Thursday, November 10, 2011

The Many Faces of Lake Effect Snow

Cleveland weather is very unique due to its proximity to Lake Erie.  Weather patterns in Northeastern Ohio from November to March can vary greatly from location to location. Chardon could be receiving 3 inches of snow per hour with 15 inches on the ground while Westlake is snow free with mostly sunny skies!  This is exactly what happened from November 9th through the 15th in 1996 when Chardon received 69", Mayfield Heights had 37" yet Medina only had 3"! Watch the video taken in Mentor back in 1996.

It might seem that with all of our lake effect history and hundreds of lake effect events, predicting lake effect snow amounts should be easier.  Yet the lake continues to fool us just when we think we have the whole lake effect snow machine figured out. Why can one lake effect snow event produce tons more snow even if they have very similar conditions? 

First, all of these ingredients must be present for lake effect to occur:

1. Wind off of the lake - (over at least 60 miles of water)
2. Wind direction - North or Northwest preferably here in Cleveland
3. Wind speed - Between 10 and 40 mph
4. The wind direction from the ground up into the atmosphere has to be the same (less than 30 degree direction difference from the ground to roughly 10,000 feet.  If not, the changing winds sheer apart the lake effect snow bands which might spread the snow over a larger area.

5. Temperature difference between lake and cold air aloft (5000 feet) has to be at least 13 degrees C.  For example, If the lake is 52 degrees, the air aloft (around 5000 feet) has to be at least colder than 26 degrees.  The greater the temperature difference, the better.

6.    Abundant moisture at the surface.

7.  The cold air at the surface should be deep.  The deeper the cold air right above the lake, the more room for clouds to develop to trigger snow growth and the greater the chance for more significant snow amounts

8.  Higher terrain inland (Geauga, Lake and Ashtabula County at elevations of more than 1200 feet compared to 700 feet along the shorline) also helps in enhancing lake effect snow. The greater the elevation difference from the lake to the "hilltop" the greater the elevation effect. If any one of the ingredients changes even slightly, the snow amounts can change greatly! 

Today, we'll concentrate on one of the bigger component: COLD AIR OVER THE LAKE

For comparison sake, let's keep the lake water temperature consistent (52-54 degrees as it is now 11/10) and examine similar lake effect events in the past keeping all other variables as equal as possible given the lack of data from the 1960s and 1970s.  Look at how these four separate lake effect events change as we change the temperature of the cold air moving over the lake. 

November 12-13, 1968: 

Air temp aloft (5000 feet) in Celsius:  -7

Wind Direction: NW, gusts to 40
18,000 feet temp (indicator of thunder snow):  -28
Surface Temp:  low of 27, high of 40

Snow Amounts: Chardon: 1/2", Clear skies in Lake County

November 11, 1977: 

Air temp aloft (5000 feet) :  -8 to -9 degrees early then down to -10 to -12

Wind Direction: NW, gusty
18,000 feet temp (indicator of thunder snow):  -28
Surface Temp: low of 34, high of 43

Snow amounts: Painesville: 12", Chardon: 10", Southern Cuyahoga Co & Akron 3-6"

November 9-16, 1996:

Air temp aloft (5000 feet): -8, drop to -14

Wind Direction: W to WNW , gusts to 25 mph
18,000 feet temp (indicator of thunder snow):  -38
Surface Temp: 25-30

Snow amounts: Chardon 69", Geneva 38" , Hopkins 19", Hudson 15", Akron 2"

November 2-3, 2006:

Air temp aloft (5000 feet): -9 to -12

Wind Direction: NW 15-25 mph
18,000 feet temp (indicator of thunder snow):  -38
Surface temp:  38,  Dew Points: 19-23 (very dry)

Snow Amounts: Ashtabula 6", Cleveland 1/2", Eastside: 4"+

All other factors being close to equal, the difference between air over the lake at 19 degrees and 9 degrees can make the difference between 3" and 15 to 20" of snow!

Be sure to watch how cold the air gets later this month and into December.  The incoming cold air over the lake will no doubt be the main factor as to how much and how quickly we get our first BIG SNOW in Cleveland, Pepper Pike, Solon, Chardon, Painesville, Thompson and Ravenna.

Tuesday, November 08, 2011

Are 60 and 70 Degree Days in November Increasing?

November stands out in my experience as one of the most changeable weather months. The Polar Jet Stream is getting stronger and pushing south while the leftover pockets of summer warmth still remain across the country.  Cold fronts become more defined due to the sharp temperature differences along them. Wild swings of nearly 40 degrees in the span of 24 hours in not unheard of.  For example, back in 2005, the high on November 16th was 67 degrees. The high the following day was.....29!

All of that said, are these 60+ degree days in November become more common?

Let's split up the month into two halves. The chart below shows the 60+ degree days from November 1st to the 15th since 1980. The average per year is 4.4 days.

The 60+ degree days for the last half of November is obviously much lower. A third of the time we don't even have temps above 60!

If we split up the time showing the average since 1995 vs 1995 back to 1980 from November 1st thru the 15th, we show a decrease...

If we split up the time showing the average since 1995 vs 1995 back to 1980 from November 15th thru the 30th, we show the average remains fairly steady...

So are 60+ degree days in November increasing? Based on these numbers, no discernible trend upward over the last 30+ years is noticeable.  So goes our up and down November weather patterns in Cleveland!