25 January Chesapeake Bay High Wind Event

During the morning of 25 January 2010, high winds developed over the Chesapeake Bay region ahead of a strong cold front. In addition to the strong gradient winds, convective winds were observed by PORTS stations. A wind gust of 52 knots was recorded by the Francis Scott Key Bridge (FSK) PORTS station located in the Baltimore Harbor shortly after 1300 UTC (0800 EST). An isolated, fast-moving shower, displayed in Figure 1, most likely produced outflow winds that resulted from precipitation loading. The storm outflow and rapid forward motion combined to produce the severe wind gust at FSK Bridge PORTS station.

Figure 1. Radar reflectivity imagery from Baltimore TDWR (BWI) between 1300 and 1330 UTC 25 January 2010.

Figure 1 displays an isolated shower moving rapidly from southeast of Washington DC to eastern Baltimore County between 1300 and 1330 UTC, passing just west of the FSK Bridge near 1320 UTC. A wind gust of 52 knots was recorded at the FSK Bridge PORTS station at 1324 UTC.



Figure 2. RUC model-derived guidance at 1300 UTC 25 January 2010 (top) and corresponding RUC sounding over FSK Bridge (bottom).

RUC-model derived guidance in Figure 2 indicated the potential for heavy showers as well as rapid storm motion due to strong winds in the lower and middle troposphere. Precipitable water values over the Baltimore-Washington corridor were greater than 40 mm (~1.5 inches) with storm motion indicated between 55 and 60 knots. The 1300 UTC RUC sounding over FSK Bridge echoed favorable conditions for high surface winds fostered by downward transport of horizontal momentum, with wind speeds near 70 knots (35 m/s) at the 850-mb level. Thus, the combination of precipitation loading, downward horizontal momentum transfer, and rapid storm motion resulted in this isolated severe convective wind event during a much larger-scale high wind event over the Chesapeake Bay region. Surface winds in excess of 50 knots posed a hazard to marine and ground transportation in the vicinity of the FSK Bridge.

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.