An unusually strong August storm threatens additional heavy, occasionally thundery downpours that are likely to continue sweeping the Chicago area—particularly the area from the I-80 to I-88 corridor northward—from time to time through midnight to 1 a.m. Tuesday morning.

Additional showers, more sporadic, are to continue beyond that into the opening hours of Tuesday. 

The storm has already dropped 1.50″ of rain at Midway and just over 0.26″ at O’Hare as of the 3:30pm post time for this message and the latest forecast.  But the rains are hardly over.  It’s widespread rainfall totals of 0.90″ to as much as 2.70″ are likely to fall with the potential for a few heavier totals of, as much as 2.50″-5″ not out of the question, given the involvement of t-storms. The Chicago area—particularly from the I-80 to I-88 corridor—is being advised of AN EXCESSIVE RAINFALL RISK by the Weather Service’s Weather Prediction Center.   

The system has dumped as much as 3.50″ on Arlington, WI — north of Madison — and 3.40″ at Beaver Dam with 2.98″ down at Waunakee, WI — all near and just north of Madison — giving you an idea of the moisture producing potential of this system. 

In addition, WELL ORGANIZED, “FULL FETCH” north/northeast winds (NOTE: WIND  are designated “FULL FETCH” when they run south the ENTIRE LENGTH OF LAKE MICHIGAN). This system’s strong winds are to build over Chicago and the surrounding area tonight through Tuesday — nowhere more so than along Lake Michigan and out over the open lake. Traveling, as FULL FETCH WINDS do, the 280 mile length of Lake Michigan,  gives these winds plenty of time to churn the lake, generating huge swells in the process. Meteorologists say this amounts to a transfer of kinetic energy from the lower atmosphere to the lake surface. 

Wind gusts over the shoreline and open waters of the lake will build as high as 30 to 45 mph at times tonight into Tuesday generating waves which could reach or exceed 5 to 9 ft. on the open lake. This IS NOT the sort of situation our mariners want to be deal with out on the lake.

THE WAVES AND POTENTIAL FOR DEVELOPING RIP CURRENTS WILL MAKE THE WHITE-CAPPED LAKE DANGEROUS FOR SWIMMERS AS WELL TONIGHT AND TUESDAY—which is why a BEACH HAZARDS STATEMENT HAS BEEN ISSUE.

The strong winds are to continue through Tuesday and finally to diminish — and rather rapidly — as the storm system moves on Tuesday night with wind velocities dropping to 3 to 7 mph away from the lake by late Tuesday night. 

Clouds and rain have held temps down tonight and the full fetch NNE winds will hold temps to the mid to upper 70s well inland as cloud breaks develop allowing sun to emerge by lunchtime and during the afternoon Tuesday.

Another chance of some t-storms may occur late Wednesday night and a portion of Thursday.

The other big weather story is heat and humidity — Perhaps some of this summer’s hottest readings — predicted to develop over the coming weekend and into the first half of next week. A series of 90-deg and hotter days is likely with dew points surging — means the heat will be accompanied by high humidities, thus likely producing triple digit heat indices from Sunday forward through next Tuesday.

Models are building a huge dome of heat over the Midwest during that period. That can cap the t-storm producing capability of the atmosphere.  But t-storms along the northern flank of such heat domes can prove finnicky and difficult top nail down with precision. T-storms are nature’s air conditioners—but their failure to develop would take the lid off the atmosphere’s ability to heat.  So the potential for this hot spell will have to be monitored.

In general, a HOTTER, DRIER (in terms of rainfall) WEATHER REGIME appears likely to take hold  Sunday through Tuesday.

To date, the city’s hottest temps have hit 93 deg this summer. Absent cooling t-storms, this air mass could produce mid to upper 90s—particularly in the Sunday through Tuesday time frame this weekend and next week.

Here’s my latest Monday Chicago metro forecast:

Flood watch through midnight through portions of the metro area, particularly areas from the I-80 to I-88 corridor and north..

A beach hazards statement is out on area beaches from tonight into Tuesday evening due to building waves and the risk to swimmers of rip currents.

TONIGHT: Waves of showers and some downpour-generating t-storms at times through midnight—then showers become more scattered. Turning windy—especially in open areas and area along and out over Lake Michigan.  Low 65.

TUESDAY: A few lingering showers possible into mid morning. Then slow clearing and windy with lowering humidities. Some gusts to 30 mph along Lake Michigan and in open areas. High 74—but 60s lakeshore.

TUESDAY NIGHT: Clouds diminish, rapidly diminishing winds. Cooler. Low 60.

WEDNESDAY: Partly sunny, warmer with much calmer southwest winds. High 86.

THURSDAY: Possible morning showers and t-storms impacting up to 40% of the area. Then becoming mostly sunny and fairly windy. High 81.

FRIDAY: A good deal of sunshine and warm with moderate humidities by afternoon. High 86—but lake breezes from the east hold beach max temps to the upper 70s,

SATURDAY through MONDAY: A good deal of sunshine with afternoon temps becoming hot and elevated humidities making the weather increasingly uncomfortable—especially Sunday and Monday.  Day to day lake breezes may temp the heat on are beaches. High Saturday 94; Sunday’s high 95 and Monday’s high around 96 inland—but 80s on the immediate lakeshore.  Peak inland head indices may reach the mid 90s Saturday and the low 100s Sunday and Monday.