Tuesday, May 04, 2021

How Many 90 Degree Days This Summer? Check Out The Northern Ohio Summer 2021 Outlook

After an up and down, La Nina driven winter (cold mainly from late January through February) with slightly below normal snowfall with nearly 50 inches (our snowfall prediction was 55-65"), the question is: Will the same La Nina drive this summer's pattern?

First the April pattern recap:  

High pressure was dominant over the northern Pacific and the north Atlantic. This drove temperatures well above normal during the first half of April. We had snow on April 1 in northern Ohio. 6-7 days late we reached 80 degrees!

Since the middle of the month, the pattern flipped.  The northern Pacific ridge shifted over Alaska. The ridge over the north Atlantic shifted north over Greenland.  Both ridges linked together over the North Pole. The trough developed underneath and across the eastern 2/3rd of the US with widespread below normal temperatures.  We had snowfall on April 20th in northern Ohio, the 3rd most this late in the season ever (2005 was the most).

Much of this was influenced by the high amplitude MJO in April rotating through phases 5 through 8.

April 2021 high temperatures vs normal in Cleveland.

Late this winter and into early April the cooler water along the equator (La Nina) has been steady but slowly warming. 

The depth of cool water is also becoming more shallow.

The MEI is another great measure of the ENSO state. It takes into account multiple oceanic and atmospheric variables (not just the ocean water temperatures) to produce an overall number either positive or negative. The current MEI is at 0.8. The MEI site's description here:

"The new version of the MEI (MEI.v2) has been created that uses 5 variables (sea level pressure (SLP), sea surface temperature (SST), surface zonal winds (U), surface meridional winds (V), and Outgoing Longwave Radiation (OLR)) to produce a time series of ENSO conditions from 1979 to present"

The evolution of La Nina in years which are similar to the current event's development.

The model projections show weakening with some intensification of the La Nina late this summer/early fall. Since it take time (months) for the ocean/atmospheric state to completely transition out of its current state, I think this will have only small effects on the overall summer pattern.
Here is a great tutorial on La Nina from NOAA:

What do some of the similar past La Nina summer look like?

These are overlapping months (May through September) showing the upper level pattern. 





Now summer temperatures (June, July and August) vs normal

What are we expecting this summer?

Based on top analogs looking at past La Nina summers along with other variables, we expect an above number of 90 degree days this summer especially in late July and August. A few 90s are highly possible into early September.

The ground moisture is running slightly below normal as of this writing (May 4). For comparison, last year we were running a rain surplus.

Over the last 3 weeks, the rainfall deficit has dropped across the state.

Crop moisture is only slightly below normal but trending to near average
...same with the longer term Palmer Drought Severity Index (PDSI)

* Wet ground/above normal rainfall in May/early June will limit the expansion of heat and ease of reaching 90 degrees for multiple days.  Drier ground over multiple states will enhance ridges of heat in summer making 90+ easier to attain deep into summer. 

What are we watching?

*  The position and strength of the Sonoran Ridge (high pressure) and the Bermuda high will dictate the position of the summer storm systems. 

*  Our analysis shows that these ridges especially the Bermuda High will develop by mid June and pulsate north frequently. The relaxation of the ridge periodically (flattening along the northern periphery) will allow storms to drift over the ridge and into the Ohio Valley especially in late June and July. 

*  We expect a more active July with storms.

Monday, April 26, 2021

Second Heaviest Snow This Late In The Season. When/How Did We Know?

I'm 5 days late with the late April snow recap.  Here are the snow amounts:  A few locations were close to 8 inches by Thursday morning.

April 21 snowfall


Seven Hills


Snow after March 20 is pretty common in northern Ohio.  Snow after April 20 is a rarity. Only two other years have had more than 2" after April 20.  1974 and 2005. The 2005 event was over a 48 hour period (see the map below).

April 23-25, 2005 snowfall.  Remember this? More than a foot of snow fell across a large part of northern Ohio.  In 3 days it was completely gone!

Signs were pointing to snow or at least a deep storm system with colder air back on April 5--two week prior.

The MJO was heading into favorable cold phases.

A recurving typhoon near the Philippines (which often times portends cooler than normal temperatures eastern US) gave us another good indication.

Eventually the long range models (back on April 8) picked up on this colder idea.

Animation of temperatures: April 10 to 22

Upper level pattern was pretty close to April 2005 snow event. That storm system was deeper.

What's the latest we've had snow? After cross-referencing hourly temperature data, the years are narrowed down to only a few.  Last year was the LATEST MEASURABLE SNOWFALL which broke the record set back in 1907.  Flurries were reported close to Memorial Day in 1907 and 1961 (Akron).

How quickly after late season snow do we see an 80° degree day? Earlier in April it took us around 7 days (6 days listed below) post snow to reach 80°.  Snow on April 1.  83° April 8.

Indications of a late month warmup were surfacing shortly after the colder mid-month pattern was showing up.  SOI composites (analog to similar situations in a La Nina) showed a weak, short-lived ridge of high pressure/warmth around April 26th.

Composite for BIG drop in SOI around April 11-13/La Nina

High temperatures across more than 15 states Monday-Wednesday this week will easily teach 80

Tuesday, April 20, 2021

Late April Snowfall: How Rare is this?

Yes, it's April 20 and we are talking about accumulating snow!

Sure we've seen late season snow before. It snowed in mid-April in 2019.

...and it snowed in May 2016 (under an inch).

In fact 16 of the last 20 years has featured measurable snowfall after the first day of spring. Over the last 30 years April has featured snowfall almost half of the time. But snow that need shoveled after April 20 is something else entirely.

Below is a calendar of each day starting on March 20 showing the snowfalls for each day. Notice how the frequency of snow drops significantly once we make it through the first 10 days of April. Interestingly we've only had 3 snow events greater than 2" after April 20th. Two of those occurred in 2005. (I remember driving home from work during that big snow on the 24th).  The other one was on May 6, 1974.  We've had snow in May but never more than 2 inches.

The late April snow in 2005 was historic for northern Ohio.  Look at the similarities between this year's pattern and 2005. Position of storm center is close.  2005 was much deeper.

Bigtime cold across the Great Lakes and New England (April 2005)

Much of northern Ohio saw more than a foot of snow that year in late April

This time the overall pattern was showing signs of stronger weather systems by the 3rd week of April (shades of blue).   My tweet from April 5th below.

The cold was building a week ago in central Canada as this temperature animation below shows. This loop is from April 15 to April 29.  Not much warmth here.

Snowfall accumulation is much different this time of year due to warmer ground and road surface temperatures.  These numbers are mainly for grassy surfaces.  Snowfall on roads and sidewalks will end up being under these levels. Sun angle is much higher so any sunshine will accelerate snow melt Wednesday and Thursday. 

Snowfall forecast could mark only the 4th time we've had more than 2" of snow this late in the season.

By the way, this snow doesn't give us any indication of what the rest of spring and summer will be like. More on the summer outlook in the weeks ahead.