Event Summary
     National Weather Service, Raleigh NC


April 6-10, 2007 Cold Weather Event
Updated 2008/04/24






Event Headlines

...A dramatic temperature swing occurred during the first two weeks of April 2007...
...High temperatures reached the lower to mid 80s on April 2-4 followed by several days of subfreezing temperatures on April 6-10...
...Morning lows ranged in the lower to mid 20s on multiple nights which produced significant damage to agricultural interests...
...The North Carolina Department of Agriculture and Consumer Services estimated crop losses at over $105 million in North Carolina...


Event Overview

A prolonged period of above normal temperatures in late March and very early April encouraged plants to flower and germinate prematurely across the southeastern U.S. Temperatures during this period averaged around 10 to 15 degrees above normal across central North Carolina.

A major weather pattern shift arrived in early April which brought cold arctic air southward into the central and eastern U.S. The cold air mass overwhelmed much of this region from April 4-10 with near-record to record breaking cold temperatures. Nighttime temperatures fell into the lower to mid 20s on multiple nights with many hours of sub-freezing temperatures. The cold temperatures produced significant damage to agricultural interests across North Carolina that exceeded $110 million dollars.


Synoptic Overview

A ridge of high pressure dominated the weather across much of the Southeast during late March and very early April. In fact, the 500 mb height anomaly from March 20 through April 3, 2007 was 180 meters above normal. During this period, high temperatures were typically 10 to 15 degrees above normal across central North Carolina ( chart of temperature departures at Raleigh  |  Greensboro). In fact, the temperature exceeded 80 degrees 6 times during the March 20 through April 3 period at both Raleigh and Greensboro.

At the end of the first week of April, a major pattern change occurred in the central and eastern U.S. On Tuesday April 3, a weakening upper level ridge was still in place across the Southeast with a series of shortwave troughs developing across the northern plains states. At the same time, a strong cold front was moving across the Midwest with an intense 1048 mb high pressure system was moving south from far northern Canada.

By Thursday April 5, the cold front marking the leading edge of the cold air mass had moved off the East Coast. At mid levels, the trough over the Great Lakes had deepened, establishing a large cyclonic northwest to westerly flow across the central and eastern U.S. By Friday April 6, the 1048 mb high pressure system had surged southward into much of the United States east of the Rocky Mountains. The high pressure system was extremely strong with a surface pressure anomaly of nearly 30 mb.

On Saturday and Easter Sunday the cut-off low over the Great Lakes intensified and the cold high pressure system remained in place. By Monday April 9, the cut-off had started to weaken and the flow across the Carolinas became more westerly allowing temperatures to moderate.

The coldest temperatures during the cold air outbreak occurred between April 6 and 10 when surface temperatures averaged 8 to 10 degrees Celsius or nearly 15 to 20 degrees Fahrenheit below normal. The average upper air pattern during this period featured a broad trough over the eastern U.S. with 500 mb heights averaging more than 150 meters below normal during this period.



Minimum Temperature Map from April 6 to 10

Minimum temperature map



Surface Analysis

A cold and intense 1048 mb high pressure system extended southward from central Canada across the central and eastern U.S. at 12 UTC on Friday, April 6, 2007. The high pressure system was unusually strong and dominated the weather across the country for several days in early April producing unusually cold weather. A loop of surface analysis imagery from 12 UTC April 3 through 00 UTC April 11, 2007 shows the evolution of the event.

Surface analysis from 12Z Friday, April 6, 2007



Historically Significant Event

The synoptic pattern during the record cold outbreak in early April 2007, featured a large, anomalously deep upper trough across the eastern U.S. The image to the right shows the mean 500 mb heights during the period of April 6 through April 10, 2007. The large trough featured a closed low across the far northern New England states which produced a large cyclonic flow across much of the central and eastern U.S. that transported cold Canadian air into this region.

The 500 mb heights during this period were as low as 530 meters which was much lower than the climatological normal, illustrating the strength and unusual nature of the trough. Comparing the observed mean during this period with the climatological normals from 1968 to 1996 yields an objective anomaly chart which shows that the 500 mb heights were more than 150 meters below normal.


The image to the right shows the surface temperature anomaly during the period of April 6 through April 10, 2007 based on climatological normals from 1968 to 1996. The temperature anomalies are shown in degrees Celsius. During this period the surface temperatures averaged 6 to 9 degrees Celsius or nearly 10 to 15 degrees Fahrenheit below normal.

Normal high temperatures in early April across central North Carolina are in the upper 60s with normal lows in the lower 40s. The normal mean temperatures are in the mid 50s.

Low temperatures during this period across central North Carolina were as low as the lower to mid 20s, nearly 15 to 20 degrees below normal. High and low temperatures for selected locations across North Carolina during the period is available here.



Cold Temperatures Had a Significant Impact

The normal date of the last spring freeze varies between the first and second week of April across much of central North Carolina. Before the cold outbreak in early April, temperatures during March were much warmer than normal. The above normal temperatures across the Southeast encouraged many plants to prematurely bloom and many crops to germinate early. The weather pattern made a dramatic change in early April, and the warm temperatures were replaced by a 4 to 7 day period of record cold temperatures across the Southeast. Temperatures during this period averaged between 10 and 15 degrees below normal across North Carolina. Many locations reported sub-freezing temperatures for several hours during multiple nights during this period.

The unseasonably cold temperatures across North Carolina during the period produced significant damage to crops and other agricultural interest. The North Carolina Department of Agriculture and Consumer Services estimated crop losses at over $105 million in North Carolina. The Agriculture Secretary reported that nursery crops suffered the greatest losses, over $55 million dollars. Fruit and vegetable farms, which include apples, peaches and various berries, suffered estimated losses in excess of $25 million dollars. Other crops affected by the freeze included corn, wheat, barley and Irish potatoes.

The maps below are provided by the State Climate Office of North Carolina in their Easter Weekend 2007 Freeze Report.


Dollars, per county, in crop losses from the April 2007 freeze

Dollars, per county, in crop losses from the April 2007 freeze - click to enlarge


Acres, per county, affected by the April 2007 freeze

Acres, per county, affected by the April 2007 freeze - click to enlarge



Archived Text Data from the Cold Weather Event

Select the desired product along with the date and click "Get Archive Data."
Date and time should be selected based on issuance time in GMT (Greenwich Mean Time which equals EST time + 5 hours).

Product ID information for the most frequently used products...

RDUAFDRAH - Area Forecast Discussion
RDUZFPRAH - Zone Forecast Products
RDUAFMRAH - Area Forecast Matrices
RDUPFMRAH - Point Forecast Matrices
RDUHWORAH - Hazardous Weather Outlook
RDUNOWRAH - Short Term Forecast
RDULSRRAH - Local Storm Reports
RDUSPSRAH - Special Weather Statement
RDUSPSRAH - Fire Danger Statement
RDUFWFRAH - Fire Weather Forecast
RDUFWMRAH - Fire Weather Forecast


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Acknowledgements

Many of the images and graphics used in this review were provided by parties outside of WFO RAH. The surface analysis graphic was obtained from the Hydrometeorological Prediction Center. Upper air analysis were obtained from the Penn State NARR data archive and the College of DuPage. Daily Mean Composites were provided by the Earth System Research Laboratory. Impact graphics across North Carolina and other information provided by the State Climate Office of North Carolina.


References

  • Easter Weekend 2007 Freeze Report from the State Climate Office of North Carolina.

  • NOAA/USDA technical report 2008-01 on the Easter Freeze of 2007.

  • NC Cooperative Extension Service 2007 Easter Freeze Information.

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