January 4 2016 marked the arrival of the first arctic air mass of the new year, and real taste of winter. It brought flurries and snow showers as we discussed last week. If you missed it, please see my video of the morning snow at the bottom of this post. But the really special thing about today was the Chesapeake Bay Effect snow band that formed late in the day and reached the lower Eastern Shore of Maryland all the way to Virginia Beach.

Lake Effect snow pushed down from the source of Lake Ontario well to our north, but a narrow plume actually developed over the Chesapeake Bay. More on that below.  The snow that reached Maryland was not purely Lake Effect, but rather Lake ‘Enhanced’ snow. Even the heavier snow in western Maryland was this ‘enhanced’ snow which is a combination of the moisture from the Great Lakes, plus lift from air rising up the mountains, and upper level support. So to be honest, even when I say Lake Effect as a general term for out snow showers, I should be saying ‘enhanced’. What is the difference?

Lake Effect Snow:


When very cold air moves over warmer water, it does not always produce Lake Effect Snow.

Temperature Difference:

First, there needs to be a large temperature difference between the air aloft and the water. The general measurements are between the air at 5,000 Ft aloft, or 850mb and the surface. The air must be 23°F (13°C)  or more between these levels to have the energy to produce the clouds and precipitation.

Air Fetch:

The flow of air also has to be long over that water source to allow for the build up of enough moisture. At least 90 miles long, which is why the Great Lakes are special to provide that in many directions. But the wind direction is key as to where those plums or bands of snow (or rain out of season) form and reach. These bands can be as small as 10 to 20 miles wide sometimes with intense snow (many inches per hour) at the core while it is sunny to partly cloudy on the edges. But they can also just be lighter showers or moderate squalls.

Chesapeake Bay Effect

ChesapeakeBayEffectSnowFor the the Chesapeake Bay to provide the same opportunity as the Great Lakes, the fetch of wind is not long enough from west to east. That is why the Eastern Shore does not have the season snow totals as Buffalo or Syracuse. The wind needs to be almost (but not exactly) due north. Based on a compass with due north being 360° or 0°, the wind flow from NNW at 340° to 350° will give the 90+ miles of flow over the water to develop the plumes like off the lakes to our north. That wind carries well south of metro Baltimore and Annapolis, and usually impacts very sparsely populated portions of the southern Delmarva in southern Maryland and Virginia. Today however, A large Chesapeake Bay Effect snow band provided snow showers for Cambridge MD and Virginia Beach.

This is a rare event. We can get a small event once or twice a year, but a district plume like this radar view usually is seen once every few years (or longer).  Usually the cold air arrive from a wind direction more from the northwest with a direction of 300° to 330°, missing the fetch across the water needed.

Wind Direction Fit:

This is a METAR report. An hourly observation for airline pilots and meteorologists. This BWI report around 6:51 PM EST shows the wind direction was at 340° at 11



Here was the wind map forecast for the same time frame


Remember this cloud formation this summer? It was a unique convergence along the Chesapeake Bay on July 17, 2015 that was over 100 miles long and made me think about this Bay Effect type events. I had to bring it out of retirement. It was one of my favorites of the year.



Video: This morning’s snow shower

Here was my view driving through Manchester, MD. I included slow motion for added drama


My next post: I will update the back edge of this arctic air, the next surge of arctic air and the storm in between.

Faith In The Flakes: Real Snow Will Arrive… Eventually

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