Late Night Heat Burst on the Lower Eastern Shore, April 25-26, 2009

In the early morning hours of April 26, 2009, dying thunderstorms and a very warm, dry lower and middle atmosphere led to the occurrence of heat bursts over sections of the Maryland Lower Eastern Shore. They were localized in a relatively small area from Dorchester County, southeast into northern Accomack county, Virginia and Worchester County, Maryland. Evidence of the bursts were detected by weather observing equipment at Cambridge, Salisbury, Wallops Island and Ocean City (noted in the table below).

A heat burst is defined as localized, sudden increase in surface temperature associated with a thunderstorm, shower, or mesoscale convective system, often accompanied by extreme drying1. The most dramtic heat bursts can also cause severe wind gusts, which can occasionally result in property damage. However, it should be noted that no damage was reported in this case, with the highest heat burst induced wind gust of 52 MPH reported by the Automated Surface Observation System (ASOS) at the Salisbury Airport.

Heat bursts usually occur in the late evening and early morning hours, and occur generally in the late spring and summer. The two primary reasons for this are the onset of the nighttime inversion and thunderstorm climatology. Heat bursts are not very common across our area, and are more often found across the Plains states.

Specific information on the heat bursts detected at each affected observing station are found below. The increase in temperature and decrease in dewpoint during this period as a result of the bursts are denoted below. The departure from the minimum temperature/maximum dewpoint BEFORE the heat burst is noted in parentheses to the right of the maximum temperature and minimum recorded dew point.

Location

Time (EDT)

Temp

Dew Point

Max Wind Gust

Salisbury

1:54 AM

87 (+13)

41 (-15)

52 mph

Cambridge

2:41 AM

79 (+6)

54 (-5)

40 mph (unofficial)

Ocean City

4:53 AM

78 (+11)

61 (-5)

(missing)

Wallops

1:54 AM

75 (+10)

59 (-2)

20 mph

Here are a few meteograms (temporal traces of temperature and dew point) from the affected stations.

This particular series of heat bursts occurred as decaying thunderstorms across the Lower Eastern Shore just before midnight. Time series radar images from Dover AFB (DE) radar are noted below.

The region had experienced unseasonably warm temeratures in the day before the heat bursts, allowing very warm, dry, and unstable air to be in place over the area. Data from the Wallops Island VA sounding (below) on the morning of the 26th showed that the warm and dry air extended from the surface to about 15,000 to17,000 feet. The following image is a Skew-T plot of the temperature and dew point data from Sunday morning’s weather balloon in Wallops, which is a depiction of the vertical temperature and dew point profile. Note how the temperature profile (the right black trace) exhibits a strong lean to the left through the bottom half of the picture; also, how far apart the dew point profile (the left black trace) and the temperature profile are throughout much of the lower half of the image.

When the thunderstorms began to decay, downdrafts formed beneath them, and due to the unstable air in place, the air accelerated rapidly. Evaporation of rain beneath the decaying thunderstorm also fueled the downward acceleration. As the downdrafts approached the surface, they warmed and dried, which caused the drastic increase in temperature and drop in dew point.

1-Glossary of the American Meteorological Society