Late Winter Squall Line

The GOES Microburst Windspeed Potential Index (MWPI) product performed effectively during a late winter squall line event that occurred on 28 February 2011.  During the morning of 28 February, an intense squall line developed ahead of a cold front over the Appalachian Mountains and tracked through Virginia, eastern West Virginia, and central Maryland.  The squall line produced numerous downbursts over the Blue Ridge mountain region of Maryland, West Virginia, and Virginia as observed by Road Way Information System (RWIS) sensors.  Downburst wind gusts of 37 to 38 knots were recorded by RWIS sensors over western Frederick County, Maryland between 1500 and 1530 UTC.  The 1346 UTC MWPI product indicated a local maximum in values northwest of Washington, DC prior to the occurrence of the downbursts.

Figure 1 shows favorable conditions for downbursts over the greater Washington, DC area during the morning of 28 February 2011.  Locations of downburst occurrence as recorded by RWIS sensors are marked as “15” and “16”.  Sensor 15 (I-70 @ Frederick / Washington County line) recorded a wind gust of 38 knots at 1514 UTC followed by a wind gust of 37 knots recorded at sensor 16 (US-340 @ MD-180) at 1528 UTC. Downburst occurrence was confirmed in doppler radar imagery in Figure 2 by the presence of spearhead echoes near each sensor. As noted in previous studies, the spearhead echo is closely associated with the development and occurrence of downbursts.  

   

The nearby RAOB sounding from Dulles Airport indicates the presence of an unstable temperature lapse rate and well-defined dry-air layers above the 850-mb level.  These conditions foster downburst generation by the interaction of dry air with the storm precipitation core in the presence of static instability.  Strong winds observed above the 850-mb level suggest that dry air was channeled into the convective precipitation cores of the squall line, resulting in evaporational cooling, generation of negative buoyancy, and the subsequent acceleration of downdrafts toward the surface.  Accordingly, a local maximum in MWPI values (orange marker) was located in proximity to Dulles Airport shortly after the time of the RAOB and corresponded to wind gust potential near 35 knots based on the regression chart shown in Figure 3.

Figure 3.  Regression chart based on the comparison of measured downburst winds gusts to proximate index values for 208 events between 2007 and 2010.

Bow Echo and Downbursts over the Gulf of Maine

During the afternoon of 21 July 2010, an intense mesoscale convective system (MCS) developed over northern New England and then tracked eastward over the adjacent coastal waters of Maine.  After several tornadoes touched down over southern Maine between 2200 and 2330 UTC, the system moved over the northern Gulf of Maine where it evolved into a bow echo that was associated with downburst winds observed by Gulf of Maine Ocean Observing System (GoMOOS) buoys on the Maine Shelf.  GOES microburst products, especially the Microburst Windspeed Potential Index (MWPI) and the imager channel 3-4 brightness temperature difference (BTD) products, effectively indicated the magnitude of downbursts produced by this bow echo system.

Figure 1.  GOES MWPI product image at 2246 UTC 21 July 2010 with locations of GoMOOS buoys plotted on the image.

 Figure 1, the 2246 UTC 21 July 2010 GOES MWPI product, indicated elevated values over southern Maine and the western Gulf of Maine near the time a tornado touched near Portland.  Index values of 20 to 24 corresponded to downburst wind gust potential near 40 knots. The radiosonde observation (RAOB) sounding profile, in Figure 2, over Portland confirmed favorable conditions for strong downbursts that included large convective available potential energy (CAPE) and a conditionally unstable temperature lapse rate between the 700 and 900-mb levels.  Embedded within this layer was a low-level jet with winds near 40 knots at the 800-mb level.   Thus, the downward transport of horizontal momentum from the 800-mb level to the surface by downdrafts initiated by precipitation loading was the most likely physical process that resulted in strong downburst wind generation.

Figure 2.  RAOB sounding profile from Portland, Maine at 0000 UTC 22 July 2010.

As the MCS tracked eastward over the Maine Shelf, an embedded supercell became apparent in radar imagery.  This bow echo was closest to the "type 3" bow echo as identified by Przybylinski (1995).  The 0015 UTC 22 July GOES BTD product image in Figure 3 with overlying radar reflectivity from Gray, ME NEXRAD identified a dry-air notch on the rear flank of the MCS with a corresponding rear-inflow notch (RIN) in radar imagery.  Due to the large distance from the GOES-13 subpoint, displacement error has resulted in about a 10 mile separation between the RIN and the dry-air notch as identified in the satellite image.  Between 0040 and 0050 UTC, a 40-knot downburst wind gust was recorded at the Central Maine Shelf GoMOOS buoy E01 (white cross) as the embedded supercell tracked directly overhead.  Note that at 0034 UTC, the RIN (white line) pointed directly toward the Central Maine Shelf buoy, thus effectively indicating the potential for strong downburst winds.  The channeling of dry mid-tropospheric air into the rear flank of the convective storm was an important source of downdraft energy that resulted in strong outflow winds.

Figure 3.  GOES imager BTD image 0015 UTC 22 July 2010 with overlying radar reflectivity.

As the MCS continued to track eastward, a well-defined comma head developed on the northern end of the MCS as shown in Figure 4.  Bow echo comma-heads are also favorable locations for downburst generation.  Between 0100 and 0110 UTC, a downburst wind gust of 40 knots was recorded at the Penobscot Bay buoy (F01) as the comma head tracked overhead.  The comma head produced several downbursts as it tracked over Penobscot Bay between 0100 and 0120 UTC, resulting in an extended period of strong winds.  The wind gusts observed at both GoMOOS buoys (40 knots) verified the anticipated magnitude as indicated by the GOES MWPI product as well as downburst occurrence as indicated by the GOES imager BTD product.

Figure 4.  Radar reflectivity image at 0106 UTC 22 July 2010.  Blue/white marker indicates the location of Penobscot Bay buoy.

References


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