Prime Feature: Significant northwest flow lake effect for metro Rochester.
While this event proved to have minimal impact on the Rochester metropolitan area, it was the first significant lake effect snowstorm of a foot or more for a major urban area in several years. Unlike similar snowfalls in Buffalo or Syracuse, lake effect of this magnitude is unusual for the Rochester area. This was especially the case this winter when lake effect snow has been a rarity.
The vast majority of this 2011-12 lake effect season has been dominated by a progressive Pacific based flow which has kept temperatures generally above normal across Western and North Central New York. While various global teleconnective patterns can be at least partially blamed for this trend, one clear culprit has been the unusual persistence of a strong Icelandic low. The presence of this climatologically favored surface low generally prevents a polar vortex from settling over eastern Canada, and this has certainly been the case for this winter as true arctic air has been bottled up over the higher latitudes.
This all changed during the first full week of February when weak ridging developed over Iceland and Greenland. The subtle pattern change over the North Atlantic promoted a short lived polar vortex over Eastern Canada, which local research has shown to be very favorable for significant northwest flow lake effect for Rochester and its eastern suburbs. A closed low remained situated over the southern half of Quebec on Sunday during the height of the lake effect, then it stubbornly drifted east across the St Lawrence Valley Sunday night and Monday, bringing an end to the accumulating lake snow.
A closer look at the thermodynamics of the event revealed temperatures ranged from about -15 to -18C at about 5000 feet, and given a wealth of low level moisture, this produced a very favorable environment for deep lake induced convection. The lift supplied by the relatively "warm" lake combined with a fairly deep dendritic snow growth area of 4000 feet to produce snowfall rates of 1 to 3 inches an hour with a west to northwest flow aiming the steadiest snow across the Rochester metro area. This process was further enhanced by a strong upstream connection to Georgian Bay, which primed the snow band long before it even reached Lake Ontario.
The event unfolded late Saturday night after an upper level disturbance dropped south from the province of Ontario. Colder air deepening in the wake of this feature produced streamers of lake effect snow off Georgian Bay which consolidated into a single plume that extended southeast to the western third of Lake Ontario. Keep in mind that during a typical winter, Georgian Bay would be largely frozen by this time. In any case, the band of lake effect snow gathered additional moisture off Lake Ontario as it curved more to the east across Monroe County. Moderate to heavy lake effect snow then fell across Northeast Orleans County a large portion of Monroe County during the early morning hours of Sunday, February 12, with accumulations of a foot or more experienced by daybreak.
The west to east oriented band of lake effect snow was then pushed to the south away from its moisture source as an Arctic cold front sagged down across the south shore of the lake. While the cold air further deepened behind this front, the airmass became hostile for accumulating lake effect as the low level shear increased dramatically and the mid levels were notably drier. This disrupted the lake effect machine for several hours from late Sunday morning through mid afternoon. After this period, a weaker connection to Georgian Bay became re-established.
While the lake effect snow machine was able to restart late Sunday afternoon, there was just enough environmental wind shear to prevent the convection from being in single band. Rather, multiple bands of moderate lake effect snow oscillated across Wayne, Northern Cayuga and eventually Oswego County from late Sunday afternoon through the first half of Sunday night. Snowfall rates were not as impressive as the previous night though as rates generally ranged from a half to one inch per hour.
Warmer air poured east from Michigan and lowered the limiting cap on the lake effect during the overnight hours, and eventually forced the lake effect to end as flurries over the Tug Hill Plateau Monday morning.
In terms of impact, the vast majority of the heavy lake snow fell at the least detrimental time for local residents. While roadways became quite treacherous at times, the heaviest snow moved through during the early hours of a Sunday morning when traffic was at a minimum. This resulted in this event earning just two ** stars due to the localized problems that it produced.
Here are some representative reports.