Sporadic light rain has replaced the morning and early afternoon snow/sleet mixture responsible for an estimated 2 to 6 inch northwest suburban accumulation. Atmospheric warming has successfully pushed temperatures above freezing broadly through the lowest 7,500 feet of the atmosphere and is behind the shift from snow and sleet to occasional rain and even a few possible thunderstorms. While some snow returns in the early hours of Thursday morning (early estimates suggest around 3 to 4 a.m. in the city proper), snowfall continues to shift north into Wisconsin this afternoon into Wednesday night.
A pullback in the powerful easterly wind velocities which have dominated since late Tuesday night, including gusts over 40 mph earlier Wednesday, is to get underway. Wind speeds are to drop precipitously to 10 mph or less for a time Wednesday night as the center of the huge late April storm behind today’s wintry weather passes over the Chicago metro area. In the same way winds fade in/near the eye of a hurricane, air movement slows dramatically toward the center of non-tropical storms as well. That’s because much of the horizontal air motion we sense as “wind” shifts into the vertical near the storm’s center where air rises on a broad scale. But, look for winds to return briskly before daybreak Thursday then continue much of the day.
Measurable snowfalls this late in the season is rare. On only 66 occasions since records began in 1884-85 (including today) has snow accumulated 0.1” (measurably) on April 11th or beyond. There have been only five 3”+ snows this late in the season—the most recent 27 years ago!
Snow in the city amounted to a few inches at the hardest hit locations but is melting quickly in the early afternoon hours thanks to an environment which includes above freezing air temperatures and warm ground and pavement readings unable to sustain a snow cover.
Preliminary snow totals include 1.9”at Midway Airport and 2.3” at O’Hare---a total there just 0.7” shy of the last 3” or greater snowfall to occur this late in the season back in April of 1982.
Snowfall had ended in all but the Illinois/Wisconsin border area at mid-afternoon Wednesday—and the the switch to rain was expected to continue north into southeast Wisconsin the remainder of the afternoon and evening. The snow and sleet earlier today started as rain in Wednesday’s pre-sunrise hours which fell into a dry atmosphere. Evaporation of the falling raindrops absorb heat in a process referred to as “evaporative cooling”. The resulting temperature drop shifted rain to wet snow, a set-up common in the cold sectors of late season storms. But, south winds just above the surface beneath the approaching storm’s eastern flank, began warming the air producing a northward shift in the area of snow and sleet, responsible for quick inch to two inch pre-dawn and early morning accumulation in some southern suburbs and a switch to rain there. That south suburban snow has long since melted and the storm’s “rain/snow line”---the well defined demarcation between rain and snow in large storm’s such as today’s---has been shifting north ever since. Chicago was positioned on this rain/snow line much of the morning, which led to the oscillation between snow, sleet and rain in the city during that period. A final west and north suburban burst of heavy snow earlier this afternoon reduced visibilites to ¼ mile at Waukegan, DePage, O’Hare and Wheeling before a shift to rain. Scattered thunderstorms have accompanied precipitation and are still likely flare from time to time into Wednesday night.
The period of relative calm near the storm’s center yields to the system’s cold backside NW winds which return snowfall to the area beginning in the hours before sunrise Thursday.
Tom Skilling
WGN-TV Chief Meteorologist
PHOTO COURTESY: C. Frystak, Antioch, Illinois
