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Don Paul: 'Significant lake effect snow for some of us'

The initial arrival of colder air by Wednesday will be modest, and give us all some adjustment time in advance of the real shakeup of our benign weather pattern to truly wintry. The first cold front will be preceded by a notable warmup and a wetdown. Here is an ensemble projection of the upper air pattern just ahead of that cold front.

In the cold front’s wake, the temperatures by Wednesday into Friday will run in the 30s during the day, turning colder during Friday and especially the weekend which follows. That is cold enough for a lake response, with a temperature dropoff/lapse rate  from the lake surface up to about 5,000 feet adequate for lake effect snow. We typically look for a lapse rate of 13 degrees Celsius from the lake temperature to around 5,000 feet to get things going. That lapse rate triggers lake effect due to upward motion tied to the lake’s relative warmth compared to colder air aloft. However, there are signs initially behind the first front the air in that layer will be fairly dry. Additionally, the winds will shift in direction with increased altitude enough to produce wind shear. That shear will tear at any lake bands and tend to keep them poorly organized midweek.

But note the sharp deepening of the eastern trough by the end of the week, responding to the sharp pumped up ridge over western North America.

This warm western ridge/cold eastern trough pattern looks to be rather persistent for some weeks afterward in extended range ensembles, meaning an lengthier period of mostly below average temperatures. There will be variations on this theme within the overall pattern, because “kinks” or short waves will be passing through the long wave trough. Each time one of those short waves goes by, the lower level winds tend to “back” in a counterclockwise direction ahead of the wave. Here, from the University of Wisconsin’s superb CIMMS NOAA Satellite blog, is a loop of the backed southwest flow which can result in delivery of significant lake snow northeast of Lake Erie.

And here is a loop of the northwest veered flow which often occurs behind the wave.

Part of the art and science of forecasting accumulations is determining how long the backed southwest flow will persist as opposed to the eventual veered west or northwest flow. In Christmas week 2001, we had lengthy periods of a southwest flow which delivered an all time record 82” of snow during that week for Buffalo.

In addition to duration of a directional flow, there must be precision in forecasting wind direction. It’s not enough to know the wind will be “from the southwest.”

If our wind directional forecast is off by 5 degrees on the compass anywhere in our most densely populated area, northern Erie County, we can warn — or not warn — the wrong 100,000+ people. If the winds are originating from 250 degrees, decades of experience tell us lake effect will focus mainly on Buffalo out to the airport and beyond to Batavia, assuming conditions for lake effect snow are favorable in terms of temperature, humidity, and lack of wind shear aloft. Just 5 degrees more veering to 255 degrees will focus on South Buffalo through Depew, Lancaster and some of the nearby Buffalo Southtowns. 245 degrees will reach parts of “southern” Amherst and northern Cheektowaga; 240 degrees reaches most of Amherst and Clarence … and so on. Just a few degrees difference in direction makes a big difference. Precision such as this has been made at least partially attainable with the advent of high resolution models such as the National Weather Service WRF.

The passage, precise timing, path, and intensity of these small short waves cannot be resolved, say, a week in advance. The high resolution zoomed-in/mesoscale models cannot run too far out in time, as opposed to lower resolution global models such as the American GFS and the European. As of this writing, there is high confidence on the overall establishment of this wintry pattern but necessarily lower confidence on the details involving lake effect  snow. Early on, the American GFS indicates a west-northwest flow Wednesday night.

But allowing the GFS to run further out in time shows some oscillations, suggesting short waves in this pattern which may allow occasional backing to a more southwest flow. The overall “look” of the pattern suggests winds will be more frequently from the west or northwest with greater snow amounts in ski country and the southern tier. There will probably be, in my judgment, some movement of the bands farther north from time to time. And the WNW to NW flow may also deliver occasional rounds of lake snow closer to Lake Ontario and suburbs north and northeast of the metro area. winter is looming on the horizon in the near future in terms of temperature and wind chill for everyone, and significant lake snow for some.

Obviously as we get closer to the pattern shift, details will become clearer. In the meantime, you can assume real winter in terms of temperature for everyone in Western New York  … and significant lake effect snow for some of us.

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