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.

Arizona Downbursts: 18 May 2009

During the afternoon of 18 May 2009, widespread convective storm activity developed over Arizona as a result of strong solar heating and the presence of an upper-level disturbance that acted as an additional lifting mechanism. The convective storm activity persisted into the nighttime hours, producing isolated strong downbursts near 0500 UTC 19 May. A downburst, with an associated wind gust of 35 knots, was recorded at Magma FRS ALERT station, 45 miles southeast of Phoenix, near 0500 UTC. A stronger downburst, with a wind gust of 52 knots, was recorded at Sedona Airport, 95 miles north of Phoenix, at 0530 UTC. The GOES imager microburst product indicated moderate risk in proximity to the location of these downbursts two hours prior at 0300 UTC. The pre-downburst environment was indicated to be favorable for dry microbursts, with significant mid-level moisture overlying a deep and dry residual mixed layer that had evolved during the previous afternoon. Interestingly, the convective downdrafts were able to penetrate the shallow stable boundary layer and produce strong wind gusts at the surface.


The images above are GOES imager microburst products at 0300 UTC with overlying radar reflectivity imagery at 0453 UTC (top) and 0529 UTC (bottom). Both images show elevated risk values in proximity to downburst producing convective storms over Magma FRS ALERT station (top) and Sedona Airport (bottom). Output brightness temperature difference (BTD) near 35K (light blue shading) was indicated immediately downwind of the convective storm over Magma FRS ALERT station. The 0300 UTC Rapid Update Cycle (RUC) sounding profile over Magma station (below, top) displayed a classic inverted V profile typically associated with dry microbursts. Especially apparent in the sounding is a nearly 500 mb deep dry adiabatic layer with a large relative humidity gradient between the 500 and 900 mb levels (just above the surface) that fostered significant downdraft instability. However, due to the time of microburst occurrence (0500 UTC), about 2 1/2 hours after sunset, a shallow stable boundary layer had developed due to radiational cooling at the surface. The depth and moisture stratification of the overlying mixed layer did promote evaporational cooling and negative buoyancy as precipitation descended in the sub-cloud layer. Downdrafts of sufficient intensity were initiated to impact the surface and produce the strong wind gust recorded at Magma station. At 0530 UTC, a stronger downburst (52 knots) was recorded at Sedona Airport. This downburst occurred in proximity to slightly higher output BTD (>40K, light green shading). The 0300 UTC RUC sounding profile over Sedona (below, bottom) displayed a shallower inverted V profile (due to the higher surface elevation) with a nearly 500 mb deep dry adiabatic layer, similar to the Magma sounding. Again, apparent in the sounding profile was a shallow stable boundary layer that was easily penetrated by the convective downdraft to produce the severe wind gust at Sedona Airport.


The parent storms of both downbursts were characterized by low reflectivity and thus could be classified as "dry" type. Although these downbursts occurred during the late evening, the deep, dry residual mixed layer that evolved during the previous afternoon promoted significant downdraft instability. The convective downdrafts penetrated the stable boundary layer to result in strong downbursts in a similar manner to previously noted nighttime downburst events.

Oklahoma Downbursts: 12-13 May 2009

During the evening of both 12 and 13 May 2009, strong convective storms developed over western and central Oklahoma, respectively, and produced severe downbursts. The GOES imager microburst product was evaluated for this significant convective wind event. The vertical environmental temperature profile for the late afternoon was characterized as a "loaded gun" profile (Johns and Doswell 1992) for both downburst days. The imager microburst product was found to effectively indicate downburst potential for both events (especially the 12 May event), with severe downbursts occurring in proximity to local maxima in output brightness temperature difference (BTD) and corresponding microburst risk.




The images above are GOES microburst products derived from GOES-12 sounder image data at 2300 UTC 12 May 2009 (top) and 2200 UTC 13 May 2009 (bottom). Measured downburst wind gusts from Oklahoma Mesonet observing stations are plotted over the images. The 2300 UTC image displays wind gusts of 54 knots that occurred at Erick and Altus at 0155 UTC and 0235 UTC 13 May, respectively. The 2200 UTC image displays a wind gust of 48 knots that occurred at Red Rock 0100 UTC 14 May. Both images show local maxima in microburst risk in proximity to the locations of the observed downbursts. The RUC sounding profiles displayed below, generated at the valid times corresponding to the above microburst products and at locations of downburst occurrence on 12 May (top) and 13 May (bottom), underscore the favorable environment for severe convective storms that included a deep, mid-tropospheric unstable layer.





