Explainer

“When will it finally get warmer?” It’s a question being posed more and more often by Chicagoans growing increasingly impatient with the area’s lengthy spell of chilly temperatures. Computer models suggest a pattern change that heralds moderating temperatures, though not a return to above-normal readings.
Today is the anniversary of the world’s greatest single-day rainfall: 73.6 inches of rain poured down on Cilaos, La Réunion, in the Indian Ocean during a 24-hour period on March 15-16, 1952. Incredibly, this amount is more than twice Chicago’s annual precipitation total (36.27 inches), and it was but one 24-hour period within an eight-day spell of intense rainfall occurring in association with the passage of a tropical storm across La Réunion. Total precipitation during that period, Mar. 11-19, 1952, was a phenomenal 162.59 inches. That’s over 13 feet of water, and also a world record for that time interval.

Although the origin of the word “blizzard” is obscure, the term appeared in the United States 135 years ago and its first use was possibly on March 14, 1870, to describe a storm that produced heavy snow and high winds in Minnesota. Technically, a blizzard is an intense winter storm with sustained winds 35 m.p.h. or higher and sufficient falling and/or blowing snow to reduce visibility below one-quarter mile for at least three hours. Colloquially, the term is loosely applied to any heavy snowstorm. Blizzard conditions figured prominently in one of the worst-ever U.S. winter storms, the “Storm of the Century” that raked the eastern quarter of the nation on March 13-14, 1993. It impacted 100 million people in 26 states from Florida to Maine and claimed 270 victims. The storm produced 11 tornadoes in Florida, a 12-foot storm surge on Florida’s west coast, 20-40” of snow and whiteout conditions from Atlanta to Maine, and winds gusting to 101 m.p.h. in Fairview, N.C.

Memories of Chicago’s mild winter are fading as the city slips ever deeper into unrelenting springtime chill. Sub-normal temperatures began nearly a month ago and now, with daytime readings projected to run 8-15º below normal through the upcoming week, there’s no apparent end to the ongoing and lengthy cold spell.
Computer models indicate that the jet stream, anchored to our south, remains in a position that favors the southward movement of cold air from northern and central Canada. That means continued, persistent cold weather for Chicago and the Midwest, as well as the Ohio Valley and the East.
A burst of light snow (such as the dusting Friday evening) has accompanied each new surge of cold air, and that scenario will dominate area weather this week. However, a stronger storm system, possibly with the ability to tap deeper moisture from the Gulf of Mexico, threatens the area with a significant rain/snow event this coming Friday and Saturday.

On a stormy day 82 years ago—March 12, 1923—Chicago recorded its all-time low air pressure, 28.70 inches.
As provided by Chicago weather historian Frank Wachowski, the following annotation appears in Chicago Weather Bureau (now the National Weather Service) records on that day: “The lowest barometric pressure in the history of the Chicago office was registered on March 12th in connection with a thunderstorm, sleet and snow and strong gales. The reading, reduced to sea level, being 28.70 inches.”
Wachowski provides this additional information: The record low air pressure value occurred at 7 a.m., with the wind roaring in from the northeast with gusts to 60 m.p.h. High and low temperatures that day were 41º and 31º and water content of precipitation was 0.67 inches, along with 0.2 inches of glaze.

After a long winter—and all Midwestern winters are long, even the mild ones—we impatiently anticipate the arrival of mild temperatures when March appears on our calendars. But lost in the lament over this spring’s persistent snow and chill is this crucial climatological fact: March is the beginning of Illinois’ tornado season, but low temperatures suppress severe thunderstorms.
The nation’s most deadly tornado, the great Tri-state tornado, killed 695 people (606 in Illinois)—and it was a March event: March 18, 1925.
So, the cliché that every dark cloud has a silvery lining is valid, and it’s applicable to the current cold spell. Consider this: The kind of upper-air wind pattern responsible for our current cold spring weather, when it occurs in January, brings sub-zero temps. In short, it’s far less painful to experience sub-normal temperatures in March than in January.

Chicago and the eastern third of the country better get used to Wednesday’s chill, since overwhelming forecast evidence points to a prolonged cold spell. With the jet stream locked south of Chicago for at least two weeks, temperatures should remain below normal through the period. Extended strings of cold, cloudy weather with flurries are not unusual in March, the most recent occurrence was just last year.
In the current regime, our jet stream remains well south, and begins to sink southward across the West, replacing the several weeks of warm weather there. The resultant cool pattern across the lower 48 is a difficult one to dislodge, hence the high confidence in extended chilly weather.
Snow Thursday could total an inch or more, and a new system with lighter amounts on Friday. The bulk of the energy with future systems should track south of Chicago.

The system that brought the cold air to the Midwest on Monday developed into another big nor’easter off New England, with interior Maine can expect over a foot of new snow. Meanwhile, frigid air by March standards settled into the Midwest. Embarrass, Minn. recorded a low of -30° Tuesday morning, with below zero readings speckled across the north.
This cold pattern is now well established across the northeast quarter of the country, and Chicago will remain along the western flank of this frigid air mass until further notice. However, just a few hundred miles to the west, more spring-like temperatures are in control, and some of this mild air approaches by early next week. Before then, the only weather maker of the next week races southeast from the Canada for some snow here on Thursday, before developing into the next big winter storm off the northeast coast on Saturday.

After a brief fling with spring on Sunday, cold air returned with gusty northwest winds and a temperature drop of 21 degrees between 9:00 a.m. and 6:00 p.m. Monday. With the return of cold weather, the connection was re-established between the Arctic and the eastern half of the country. At upper levels, the jet stream has once again slipped south and west of Chicago, bringing in northerly flow from the coldest regions of the hemisphere. The bitter cold that had blanketed that region for much of the winter has eased in recent weeks, so the extended cold spell under way here should not be that extreme. Meanwhile, the coldest air in this regime should settle east of Chicago. Nonetheless, temperatures will remain below normal for the next week, with occasional bouts of light snow from Thursday through Saturday, and another brief warm-up possible early next week.

Gusty west and southwest winds drove temperatures in Chicago higher all afternoon, topping out at 64° at both Midway and O’Hare airports. Even Northerly Island along the chilly lakefront reached 63°, making Sunday the warmest day in four months—since the 65° Nov. 6.
But the party’s over for the mild spring weather. A sharp cold front will sweep south through the region Monday morning with temperatures falling all the way to the teens overnight into Tuesday. Northwest winds gusting past 30 m.p.h. will add intensity to the temperature drop.
This front signals the beginning of a long string of below- normal days as a powerful northerly jet stream locks in place along the western edge of a blocking pattern that stretches all the way to Europe. Stellar upper air credentials suggest it will be even colder than forecast, with a major storm late in the week—but computer models vary wildly on its timing and location.

Saturday temperatures soared into the 60s and even 70° in snow-free areas of the Dakotas. This surge of warmth, headed toward the Midwest Sunday, pooled ahead of a cold front gathering strength in southern Canada. As a result, Sunday’s highs around Chicagoland should easily make it into the mid 50s with breezy west to southwest winds, and rain holding off until the wee hours of Monday.
But the shoe drops when that cold front sweeps through Chicago around noon on Monday. Falling afternoon temperatures turn the widespread light rain to snow as evening temperatures plummet and overnight lows bottom out around 20°. This return to chilly winter weather is a result of the blocked pattern in the Atlantic, one which appears locked in place for the next two weeks. Chicago sits towards the western edge of the cold air, with the brunt of the chill destined for New England.