CAPITAL OF THE U.S. WHAT THAT TITLE MEANS FOR THE NEW MAYOR'S REFORM EFFORTS.

>>> FIRST, OUR TOP STORY TONIGHT.

IF YOU ARE HEADING TO THE LAKE, NOT SO FAST.

GRAB A SWEATER BEFORE YOU LEAVE.

THAT IS BECAUSE THE NATIONAL WEATHER SERVICE IS CALLING FOR A SO-CALLED PNEUMONIA FRONT SET TO HIT CHICAGO LAKEFRONT SHORTLY.

TEMPERATURES ARE EXPECTED TO DROP FROM 80 DEGREES TO THE LOW 50s WITHIN MINUTES.

JOINING US TO EXPLAIN THIS WEATHER PHENOMENON IS PROFESSOR OF METEOROLOGY AT THE COLLEGE OF DUPAGE.

THANK YOU FOR JOINING US.

>> THANK YOU.

>> STILL WARM THERE.

WHAT EXACTLY IS THE SO-CALLED PNEUMONIA FRONT ?

>> AND PNEUMONIA FRONT IS SORT OF A SLANG NAME THAT WEATHER FORECASTERS BACK IN WISCONSIN AND MILWAUKEE BACK IN THE '60s CAME UP WITH.

IT INDICATES WHAT HAPPENS VERY OFTEN IN THE SPRINGTIME WITH COLD AIR COMING IN OFF OF THE LAKE.

THE WINDS SHIFT AROUND IN THE NORTHEAST.

THAT COLD AIR OFF OF LAKE MICHIGAN JUST SPREADS INLAND.

AND YOU GET SOME REALLY RADICAL TEMPERATURE DROPS .

>> THIS IS SAID TO BE HAPPENING AROUND 5:40.

THAT IS IN A FEW MINUTES.

WE KNOW IT IS STILL IN THE 80s FOR A HIGH.

JUST A FEW MINUTES AGO.

THAT IS GOING TO LURE PEOPLE OUT TO THE LAKE.

WHAT KIND OF WEATHER AND WE EXPECT FOR THE REST OF THE NIGHT?

>> JUST TO GIVE YOU SOME IDEA, I HAVE BEEN LOOKING AT CHICAGO AIRPORT.

IT USED TO BE KNOWN AS PAUL LOCKEY AIRPORT.

THEY DROPPED 17 DEGREES IN 15 MINUTES.

81 DEGREES AT 5:05.

5:20 IT IS 64 DEGREES.

THAT COLD AIR IS RUSHING SOUTHWARD.

O'HARE WILL BE THE NEXT ONE.

YOU AT THE STUDIO AT WTTW .

WINDS WILL SHIFT FROM THE NORTHWEST AT 10 TO THE NORTH-NORTHEAST AT ABOUT 15 TO 20 MILES PER HOUR.

YOU WILL KNOW INSTANTLY WHEN IT PASSES.

THAT SHOULD BE SCREAMING THROUGH THE CHICAGO AREA IN TENS OF MINUTES.

>> OF COURSE IT LEGITIMIZES THE REFRAIN WE ARE USED TO HEARING IN CHICAGO.

IF YOU DON'T LIKE THE WEATHER, JUST WAIT FIVE MINUTES.

HOW COMMON IS THIS?

>> WELL, WE GET THIS QUITE OFTEN IN THE SPRINGTIME.

ANYWHERE BETWEEN APRIL AND MAYBE EARLY JUNE.

TEMPERATURES ARE RATHER COLD.

IT TAKES MUCH LONGER TO HEAT UP THAT COLDWATER .

AND THEN WHEN WE HAVE DAYS LIKE TODAY WITH DRY CONDITIONS AND IT IS SUNNY AND WARM AND BEAUTIFUL, THAT SETS UP A REALLY BIG TEMPERATURE DIFFERENCE FROM THE WATER TO THE LAND.

AND THAT ALLOWS THE PRESSURE TO BUILD UP OVER LAKE MICHIGAN .

EVENTUALLY THAT COLD AIR HAS AN UPPER LEVEL DISTURBANCE THAT STARTS TO PUSH ENGLAND.

YOU WILL SEE IT PUSH RACINE, WISCONSIN.

KENOSHA.

THEN YOU GO TO WAUKEGAN AND MILWAUKEE AIRPORT.

O'HARE.

DUPAGE AND MIDWAY AIRPORT.

IT WILL CONTINUE TO PUSH SOUTHEAST.

MAYBE AS FAR AS JULIET .

USUALLY WHEN YOU GET OUT FAR ENOUGH SOUTH AND FAR ENOUGH WEST, IT IS NOT SO BAD.

>> IT LOOKS PRETTY OUT THERE.

15 SECONDS BEFORE WE LET YOU GO.

IS IT VERY DIFFICULT TO PREDICT THIS KIND OF AFFRONT?

>> NO.

THINGS ARE PRETTY WELL HANDLED BY THE MODELS.

LAKE MICHIGAN -- WHEN WE LOOK AT OUR NUMERICAL MODELS, THE TEMPERATURE SHOWS REALLY COLD.

HIGH RESOLUTION MODELS.

WHEN YOU HAVE THESE LIGHTER WINDS IN THE AREA AND I AN UPPER LEVEL DISTURBANCE COMING OUT OF CANADA , WE MIGHT HAVE SOME GORGEOUS SUNSETS TONIGHT.

BUT THOSE ARE USUALLY THE DAYS WE EXPECT THE BACK DOOR COLD