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DECEMBER DERECHO FIRST IN U.S. HISTORY...

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FIRST DECEMBER DERECHO EVER RIGHT HERE IN IOWA...

Meteorologists continue to analyze data from last Wednesday’s severe weather outbreak and have concluded that preliminary information does confirm the storm system was a derecho. The thunderstorms that spun off at least 21 tornadoes in Iowa last week were part of the first December derecho ever recorded in the United States, according to the National Weather Service. Severe Thunderstorm or tornado warnings were issued in 97 of Iowa's 99 counties and the entire state was under a tornado watch at one time or another.

A derecho is defined as a widespread, long-lived wind storm associated with a line of thunderstorms. To officially be categorized as a derecho, winds must be 58 mph or greater, extending for 240 miles or more. Last Wednesday’s line of severe weather met both the wind and distance criteria but the storms were far different than last years August 2020 derecho. This years storm is now classified as a serial derecho. Whereas the August 2020 event was what's known as a progressive derecho.

Serial derechos are defined as an extensive line of storms with multiple bow echoes - or curved line of storms with strong winds - which give it a scalloped or arc-like appearance. Serial derechos are a line of storms whose movement is based on the direction of the winds produced by the larger storm system, in this case a low-pressure that moved rapidly northeast. The environment in serial derechos is more favorable for the development of tornadoes. At least 12 tornadoes touched down in Iowa last Wednesday. The strongest winds were 88 mph compared to 140 in the 2020 event.


The extreme August 10th 2020 derecho was categorized as a progressive derecho; narrow in path but traveling a long distance. The system advanced as additional storms developed ahead of the initial cluster. Progressive derechos are most often seen in the summer and and associated with hot humid conditions.

Climatology shows that the state of Iowa is impacted by a derecho every one to two years. Derechos are pretty common features but the term really wasn’t well known until the extreme event of 2020 ravaged a large swath of central Iowa. Even though derechos occur with some regularity, the 2020 event was unique in intensity and was unlike anything experienced before and remains the most expensive thunderstorm in U.S. history. More analysis will be done on last week’s system to help experts better understand and predict the phenomena. The big take-away in all this is that thanks to the recent derecho, Iowa has been a part of two historic weather events unprecedented in U.S. history in back to back years. Wow!

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TRAVELING CONDITIONS THIS CHRISTMAS...

High pressure is situated over the Midwest and will see to it that the weather is cool, calm, and collected Wednesday. Under sunny skies highs will be seasonal in the low to mid 30s after a rather chilly start in the teens. Thursday, the high moves off to the east and return flow commences and so does another round of above normal temperatures. Highs Thursday should reach the mid 40s north to the low 50s south. We'll up those numbers even more Christmas Eve day with highs that are way above normal well into the 50s, perhaps near 60 in the far SE Iowa and WC Illinois. Only 11 out of the past 149 years has the Quad Cities seen a high reach the 50s on December 24th (7 percent of the time). Here's what the EURO is showing for highs.

Those readings are a slightly cooler than what was shown yesterday but still above normal by 15-20 degrees as you can see in the temperature departures below.

Precipitation chances are very low through Friday although a few stray showers are possible Friday afternoon along a late day cold front. Amounts are only a couple hundredths of an inch (if that).


Christmas day has shown a recent trend for some rain showers. There is still low confidence on amounts but the latest guidance shows light totals with the northern half most favored. The EURO is essentially dry.


Another disturbance late Sunday or Sunday night appears to be better organized as it passes though the region. The GFS is colder, slower and further south with the system and produces some snow in my area. The EURO and GEM are warmer and further north making the disturbance a rain producer. With the wave lengths being so short models are struggling a bit to get a handle on both weekend systems but one common thread as of now is that there would be no snow to deal with until the close of the weekend Sunday night. Even that's questionably as the GFS is the only model showing a snow threat. We are going to need more time to see where future trends lead us.


At least for now, here's what modeling is indicating for precipitation totals for all events combined through Monday.


The EURO

The GFS

The GEM

Overall the mild weather should last the rest of December but models are now beginning to lean toward a significant cool-down in early January. The EURO shows the 500mb flow much colder by January 4th.

However, the above normal temperatures of December will continue to dominate until then. Here are the 5 day average temperature departures the next 15 days on the EURO


Days 0-5 Dec 22-27th

Day 5-10 Dec 27- January 1st

Day 10-15 January 1-6th

The CFSv2 shows this for 30 day temperatures January 5th-February 4th. Cold is on the table but the way this year has gone I'll truly believe it when I see it.

With that I shall call it a post. Only 2 more shopping days after today. It's coming! Roll weather...TS

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