May 2011 Precipitation Summary

Analyzing The Large Scale Pattern: May 2011

Analysis At 300-mb
The chart of mean 300-mb heights (below, middle) for May 2011 shows a deviation from the long-term mean (below, left) in the form of an undulating pattern (annotated image) consisting of a trough over the Pacific Northwest, a ridge over the northern Rockies, a trough dipping into the upper Midwest and a strong ridge over the Canadian Maritimes. The chart of April 2011 anomalies (below, right) indicates that the trough over the Midwest was associated with a -30 to -60 meter height anomaly, while the ridge over the Canadian Maritimes was a remarkable 100 to 150 meters above normal. Ohio was located between these two upper-air features.

Analysis at 300-mb

Mean 300-mb heights for the month of May -- 1968 to 1996. (Larger Image)	Mean 300-mb heights for May 2011 (Larger Image)	300-mb height anomalies May 2011 to long-term mean. (Larger Image)

Analysis at 500-mb
The trough and ridge pattern evident in the 300-mb analysis, is also visible in the 500-mb data. The chart of 500-mb height anomalies for May 2011 (below, right) shows a negative anomaly of 20 to 60 meters over the Midwest and higher than average heights (60 to 120 meters) extending from New England to the northeast.

Analysis at 500-mb

Mean 500-mb heights for the month of May -- 1968 to 1996. (Larger Image)	Mean 500-mb heights for May 2011 (Larger Image)	500-mb height anomalies May 2011 to long-term mean. (Larger Image)

Analysis at 850-mb
The chart of mean heights at 850-mb (below, middle) for May 2011 shows an elongated trough over the upper Midwest and a corresponding ridge over the Canadian Rockies and eastern Canada. The trough deviates from the long-term mean by -30 to -40 meters while the eastern ridge exceeds the long-term mean by 30 to 70 meters (below, right). The pattern at 850-mb mimics the 500-mb and 300-mb pattern, but was shifted slightly to the east.

Analysis at 850-mb

Mean 850-mb heights for the month of May -- 1968 to 1996. (Larger Image)	Mean 850-mb heights for May 2011 (Larger Image)	850-mb height anomalies May 2011 to long-term mean. (Larger Image)

A Blocking Pattern
The undulating upper-air pattern indicated on the mean heights at 500-mb during May suggests the recurrence of a blocking pattern where the normal west to east flow of weather features across the country is disrupted. This causes an area to be under the persistent influence of a 500-mb ridge or 500-mb trough for several days or up to a week or more. In some circumstances a closed 500-mb low or 500-mb high forms in the base of the trough or at the peak of the ridge respectively. An area under the influence of a 500-mb low can receive a significant amount of rain, particularly if the pattern persists for several days.

Blocking Patterns

May 1-3, 2011 500-mb pattern. (Larger Image)	May 12-18, 2011 500-mb pattern. (Larger Image)	May 22-26, 2011 500-mb pattern. (Larger Image)

There were three distinct periods during May 2011 in which a blocking pattern setup over the United States and caused a 500-mb trough and boundaries associated with surface low-pressure systems to linger over Ohio. The first was May 1st to 3rd which produced 1.35" of rain at Cleveland-Hopkins Airport, the second period was May 12 - 18 which produced 3.12" of rain and last, a four day period from May 22 to 26 which resulted in 2.6" of rain. Combined, these three periods produced 7.07" of rain, nearly 91% of the total received in May.

Analyzing The Impact At the Surface
The mean sea level pressure for May 2011 (below, middle) for Ohio was approximately 1012-mb. A review of the anomalies for May (below, middle) shows that the surface pressure in Ohio was nearly 6-mb below normal and was located in a large area of below normal pressure affecting the eastern half of the country. This chart also shows a significant positive sea level pressure anomaly of 8-mb over Canadian Maritimes associated with the 850-mb, 500-mb and 300-mb ridge discussed above.

Analysis at the Surface

Mean sea level pressure for the month of May -- 1968 to 1996. (Larger Image)	Mean sea level pressure for May 2011 (Larger Image)	Sea level pressure anomalies May 2011 to long-term mean. (Larger Image)

The surface charts showing the conditions on the three days in May having the highest total precipitation (May 14 -- .82", May 23 -- .84" and May 25 -- 1.22") are presented below. Not suprisingly, all of the charts show northern Ohio under the influence of a low-pressure system and associated boundaries. The rain contributing to the highest daily total (May 25 -- 1.22") was associated with a stationary boundary draped across northern Ohio.


Surface Analysis at 18Z on May 14, 2011. Daily precipitation was .82 "	Surface Analysis at 18Z on May 23, 2011. Daily precipitation was .84"	Surface Analysis at 18Z on May 25, 2011. Daily precipitation was 1.22"
Click here to view surface analysis charts for all of the days in May where precipitation was greater than .10"

Transporting Moisture To Northern Ohio
Similar to April long-term mean, the mean 850-mb vector wind for all Mays (below, left) shows a consistent flow of air from the western Gulf of Mexico north into the Midwest and then northeast into New England. However, the chart of 850-mb vector wind (below, middle) during May 2011 shows a shift to the east and southeast in this flow. The chart of May anomalies (below, right) suggests a positive anomaly (2-3 m/s) along the Gulf Coast. More interesting is the positive anomaly over the northeastern United States indicating stronger southeasterly winds at 850-mb.

850-mb Vector Winds

Mean 850-mb vector wind for the month of May -- 1968 to 1996. (Larger Image)	Mean 850-mb vector wind for May 2011 (Larger Image)	Mean 850-mb vector wind anomalies May 2011 to long-term mean. (Larger Image)

The stronger than normal southeasterly 850-mb winds during May over the northeastern United States enhanced the precipitable water values over the area by transporting moisture from the Atlantic Ocean. The chart of precipitable water anomalies (below, right) shows a bulls-eye of positive values approaching 8 kg/mˆ2 over Pennsylvania with an anomaly of approximately 7 kg/mˆ2 spreading back into Ohio. As a percentage of the long-term mean the anomaly in May is less than April, but significant nonetheless.

Precipitable Water

Mean precipitable water for the month of May -- 1968 to 1996. (Larger Image)	Mean precipitable water for May 2011 (Larger Image)	Mean precipitable water anomalies May 2011 to long-term mean. (Larger Image)

A complete collection of the 300-mb, 500-mb, 850-mb and surface charts for all days in May 2011 where the total precipitation at Cleveland-Hopkins Airport was at least .10" can be found here.

Summary
The total precipitation at Cleveland-Hopkins during May 2011 (7.74") didn't set a new record (9.14" in 1989), but it ranks as the second rainiest May in history. The vast majority (91%) of the rain was produced during periods in which a mid- and upper-level blocking pattern was in place over our region. May across northern Ohio was simply dreary.

Note: All mean and anomaly imagery courtesy of the Earth Systems Laboratory.

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