Chicago enters an ABOVE NORMAL TEMP REGIME for the time of year—a pattern likely to hold over the coming 2 weeks. Plus precip will be BELOW NORMAL—with the heavy rainfalls held well to the city’s west over an area near and west of the Mississippi and in the Plains.  There, some heavy rain totals are expected—but NOT HERE.

A persistent easterly low level wind flow will keep winds coming in off the lake over the coming week.  But the overall air mass is WARMER THAN NORMAL by latest September standards. So while there will be some cooling each day as winds come ashore off the 68-deg southern Lake Michigan waters, inland readings will first with 80-deg again Friday and Saturday and in the mid 70s Sunday—ABOVE THE NORMAL HIGHS IN THE LOW 70S THIS TIME OF YEAR.


This week is to finish 3-deg above normal and next week is likely to average 4-deg ABOVE NORMAL.

The clock is ticking toward the AUTUMNAL EQUINOX—which is the astronomical start of the fall 2023 season. The EQUINOX takes place at 1:50 am CDT Saturday morning—in other words it arrives tomorrow (Friday) night. It’s the moment at which the sun’s most direct rays are falling on the equator. The equinox is also the day where both the northern and southern hemispheres see approximately equal days and nights. 

We’re losing just over 2 min and 40 sec of daylight each day—and will continue to do so well into October, which begins a week from this Sunday.

Since our longest day—June 21st—Chicagoans have watch 3 hours on daylight disappear—and the coming 30 days will see the loss of another 83 minutes of daylight (between today and October 21st).

The coming weekend is the final FULL weekend of September and the persistence of an ABOVE NORMAL, COMPARATIVELY LOW PRECIP PATTERN across the GREATER CHICAGO AREA is because the North America weather pattern is getting tied up in a developing ATMOSPHERIC BLOCKING PATTERN. The type of block involved in the current weather pattern—and likely to remain an important factor in the coming week is what’s known in the meteorological trade as a “REX BLOCK”–named for the meteorologist Daniel F. Rex, who in 1950, first described the phenomenon. 

You CAN hear REX BLOCKS referred to as ‘HIGH OVER LOW” blocking patterns—since that’s what happens when these blocks take shape—THERE’S A HIGH PRESSURE sprawled above a LOW PRESSURE SYSTEM. The jet stream splits and moves around the BLOCK

Blocking patterns slow weather system movement to a crawl.  The split flow which develops aloft around the atmospheric block means weather systems are essentially routed around the block. In THIS particular REX BLOCK, it appears a sprawling surface high pressure system is to take up residence over Quebec in Canada and ridge southwestward into the Midwest.  This is to keep a low level easterly flow blowing into Chicago over the coming week which will greatly restrain the flow of deep moisture into the Chicago area—and that will limit precipitation.  

There are light showers scattered in the area Thursday evening and night. There may even be an isolated t-storm in spots. But BIG RAINS are being held at bay by the blocking pattern—so rain tallies will be light.

When you sit beneath an atmospheric block—as the Chicago area will over the coming week—the temp pattern you’re in holds—it’s more of less locked in.  Today’s Chicago temps come in about 10-deg above normal—and day to day temp surpluses through next Wednesday will average frok 4 to 8-deg ABOVE NORMAL.

There will be several spells of scattered light showers in coming days. There are likely to be impressive cumulative rainfalls 

The anticipated development of Tropical Storm Ophelia off the Eastern Seaboard will only reinforce the block. Air venting out the top of the windy, heavy rain producing storm system expected to impact an area of the East Coast from North Carolina northward, including big cities like Washington, Baltimore, Philadelphia and ultimately New York with local 4-7″ rain, a pounding Atlantic surf and ABOVE NORMAL TIDES with coastal flooding, migrates away from the system and sinks—strengthening the blocking pattern.

TONIGHT: Cloudy, some scattered mainly light showers—though an isolated thunderstorm can’t be ruled on in spots.  Mild and hazy.  Low 66.

FRIDAY: Sporadic cloud breaks allow SOME mixed sunshine. Warm for the season.  Chance of a few spotty sprinkles or a shower or two mainly west of the city and with very limited areal coverage. (i.e. less than 10 to 15% of the area)  High 78—but low to mid 70s lakeshore.

FRIDAY NIGHT: Becoming cloudy. Mild. A shower or two possible west of the Fox Valley. Low 65.

SATURDAY: Turning partly sunny, similar daytime temps. High 79—but low to mid 70s again on area beaches.

Full forecast details and more at the WGN Weather Center blog

SUNDAY: Mixed sun gives way to more cloudiness in the afternoon. Sprinkles possible at night. High 76—but upper 60s to near 70 along the lake.

MONDAY: Mostly cloudy, a bit breezy from the east/northeast.  High 74—upper 60s lakeshore.

TUESDAY through THURSDAY: Partly sunny days and partly cloudy nights. Temps continue 4 or 5-deg above normal.  High Tuesday 74; Wednesday’s high 73.  High Thursday 77—-modestly cooler each day along Lake Michigan.