By Vince Condella Published Apr 02, 2003 at 5:07 AM

The month of April is upon us and the fun is just beginning when it comes to interesting weather. We are not likely to get bored during any month of weather around here. But April presents its challenges and keeps things interesting. This is when the severe weather season begins in southern Wisconsin, yet winter leaves a few reminders from time to time.

Consider the warming of spring to continue as the normal high and low temperatures climb from 48/32 on April 1 to 54/36 on the 15th and then on to 60/41 on the 30th. Contrasts? Look no further than the weather records for April 2. In 1975 a late season snowstorm dumped 10.4 inches on Milwaukee. Two years later a tornado hit Brookfield, destroying eight homes and damaging 166. April is full of such side-by-side contrasts. The West Bend tornado of April 4, 1981 killed three people, injured 51, and destroyed 29 homes. The twister struck at 12:15 a.m, a dangerous time because many people may not hear the late-night warnings on TV and radio. April 9, 1973 saw a huge snowstorm paralyze Milwaukee with a foot of snow, 60mph winds, and 10-foot snowdrifts. Four years later it was Easter Sunday on April 10, 1977, and a high temperature of 83!

In a previous weather column, I wrote about the wavering jet stream and how it alternates between plunging southward with cold air and rising northward with warm air. What causes this meandering is still a mystery. I often imagine air masses like giant globs of molasses or Jell-O. (Hmm, I detect a food theme here.) The air masses cover hundreds of miles and contain either warm and humid air, cold and dry air, cool and wet air, etc. These areas move slowly across our country mainly from west to east, but sometimes north to south, too. As a warm and humid glob of Jell-O moves north from the southern Plains, it may push aside a cold and dry glob of molasses in the northern Plains. The jet stream is the great divider between these two air masses in the upper atmosphere. At the Earth's surface, either warm or cold fronts separate the globs.


If forecasters can know the future location of the jet stream, forecasts would reach an accuracy of 100%. The wavering and meandering nature of the jet stream is the biggest cause of a busted forecast. Sometimes the jet stream will strengthen in one section and produce what is called a "jet streak" or "jetlet." This is an area of faster winds embedded within a region of fast winds. For example, an area of wind with a speed of 100mph may be 500 miles long and 100 miles wide. Encased within this region is a jet streak with a speed of 130mph in an area 100 miles long and only 50 miles wide. These winds may occur at 30,000 feet above the ground, but their momentum and energy transfers down to weather systems on the ground, causing a cold front to speed up or a light rain system to strengthen to a line of thunderstorms.

Speaking of thunderstorms and the jet stream, the two seem to go together nicely, especially this time of year. So it's only appropriate that the week of April 7 -- 11 is Tornado and Severe Weather Awareness Week. Thursday, April 10, will be the day that the National Weather Service conducts their severe weather drills. They will issue test watches and warnings, and schools will put their students and staff through tornado safety procedures. It's a good time for all of us to consider the way we receive severe weather information at home or at work. Radio and TV stations are responsible for passing these watches and warnings on to you. But what if you aren't watching and/or listening to these media outlooks? What if severe weather warnings are issued in the middle of the night? The time is right to own a NOAA Weather Radio. More on this important piece of weather equipment in my next column.