BIG WEATHER CHANGE LOOM FOR CHICAGO AS A POWERFUL SPRING STORM TAKES AIM ON THE MIDWEST WITH THE POTENTIAL FOR A SIGNIFICANT OUTBREAK OF SEVERE WEATHER LATE FRIDAY INTO FRIDAY NIGHT—-THEN A CLASSIC, NON-STORM HIGH WIND RISK BEGINNING IN SATURDAY’S PRE-DAWN HOURS AND CONTINUING INTO SATURDAY. 50+ MPH WIND GUSTS THREATEN AT THAT TIME

Northbound warm front sweeps across the Chicago area

Strengthening south/southwest winds send a flood of milder tropical air into the area and begins a surge in dew points which by late Friday going into Friday night will introduce the most humid air to the city (low 60° dew points) in more than 5 months—since October 11.

Low 60-degree dew points bring with them a ‘WARMER FEEL” to the air.  Combined with the mid to upper 60° temps predicted Friday, it means Chicagoans are in for the WARMEST FEELING AIR of 2023.

But the POWERFUL SPRING STORM responsible for the south winds and warming brings with it growing prospects for clusters of t-storms Friday and SETS UP A POWERFUL WIND REGIME IN CHICAGO.

It’s into POWERFUL UPPER AIR WINDS, predicted be in place above Chicago later Friday and Friday night, that Friday’s warm air will ascend. Computer generated wind profiles at that time suggest a highly volatile atmosphere is to develop—one in which the ascending air will cool to saturation forming t-storms, rotating as this occurs. IT’S AN ATMOSPHERE PRIMED TO PRODUCE ROTATING SUPERCELL T-STORMS. It’s from such storms damaging winds and even tornadoes can occur. 

While sporadic waves of showers and t-storms begin sweeping into the area from the southwest late Thursday night and Friday—they’ll come and go—producing passing downpours and lightning—but with dry intervals mixed in.  BY LATER FRIDAY AND FRIDAY NIGHT—from current indications, in the 6 to 10 pm time frame Friday evening and night, that conditions may be favorable for a squall line or set of powerful t-storms to develop, some of these storm potentially severe or tornadic, and to sweep into the area from the west.  TORNADO WATCHES may be required.  

Modeling suggests a squall line could flare in eastern Iowa later Friday afternoon and race at highway speed (60 to 65 mph) eastward toward Chicago—reaching the Fox Valley 6 to 8 pm and proceeding to Chicago and into northwest Indiana in the hours which ensue. 

Round 1—overnight/early Friday morning—non-severe

Round 2—Early afternoon Friday—isolated severe storms

Round 3—Arrives between 6-10 p.m., strongest storm dynamics in place-—greatest risk for super-cellular thunderstorms capable of damaging wind gusts and possible tornadoes. 

• The passage of those t-storms will be followed by the POWERFUL “BACKWASH” WINDS TO HIT IN SATURDAY’S PRE-DAWN THEN INTO SATURDAY. These NON-T-STORM WINDS may gust to 50 mph and higher—the threshold for damage to vulnerable trees and unsecured outdoor objects, like lawn furniture. 
• TEMPS FALL BACK to the 40s—even the upper 30s Saturday—but surge back to the upper 50s to near 60 Sunday. 
• CHICAGO’s WEATHER PATTERN REMAINS ACTIVE NEXT WEEK with another large, windy spring storm likely to sweep with more showers and t-storms and a return to 60-plus-degree warmth likely Tuesday into Wednesday.  It looks from this distance like another system with potential severe weather producing capability.

Official Chicago temperature records since 1942 show that the month of April has produced 70-degree temps in 100% of the years; 75-degree temps in 94% of years, 80-degree temps in 68% of years; 85-degree temps in 30% of years—we’ve even seen 90-degree temps in 5% of years. That doesn’t mean there won’t be some chilly spells—even some snow. But these cold spells grow less intense and less frequent with the passage of time.