14 July 2004: Southern Maryland Downbursts

During the early afternoon of 14 July 2004, a cluster of convective storms developed over the Appalachian Mountains in northeastern West Virginia and northwestern Virginia. The cluster of storms merged to form a squall line over northern Virginia, which then tracked eastward and southeastward over southern Maryland, producing convective wind gusts 0f 65 and 75 knots at Solomons Island PORTS station and Patuxent River Naval Air Station, respectively. The GOES Sounder-derived WMSI product indicated high WMSI values over northern Virginia and southern Maryland during the time that the squall line was developing. Large WMSI implied potential for the development of convective storm activity with strong updrafts and heavy precipitation and the subsequent development of intense convective downdrafts (Pryor and Ellrod 2004). In addition, the GOES imager microburst product (Pryor 2009a), generated with GOES-12 sounder image data, indicated elevated values over lower southern Maryland, downstream of the squall line.

1800 UTC GOES WMSI product above displays the cluster of convective cells over northern Virginia, merging in a region of WMSI values well in excess of 100. The strong instability and favorability for intense, deep convection was indicated by the presence of enhanced cumulus immediately south of the convective cluster. Regional radar imagery (not shown) displayed the convective cluster merging to form a squall line between 1645 and 1845 UTC in a similar manner to the broken areal evolution described by Bluestein and Jain (1985). The corresponding MODIS microburst product (Pryor 2009b) (above) displayed high risk values (red shading) over lower southern Maryland, eastern Virginia, and the Eastern Shore. The highest MODIS microburst risk values were co-located with a plume of mid-level moisture as indicated in water vapor imagery (below) where the location of the 75-knot wind gust at Patuxent River NAS is plotted in the image. Similar to the GOES imager microburst algorithm, the MODIS microburst algorithm incorporates BTDs between bands 27 (6.535 - 6.895 μm), 31 (10.780 - 11.280 μm) and 32 (11.770 - 12.270 μm). MODIS data is desirable due to its high spatial resolution (1 km).

By 2000 UTC, the squall line had tracked east- southeastward over northern Virginia and into southern Maryland. 2000 UTC GOES WMSI shows the squall line extending from the upper Chesapeake Bay to north-central Virginia. WMSI values had increased in the region, especially over southern Maryland. A WMSI value of 209 was indicated just east of Patuxent River NAS. Based on previous validation, WMSI values in excess of 200 signify the potential for convective wind gusts (downbursts) of greater than 65 knots (Pryor and Ellrod 2004).

In a similar manner, the GOES imager microburst product above displayed moderate to high risk over lower southern Maryland. The location of the 65-knot wind gust at Solomons Island is plotted on the image.

By 2000 UTC, a supercell developed along the squall line over the Potomac River near Quantico, Virginia. The supercell then tracked east-southeastward toward the mouth of the Patuxent River. As shown above in the radar reflectivity image from Sterling, Virginia (KLWX) NEXRAD overlying the 2000 UTC imager microburst risk product, the supercell had evolved into a bow echo structure, in the region of strong instability, between approximately 2050 and 2130 UTC. A strong low-level reflectivity gradient developed along the leading edge of the bowing line segment, while a rear inflow notch (RIN) developed along the trailing edge. The strong low-level reflectivity gradient signified the location of the convective updraft center while the RIN signified a region of evaporatively-cooled, lower theta-e air being channeled toward the leading edge of the bow (Przybylinski 1995). The RIN signature indicated the location of damaging downburst winds. During this time, at 2100 and 2105 UTC, downburst wind gusts of 65 and 75 knots were observed at Solomons Island and Patuxent River NAS, respectively. As shown in the 1815 UTC water vapor image above, the severe downbursts occurred in region of mid-tropospheric dry air.

This downburst event is one of the strongest recorded in the continental U.S. during the 2004 summer season, resulting from the simultaneous presence of favorable conditions for severe convective storms. The GOES microburst products featured in this study effectively indicated favorability for strong downbursts over southern Maryland (Pryor and Ellrod 2004; Pryor 2009a): Large CAPE, a mid-troposperic dry air (low theta-e) layer, and a steep low-to-mid-tropospheric temperature lapse rate. The steep lapse rate, as inferred from the GOES imager and MODIS microburst products, fostered downdraft instability and the resulting intensity of the downbursts.

References

Bluestein, H.B., and M.H. Jain, 1985: Formation of Mesoscale Lines of Precipitation: Severe Squall lines in Oklahoma during the Spring. J. Atmos. Sci., 42, 1711-1732.

Pryor, K.L., and G.P. Ellrod, 2004: WMSI - A New Index For Forecasting Wet Microburst Severity. National Weather Association Electronic Journal of Operational Meteorology, 2004-EJ3.

Pryor, K.L., 2009a: Microburst windspeed potential assessment: progress and developments. Preprints, 16th Conf. on Satellite Meteorology and Oceanography, Phoenix, AZ, Amer. Meteor. Soc.

Pryor, K. L., 2009b: Assessment of GOES imager microburst product over the southwestern United States. arXiv:0904.0446v1 [physics.ao-ph]

Przybylinski, R.W., 1995: The bow echo. Observations, numerical simulations, and severe weather detection methods. Wea. Forecasting, 10, 203-218.