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April 30, 2008

May brings temperature surge, storm threats

The surging temperatures and gusty winds that mark the start of May on Thursday
bring two days of lake-cooling to a precipitous end. An accompanying moisture
increase, which will give the air a decidedly more spring-like feel, primes the
atmosphere for several showers and thunderstorms Thursday and a potentially more
important outbreak of storms and possible severe weather later Friday. Computer
wind trajectories indicate Thursday’s predicted 70s were over Tennessee, Mississippi
and the Gulf Coast near New Orleans only 24 hours ago.
May is the city’s third fastest warming month, with “normal” highs jumping from 64
degrees to 75 degrees.
Wednesday’s 31 degrees was the second sub-freezing low at O’Hare. Back to back
readings below 32 degrees have occurred this late in the season here in only three
other years in the past half century.

FROM 82 DEGREES TO A BIG SNOW—IN ONLY THREE DAYS!
Western South Dakota is bracing for near blizzard conditions. It was only Tuesday
that Rapid City topped out at 82 degrees. Up to 8 inches of wind-driven snow is
predicted there Thursday/Friday—and up to a foot in the Black Hills.

80 degrees may set stage for second consecutive Friday storm outbreak

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Major storm system’s path carries it over Chicago Friday

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When is the danger of frost and freezing temperatures in Chicago is past for good?

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Dear Tom,
When can I be assured the danger of frost and freezing
temperatures in Chicago is past for good?

Jeffrey Rayner, Chicago

Dear Jeffrey,

Light frost can form on plant leaves on a clear, calm night even when air
temperatures a few feet above the ground are as high as 38 degrees. A heavy, killing
frost usually requires a temperature of 32 or lower.

Because overnight temperatures on any given chilly spring night can, in
extreme cases, vary by 20 degrees across the Chicago area, it is difficult to pin down
an exact date. It really depends on your location.

Here are some guidelines: Near the lake, light frost (38 degrees) has never occurred
beyond May 31; heavy frost (32) never beyond May 14. Within about 2-6 miles inland,
the corresponding dates are June 4 and May 13; 7-15 miles inland, June 22 and May
29; farther inland, add seven days.

A second day of spring thunderstorms in Scotland

Stormy weather continues to sweep Scotland. Mark Vogan has sent us new
photographs of thunderstorms scattered across the beautiful Scottish countryside. He
tells us, “I took these shots this afternoon from our local hills overlooking our valley,
note the low-hanging dark clouds with black scud clouds which raced underneath the
slower moving main cloud base with a distinct rain shaft in the distance. Also note
the large collection of water on roadways.”

Thanks Mark for the excellent update and for the chance to see more of your
beautiful country! Stay dry!

-Tom Skilling

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Photos courtesy of Mark Vogan, Glasgow, Scotland

• View all April 2008 entries

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