Fri-Sun, Jan. 18-20th Weekend Winter Storm Update #4

​(1:15PM -1/17/19) It is now Thursday afternoon and model guidance continues to slowly get their story straight with this system, but even as we draw within 24 hours of first flakes falling across portions of the area, we continue to see some differences among model guidance. The difference still at this time are regarding the handling of the strength and track of both the upper level low and surface low as it crosses the plains. The trend however over the last day or so except for a few models has been for a further north and stronger track and this has led to increasing chances of a significant winter storm across portions of central and northern Illinois.

I dig into some new details and thoughts below, including arrival time of snowfall tomorrow...

At this time, our system of interest is just beginning to come ashore northern California this afternoon and will continue to do so over the next several hours and into the evening as well. This will hopefully lead to model guidance tonight getting full sampling of this system and helping them to finally fine tune several things.

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Over the next 24 hours, our system will continue on mostly easterly jog as it approaches the Rockies by the afternoon tomorrow, eventually crossing and ejecting out into the plains. It will also be slowly strengthening/amplifying as it does so by late tomorrow afternoon and evening as it moves into the southern and central plains. ​

As mentioned in previous posts, this lee side cyclone will lead to developing strong warm air advection and isentropic ascent to the east of it as strong low level flow goes up and over the cold air to the north. This will lead to the develop of a widespread area of snowfall to the north of the developing surface low by tomorrow morning and afternoon across the central and northern plains. ​

As the upper low and surface low continue to move towards the southern Midwest late Friday night, the large area of snow will eventually move eastward into portions of northern and central Illinois by the late afternoon to mid evening hours as things look now. ​

​Given the potential for mesoscale banding of snow, there is a good likelihood that snowfall rates in this somewhat narrow band Friday night across northern Illinois could produce snowfall rates exceeding 1"/hr as it continues into the overnight and morning hours of Saturday. Model guidance continues to differ somewhat on the placement of this initial band of potentially heavy snow with some placing it across southern Wisconsin and some further south along the I-88 corridor in northern Illinois. 

Below is a quick comparison of two high resolution models, showing the difference in place of the snow band across that area tomorrow night and into the morning hours on Saturday. Forecasting the placement of the heavies amounts is quite tricky and the difference of 25-50 miles could mean the difference between just a few inches and amounts exceeding 6". 

Snow will continue through the morning and into the afternoon across portions of northern and central Illinois with further downstate seeing rainfall initially and possibly some mixed precipitation as well. As the area of low pressure continues off to the east later Saturday, colder air will advect in on the backside, potentially flipping rain over the snow across southern Illinois with accumulations possible. ​

Overall this looks to be a very significant winter storm for portions of the state, favoring the northern half of Illinois with the highest expected totals across extreme northwest Illinois and across northeast Illinois from added lake enhancement/lake effect snow. We will continue to have updates going forward as this impressive storm system approaches over the next 24 hours. ​

MP