setting up. From this Saturday forward through the following weekend and into the early days of the week which follows, daily average temps are predicted to average 4 to 12-deg below normal which translates to a majority of daytime highs hold to the 50s at a time of the year when mid 60s are more common.