We got a bit of a (cold) respite during the weekend from any precipitation, but all that is about to change in a big way tonight and into tomorrow. A complex storm system that is set to bring snow across a wide swath of the Midwest will be approaching the area tonight and into tomorrow. Various factors make this storm even harder to forecast than the Nor’easter last week, including a still uncertain storm track and a very tight temperature gradient leading to the potential for a mixed bag of all precipitation types in the NYC region.

Impacts

This storm is set to make Monday a downright nasty day to be out. Snow is forecast to begin falling tonight, with areas to the north of the city expected to pick up the most snow through this storm. During the AM rush, temperatures are expected to rise just enough in the lower atmosphere to allow for the possibility of sleet, ice pellets, and a period of freezing rain along the coast near the city, with rain possible further east in Long Island. Then, to close out the day, temperatures will drop rapidly below freezing, such that any slush or melted snow or wet surfaces could flash freeze. Because the temperature gradient is so tightly packed, it will again literally be a matter of miles between a spot that picks up mostly snow and perhaps 8″+, and another location that picks up almost no snow, but gets a significant coating of ice.

Below are some probabilistic forecast graphics that will give you a good sense of the degree of uncertainty within this storm forecast. First, with respect to snowfall totals over the next 48 hours.

Here’s the same time window, but with probabilities for ice accumulation.

In the snowfall probability maps, you can see just how tightly packed the different bands of probability for snowfall are. A matter of 50 miles means the difference between being in an area of 40%-50% probability of receiving more than 4″ of snow, and an area with a greater than 80% chance of receiving the same total.

Timing

Snow will begin to fall later this evening and pick up in intensity overnight. Depending on the area you’re living in, you could see between 3″-8″ overnight, with the highest totals most likely north of the city, and the lowest totals along the coast and in eastern Long Island. Around the morning rush, areas along the coast and eastern Long Island should begin to see a transition from snow to freezing rain, or some type of wintry mix of ice pellets and sleet. Some parts along the coast may even see a changeover from rain/snow to plain rain. In the afternoon, temperatures will drop quickly below freezing, allowing for flash freezing of slush and other wet surfaces. Winds will begin from the east initially before turning northeast and then north through the day.

Why the Uncertainty?

The reason behind the complexity of this storm with respect to precipitation types, uncertainty of total snowfall amounts, rests with storm’s track and the dynamics in the lower atmosphere accompanying this storm. This storm, unlike a Nor’easter, has its genesis over the Central Plains. However, as it approaches the coast, it will still be able to pick up a good amount of moisture off the ocean and from points south.

Recalling that air around a low pressure center in the northern hemisphere in a flows in a counterclockwise motion, you can better understand what is going on here. The low pressure center is forecast to pass just to our south. To the north of the storm center, colder air will dominate, since the cyclonic flow would be pulling air from north to south. Meanwhile, on the south side of the storm center, warmer air is being wrapped into the storm from the south and east. This onshore flow bringing relatively warmer air in close proximity to colder (sub-freezing) air is the reason why there is such a dramatic range in forecast snowfall totals across the area, and also the reason why there is a possibility for a range of different precipitation types.

Above is a forecast map that shows the forecast position and track of the storm. The colored points represent the individual forecast ensemble members (different forecast models with slightly different inputs) predictions for the storm. Notice that there is still a nontrivial spread between these ensemble members. You may also observe that they seem to cluster to the north of the official forecast track (the black line). Indeed, forecasters have been making slight northward adjustments in response to this trend. Should this bear out, and the low pressure center does move a bit further north, we would end up with less snow, more chance for mixed precipitation and even rain. On the opposite end, if the storm track deviates south, we would get more snow, and less chance for mixed precipitation. A very tricky forecast indeed!