Winter's biggest snowstorm threatening 8 to 14" totals, near-blizzard conditions

A February snowfall of 10″ or more occurs on average only once every 16 years in Chicago. Only five such storms are on the books at Midway Airport in the 81 years since 1929.  Tuesday’s storm may well become the sixth!  Before it finishes with the Chicago area, 8 to 14 inches of snow may accumulate–the season’s heaviest to date and the biggest storm to occur here since nearly a foot fell here over two days just over a year ago in late January  

The current system’s impact on the Chicago area is only in its early stages as Tuesday gets underway. Three rush hours may ultimately be impacted by the system–none more than Tuesday evening’s when snowfall will be at its height and winds will be gusting to 25 mph–strong enough to begin sending the additional 4 to 7″ of snow predicted to fall Tuesday airborne in some open areas surrounding the city. That’s also the period in which an infusion of lake moisture is to begin, supplementing snowfall. Lake enhancement of snowfall could end up spanning 10-14 hours, extending into the opening hours of Wednesday morning even as non-lake-effect snow tapers to passing flurries at inland locations. It’s an important reason this storm’s heaviest snow totals are predicted to occur in lakeside counties of Illinois, Wisconsin and Indiana.  

Explosive intensification of the storm is predicted by computer models later Tuesday as a secondary center forms off the mid-Atlantic Coast. Storms intensify when air rushes aloft at increasing speed. Ground-level winds strengthen as part of this intensification process. The rate at which air ascends increases, encouraging air to rush in from the storm’s periphery at faster speeds to replace the upwelling of air at the heart of the storm.

The impact of the current storm’s intensification across the Chicago area could become most noteworthy Tuesday night when winds approach their peak, gusting as high as 30 to 40 mph, particularly in areas surrounding the city.  Winds of that strength should easily lift snow and hurl it through the air, producing near-blizzard or blizzard conditions. A blizzard is defined by the National Weather Service as three or more hours with winds gusting to or above 35 mph, and during which blowing snow reduces visibilities to a quarter mile or less. The density of structures such as homes and buildings in the city and close-in suburbs often generates enough drag on the moving air to subdue velocities there.  But this doesn’t happen in open areas surrounding the city, and these areas appear particularly vulnerable to potentially significant blowing and drifting snow Tuesday night.
Latest storm producing 1,300-mile corridor of snow; “Bosnywash” corridor being hit hard again

As unbelievable as it may seem, the Nation’s Capital–where a snowfall of 1 to 3″ can cause serious travel problems–is under the meteorological gun once again. Just three days from one of the region’s worst blizzards in history, forecasters are predicting 10 to 20 inches of new snow may fall as winds strengthen later Tuesday. This promises a rash of new travel problems.

Lines on weather maps

Dear Tom,
What do the lines on weather maps mean?

–Jenna Jamieson (5th grade), Bloomingdale, Ill.

Dear Jenna,
Meteorologists use weather maps to display information such as temperatures, air pressure and winds over large areas. The maps give forecasters the ability to see the big weather picture and, at a glance, to understand “what’s going on” with the weather. That information is the starting point in the preparation of weather forecasts. Many different lines are drawn on weather maps, but the most common ones are called “fronts.” Fronts show the boundaries between different kinds of air masses (such as hot and cold, or humid and dry). Lines called isobars frequently appear on weather maps as well. Isobars connect locations that have the same air pressure, and they show where the air pressure is low (usually with stormy weather) or high (usually with fair weather).

Patchy flurries in the Chicago-area of heavier snow moving north from central Illinois

Shortly after 9pm scattered flurries were reported in the Chicago area but radar indicatd a larger area of steady light snow spreading north from central Illinois.  Snow should become widespread across the region in the next few hours and once it begins will continue steadily through the night.  By morning accumulations are expected to be in the 2 to 4 inch range, just the opening shot of a major snowstorm that is expected to continue through Tuesday and Tuesday night with snowfall totals reaching 8 to 14 inches before the snow finally ends early Wednesday morning.

The heaviest snow totals are expected to occur in areas close to Lake Michigan where lake-enhanced snowfall is expected to contribute several inches to the storm totals.
A late report indicated about 3/4 of an inch of snow down in the Rockford area. 

Light snow slowly moving into the Chicago area- Heavy snow in Iowa

At 8 pm radar showed an area of light snow working north and east into the Chicago area. The snow has been slow to develop across northeast Illinois as it has been necessary to saturate a layer of dry air over the area before the snow can reach the ground.

Snow was being reported at Aurora. West Chicago, Morris  in northeast Illinois at 8 pm and as falling across nearly all of central and western Illinois. The snow should gradually increase across the Chicago area in the next few hours with steady snow falling in most area by midnight. 
Reports coming in from north-central Iowa indicate heavy snow fell there today.
Algona 9.0 inches
Grundy Center 6.0 inches
Webster City 8.0 inches
Eldora 8.0 inches