Daily Weather Explainer

• 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.

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• 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-...

• Mid-Atlantic hit with blizzard conditions/three-foot snowfall

  As low pressure moved out to sea Saturday, the mid-Atlantic states of Virginia, Delaware, Maryland, New Jersey and Pennsylvania were digging out of record snowfalls. Totals in the Washington,D.C. area ranged from a low of 17.9 inches at Ronald Reagan National Airport to 40 inches in the northern...

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Feature Graphic

• Long-lived 8-14" storm may join 5 other 10"-plus February snows here

  

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Ask Tom Why

• 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,...

• The speed of falling raindrops

  Dear Tom,
  You've noted more than once that large raindrops fall faster than small ones. What circumstances cause this apparent contradiction to what Galileo learned, namely objects of different weights fall at the same speed?
  -- Dennis McGann, New Lenox
  
  Dear Dennis,
  
  At the Leaning Tower of Pisa, Galileo...

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