Lake Erie Convergence Zone Strikes Back!

The Lake Erie Convergence Zone isn't a magical place, where snow shovels itself and it's always a balmy 80 degrees.  From a forecast perspective, however... It will either make you or break you.

Allow me to explain:

Winds are driven by high and low pressure systems.  They don't just move clockwise and counter-clockwise, however but spiral inward and outward, due to the external forces acting upon them.  These forces include the Coriolis and Pressure Gradient Force (PGF).  To a lesser-extent, friction also influences wind direction at the surface.  Friction is simply the result of topography.  The more obstructions, the more friction becomes a factor.

Take the lake shore for example.  When a strong southerly wind is present, the City of Erie can be several degrees warmer than locations further inland.  This is caused by warm air advection and friction from a sloping Lake Erie shoreline.  The same process occurs on the leeward side of the Rocky Mountains.  As air travels toward sea-level, it accelerates and warms adiabatically- a phenomena known as the Chinook.

Obviously, there are no such obstructions in the mid and upper levels of the atmosphere.  Therefore, friction is non-existent here.  To a lesser-extent, this is also the case over vast, open bodies of water.

Here's where the Lake Erie Convergence Zone comes into play!

Winds are mainly synoptically driven.  That is, winds are driven by high and low pressure systems.  Winds over open water, however can vary by as much as 45-90 degrees than those over land.  One reason: There is less-friction present over water than on land.  Add a strong 500mb trough to the equation (see below), and you have increased wind speed with height over a virtually frictionless body of water.

Where these northwest and southerly winds meet, is called a convergence zone.  This typically occurs along the lake shore.  Thus, the textbook definition of "lake effect" can be reversed for places like Erie, where a mainly southerly or even calm wind is present at the surface.  All it takes is a -13 (on average) degree difference between the air and lake water temperature, and a mainly northwest flow aloft.

So, you see how Mother Nature deals us a wild card every now and then.  A textbook lake effect event in Erie is a west-to-north flow, with almost no wind shear present.  In this event, however Erie and the lake shore saw a mainly southerly flow, with some wind shear (winds backing to the northwest with height).  That, combined with moisture from a relatively warm Lake Erie provided enough instability aloft to produce a 3-5 inch swath of snow along and north of I-90, with lesser amounts south across the traditional snow belts.

I might also add that a similar process occurs during the spring and fall months.  Anytime wind direction changes with height, you have a highly sheered environment.  When storms encounter this "twisting" with height, brief tornadoes can and often do with storms moving onshore.

I plan to spend more time talking about this during the 2012 Severe Weather Workshop and Symposium, January 10th at Penn State Erie- The Behrend College.

Tim Samaras and I hope to see you there!