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By Meteorologist Paul Merzlock

With less than two weeks of meteorological winter to go, it appears this season will remain well below normal in terms of snowfall. As of Friday, total snowfall at O'Hare International Airport stood at 15.7 inches, with Midway Airport having recorded 18.8 inches. These totals are 11 inches and 8.6 inches below normal, respectively. Current forecasts suggest there is little chance of making up this snowfall deficit over the next week. On Friday, a developing storm system over south Texas spread flooding rains across the western Gulf Coast region. This moisture laden storm will move across the South laying a swath of heavy snow across the central Appalachians and mid-Atlantic, while Chicago can expect brisk north winds and a few flurries Saturday into Saturday night. Weaker storm systems are forecast to move through the area Tuesday and Thursday night into Friday. These systems are not expected to bring significant snow to Chicago.

By Meteorologist Paul Merzlock

On Sunday the high temperature at O'hare airport will remain below freezing, marking only the 12th such occurrence this winter season. Normally, Chicago experiences 42 sub-freezing days over the course of the winter. This is just another statistic measuring how mild the winter of 2011-2012 has been. Intrusions of polar air into the continental U.S. have been minor, with the core of arctic air remaining bottled up over northwest Canada and Alaska. Cold spells have been brief as well. The longest stretch of consecutive sub-freezing days was a 4-day period from the 18th through the 21st of January. Our current brush with wintry weather will be no different. Mild air will once again sweep across the Midwest on Monday. Stout south to southwest winds will send temperatures soaring well into the 40s both Monday and Tuesday, with some 50 degree readings not out of the question. A brief cool down is expected later in the week, but the long term trend through February will favors mild temperatures.

By Meteorologist Paul Merzlock

A one-two punch of colder air will sweep across the Chicago area this weekend. The first will arrive in the wake of Friday night's light snowfall as blustery west winds usher more seasonable temperatures into the Midwest. Despite this cool down, Saturday's temperatures will still run a bit above normal.

A second, more potent shot of polar air is due to arrive Saturday night, with the core of coldest air passing across the area during the day Sunday. The arrival of this colder air will be accompanied by another band of light snow or snow showers Saturday night into Sunday morning. This snow will be minor, with accumulations of less than an inch expected. Nonetheless, the combination of subfreezing temperatures, blustery northwest winds and flurries on Sunday will make it seem more wintry.

As has been the case this winter, this brush with polar air will be transitory. Temperatures will begin to moderate Sunday night, leading to a significant surge of mild air on Monday.

Radar imagery since 630 PM has shown a steady decrease in areal coverage and intensity of snow over northern Illinois.  Though light snow may continue this evening, any additional accumulations will be less than an inch based on the marked improvement in visibilities across the area.  

 

The RUC (Rapid Update Cycle) model has been fairly accurate in its positioning/timing of snow across the Chicago area this afternoon.  The latest run of this model (21 UTC) suggests that moderate to heavy snow will continue across much of northeast Illinois and far northwest Indiana through 6 PM CST.   

Below is the latest forecast of mid level relative humidity and vertical motion fields valid for 6 PM this evening.  Heaviest snow will tend to line up where strongest upward motion is combined with saturated air.  Based on this guidance, moderate to heavy snow will likely be ongoing across the Metro area.

   Snow has been falling at the rate of 1 to 1.5 in/hr at my observation site on the west side of Joliet since about 2 PM.  As of 345 PM CST,  4 inches of new snow has accumulated.   

Snow has increased across SW suburbs since 115 PM.  Snowflakes have increased  to about the size of nickels with visibility less than 1/4 mi. Snow is falling at the rate of 1 in/hr with total new snow accumulation (2.5 N of KJOT) up to 3 inches as of 215 PM.  Travel on I-55 has become more impaired due in part to the low visibility.  The road surface itself is still OK in the outside lane but passing lanes are partailly to entirely snow covered.          

         Below is a radar image from 236 PM CST.  The magenta, gray and white cores are areas of moderate to heavy snow. Movement of this activity is east at 20 to 25 mph.  

 

 As of 115 PM:  Joliet, west side:  1.4 in new snow.  Snowing, vis variable 1/2 to 3/4 mi.  Snow has been fine (small flakes) up to this point but the density of the fall is very high leading to the low visibility.  Other stats:  temp 16, dew point 12, wind NE at 7.  Road report:  Most roads snow covered.  I-55 is OK.  Partailly snow covered in passing lanes. 

Radar imagery continues to indicate an area of moderate to heavy snow from east central Iowa to Chicago.  This snow is occurring as a result of mild air to our south overriding the polar in place over Chicago.  Based of the latest (17 UTC) run of the RUC (Rapid Update Cycle) model, this area of snow will continue east across northern Illinois through much of this afternoon.

 

The 700 mb (~10,000 ft) forecast for mid afternoon shows saturated air over northern Illinois with a respectable amount of vertical motion leading to continued snow production.

      In addition,he forecast for the 850 mb (~5,000 ft) level valid at 2 PM CST  shows a strong influx of mild air from the lower MS valley overrunning much colder air over northern Illinois.   This overrunning process is forecast to continue into the evening rush hour as indicted by the 23 UTC 700 mb forecast below:

Very light snow began on west side of Joliet at 0955.  Has increased in intensity since with visibility down to about 2 miles.  As of 1115 have only accumulated 0.3 inch but flake size has begun to increase as column saturates in low levels. Surface dew point has risen to 11 F; RH now up to 81%. My weather instruments are located about 2.5 mi due north of KJOT.

 

By Meteorologist Paul Merzlock

Under cloudless skies, temperatures on Sunday rebounded into the lower and middle 40s, bringing an end to winter's brief visit to Chicago.  Monday will bring similar conditions, with mild southwest winds again boosting temperatures above 40 degrees.  However, an increase in cloud cover will limit daytime heating just a bit. Monday's increase in cloud cover is but a vangard of major weather changes to take place through the week. A storm system is forecast to develop over the southern Plains on Tuesday.  This system will bring unseasonably mild air into Chicago by Wednesday as it rapidly intensifies. Another significant aspect of this storm will be heavy rain and perhaps a few embedded thunderstorms. Rainfall amounts in excess of an inch Wednesday and Wednesday night will be widespread, with some computer models suggesting that as much as 3 inches of rain may fall. The midweek warm up will be followed by a sharp downturn in temps on Thursday.