Deadly December Tornado Outbreak Recap

The clean up continues across the south after Saturday's deadly tornado outbreak.

people lost their lives as 71 tornadoes hit after dark between  December 10 and 11 across parts of 4 states. This LINK HERE lists multiple resources on the damage assessments.

More than 48 hours prior to the event, the Storm Prediction Center started to plant the seeds of potential severe weather.

By Friday, the SPC narrowed and upgraded their outlook from Enhanced to Moderate:

Chicago NWS graphic below shows the HUGE differences in temperature.

The severe storm parameters were present. Graph from NWS Little Rock, AR.

Storms were building in the distance.  Notice the sunshine ahead of the storms. This can create a false sense of security.

Nathan Scott, Little Rock, AR

Look at the wide variation in weather occurring simultaneously on December 10-11.  Winter weather watches across the upper midwest.  Rapidly deteriorating conditions in the south as the line of severe storms tracked east.  Notice the tornado watches/warnings in red.

This line which evolved into two separate lines produced large scale damage from western Ohio to southern Arkansas.

Meteorologists at the SPC and the various other NWS offices across the central US did an amazing job in giving advanced warning. Look at the accuracy of the outlook vs the tornado reports.

Total path exceeded 200 miles

This outbreak could end up breaking the tornado track record set back in 1925

For perspective, here is a map of the tornado outbreak in April 1974

More details will be forthcoming on the strength of the tornadoes. Here is the list and locations of ALL EF4 & EF5 tornadoes.  The last EF5 was May 2013.

How Warm Was This Summer? Complete Summer Weather Recap - Part I

Credit:  Nadine Maruszczak, Getty Images

This past summer featured significant warmth across the northern states, New England and western US.  Temperatures were below normal across the deep south.

The next 3 maps below show temperatures vs normal for JUNE, JULY and AUGUST.

When we examine each climate division and rank the OVERALL summer temperatures for each division (out of  129 years) we get this picture below.  The lower the number (out of 129 years) the warmer the period (JJA).

Overall summer temperature trends across the US are increasing

Once we separate the divisional high temperatures from the low temperatures, we can see significant difference.  Divisional high temperature ranks are similar to the overall temperatures. 

Yet the overnight low temperature ranks are far higher in many areas.

The well above normal low temperatures correspond to the well above normal rainfall areas

Summer rainfall vs normal below .  Green and blue areas indicated above normal.  Warmer colors show .well below normal rainfall

The Ohio Valley and upper midwest breakdown for JUNE, JULY and AUGUST on PART II (NEXT POST)

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. 

2018

2012

1999

1996

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.