WGN Weather Center Blog

Chilly, wind-driven rain is expected to pummel the Chicago area Sunday, forcing
Mother’s Day celebrations indoors.

More than an inch of rain is likely to fall in many locations in heavy downpours before
the rains diminish to light showers in the afternoon. High temperatures will struggle
to reach the lower 50s, and as strong northeast winds gust in excess of 40 m.p.h. in
the afternoon, readings should drop back into the 40s, making 2008 the coolest
Mother’s Day here since a 50-degree high back on May 12, 2002.

DEADLY TORNADOES SWARM SOUTHWEST MISSOURI

Numerous tornadoes struck southwest Missouri late Saturday. Early reports indicate
three were killed, with many injured and extensive damage. Towns hit included
Neosho, Newtonia and Seneca.

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Dear Tom,
What is the difference between a hurricane, a cyclone and a typhoon?
—Jessica Vega

Dear Jessica,

There is no difference. Unfortunately, meteorological jargon and popular usage
variously apply different or overlapping meanings to those three words. In
meteorological parlance, a cyclone is a lowpressure system. Hurricanes and typhoons,
too, are low-pressure systems, but they are a specific kind of cyclone—tropical
cyclones (logically, cyclones that form in the world’s tropical and subtropical zones).
Hurricanes (in the Atlantic Ocean), typhoons (western Pacific Ocean) and cyclones
(Indian Ocean and Australia) are different names for the same type of storm. In
popular usage in the United States, “cyclone” is applied loosely to tornadoes,
waterspouts, dust storms, hurricanes and even to any strong wind.

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Saturday's sunny open isn't to last. Clouds marking the approach of a windy Mother's Day
storm expected to soak the area Sunday arrive as the day proceeds, but not before
Chicago temperatures reach the 60s away from the lakeshore. However, cooling east
winds are expected to restrict shoreline highs to the 50s.
LATE SHOWING OF BACK-TO-BACK 50s
The chill that has gripped the area since Thursday is more than a bit unusual by historic
standards. The 59- and 58-degree highs here Thursday and Friday are the first
back-to-back 50s to occur on a May 8 and 9 here in a quarter century. In addition, 80
years of weather records dating back to 1928 at Midway Airport indicate that fewer than a
third of mid-May highs fail to crack the 60-degree mark.
IT WAS WARM THIS TIME A YEAR AGO
By contrast, Chicago was in the midst of early-season warmth at this time last year.
Saturday's predicted 63 degrees falls 21 degrees short of the 84-degree high a year ago.
Interestingly, last year's first 90-degree day was to occur in only a week's time on May 14.
May 2007 ended up with 13 days of 80 degrees or higher.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune

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Dear Tom,
A lot of Chicago's moisture comes from the saltwater Gulf of Mexico, so why isn't our
rain salty?
Bob Johnson, Oak Park
Dear Bob,
Since the beginning of time, the saltwater oceans have been the reservoir of about 97
percent of this planet's water supply. The water is constantly being recycled,
evaporating into the atmosphere and returning to Earth as rain or snow in an ongoing
hydrologic cycle. Sea water is indeed salty, with about 35,000 parts of salt per million
parts of water. However, the salt in sea water is only dissolved in it and not chemically
bonded, so it is left behind when the water evaporates, like when a pot of saltwater is
boiled dry on a stove. That is the reason sea air is salty when the ocean's spray
evaporates, and also why precipitation falls as fresh water. When salty ocean water
surges inland as in a hurricane storm surge, it can kill vegetation.

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Runaway Runoff

Pavement, buildings and other waterproof surfaces cover a large portion of the land in urban areas like metropolitan Chicago. That means rain water cannot soak into the ground. Instead, it gushes into storm drains that discharge directly to local rivers and streams.

About 55 percent of rainwater that falls on a city runs off (and in intensely urbanized locations such as the Loop, runoff approaches 100 percent); only 15 percent of rainfall sinks in to recharge groundwater supplies. The remaining 30 percent evaporates back into the atmosphere.

For comparison, only 10 percent of rain that falls on natural ground cover (forests, wetlands or other natural areas) runs off, and about 50 percent soaks into the ground.

As rainwater runs over city surfaces, it picks up pollutants encountered along the way -- soil, oil and grease from cars, pesticides and fertilizers, pet waste and much more. Not only do such pollutants harm fish and wildlife, they also degrade drinking water supplies and often require that recreational waterways be closed.

What can you do?
If you plan to replace a walkway, driveway or patio in your yard this spring, consider using water-permeable surfaces such as wood decks, spaced bricks or concrete lattice instead of solid concrete or asphalt. These surfaces allow water to soak through to the ground, thereby reducing runoff from your property and helping to replenish groundwater supplies.

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Temperatures on Chicago's North Shore failed to break out of the 40s Thursday. Such
readings are more typical of late March than May. Wilmette topped out at just 48
degrees while Highland Park struggled to 49 degrees and Glencoe only made it to 50
degrees. Chicago's official 59-degree high at O'Hare International Airport was 8
degrees below normal and a far cry from an 82-degree high only a year earlier. It
marked the chilliest May 8 high here since the 51-degree high for the date 24 years
ago in 1984.

The cool weather continues Friday beneath increasing clouds and ahead of an
approaching disturbance that could spark a shower in spots late Friday.

LATEST SOAKER HITS DOWNSTATE

Unwelcome rains drenched the southern Midwest on Thursday. As much as 1.59
inches fell at Freeburg while 1.54 inches was measured at Flora—both east of St.
Louis in Downstate Illinois. The same storm spawned half a dozen twisters across
Tennessee, Alabama and Mississippi. Meantime, still another storm entering the
western Plains produced 68,000-foot-tall thunderstorms responsible for 70 m.p.h.
gusts at Lewis in southwest Kansas.

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