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Active April With Record Rainfall and Severe Weather
No matter the location, it seems that April 2011 will not be missed by many east of the Rockies due to the unpleasant and, in some cases, deadly weather that was seen. From lingering cold and snow in the western Great Lakes and northern Plains, to record rainfall in the Ohio Valley, to destructive tornadoes in the South, there wasn't much about April 2011 that we would like to remember.

There is no way around it, April was a busy month. Particularly from the Ohio Valley to the Gulf of Mexico, it seemed to rain every day, with more days than not featuring flooding, severe thunderstorms, tornadoes, or high winds.

What, though, made April 2011 so much different from previous Aprils to bring such hectic weather? In an attempt to answer this question, a composite look (averaging the first 27 days of April together) of different levels of our atmosphere reveals some clues to the longer term pattern that was in place, driving the day-to-day weather.

Jet Stream

In the following image is a composite of the deviation from normal of jet stream level winds for much of the month of April. It is seen very clearly, that larger scale west-to-east flow across the country was much above normal (warm hues - black arrows). This faster than normal, and more energetic stream of upper-level winds was reponsible for tracking many strong systems across the country. Not only did this increase the frequency of weather systems across the country, but the stronger winds aloft led to stronger wind shear in the mid and upper levels of the atmosphere, which is a key ingredient for thunderstorm organization, longevity, and severity.



Temperatures

In the following image is a composite of the deviation from normal of temperatures at 850 millibars (about 5000 feet) in our atmosphere, and is a good proxy for how our temperatures deviated from normal at the surface. This image clearly shows a battleground in place across the country, from much colder than normal air in the north to much warmer than normal temperatures in the south. While not atypical for April to see a significant push/pull of the cold and warm air, the combination of colder than normal temperatures in the north, and warmer than normal temperatures in the south, meant that every weather system crossing the country (see fast jet stream above) likely pulled very warm air to the north where it clashed with very cold air following in behind. There's a second ingredient for better than normal chances of storms.



Moisture and Low Level Winds

In the following image is a composite of the deviation from normal of atmospheric moisture (called precipitable water) for the month of April. This image clearly shows that within the larger region of warmer than normal areas of the southern United States, there was region of enhanced moisture pooling centered mainly over the Ohio River Valley.



But why was that moisture pooled over the Ohio Valley? As a result of the larger scale global weather circulation, this area became a battleground between the push/pull of the warmer and colder temperatures, underneath faster and stronger jet stream winds. Moisture tends to pool in vicinity of these frontal zones, and when considering the above normal southerly winds at 850 millibars (5000 feet) as seen in the image below, there was an ample of supply of above normal moisture transport off the Gulf of Mexico into this battleground/frontal zone for much of the month.



Instability

So what happens when you bring above normal temperatures and moisture into a frontal zone, in the vicinity of stronger than normal winds aloft? You get a much more volatile atmosphere, with increased instability and wind shear, the two primary ingredients for thunderstorm severity, and given the duration of the pattern, a higher than normal probability of record rainfall and massive flooding.

In the image below, is a composite of the deviation from normal of the atmospheric instability for the month of April. Cooler hues (purple and blue) indicate areas where atmospheric instability is higher, with greater threats for thunderstorms considering the above normal temperatures and moisture in place.

The End Result = Too Much Rain

In the image below, is rainfall totals across the Ohio Valley from April 1st through April 29th, 2011. Note the maximum which aligns perfectly with the main stem Ohio River, an ingredient for major flooding. Follow the major flooding on the lower Ohio River here and here.



Departure From Normal and Percent of Normal Rainfall

These images (courtesy of Midwest Regional Climate Center) clearly show this axis of excessive and record precipitation for communities along the Ohio River.


Wettest April of All-Time

Below is a table of some of the more notable cities breaking the record for all-time wettest April. There were many more cities than this, especially across Kentucky, southern Indiana, southern Illinois, and southern Ohio, but this table gives a sample of the excessive rainfall which fell over the area in April 2011.

April 2011 Record-Breaking Rainfall
LocationNew April Record RainfallOld April Record Rainfall
Cincinnati, OH13.52"9.77" (1998)
Columbus, OH7.14"7.08" (1893)
Frankfort, KY13.25"9.28" (1948)
Lexington, KY12.70"9.30" (1970)
Louisiville, KY13.97"11.10" (1970)
Jackson, KY10.21"10.00" (1998)
Paducah, KY15.91"14.54" (1983)


The tables below show record April rainfall for Cincinnati, Columbus and Dayton, OH. April 2011 was the wettest April on record in Cincinnati and Columbus, and the second wettest in Dayton.

April Rainfall Comparison
LocationApril Record Rainfall YearApril Record Rainfall Amount
Cincinnati, OH201113.52"
19989.77"
19478.62"
19968.20"
18537.70"
18597.58"

April Rainfall Comparison
LocationApril Record Rainfall YearApril Record Rainfall Amount
Columbus, OH20117.14"
18937.08"
19986.51"
19966.39"
19646.36"
19406.09"

April Rainfall Comparison
LocationApril Record Rainfall YearApril Record Rainfall Amount
Dayton, OH19969.20"
20118.72"
19936.78"
19476.69"
19896.52"
19946.32"


The tables below show record monthly rainfall for Cincinnati, Columbus and Dayton, OH. April 2011 was the second wettest month on record in Cincinnati. Farther north and away from the heaviest rainfall axis, April 2011 amounts were not among the top five in either Columbus or Dayton.

Record Monthly Rainfall Comparison
LocationRecord Rainfall Month/YearRecord Rainfall Amount
Cincinnati, OHJan 193713.68"
Apr 201113.52"
Mar 196412.18"
Aug 187911.72"
Jun 184511.50"
Sep 186610.88"

Record Monthly Rainfall Comparison
LocationRecord Rainfall Month/YearRecord Rainfall Amount
Columbus, OHJul 199212.36"
Aug 200311.46"
Jan 193710.71"
Nov 198510.67"
Jun 200810.39"

Record Monthly Rainfall Comparison
LocationRecord Rainfall Month/YearRecord Rainfall Amount
Dayton, OHJan 193712.41"
Jun 195810.89"
Dec 199010.04"
Apr 19969.20"
May 19959.05"


Record Start to the Severe Weather Season

As shown very nicely on this webpage from the Storm Prediction Center, the pattern laid out above has led to the fastest start to a severe weather season in terms of number of tornadoes, wind damage reports, and hail reports.

Below is an image of the severe weather reports just for the month of April, with a daily tally below the image. (Courtesy Storm Prediction Center) Blue dots indicate wind damage, green dots indicate large hail reports, and red dots indicate tornadoes.



- Seth Binau, Science and Operations Officer (seth.binau@noaa.gov)

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Page last modified: May 3, 2011.
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