Winter has arrived in Ithaca: Forecast for week of October 25 2016

Minor edit: replaced Ned Stark with a Whitewalker

The cold promised by a northeast winter is finally here. The storm that dropped 2-4 inches of rain on Thursday night has exploded into a large midlatitude cyclone and is still slowly meandering to the NE over NE Canada. Cold air will continue flowing in behind the storm, dropping temperatures below normal.

Infrared satellite imagery of the Northwestern Atlantic showing a large low pressure system over Eastern Canada with cold front associated convection (thunderstorms extending into the Caribbean

This is our best shot at a freeze so far this season. Model projections predict that lows will dip into the mid 30s tonight and just below freezing by Tuesday night into Wednesday morning. Highs on Tuesday and Wednesday will be  in the mid 40s. Lows Wednesday night into Thursday will be slightly above freezing as cloud cover ahead of a approaching storm traps heat radiated by the Earth and winds switch to the SE.  Thursday looks to be wet and raw, with a high in the upper 40s. We should dry out and warm up slightly just in time for the weekend.

So about that snow:
The upper levels of the atmosphere are well cold enough for snow. The problem is available moisture and lift. Both are lacking on the back side of a low pressure system, where dry continental winds push out of the NW. So how are we going to get moisture? It lies with the Great Lakes and its famous lake effect snow.

Wind and water are important to lake effect. A cooler, ice covered lake in the winter will require air to travel over it for a longer time to pick up the same amount of moisture as wind traveling over a warm lake. Wind that is perpendicular to the lake will move air over the lake for too little time to pick up enough moisture but strong flow parallel to lake length is the best setup for lake effect. That is why cities at the eastern ends of the lakes, like Buffalo and Watertown, NY are such good places for lake effect.  In cold winters, when the lake finally freezes over, the lake effect tap is turned off for the rest of the season.

How lake effect works

This week, hit or miss lake effect streamers will continue for the next two days before diminishing as the winds relax. Temperatures aloft are well cold enough for snow. It's the surface temperatures that are an issue, thus the highest chance for snow will be at night and the early morning hours, before it warms up. The chance for seeing flakes at any one place is low, around 30% so we have to get lucky. The one thing that is working for us is timing. Since it is still early in the season, the Great lakes are still very warm.The dry northwest winds carry moisture off the lake surface and carries it towards us. As it is forced upwards by topography, condenses and forms lake effect streamers. Although streamers are not long, the warm waters will "supercharge" the bands this time around. Any precipitation that falls will originate from lake Huron and Superior as opposed to Lake Erie or Lake Ontario, which are closer. The problem is that air temperatures are just a bit too warm for snow.

NAM model at 1AM Tuesday depicting two lake effect precipitation bands (mostly rain) coming off Lake Huron with some enhancement by lake Ontario.

The next chance for snow will come as the cold air mass retreats ahead of the next storm early Thursday morning. Moisture will overrun the cold airmass. There is a good chance of some snow showers ahead of the main slug of moisture warms the atmosphere enough to change over to rain. Unfortunately, (or fortunately, I'm a snow nut) the approach of the low will direct southerly winds at us, warming us well above freezing, but not past 50 on Thursday. Slight changes in moisture transport and storm track could have visible effects on precipitation types. In my experience, (though confined to DC area snowstorms) precip tends to start a few hours earlier than predicted. However, since it is still early in the season, it's going to be hard to get substantial snow. Even then, some good snowTV would be pretty to go with the cold.

GFS model prediction for 8AM Thursday showing a bit of snow before changing over to rain

Thus concludes my first forecast for Ithaca. Please feel free to point out anything that seems out of place (especially geography) and ask any questions you have. Thanks for reading!

-Alex