Friday, September 11, 2020

How Was The Labor Day 2020 Rain Event Different From The 1996 Record Setting Rain Event?

The Labor Day Rain is a weather event that we will remember for some time. Not only did it occur on a holiday, it came very close to breaking the all-time single day rain record in Cleveland set back in 1996, 24 years to the day earlier!

How are these two rain events different? 

I downloaded the radar imagery from the National Climate Data Center and created an animation for September 6-7, 1996 when the well organized remains of Hurricane Fran passed over Ohio. Notice how long the rain stayed over the area. Rain was reported at the NWS at Hopkins Airport for almost 24 straight hours! 

Now compared it to the radar loop from the Labor Day 2020 rain event:

The main difference between the two events was the duration and intensity of the rain. The 1996 event featured more consistent and steady rain as indicated by the yellow radar returns from the tropical system (Fran). The 2020 event was heavier (orange and red radar returns) with elongated bands along with bursts of higher intensity rain from a stationery front.

Many people have also asked how the rain on Labor Day will impact the Lake Erie water levels. Since we haven't had an event like this since 1996, we'll use it as a base for comparison.

The graph shows the Lake Erie water level from September 6 to September 9. 

By marking the start and end of the rain event (green line) gives some perspective on how the lake responded to the volume of water

Now the Labor Day event over the same time frame.

Interestingly, the water level increase was similar with both events. 2020 event took roughly 9 hours less time to rise the same amount! 

        Duration of Rain    Lake Erie Level Change    Time to Peak Level

1996:     ~24 hours                  1.19 feet                       27 hours

2020:     ~10 hours                  1.14 feet                       18 hours

The 1996 levels fell quickly after the tropical rains.  As of this writing, the levels have remained pretty high. I surmise that this is due to the orientation of the 2-4" accumulated rain region which parallels the lake.  Perhaps the runoff into the lake has been more uniform keeping levels higher for longer.  

Here is a loop of the accumulated rainfall amounts over the entire event

Final doppler rain estimate rainfall totals from the 1996 events were impressive. Even with it's highly pixelated resolution you can see the higher rain amount regions. Pink colors indicated 3" or more. Blue indicates 4"+. A bullseye in southwestern Cuyahoga county of 5-7"!

Compare this with Labor Day 2020. The color scheme is different because this map is newer. It shows the orientation of the higher rain totals more west to east. 70% of the area received more than 2 inches of rain.  That equates to more than 3000 square miles!

A larger look at the rainfall across the state for both events gives a better look on the spatial coverage.

What about rainfall in the weeks prior to each event? The rainfall departures for the 3 weeks leading up to 1996 event were well below normal. This year, rainfall from mid-August has been above normal for most areas.

Another way to look at comparing 1996 to 2020 is the Arridity Index. This combines the high temperatures departures and rainfall departures into one number. Above zero is considered drier than normal, below zero wetter than normal. Each year saw a significant drop after the heavy rain.  Remember that a few weeks ago, we had another big rain event on August 28th which set the stage for wetter conditions before the Labor Day rain.

Both events were significantly different in many ways. The 1996 event was due to tropical system and lasted longer.  The 2020 event was from a stalled front which featured bursts of heavier rain over a shorter period of time. Conditions were much drier in 1996 versus 2020. The rain coverage was further south and more localized in 1996. The 2020 rain event was more extensive covering a larger area.  

Monday, September 07, 2020

Labor Day Heavy Rainfall in Ohio. Summer heat to snow out west

 Stalled front between 6am and mid afternoon created training of storms across most of northern Ohio.

Radar loop from 5am to 7pm Labor Day

More than 90% of northern Ohio received 1" or more (green or yellow colors) above

I calculated that more than 80 BILLION of gallons of rain fell over a 12 county area between 6am and 6pm. 

Labor Day's rainfall ranks 3rd all-time at Hopkins? 

Here is the top 30 rainiest days. I boxed in red the events that occurred in September.

Today's rain is exactly 24 years after the remains of Hurricane Fran passed over eastern Ohio setting our single day rainfall record of 4.59"

I dug up some of the NWS Cleveland forecast messages from the Iows Mesonet Database (check it out. It's a great resource) issued from that day in 1996:

Message from 8:30am 9/7/1996
8:30am message from NWS Cleveland on 9/7/1996

10:40am NWS message from NWS Cleveland on 9/7/1996

All of this here in Ohio doesn't compare to the dramatic changes parts of Colorado will go through weatherwise over the next 24 hours. Mid 90s to ower 30s with accumulating snow!

Per "Maxar_Weather" on Twitter: "...Years in which Denver recorded both a 100° high and measurable snow. The shortest time in between was 25 days in 1962, with 100°+ on 8/14 and snow measured on 9/8. Denver reached 101° on Saturday and is expected to record measurable snow three days later--Tuesday."