Both sounding profiles above are variants of the "loaded gun" sounding described in Johns and Doswell (1992). These profiles feature a deep, dry adiabatic layer based in the mid-troposphere overlying a shallow, capped moist boundary layer. However, the 13 May profile
over north-central Oklahoma near Red Rock mesonet station exhibited a slightly deeper and drier mixed layer. Interestingly, higher microburst risk and resulting convective wind gusts were associated with the 12 May sounding profile. This finding indicates the importance of the mid-tropospheric dry adiabatic layer (> 300mb deep) in forcing strong convective downdrafts, especially with the supercell storm that produced the severe downburst at Erick during the evening of 12 May. The weaker downburst (48 knot wind gust) that occurred at Red Rock during the evening of 13 May was produced by a multicell storm in an environment with a more shallow mid-tropospheric unstable layer. Thus, as apparent with this two-day convective storm outbreak, the mid-tropospheric thermodynamic structure had a greater influence on downburst strength than did the boundary layer structure.

REFERENCES

Johns, R.H., and C.A. Doswell, 1992: Severe local storms forecasting. Mon. Wea. Rev., 121, 1134–1151.

Assessment of GOES Sounder Microburst Product

During the afternoon of 4 May 2009, supercell convective storms developed over the coastal plain regions of Virginia and North Carolina. A supercell produced strong downbursts in the Norfolk, Virginia area near 1800 UTC 4 May. Measured downburst wind gusts ranged from 42 to 48 knots. Elevated index values were indicated by the new Geostationary Operational Environmental Satellite (GOES) sounder microburst product in proximity to the downbursts in the Norfolk area about one hour prior (1700 UTC). The GOES sounder microburst product was introduced in the previous entry, and was found to perform effectively for the 4 May event in a similar manner to the 26 April event. National Ocean Service (NOS) Physical Oceanographic Real-Time System (PORTS) observations were utilized as verification for this downburst event.


The images above are a Geostationary Operational Environmental Satellite (GOES) sounder microburst products at 1700 UTC: McIDAS-V version with overlying radar reflectivity imagery from Wakefield, Virginia NEXRAD (KAKQ)(top) and McIDAS-X version (bottom), available on the microburst product web page, with the location of the downburst wind gust observation of 48 knots plotted on the image. The microburst product images display elevated risk values over southeastern Virginia, with a local maximum over the Norfolk area. Overlying radar reflectivity imagery displayed scattered supercell storms over eastern Virginia and North Carolina. A particularly large supercell with a well defined hook echo was indicated over the city of Norfolk. This storm produced strong downbursts between 1800 and 1810 UTC, with peak wind gusts of 45 and 48 knots observed at South Craney Island PORTS station (below) and Norfolk Naval Air Station (NAS), respectively. The wind histogram at South Craney Island clearly indicates the occurrence of a downburst as a sharp peak in wind speed near 1800 UTC.



Elevated output BTD greater than 40K (orange shading) was indicated by the GOES sounder microburst product in proximity to the downbursts about one hour prior to downburst occurrence. Previous validation has identified that BTD greater than 40K is strongly correlated to wind gust potential of 40 knots or greater (Pryor 2009). The microburst products and the 1700 UTC Rapid Update Cycle (RUC) model analysis sounding profile displayed below show that the preconvective environment over southeastern Virginia was characterized by a steep temperature lapse rate and a well-mixed boundary layer that favored the development of intense convective downdrafts and resultant downburst generation. This sounding profile can be identified as a modified "inverted V" profile.


The relatively deep mixed layer (depth near 6000 feet) present over southeastern Virginia resulted in outflow-dominated supercell storms that produced strong downbursts rather than tornadoes. McCaul and Cohen (2002) noted that the combination of a relatively high Level of Free Convection (LFC) and Lifting Condensation Level (LCL) favors outflow dominated storms that could produce strong convective downdrafts and subsequent downbursts. Precipitation loading and downward momentum transport from near the top of the boundary layer to the surface were also factors in strong downburst generation.

References

McCaul, E.W. and C. Cohen, 2002: The Impact on Simulated Storm Structure and Intensity of Variations in the Mixed Layer and Moist Layer Depths. Mon. Wea. Rev., 130, 1722-1748.

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