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April 2012 Finishes Cooler than March 2012
Some might argue that this April felt rather chilly, but that would be a misperception that likely stems from the
onslaught of spring fever, brought on by the record-breaking warmth the region experienced in March. Climatologically
speaking, April 2012 was actually pretty close to normal in terms of temperatures, but it did end on an unusual note...with
an average temperature cooler than that of March 2012.
Since climate records began in the late 1800s, the only other recorded instance of an April averaging cooler than March in a
given year at each of our three climate stations was in 1907.* Average temperatures for April 2012 were about 1 to 2 degrees
cooler than average temperatures for March 2012 at Columbus, Cincinnati, and Dayton. Despite running around 10 degrees above
normal, the warm temperatures on April 30th did not appreciably impact the average monthly temperature. Thus, this April
finished cooler than March, only the second time that this feat has occurred at all three climate stations since record
keeping began.
Examining the table below, it is interesting to note the difference in average temperatures between March and April 1907 compared
to March and April 2012. April 1907 averaged about 4 to 7 degrees cooler than March 1907, and there were a handful of days that
April with highs only in the 30s and several nights with lows in the 20s. In contrast, the coldest highs during April 2012 were
in the upper 40s (just one occurrence at each station), and there were only a couple instances of temperatures dropping below 30
degrees at night.
*April 1946 finished slightly cooler than March 1946 in the Columbus area (52.3 vs. 52.6 degrees), but that
April averaged slightly warmer than March at both Cincinnati and Dayton.
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1907 Average
Temperatures
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2012 Average
Temperatures
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Normal
Temperatures
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(degrees F)
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March
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April
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March
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April
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March
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April
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Columbus
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46.8
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43.0
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53.8
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53.1
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41.9
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53.1
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Cincinnati
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51.3
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45.5
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55.3
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54.1
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43.6
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54.2
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Dayton
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49.4
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42.8
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53.6
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51.8
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40.4
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51.6
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So why the unusual flip-flop in temperatures so far this spring? The main reason has been a change in the large-scale weather
patterns across the nation since about mid-March. The latter half of March was dominated by a strong trough/ridge structure in the
upper levels of the atmosphere. A deep trough was positioned over the western United States while a strong ridge had anchored
itself over the rest of the country. As a result, the jet stream was positioned much further north (well into Canada!) than it
usually would be for that time of year, allowing record-breaking warmth to surge northward into the Ohio Valley and Great Lakes
regions. The plots below show average 300 millibar height patterns and the average jet stream position across North America
during the entire month of March, as well as during the second half of the month. Note how the upper atmospheric flow was more
amplified during the unseasonably warm second half of March.
Mean 300 mb height contours and average jet stream position across North America for March 2012.
Mean 300 mb height contours and average jet stream position across North America for March 15-31, 2012.
In contrast, for the first three weeks of April 2012, the jet stream's average position was shifted further south (just about on
top of our forecast area), and the Ohio Valley was generally located between a broad, weak upper level ridge over the central U.S.
and a trough over the Northeast. This resulted in much more seasonal temperatures for the region. So, coming off of such a mild
winter and the warmest March on record (with temperatures that averaged more than 10 degrees warmer than normal!), it is not
surprising that this April has been perceived by some as "chilly," even though temperatures have in fact been averaging very
close to normal.
Mean 300 mb height contours and average jet stream position across North America for April 1-20, 2012.
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Temperature Graphs For March and April 2012
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Columbus
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Columbus
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Cincinnati
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Cincinnati
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Dayton
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Dayton
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Michael Kurz, Meteorologist Intern
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www.nws.noaa.gov
