Northwest Flow Snow: How The Great Lakes Bring Snow to The Carolinas

The perfect set-up for northwest flow snow will continue into the evening hours. This set-up includes very cold air plus very warm water over the Great Lakes. Then a little mid-level energy to help the lift never hurts. The primary mechanism for this type of snow is taking cold air and moving it over large bodies of water like the Great Lakes.

Then moving it inland where it is then lifted by the higher elevation of the Appalachians which cause snow to fall. The typical distribution of a northwest flow event is seen below.

(Credit ASU & GSP)

It really helps to have open(ice free) and relatively warm water over the Great Lakes too. This difference in the air temperature and water surface or delta T. This causes a lot of moisture to be picked up and transported downwind. The first part of this is usually lake effect snow near the lake shores but further downwind the lift caused by the higher elevation of the Appalachians can “squeeze” even more moisture out. This all starts with some lake effect as seen below.

Current Lake Water Temperatures:

Notice all the lakes are ice free and above freezing or 0° C.

The amazing thing has been today and last night the impacts of multiply lakes added moisture and the results are some convective type snow bands. These bands have stayed together for very long distances downwind of the lakes. Causing snow around Baltimore and parts of the DelMarVa peninsula.

The results today as seen via radar and visible satellite imagery.

Forecasting these events:

The model guidance from the higher resolution NAM4 did a relatively good job. The models still always underestimate the amount of northwest flow snow especially when the air is this cold and the lakes are this warm. This is where experience and pattern recognition will help a forecaster.