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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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| April 2011 Record-Breaking Rainfall |
| Location | New April Record Rainfall | Old April Record Rainfall |
| Cincinnati, OH | 13.52" | 9.77" (1998) |
| Columbus, OH | 7.14" | 7.08" (1893) |
| Frankfort, KY | 13.25" | 9.28" (1948) |
| Lexington, KY | 12.70" | 9.30" (1970) |
| Louisiville, KY | 13.97" | 11.10" (1970) |
| Jackson, KY | 10.21" | 10.00" (1998) |
| Paducah, KY | 15.91" | 14.54" (1983) |
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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.
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| April Rainfall Comparison |
| Location | April Record Rainfall Year | April Record Rainfall Amount |
| Cincinnati, OH | 2011 | 13.52" |
| 1998 | 9.77" |
| 1947 | 8.62" |
| 1996 | 8.20" |
| 1853 | 7.70" |
| 1859 | 7.58" |
| April Rainfall Comparison |
Location | April Record Rainfall Year | April Record Rainfall Amount |
| Columbus, OH | 2011 | 7.14" |
| 1893 | 7.08" |
| 1998 | 6.51" |
| 1996 | 6.39" |
| 1964 | 6.36" |
| 1940 | 6.09" |
| April Rainfall Comparison |
Location | April Record Rainfall Year | April Record Rainfall Amount |
| Dayton, OH | 1996 | 9.20" |
| 2011 | 8.72" |
| 1993 | 6.78" |
| 1947 | 6.69" |
| 1989 | 6.52" |
| 1994 | 6.32" |
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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.
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| Record Monthly Rainfall Comparison |
Location | Record Rainfall Month/Year | Record Rainfall Amount |
| Cincinnati, OH | Jan 1937 | 13.68" |
| Apr 2011 | 13.52" |
| Mar 1964 | 12.18" |
| Aug 1879 | 11.72" |
| Jun 1845 | 11.50" |
| Sep 1866 | 10.88" |
| Record Monthly Rainfall Comparison |
Location | Record Rainfall Month/Year | Record Rainfall Amount |
| Columbus, OH | Jul 1992 | 12.36" |
| Aug 2003 | 11.46" |
| Jan 1937 | 10.71" |
| Nov 1985 | 10.67" |
| Jun 2008 | 10.39" |
| Record Monthly Rainfall Comparison |
Location | Record Rainfall Month/Year | Record Rainfall Amount |
| Dayton, OH | Jan 1937 | 12.41" |
| Jun 1958 | 10.89" |
| Dec 1990 | 10.04" |
| Apr 1996 | 9.20" |
| May 1995 | 9.05" |
- Seth Binau, Science and Operations Officer (seth.binau@noaa.gov)
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www.nws.noaa.gov
