Friday, December 04, 2020

What's The Most Underrated Lake Effect Snow Variable?

The snow event on December 1st makes a great case study on lake effect snowfall.  In many instances, its difficult to separate general synoptic/system snow from actual lake effect as they tend to overlap. I like to classify the events into three phases.  First is system snow. Second--the most tricky of the three--is system snow+lake enhanced snow. Third is only lake effect snow as the storm system moves out.

Lake Erie water temperature was 48 degrees F (9C). 850 mB temperature (5000 feet) was -9C so temperatures difference was around 18 C. You need a temperature difference of at least 13C to create lake effect snow so this was good enough. After checking my lake effect snowfall diary (yes I have one of these. It shows various lake effect snow events back to the 1960s/70s) a basic setup like this could easily create decent snowfall!  Yet Bufkit (NWS forecasting application) showed only conditional instability which meant the ability for the air to rise creating distinct bands of lake effect snow would be on the low end (see the radar loop above).   Hmmm.  

So why did the models, the NWS and other local meteorologists forecast significant snow?

Initial NWS Cleveland forecast

My initial forecast. Snowfall was conservative

I checked the system moisture to see if this was shallow or deep.  Sure enough, it was BOTH! Look at the coverage around the Great Lakes. The moisture depth was up to 10,000 feet. 

The final snow totals were extremely impressive. 20-25" in the snowbelt. 10-15 south and west of Cleveland.

This weekend, we have a few clipper cold fronts passing through.  Each one will produce periods of colder air. 850 mb temperatures (5000 feet) will drop to -9C late Sunday/Monday.  Lake Erie water temp is between 45 and 48 degrees F (7-9C).  Instability per Bufkit (NWS forecasting tool)  will be conditional.  So far it looks very similar to the big event earlier in the week!

So why is this weekend's snowfall forecast so much different than the December 1 event?

Answer is simple:  TOO LITTLE MOISTURE

The overall system moisture is not widespread nor is it deep as Tuesday's storm.  See how the dry air denoted by tan colors is pulled in from the west.

Here is my initial snowfall forecast for December 5-7 and the NWS snowfall forecast.  HUGE difference compared to the snow earlier in the week

In my opinion, one of the most underestimated components in forecasting lake effect snow is moisture.  You can have temperature conditions that are very similar yet the snowfall results can be hugely different. 

So don't forget to check the depth and spatial coverage of the moisture before each snow event. 

Wednesday, December 02, 2020

First BIG snowfall of the season!

My previous post written before Thanksgiving illustrated the strong possibility of a significant snow. The pattern has been ripe for this type of event. Sometimes the large scale pattern doesn't always translate to a big weather event locally so I'm glad the outlook and subsequent forecast worked out. 

I try to enjoy forecasting success. I'll remind everyone on social media when we get it right.  I also know full well that long range and short range forecast failure will eventually give me a reality check. When that happens I always own it. I find out what I missed or didn't utilize. It's filed away so I can use it as a forecasting tool in the future.)

Here is the entire radar loop showing the widespread snow December 1st and the lake enhancement/lake effect at the end.

The 9.5" at Hopkins Airport was the 12th most single-day snowfall since record keeping began at the airport around 1940 and 15th since 1900. The list below shows Hopkins Airport snow numbers AND the downtown snow numbers which is from ~1890s to roughly 1940.

Here is the entire snowfall total list for northern Ohio