Above: Radar mosaic from 8 pm EST Thursday, February 28, 2020, of the intense lake-effect snowband that stretched the entire length of Lake Ontario, heavily impacting the Tug Hill Plateau east of the lake for more than a 36-hour period of time. (NWS)
This past weekend, we saw what can happen when the Great Lakes snow machine kicks in. The areas just east of Lake Erie, and especially Lake Ontario, got plastered with heavy snow on February 27–29. Why was this set-up such an effective snow producer? The main engine of lake-effect snow is cold winter air passing over the lakes, especially when the lakes are largely unfrozen, as is still the case this year. However, I have always said “you need more than cold air over a warm lake to produce lake-effect snow”. Otherwise it would be snowing most of the time off the Great Lakes in winter.
For lake-effect snow to develop you also need:
- A moist large-scale atmospheric environment (if it’s too dry, it’s tough to produce snow)
- A deep enough layer of cold air to develop clouds thick enough to make precipitation
- A temperature/moisture profile that ensures excellent snow crystal production
If any one of those factors are not present, then you may only get some thin clouds, or possibly even clear skies, in the presence of very cold air over those warm lakes.
Below is an example of mid-February in which surface air temperatures were in the single digits Fahrenheit while the lakes were some 25-30°F warmer. Yet, there was barely any cloud cover over or downwind of the lakes, much less snow. High pressure entrenched over the lakes provided strong subsidence in the atmosphere, which warmed and dried the air, producing a temperature inversion and limiting any cloud growth across the area.
On the flip side, if you can maximize those factors listed above, then powerful snowbands can develop and pummel the downwind locations with very heavy snowfall. That’s exactly what happened near Lake Ontario on February 27–29. Forecasters in the Great Lakes region sometimes use the phrase “weather clears up stormy” downwind of the lakes during fall and winter. Long after the parent large-scale low-pressure system moved east of the region, the cold air that built in its wake moved across those warmer waters to produce clouds and snow. The snow was especially heavy in the Tug Hill region near Watertown (pictured in the tweet below), where snowfall rates of 3” to 4” per hour were common.
Anatomy of a lake-effect blast
The setup for this mega-snowband was pretty classic. On a large scale, a very strong short wave at the 500-mb level (about 18,000 feet) deepened rapidly as it crossed over the lower Great Lakes. The reflection of that upper-level feature at the surface was a fast-moving, deepening low pressure system that brought first rain, then snow, and finally very strong winds to upstate New York in its wake. As the 500-mb short wave deepened and eventually “closed off” into the upper level Low, it slowed down and became vertically stacked over the advancing surface low, north of Lake Ontario—a climatologically favorable position for a long-lasting lake-effect snow east of the lake.
This setup produced a thick layer of cold air, satisfying one of the conditions for deep cloud growth over and downwind of the lake. The North American Model (NAM) 24-hour forecast sounding for 7 am EST Friday, 28 February, is shown below. It compares reasonably well with the actual sounding taken at Buffalo at the same time, with the expected modification east of the lake in the heart of the snowband. The forecast sounding showed the very deep layer of cold air, with steep lapse rates through the first 10,000 feet of the atmosphere (roughly the 700mb level on the figure). That deep layer of cold air with steep lapse rates ensured strong updrafts that would grow quite tall for a lake-effect snowstorm.
In addition, the temperature and dewpoint were very close together on the forecast sounding, signifying a very moist environment right up through about 10,000 feet. In fact, there were reports of thundersnow on Thursday evening in the heart of the band east of Lake Ontario.
Now for the “icing” on the lake-effect snow cake. The temperature and moisture profile were very favorable for efficient production of snow crystals in a region referred to as the dendritic growth zone (DGZ), shown as the DGZ on the left-hand side of the sounding. The DGZ occurs within a temperature range of –12°C to –18°C (10°F to 0°F), which is the most efficient temperature range for the development of snow crystals. If that area of favorable growth coincides with the area of maximum vertical motion, then you maximize snow crystal production. In fact, in that forecast sounding, the strongest vertical motion (colored bars on the left-hand side of the sounding) is coincident with the DGZ. (A side note for weather geeks: did you know that the saturation pressure difference between ice and water maximizes between roughly –12°C and –18°C? That’s why you get such good growth of snow crystals in that temperature range.)
In short, the temperature and moisture profiles were very favorable for deep convection (deep updrafts within unstable air) and excellent snow crystal production. But it was the position of the vertically stacked, nearly stationary upper level and surface Lows that resulted in westerly winds directed down the longest fetch of elliptically shaped Lake Ontario, producing a single, intense band of snow for an extended period of time. In fact, those well-aligned winds actually had a connection to the upwind lakes—something not all that uncommon for Lakes Erie and Ontario.
If the winds take on the configuration of a cyclonically curved trajectory that comes from Lake Huron and Superior, then the Lake Ontario band can actually be enhanced. A convergence zone often develops as northwest winds track down the narrow fetch of the northwest portion of Lake Huron, which leads to the development of a single intense band of snow. That band can stretch hundreds of miles downstream and as the winds “channel” that band down the longest fetch of Lake Ontario, the result is a narrow but very strong band of snowfall. This happens perhaps once or twice each winter season on average.
Put it all together and you end up with a narrow, deep band of convection over and downwind of Lake Ontario featuring very efficient snow crystal production. Moreover, the westerly winds were forced upward as they passed east of the lake and reached the Tug Hill Plateau. The 1700-foot orographic lift provided by the plateau gave that extra boost in lift, and the result was a snowband that produced rates of 3-4” per hour.
A summary of snowfall reports east of Lake Ontario showed that the town of Carthage in Jefferson County received 48” of snow, while Croghan in nearby Lewis county recorded 42.5”. A bit farther south, the town of Redfield reported 31.2”. Although this was by no means a record-setting snowstorm, it did provide snow lovers with a nice (albeit late) gift for a winter that has seen very little snowfall in the Northeast.
