Tim's Weather World: Quiet Start To Severe Season So Far…

We made it through the first couple of weeks of March without one single report of severe weather in Illinois.  There are signs our weather pattern is becoming a bit more “progressive” or volatile in the coming weeks and that will increase the chances for severe weather.

In the meantime, while we wait for more active weather, we can prepare.  Chicago’s National Weather Service office is offering severe weather spotter training over the coming weeks.  Check out their schedule to find training near you.


supercell.jpgIf you can’t make it out to the training, you can train online.  Skywarn has provided some online severe weather training guides.

You can also print out a preparedness guide offered by NOAA and the Red Cross.

Last but not least, the Storm Prediction Center is on the front lines when it comes to preparing for severe weather.  They issue “convective outlooks” every day that outlines areas with potential risk for severe weather.




Jarring shift from 70-degrees to possible wet snow ahead

March is to put on quite a meteorological show in coming days, strutting the weather of two distinctly different seasons before Chicagoans over a span of just days. Unseasonable warmth Thursday and Friday—including the year’s highest daytime readings to date—is to yield to a wintry temperature downturn this weekend which may well set the area up for wet snow. It’s sure to be a jarring change for many in the Chicago area with the wintry tone of the weather shift in conflict—at least in spirit—with spring’s 12:32 p.m. Saturday arrival.

The year’s mildest temperatures are due first—including a possible 70 degree high Thursday. It’s a development which would propel this month into the ranks of the 70 percent of Marches over the past 140 years which have produced a 70-degree temperature. Not since a 73-degree high on Nov. 8 has the mercury warmed to that level here. The production of such warmth this early in the season always depends on the  perfect alignment of a host of factors.  West winds must overcome the tendency for the 2,000 ft. deep dome of cold, dense air, which hugs the chilly lake surface this time of year, to surge inland. And clouds must not seriously screen out sunlight which is especially critical in producing early season warmth.

It appears both conditions will be met Thursday—though the wild card will be high clouds expected to move across area skies as the day proceeds. Air sinking on a broad scale beneath a band of powerful jet stream winds is expected to produce enough warming to offset at least some of the impact of these clouds.

A 70-degree temperature would come one week earlier than the most recent 50 year average date of March 24 and nearly a year to the day of last year’s first 70, which occurred last St. Patrick’s Day.
Set-up behind Chicago’s Thursday warm-up produced high winds, record-breaking temps in Montana
Sinking air beneath the same pocket of powerhouse jet stream winds expected to traverse Chicago airspace Thursday and Thursday night, helped produce record highs and powerful downsloping winds in Montana Wednesday. Record highs resulted at Billings (73-degrees) and Miles City (71).
March 2010 more the 5-degrees above normal
Chicago’s March temperatures are running more than 5-degrees above normal to date and more than 3-degrees ahead of the same period a year ago. That’s warm enough to place its opening 17 days among the warmest 13 percent of comparable periods since 1871.
Models hint at sticking weekend snow here or nearby—but stay tuned
Computer models suggest sticking snow may accompany this weekend’s temperature crash over at least portions of the Chicago area. A suite of 29 different model forecasts put average snowfall Saturday afternoon and night around 4 inches. But it’s a forecast which is hardly carved in stone. Predicting accumulations in late season snow systems, particularly when they follow air masses as mild as the one in place Thursday and Friday, is extremely challenging.

109 of the past 125 years—that’s 87 percent of them—have hosted accumulating snow beyond March 18.  It’s a sign winter isn’t over yet.


What is the earliest date for the last measurable snowfall of a season in Chicago?

Dear Tom,
What is the earliest date for the last measurable snowfall of a season in Chicago?

–Arleen Hambach

Dear Arleen,
It seems hard to believe, but Chicago’s earliest last measurable snow has twice occurred in February. The earliest finish to a snow season was Feb. 27, 1997, when 0.5 inches of snow was measured. Beyond that only traces were recorded in March and April with the season’s last flakes on April 13. Close behind was the winter of 1993-94 when the 0.8 inches that fell Feb. 28 was the season’s last measurable snow. That year the final flurries were observed April 30. Despite the early end to those snow seasons, the seasonal totals were close to the city’s average of about 40 inches. The winter of 1996-97 produced 40.6 inches while the winter of 1993-94 racked up 41.8 inches.

Dense Fog Advisory until 10AM for the far western suburbs

At 5:45 this Wednesday AM, dense fog covers much of western and central Illinois.  The fog has crept into some of Chicago’s far western suburbs, and a Dense Fog Advisory is in effect until 10AM for Boone, DeKalb, LaSalle and Grundy Counties.

Motorists traveling from Chicago into western and central Illinois along I-88, I-80 or I-55, will be forced to slow down due to visibility of 1/4 of a mile or less.

The fog will begin to ease in many areas by 9AM. 

Operations at both O’Hare and Midway should not be impacted by the fog. 

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Area headed for 2010's warmest temps yet; first 70 in 4 months possible

Bountiful St. Patrick’s Day sunshine isn’t to be the last in coming days. There’s more great weather on deck—though there are also signs Chicago’s weekend weather could become dicey with rain and even the threat of wet snow. Storms must be monitored closely this time of year. Their varied temperatures can take rain to snow despite a period of warmth. This storm’s impact on the Chicago area is still days away and will depend on a variety of factors including its future track.

Snow is to be the last thing on people’s minds Wednesday. Temperatures surged past 60-degrees Tuesday– topping out at 61 at O’Hare and as warm as 69 at Elgin, 68 Itasca and 67 in Oswego and 66 at Blue Island. These readings are 18 or more degrees above normal—and Wednesday looks even warmer. The 65 predicted at O’Hare Wednesday afternoon would immediately become 2010′s warmest to date and would arrive nearly a year to the day of last year’s first 70-degree reading—a 74-degree high which occurred on today’s date in 2009. 

But it’s Thursday which stands a good chance of becoming the warmest of the current mild spell. That’s when the city’s official high is predicted to move to within striking distance of 70-degrees. Large scale subsidence of air beneath the incoming nose of a powerful jet stream is to combine forces with Thursday’s generous sunshine and strong west winds capable of overcome lake cooling to produce the first 70-degree reading of 2010 nearly 4 months after the Chicago area’s last 70 on Nov. 8.

Warm weather isn’t surprising this time of year nor is it mis-timed. In the past decade, March 22 has been the average first date for a 70 here—and March 24 has been the average over the past half century of readings at O’Hare.
Tuesday becomes one of only 10 days in the past 140 years with relative humidities in the teens
Remarkably low relative humidities (RH) across the metro area played a role in producing Tuesday’s warm highs. Dry air heats expeditiously. A plunge in RH readings observed just after noon Tuesday coincided with rapid warming. The appearance of low humidities occurred as winds helped mix exceptionally dry air, which had hovered just above the surface since Monday, down to ground-level.

Chicago recorded 90 percent of its possible sunshine Tuesday bringing the March average to 43 percent—52 is considered “normal” here in March.

Windy Seattle storm could head toward the Midwest
The storm system which may impact the Midwest this swept hit Seattle and Pacific Northwest with 60+ mph winds Tuesday, forcing bridge closures and producing power outages. It produced wind gusts of up to 69 mph at Tatoosh Island WA and 46 mph gusts in Seattle proper. At least 6,000 were reported without power in the region.

Despite the unseasonably mild weather predicted in Chicago over coming days, the weekend storm is to bring rain to the area which could mix with or change to wet snow Sunday. March in Chicago has never closed without at least a trace (0.1 inch or less) of snow.