October 2009 Chesapeake Bay Downburst

A week after the passage of an intense coastal storm and associated winter-like conditions, a more spring-like regime over the Chesapeake Bay region resulted in a significant downburst event. During the afternoon of 24 October 2009, a rain band with embedded convective storms developed ahead of a strong cold front over the Blue Ridge Mountains. Similar to the 15 November 2008 event, temperatures were above normal ahead of the cold front, especially over the Delmarva Peninsula. A convective storm on the eastern flank of the rain band produced a strong downburst at Tolchester Beach, Maryland. The GOES imager microburst product, derived from from both the GOES-12 sounder and GOES-11 imager (full disk mode), indicated favorable conditions for downbursts three hours prior to the event.

Figure 1 compares the GOES-12 sounder and GOES-11 imager products. At 1746 UTC, the sounder derived product image displayed a small break in the large frontal cloud band over the upper Chesapeake Bay and northeastern Maryland. Brightness temperature difference (BTD) between 30 and 35K indicated wind gust potential of 30 to 35 knots. By 1800 UTC, the GOES-11 image product showed a small break in the frontal cloud band over the Chesapeake Bay near Tolchester. Again, output BTD in this region ranged from 30 to 35K, indicating wind gust potential of 30 to 35 knots. Overlying radar reflectivity imagery from Dover Air Force Base NEXRAD at 2059 UTC displayed the downburst-producing convective storm as a spearhead echo over Tolchester Beach, with reflectivities greater than 50 dBZ. At 2100 UTC, the Tolchester Beach PORTS station recorded a wind gust of 39 knots, well-marked in the wind histogram in Figure 2. In a similar manner to the 15 November 2008 event, steep low to mid-level temperature lapse rates as inferred by elevated BTDs and precipitation loading as inferred by high radar reflectivity in the parent storm favored strong downdraft instability.The RUC sounding profile displayed in Figure 2, over Tolchester Beach at 1800 UTC, suggests that downward momentum transport was also a factor in downburst generation with winds near 40 knots near the top of the mixed layer at 955 meters above the surface.

Figure 1. GOES-12 sounder microburst product at 1746 UTC 24 October 2009 (top) and GOES-11 imager microburst product at 1800 UTC (bottom). Radar reflectivity from Dover AFB NEXRAD at 2059 UTC is overlying both images.

Figure 2. Wind histogram for Tolchester Beach PORTS station (top) and RUC sounding profile over Tochester at 1800 UTC 24 October 2009 (bottom).

Comparing product images in Figure 1 reveals that the GOES-11 imager product in full disk mode provides a higher spatial resolution and a more precise display of output BTD than that produced by the GOES-12 sounder. Thus, the GOES-11 imager product may provide forecasters with more useful guidance pertaining to downburst risk over the Chesapeake Bay region. More information pertaining to downburst activity over the Chesapeake Bay region is available in this paper.