May Snow & Cold in Cleveland/Northern US

Here in northern Ohio we typically say that if you make it to Tax Day, the chances of seeing snow are pretty remote. Not this year. Late April cold invaded the eastern half of the US and again in early May!  A ton to unpack here so let's get to it.

~2" snow in Auburn Township - May 9, 2020

The month didn't start off super-cold. We had a few days in the 70s during the first weekend. Watch the progression of the cold across the northern US via the RTMA temperatures between May 2nd and 5th.

Historically this is what May should look like.

There have 19 years since 1950 with at minimum snow flurries in May and only a handful of years with measurable snow. I remember May 15, 2016 because I was at a baseball tournament coaching. It was a Sunday and there was snow on the ground at 7am!

Earlier last week, the models were showing a strong surge of cold for Friday/Saturday, May 8-9th. Based upon both the GFS and EURO long range data for Detroit and Pittsburgh--IF verified--this would tie or possibly break the all-time upper level sounding data record for 850mB (5000 feet) cold !set back in 1966 While this isn't where we work and live, this level is critical for determine lake effect snow and surface temperatures.

The sounding history for Detroit shows the EURO forecast for both temperature and 500mB heights at record levels for early May

This is what the 850 mB temperatures read at 8am Saturday morning, May 9th:

Temperatures at 500mB (18,000 feet) were also extremely cold for early May.

Our forecast called for "thundersnow" due to the large temperature change from Lake Erie (45 degrees or 7.5°C) to the temps above (-32°C)

Radar loop Saturday morning showed definitive areas of lake effect

North Royalton

Highest amount: Auburn Township - ~2"

Northern Ohio snow reports - courtesy NWS Cleveland

Northeast snow Saturday May 9, 2020

Check out the progression of record lows Saturday morning across the US.

Core of the cold over the Ohio Valley, Great Lakes and New England Saturday morning.

Wind chill at NWS Cleveland dropped to 20 at 7:51am Saturday May 9th

Has this happened before?  Here are two composite years:  1986 and 1966

After a break Sunday with temperatures near 60 with storms/shelf cloud passage in the early evening...

Vermilion - May 10, 2020

...another clipper front passed through early Monday morning, May 11th. Yet another shot of frontal/lake effect snow!

Chardon - ~2" of snow - May 11, 2020

Hambden Township - May 11, 2020

Mayfield Heights - May 11, 2020

* 2020 marks the only year in Cleveland (NWS Cleveland-Hopkins) since 1954 with multiple days of snow flurries and/or measurable snowfall in the month of May

* New record snowfall this late in the season at NWS Cleveland: 0.2" on May 11th which breaks old record on 5/10/1907

* 5/11/2020 high of 45 was also the COLDEST high temperature in the last 100 years this late in the season (tied 5/11/1996)

We still haven't reach 80 degrees this year. A warm up is coming! (regression to the average which is now nearing 70)

Potential high temperatures Thursday, May 14th

Here are the years when we touched 80 or better after May 15th (since the 1930s). The list is short:

Lake Erie Sunset 5/11/2020

Why The Abrupt Cooler Pattern in the Northeast?

After the warmest April on record in places like Cleveland, Ohio and across much of the Ohio Valley and much of the eastern US, the pattern has not only reverted back to spring, it took several steps back resembling elements of winter. Throughout April, this high pressure ridge stayed east which allowed the ridge across the eastern US to build allowing above normal temperatures. Many were fooled into thinking that summer was here and we were in the clear to plant gardens. Not so fast!

Look at April temperatures versus the 30 year average:

Now look at the first week or so of May per the American model (GFS).

Why the sudden flip?  In this case, we have to look over Greenland for the answer.

Remember that across the mid-latitudes, troughs and ridges (valleys and peaks in the upper levels of the atmosphere) travel like a fluid through the atmosphere. When one location is lucky enough to be blanketed by a ridge, fair weather typically is the result. Once the succession of trough and ridges is changed by an interruption in the wave pattern, the weather will change at the surface. These changes (too much to go into here. See Southern Oscillation Index) often originate over the Pacific Ocean and propagate east. Currently, these changes are reflected over the northern Atlantic and Greenland.

Notice the red (high pressure) at the top inching west blocking/stalling the blue (low pressure systems) over the NE

We can quantify the strength of this ridge using the NAO of the North Atlantic Oscillation.  The more negative the index goes, the stronger the Greenland ridge. The NAO has been forecasted to drop close to -300. Historically this has only happened once in the month of May since record keeping began.  That was back in 1993.

NAO drops to -300 soon

But be warned, historically, this pattern over Greenland in May doesn't always produce a cold and rainy May. Look at the high temperatures in Cleveland during these years of record low May NAO:

Look at the resultant upper level pattern across the US in each of these years. All but 1948 and 2008 show dominant warmth.

May 1948 - DOMINANT COLD OVER GREAT LAKES/NEW ENGLAND

MAY 1951 - WIDESPREAD WARMTH ACROSS US. COLDER SOUTHWEST

MAY 1962 -  WIDESPREAD WARMTH ACROSS US

MAY 1980 - WIDESPREAD WARMTH ACROSS US

MAY 1993 - MORE US WARMTH. COLDER DEEP SOUTH

MAY 2008 - MUCH OF US BELOW NORMAL

The NAO this low (or any other index) isn't always a great predictor of temperature and the overall pattern. 

Unfortunately, unlike any of the previous instances of strongly negative NAO in May, this cold is locked in for at least another week.  So get ready for rain, scattered frost and perhaps some light snow!