Timing The Winter Storm - Update #7

Good morning everyone. It is Sunday, November 25th and our approaching winter storm is taking shape and slowly moving our way. Latest model trends continue so slow the system down ever so slightly, which may actually help boost snowfall totals. 

The system will not be an all snow maker and will start out as rain. Using some of the latest high resolution (HRRR) model guidance we will try and lay out the changeover to snow and break it down for you all so we know what to expect, where and when. Keep in mind the times won't be EXACT.

Currently (9am at the time of this writing) we see a band of light rain moving slowly north across N IL. ​

This is well sampled by the model, an indication it (hopefully) has a good handle on the situation and goes in line with our general thinking.

As the system begins to strengthen and continue east, colder air will wrap around the backside - which is common evolution - and rain will change over to snow. First across parts of northwest Illinois. ​

The changeover will slowly move to the east, while at the same time dipping south as the cold air continues to wrap around. By early evening as the sun is getting to set, parts of northeast Illinois will begin to see a changeover. The Immediate Chicago area will continue to see rain longer due to a combination of on-shore flow from the lake, and synoptic placement of the weather system. ​

Later in the evening the change continues east. Timing the change for the Chicago area is one of the more difficult aspects of this forecast. Various contributing factors such as flow off the lake, urban heat island and system placement could delay the onset even later. Model runs go back and forth about the exact time which is impossible to pin down, but we are fairly confident it won't be until 8pm or later. Its worth noting that the precipitation will be quite heavy at this time. Thunder and lightning may also be possible.

Just a couple hours later, our system has fully strengthened and is now raging across the state. Chicago is potentially changed over to snow and the heaviest snowfall rates - possible blizzard conditions - are now occurring. Snowfall rates could exceed 1 inch per hour. The heavy rate of precip could make up for the later start time across parts of northeast and north central Illinois. Snow is beginning to wind down across West Central and northwest Illinois. ​

As we move into Monday morning the storm rages on, battering north central and parts of northeast Illinois with heavy, wet snow. Areas further south of Chicago area, along the I-55 and I-57 corridors, will changeover from rain to snow around this time. The snow will come down fast, but the late changeover will reduce accumulations in these areas (see TAM below)

As the system continues east throughout the morning, it will take the heaviest rates with it and snow will begin to slowly wind down across northeast and central Illinois. Current indications are the system should exit the state by Noon on Monday, probably earlier. We are still continuing to monitor progress, and if the system continues to slow down these times may be delayed so stay tuned for further updates!

Below is a look at our current TAM. This map may be refined as the day progresses. We are still anticipating upgrades to blizzard warnings as well. Regardless if that occurs, conditions will be dangerous and travel is not advised. Hunker down, and stay safe today!

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