By now, many of you have probably heard about a big storm coming next week and talk of it possibly being a big snow, but what is important to remember is that any forecast of a storm that far out holds a LOT of uncertainty. This storm is currently over 5000 miles away from Chicago located over data sparse regions of the northern Pacific Ocean, which has big implications on the ability of our forecast models to predict the storm. Our forecast models are fed by and rely upon observational data, so when a storm is out over the ocean the amount of observational data fed into the models is more limited, which can result in models producing less accurate forecasts than when the storm is over land.
The map below shows a range of possible locations of the low pressure by next Thursday night based on computer model forecasts generated Friday. The smaller blue "L"s are different possibilities generated from one model, while the larger flesh colored "L"s represent forecast locations from some of the main longer range forecast models. The main theme of the image below is to illustrate that there is nearly a 1000 mile range in the east-west possibilities of where this storm will be and over 500 miles north-south, with the range of forecast possibilities for us being anything from nearly missed by the storm, to being hit with a big snow, to getting all rain and possible thunderstorms. Not only is there a wide array of possible tracks of the storm, but the forecast intensity varies from a fairly run of the mill storm to a powerhouse wind & heavy precipitation maker.
Meteorologists often look at connections between hemispheric patterns, referred to as teleconnections, for forecasting general patterns one to two weeks or even further in advance. For this coming week, it appears that there are strong signs there will be an increasingly negative Pacific/North American (PNA) pattern and a negative North Atlantic Oscillation (NAO) pattern, which is shown below on a large scale forecast model. Teleconnections of a negative PNA support a mean trough setting up across the western half of the CONUS and in general can mean a wetter than normal pattern across much of Illinois and Indiana. In the case of these two concurrent negative phases of the PNA and NAO, this can lead to a more active southern stream track across the central U.S. further supporting above normal precipitation somewhere in the Midwest.
Image reflects 500mb Height Anomalies (warm colors = above normal, cold colors = below normal)
Ultimately, there certainly appears to be a fairly high probability of precipitation of some sort moving into the region late next week or early next weekend, but any forecasts of rain or snow amounts or even suggesting the precipitation will be all rain or all snow should be viewed with skeptism as the range in possibilities for how this storm will evolve over the next week are huge. The best bet at this point is to pay attention to later forecasts, because there certainly is the potential for stormy and possibly wintry weather depending on how the storm evolves.