New Graphical Microburst Guidance Product

A new microburst graphical guidance product has been developed that employs data from the Rapid Update Cycle (RUC) model. Prototypical conditions for microbursts include a steep temperature lapse rate and decreasing humidity with decreasing height in the boundary layer. Thus, the graphical guidance product incorporates boundary layer temperature lapse rate and vertical relative humidity difference, important factors in initiating and sustaining a convective downdraft. The new guidance product demonstrated effectiveness in indicating favorable conditions for downbursts over the northern Chesapeake Bay region during the afternoon of 5 November 2009 in which a convective storm produced a strong wind gust of 38 knots at Tolchester Beach, Maryland (See 6 November blog entry) . The 1800 UTC RUC graphical microburst product indicated high downburst risk in proximity to Tolchester Beach.

Figure 1. RUC graphical microburst product at 1800 UTC November 5, 2009 with radar reflectivity from Dover Air Force Base NEXRAD at 2237 UTC overlying the image (top) and RUC model analysis sounding over Tolchester Beach, Maryland at 1800 UTC (bottom).

Figure 1 compares the new Rapid Update Cycle (RUC) graphical guidance microburst product to a corresponding RUC sounding over Tolchester Beach, Maryland at 1800 UTC, 5 November 2009. At 1800 UTC, the image product showed a large area with steep boundary layer lapse rates, greater than 8.5 K/km, extending from eastern Pennsylvania and New Jersey to Virginia. The product image also displayed a local maximum in vertical humidity difference over northern Maryland and southeastern Pennsylvania. The highest microburst risk was indicated where the highest vertical humidity difference was co-located with steep lapse rates in the 850-1000mb layer. Srivastava (1985) noted that microbursts are likely with lapse rates greater than 8.5 K/km. Overlying radar reflectivity imagery from Dover Air Force Base NEXRAD at 2237 UTC displayed a downburst-producing convective storm as a spearhead echo over Tolchester Beach. At 2242 UTC, the Tolchester Beach PORTS station recorded a wind gust of 38 knots. The corresponding RUC sounding profile echoed favorable conditions for downbursts in the Tolchester Beach area with the presence of a 5000-foot deep mixed layer and steep temperature lapse rate below 850mb. Although radar reflectivities with this storm were not impressive (25-40 dBZ), the steep lapse rate was a major contributor to downdraft instability. As demonstrated in this case study, the RUC graphical guidance product, visualized by McIDAS-V software, effectively highlighted a region favored for strong convective winds. The ability to overly radar reflectivity on the microburst product shows the utility of the product in the downburst nowcasting process.