USA Weather Summary: April 2011

April was a month of historic climate extremes across much of the United States!

april_temps

April temperatures averaged below normal across the northwestern half of the country and above normal from the southern Plains to the Northeast, reflecting a westerly circulation that extended further south — but was stronger — than normal.

The three states in the Pacific Northwest (Washington, 2nd coldest; Oregon 5th; Idaho, 10th) had the 10th coldest, or colder, April, while 9 states in the South and East ranked in the top 10 warmest category, with Delaware claiming its warmest April on record.

This temperature pattern reflected a storm system orientation that contributed to the stormy weather east of the Rockies.

On a national scale, April 2011 ranked as the 39th warmest and 10th wettest April in the 1895-2011 record.

Cold fronts and low pressure systems moving in the storm track flow are influenced by the broadscale atmospheric circulation. Four such large-scale atmospheric circulation patterns were dominant during April.

The first was a weakening La Ni??a, which is the phenomenon created by cooler-than-average sea surface temperatures in the eastern tropical Pacific Ocean.

At this time of year, La Ni??a is typically associated with cold temperature anomalies in the Northwest, warm anomalies in the South, and dry anomalies across most of the country.

The second atmospheric circulation index was the Pacific/North American (PNA) pattern, which was negative for most of the month.

A negative PNA this time of year is typically associated with warmer-than-normal temperatures over the southeast third of the U.S. and colder-than-normal temperatures in the Northwest.

April_precip

Precipitation is not strongly correlated.

The third atmospheric circulation index was the North Atlantic Oscillation (NAO) pattern, which was positive for most of the month.

A positive NAO this time of year is typically associated with warmer-than-normal temperatures across most of the country and drier-than-normal conditions in the Southeast and parts of the central and western U.S.

The fourth atmospheric circulation index was the Arctic Oscillation (AO) pattern, which was also positive for most of the month. A positive AO this time of year is typically associated with warmer-than-normal temperatures across the central U.S. and drier-than-normal conditions across parts of the central and southeastern U.S.

The pattern of observed temperature anomalies for April 2011 generally matched the La Ni??a and PNA patterns for April (mid-spring), with some correlation to the NAO and AO patterns.

The April precipitation pattern generally matched the mid-spring La Ni??a, NAO, and AO patterns in the southern Plains and Southeast. But the above-normal precipitation pattern for April 2011 in the Midwest and Northwest is closer to the mid-winter instead of mid-spring La Ni??a, PNA, and AO patterns.


Climate Highlights – April

The average U.S. temperature in April was 52.9 ??F (11.6 ??C), which is 0.9 degrees F (0.5?? ??C) above the long-term (1901-2000) average.

April precipitation was 0.7 inch (18 mm) above the long-term average, the tenth wettest such month in 117 years of data.

April was a month of historic climate extremes across much of the United States, including: record breaking precipitation that resulted in historic flooding; recurrent violent weather systems that broke records for tornado and severe weather outbreaks; and wildfire activity that scorched more than twice the area of any April this century.

Nationally, the overall drought footprint across the contiguous U.S. remained above average, but decreased slightly from the beginning of the month to about 22 percent. The area of the country affected by the two most intense drought categories (D3, Extreme and D4, Exceptional) has increased for ten consecutive weeks, a streak dating to mid-February. Much of this very intense drought is focused in the Southern Plains and Southern Rockies.

Ideal wildfire conditions prevailed across portions of the Southern Plains during April, and a record breaking 1.79 million acres burned across the country during the month. Texas, where over 2.2 million acres have burned since January, again bore the brunt of the wildfire activity.

Several violent tornado outbreaks affected the country during April. According to data from NOAA’s Storm Prediction Center, the number of confirmed tornadoes for the month may approach the previous all time monthly record of 542 tornadoes which struck the U.S. in May 2003.

  • Much of the southern and eastern U.S. experienced above-normal warmth in April. Based on preliminary data, Delaware had its warmest such month since records began in 1895. It was the fourth warmest April for Virginia and the fifth warmest for Texas. Other states with much above normal warmth include Florida and Louisiana (7th warmest), New Mexico and West Virginia (8th), New Jersey (9th) and Maryland (10th).
  • The Northwest was much cooler-than-normal during April. Washington (2nd coolest), Oregon (5th coolest) and Idaho (10th coolest) were all much cooler than normal. Washington had the greatest negative anomaly, 5 degrees F below their long-term average.
  • Climate Highlights – Rolling Three-, Six-, and 12-month Periods
  • Regionally, February-April 2011 was the wettest such period on record for the Northeast and Central climate regions. Within those regions several states experienced statewide precipitation records: Indiana, Kentucky Ohio, New York, and Pennsylvania. Other states with much-above-average precipitation for the period were: West Virginia (2nd wettest), Illinois and Vermont (3rd), Michigan (5th), Missouri(6th), and Washington (9th).
  • Temperatures during the past three months, when averaged across the nation, were near the 20th century average. However, much like April, the Northwest was cooler-than-normal while a large swath of the southern and eastern U.S. were above- to much-above normal.
  • According to NOAA’s Climate Prediction Center, La Ni??a conditions continued through April 2011 and are expected to transition to neutral conditions by early summer. The February-April U.S. temperature and precipitation patterns were largely consistent with those commonly associated with an early spring La Ni??a episode.
  • The persistent dryness in Texas can also be traced back to the six-month period – their second driest such period. It was the sixth driest November-April period for New Mexico and the tenth driest for both Louisiana and Oklahoma.
  • The rolling 12-month (May-April) period show record wet for Minnesota, North Dakota and Wisconsin. The Great Lakes area had their wettest such period and it was much above normal in the Northeast, Central, Upper Great Plains, Northern Rockies and Northwest regions.
  • Above average warmth has occupied much of the nation during the 12-month period. Only Montana, Oregon and Washington experienced average temperatures that were below the 20th century average.
  • Climate Highlights – Year-to-Date Period
  • The four-month average that makes up the year-to-date period has been above to much above normal for precipitation across the northern tier of the country. Record precipitation averages in New York and Pennsylvania contrasted with the third driest such periods in Texas and New Mexico.

For additional details about recent temperatures and precipitation across the U.S., see the Regional Highlights section below and visit the Climate Summary page.

For information on local temperature and precipitation records during the month, please visit NCDC’s Records page.

For details and graphics on weather events across the U.S. and the globe please visit NCDC’s Global Hazards page.


Regional Highlights:

These regional summaries were provided by the six Regional Climate Centers and reflect conditions in their respective regions. These six regions differ spatially from the nine climatic regions of the National Climatic Data Center.

See The?? NCDC’s Monthly Records web-page for weather and climate records for the most recent month.

For additional national, regional, and statewide data and graphics from 1895-present, for any period, please visit the Climate at a Glance page.


PLEASE NOTE: All of the temperature and precipitation ranks and values are based on preliminary data. The ranks will change when the final data are processed, but will not be replaced on these pages.

Graphics based on final data are provided on the Temperature and Precipitation Maps page and the Climate at a Glance page as they become available.


Citing This Report-This article was extracted and edited from the original by NCDC. The original, online report is:

NOAA National Climatic Data Center, State of the Climate: National Overview for April 2011, published online May 2011, retrieved on May 12, 2011 from http://www.ncdc.noaa.gov/sotc/national/2011/4.


Questions?

For questions on technical or scientific content of this report, please contact:

Chris Fenimore: Chris.Fenimore@noaa.gov

For general climate monitoring questions, please contact: Contact@noaa.gov

For climate data orders, please contact the National Climatic Data Center’s Climate Services and Monitoring Division:

NCDC.Orders@noaa.gov


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