When Will This Cool/Unsettled Pattern Change?

It's not uncommon for April to feature stalled low pressure with long stretches of "cool" air across the Great Lakes/New England.  Our long stretch of warmth a few weeks ago makes this cooler/cloudy/rainy pattern that much harder to deal with.  What does the long range (2-3 week outlook) say?

The southern oscillation index went through some significant changes over the last week and a half. The greater these day-to-day changes the stronger the effect across North America, Here is the peer reviewed research:  PAPER HERE

SOI analog forecast show the Great Lakes/Eastern US trough backing off by mid month.  How quickly will the Southeast ridge build back is the big question.

Another element going forward this summer and fall will be the building El Nino. I'm not going to go into the specifics of that here. Perhaps a blog post on this in the weeks and months ahead. But as La Nina decays, the overall effects will be cascading across the Pacific augmenting the jet stream and the weather across North America.

Watch the decaying La Nina as shown through the weekly sea surface temperature anomalies from December 2022 to mid April 2023:  Watch the blue area disappear slowly.

Sea Surface Temperature Anomalies

The big drivers of the pattern change have been the ridge of high pressure in the Gulf of Alaska and the western corners of North America along, the deep low pressure over the Aleutian islands and the stalled ridge over Greenland (negative North Atlantic Oscillation). The position of these high pressure ridges have allowed low pressure to undercut them through the middle of the US. This has significantly suppressed the Bermuda high which brought us the warmth a couple weeks ago locking in below normal temperatures across the eastern 2/3 of the US.

Based upon all of these factors, I don't foresee any long stretches over warmth across the Great lakes and Ohio valley through at least the middle of May.  

The reinforcing front Sunday/Monday of next week could produce rain/snow mix.  Here was my forecast tweet from April 20 on the early May rain/snow mix potential.

As of this writing (April 25) the new EURO and GFS models show the Greenland Ridge & western North America ridge (red areas) holding strong until the second week of May then gradual weakening. This flattens the trough across the central US bringing an end to the unsettled pattern. These solutions were indicated on the SOI analogs more than a week ago.

Temperatures per the GFS also corroborate what the SOI analogs indicated 2 weeks ago.  That is gradual improvement across the central US and Ohio Valley but still no big signs of any long stretches of warmth.

Rainfall 2021 vs 2002: April 1 to May 5

Rainfall this spring has overall been above normal.  Using NOAA MRMS radar data from the Iowa Mesonet Site here is a county by county breakdown of total rainfall over this 35 day period comparing this year to last year

Legend for each graphic below (2" to 7")

Mansfield, Ashland, Wayne, Holmes Counties

Lake, Ashtabula Counties

Cuyahoga County

Erie County

Geauga County

Huron County

Lorain, Medina Counties

Summit, Portage Counties

Stark County

Tuscarawas County

Snow to 80s in April. How Often Does This Happen?

Snow showers this morning. Yes, it's April 19th

Hinckley

Twinsburg

April snow is not that uncommon. Look at the snow history from Late March through April. These are the individual instances of snow for EACH DAY since April 3rd color coded. 

Yet this week temperatures will be jumping significantly heading into the weekend. (SOI forecast from April 5th seems to be working out). 

SOI Changes in early April connect with these historical weather conditions

All of this means we could go from accumulating snow to 80 in a little over 4 days. When was the last time this happened?  LAST YEAR

I was surprised to find this has happened almost 60 times since 1950! The fastest SNOW to 80s was 19 hours in April 3-4, 2007

How about 80s to snow?  Only 10 times. Most recently in 2018

Big temp swings in March and April occur with high frequency vs other months. These big changes occur in the central US more than in the Great Lakes.

Angry Lake Erie and Unseasonably Cold Temperatures

 

Strong NE winds developed Thursday and Friday which piled up water in the western basin of Lake Erie. Video above is from Kelley's Island Friday courtesy Todd Neu.

Wave heights reached 10-15 feet before they settled down Saturday and Sunday

The low resembled a late fall, cut-off system. You can see how it developed over the upper midwest and then stalled in the southern Great Lakes.

Temperature started out 20+ degrees below normal Saturday morning across the Great Lakes.

High temperatures Saturday stayed in the lower 50s north and upper 50s south. Temperatures this cool (under 55) don't happen very often this late in the year.

Spring Rainfall/Temperatures/Cloud Cover

April/May 19th cloud cover for each day in Cleveland and Akron

* Bottom of each graph is the ground. Top of graph is 30,000 feet
* Light blue color indicates clear skies
* Black/Grey color indicates cloud cover

Notice more cloudy days this year compared to last year over the same period. Also, Akron has had less cloud cover overall.

CLEVELAND: April-May 2018

CLEVELAND:  April-May 20, 2019

AKRON:  April-May 2018

AKRON: April-May 20, 2019

Mid April-Mid May: Ohio rainfall and temperatures vs normal since 2015

2019

2018

2017

2016

2015

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!