RUC Model Guidance Indicates Downburst Potential over the Chesapeake Bay

During the late afternoon and evening of 9 June 2009, a strong squall line tracked through the Chesapeake Bay region of Maryland. The convective storm line produced a severe downburst at Tolchester PORTS station on the northern Chesapeake Bay shore. The squall line produced strong winds farther south on the bay at Cove Point PORTS station 30 minutes later. The stronger downburst, observed at Tolchester, was associated with a more favorable environment that included a well-developed mixed layer and a larger precipitable water (PW) content. Parameters considered were the temperature lapse rate and vertical relative humidity difference between 1000 and 850mb, and precipitable water. The following is a summary of downburst activity and associated environmental parameters: Time (UTC)/Location/Wind Gust (kt)/Lapse Rate (K/km)/dRH (%)/PW (mm)
2206/Tolchester/64/7.7/15/39
2236/Cove Point/40/8/15/29

Figure 1. RUC model analysis soundings at Tolchester (top) and Cove Point (bottom) at 2100 UTC 9 June 2009. A comparison of RUC soundings in Figure 1 reveals a more favorable environment for severe downbursts over the northern Chesapeake Bay near Tolchester. Apparent in the Tolchester sounding profile is a well-defined "inverted V" that signifies the presence of a convective boundary layer with a steep temperature lapse rate. At Cove Point, although mid-level relative humidity is lower, precipitable water content is lower, as well. These conditions were effectively captured in the RUC graphical microburst product images displayed below. Figure 2. RUC graphical microburst guidance product at 2100 UTC 9 June 2009 with vertical humidity difference (top), preciptable water (bottom), and radar reflectivity from Dover AFB NEXRAD overlying the image. Location of Tolchester PORTS station is marked with an "X". The product images in Figure 2 echo the sounding profile at Tolchester displayed in Figure 1. It is evident that precipitation loading, indicated by both high PW and reflectivity values (> 50 dBZ) over Tolchester, was a major factor in downburst development. Although lapse rate and dRH values at Cove Point were similar in magnitude, lower PW values, as displayed in Figure 3, most likely were associated with a weaker downburst. Figure 3. RUC microburst guidance product at 2100 UTC with overlying radar reflectivity from Dover AFB NEXRAD at 2234 UTC. Location of Cove Point PORTS station is marked with an "X". As displayed in the sounding profile over Cove Point in Figure 1, the boundary layer was more stable with a deeper, surface-based inversion. The stronger downburst at Tolchester, thus, was associated with a more unstable boundary layer, steeper lapse rate, and higher atmospheric water content. This suggests that in humid environments, precipitation loading, sub-cloud temperature lapse rates, and a positive humidity difference between the surface and the cloud base, rather than mid-tropospheric humidity, are all forcing factors for strong convective downdrafts.