Halloween snow showers are in the offing as a vigorous upper disturbance sweeps into the Chicago area Tuesday. That’s not something which happens all that often. In fact, since 1884, snow Tuesday would mark only the 8th time snowflakes have fallen on a Halloween. If snow sticks—even modestly—it would make only the 2nd time since over the city’s 138 year official snow record—that a Halloween has managed snow which sticks, even if only briefly. A 3.4″ snowfall on Halloween, 2019 is the heaviest snow which has occurred on Oct 31st here in Chicago.

The anticipated snowflakes Tuesday would dovetail nicely with Chicago snow climatology (i.e. historic snow occurrence here). The average date of the first snowflakes annually in Chicago is on or about October 31st—-which is tomorrow’s date.

The meteorological set up Tuesday is an interesting one and coincides with the onset of the chilliest temps of fall and the coolest spell of weather in Chicago of the past 6 months. Monday morning’s official 33-deg low at O’Hare is the coldest morning low temp since a 31-deg minimum on April 26th.

Tuesday’s snowmaker is the product of a vigorous upper wave sweeping into the area amid an especially steep vertical temp drop through the atmosphere. When temps drop rapidly with height, air is encouraged to rise. Add the “heat” given off by still warm lake waters (they currently average 56-deg across southern Lake Michigan), and the buoyancy of the air which will result could generate t-storms out over Lake Michigan and even waterspouts there. Steering winds would keep the waterspouts off the Illinois shoreline but might encourage any which form to drift toward the Michigan or possibly Indiana shorelines. Projected winds would also favor the formation of lake effect rain and snow squalls (the term “squalls” speaks to the gusty winds with those lake effect showers). Those rain and or snow showers could blow into sections of northwest Indiana later Tuesday and Tuesday night.


While the ground is warm, bursts of snowfall could be significant enough to produce the season’s first sticking snow at least temporarily (since it would ultimately melt given the warmth of the ground) on SOME colder outdoor surfaces—grassy surfaces come to mind as the most likely for any accumulation. There’s no guarantee that will happen—but if any snow bursts which occur produce vigorous rates of snowfall, that can overcome the rate of snowmelt once the snow hits the warm ground—at least for a time.
The concern here isn’t huge accumulations—though THE FIRST STICKING SNOW of the season is always an eye-catcher. More significant may be the potential for brief periods of visibility reduction in the bursts of snowfall.

SNOW’S ON THE WAY HALLOWEEN: Then snow’s arrival is right on the money in terms of Chicago climatology. Historically, the first flakes have flown on or about October 31st over the years. Modeling suggests there may even be enough to stick on colder outdoor surfaces. It wouldn’t last long given the environment in which its falling, and any accumulation would likely be limited to colder outdoor surfaces–like grass.


Also of note—and a jarring change from the 83-deg high temp recorded on Tuesday a week ago—will be quite noticeable to all in the Chicago area Tuesday. Halloween day’s predicted high temp of just 40-deg mean temps will come in 43-deg colder than a week ago and well below the normal October 31st high of 56-degrees. Plus, gusty winds as high as 40 mph at times by afternoon mean daytime wind chills will hold in the 20s. So, in effect, tomorrow is to feel nearly 60-degrees colder than the 80s experienced last Tuesday.
A high of 40-deg tomorrow (Halloween) would make this one of the three chilliest Chicago Halloweens since 2000—and one of the 11 coldest Halloweens of the past 153 years.
BEYOND TUESDAY’S CHILL—temps rebound to the 50s Thursday and Friday and to 60 Saturday. Chances for additional precipitation this work week remain nil. The coming weekend may see the next precipitation. A far wetter weather system appears in the offing early next week. Both of these systems look likely to be rain producers.

National Snowfall Analysis: Accumulation from September 30 to October 30, 2023

CHECK OUT THE SNOW COVER IN THE COUNTRY–which sits atop 17.9% of the Lower 48 Monday morning. That’s a 942% in crease in area covered by snow in just the past two weeks.

Snowfall Season-To-Date-Winter 2023-24


Hurricane Otis’ unpredicted, explosive development from a 70 mph tropical storm to a TOP-TIER 165 mph CAT 5 hurricane prior in the 12 hours prior to its 1:25am CDT landfall in Acapulco, Mexico last Wednesday (October 25th), is the subject of much discussion in meteorological circles. Otis was the most powerful hurricane to strike the west Mexican coast.
The NASA animation you see here is satellite derived view of Otis’s rains and structure the day before landfall (last Tuesday) off the Mexican coast, 175 miles south/southeast of Acapulco as it drifted slowly northward over 31-deg C (88-deg F) Pacific waters.
READ THE FULL NASA RELEASE HERE: https://svs.gsfc.nasa.gov/5181


The widely respected Associated Press science writer Seth Borenstein posted the following story last Thursday on Hurricane Otis’ surprise intensification prior to roaring ashore last Wednesday: https://apnews.com/article/otis-mexico-acapulco-hurricane-warming-oceans-pacific-18a5160b0d90caf693b41273647bd076?fbclid=IwAR3zR8sR8o42Efos6-HVjvwWJKdYtn3YqLnQI0DJnWcUjKh6BWUAsK_xZmY