How Often Do Conditions Develop To Produce Lake Effect Snow?

One of the many variables to produce lake effect snow is a significant enough difference between the water temperature and the temperature 5000 feet off the ground. Typically you need a difference of at least 13 degrees Celsius. So if the water temperatures is say 50 degrees F (10 Celsius) and the air temperature at 5000 feet is 27 degrees (-3 Celsius) that would be the lowest difference to allow evaporation and eventually the condensation of water vapor to occur as the colder air moves over the warmer lake.  The greater the difference, the faster the air rises and the more unstable the lake effect snow becomes.

This satellite image shows the uniform bands of lake effect cloud cover off every Great Lakes with a strong northwest wind. Notice the lack of cloud cover on the windward (western) side of each lake. The condensation of the water vapor occurs more than 20 miles offshore as the warm water vapor rises into the colder air aloft. 

How often does a difference of 13 degrees Celsius occur in northern Ohio in the prime lake effect snow time of the year?

I cross referenced the Lake Erie water temperature and the temperature at 5000 feet for five straight winters (Nov 2014 through January 2019). Each instance where the difference meets of exceeds 13 degrees Celsius I shaded that day in blue.

We average about four instances where the Lake Erie/5000 foot temperature difference exceeded 13 degrees (high enough for lake effect snow) each winter between 2014 and 2019.  How many of these produced significant snow?  More on that later.

What Happened to the "Warm" Lake Erie?

It was only a little over 3 weeks ago (November 18) when Lake Erie was the warmest ever at this point in November. Satellite water temperatures go back to the mid 1990s. This was undoubtedly in response to the very warm early November.

The warmth started in October across the central US and Great Lakes.

When you look at just the first 3 weeks of November the warmest expanded and intensified.

November 14 through the 19th warmth continued to move into the Midwest.

Once we moved past Thanksgiving, the overall pattern became dominantly colder across areas that were breaking record high temperatures. Here is the progression of the cold from central Canada starting in late November. Each image is a 10 day period. First image starts November 19th.

The first half of December has been below normal overall across much of the Great Lakes even after the core of the cold retreated back north.

The graph below illustrates the Lake Erie water temperature each year (1995 through 2024) starting on November 28 and ending December 17. Notice how this year was the warmest 3 weeks ago. Look how the Lake Erie water temperatures responded to the below normal temperatures. Compared to the last 30 years, the water temperature now is near average. A huge drop of roughly 10 degrees in almost 3 weeks!

Given the 2-3 day period of cold this weekend then the warmer pattern between Christmas and New Years, the water temperature drop will probably slow.  Look at the new Canadian long range model output for early January. Ridge development near Greenland, Gulf of Alaska/Aleutian island low shifts west. Ridge/warmth over the central US shifts west. Trough/cold settles into the Great Lakes/eastern US the first week of January. (This setup was highlighted on the the FOX 8 Morning Newscast December 9). If this verifies, Lake Erie water temperature will continue to drop. Good bet we will have some ice coverage by the second week of January.

First San Francisco Tornado Warning Ever!

Tornadoes in California are a rarity but not unprecedented. 

 Here is the short list of tornado warnings issued by the NWS office in San Francisco since 1989:

Composite map showing all 9 tornado warnings since 1986.

Each tornado warning and the radar composite for each event:

December 14, 2024

The warning went out before 6am Saturday and ended at 6:15am. Damage assessments from the NWS said the tornado had peak winds of 90 mph

A team from the National Weather Service San Francisco Bay Area conducted a storm survey in Scotts Valley. The team identified a swath of tornadic damage that began near the intersection of Mount Hermon Rd and Lockewood Ln and continued southeast along Mount Hermon Rd until just short of Kings Village Rd. A wide range of damage was observed including: downed trees, downed power poles, trees stripped of branches, numerous overturned vehicles, and damaged street signs. Based on these damage indicators the survey team assessed a maximum wind speed of 90 mph, which is a weak EF1 tornado.

March 2, 2024

March 18, 2011

January 20, 2010

January 23, 2010

September 18, 1989