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.

December Begins with a Windstorm

The windstorm that evolved along and ahead of a cold front over the Mid-Atlantic region during the morning of 1 December 2010 culminated in the development of a squall line that produced downburst winds over the Tidal Potomac region.  Between 1400 and 1430 UTC, a line of convective storms developed along the cold front over northern Virginia and then tracked east and northeastward into central and southern Maryland between 1430 and 1500 UTC.  Between 1430 and 1445 UTC, several strong downbursts occurred over the Tidal Potomac River and were recorded by WeatherFlow observing stations.  Similar to the 17 November downburst event, the channeling of mid-tropospheric dry air into the core of convective storms provided significant downdraft energy that resulted in high winds.  

Figure 1.  GOES-13 VW-IR brightness temperature difference (BTD) image at 1445 UTC 1 December 2010.

    Figure 1 shows a GOES-13 BTD microburst risk image over Maryland and Virginia near the time of downburst occurrence over the Tidal Potomac River.  At 1430 UTC, a wind gust of 41 knots was recorded by Potomac Light 33 WeatherFlow station associated with a squall line downburst.  Five minutes later, at 1435 UTC, a stronger downburst wind gust of 43 knots was recorded by Cobb Point station.  Note that the dry-air notch identified in Figure 1 on the southwestern flank of the squall line was pointing toward Cobb Point, indicating that the injection of dry air into the convective storm was resulting in evaporational cooling and the generation of negative buoyancy that would result in the downburst winds over the Tidal Potomac.

Figure 2. Radiosonde observation (RAOB) from Dulles Airport, VA at 1200 UTC 1 December 2010.

 Figure 2 displays favorable conditions for strong convective winds over the Tidal Potomac and Chesapeake Bay region.  The sounding profile identifies a dry-air layer near the 800-mb level with winds near 45 knots, very close to the wind gust speed observed at Cobb Point.  This provides evidence that in addition to precipitation loading and evaporational cooling, the downward transfer of horizontal momentum from the dry-air layer to the surface by heavy rainfall was also an important forcing factor in the strong downburst winds.
As the squall line continued to track eastward, strong downburst winds were observed over the Chesapeake Bay between 1500 and 1530 UTC.  At 1517 UTC, Thomas Point Lighthouse Coastal-Marine Automated Network (C-MAN) station, displayed in Figure 3, recorded a wind gust of 50 knots with the passage of the squall line.  

Figure 3.  Thomas Point Lighthouse, Maryland.

By this time, the dry-air notch on the southwestern flank of the line had become more pronounced in both satellite and radar imagery with a southwest to northeast orientation toward Thomas Point Light (white cross in Figure 4).  The peak wind gust recorded at the lighthouse was from a southwesterly direction with a local maximum in radar reflectivity (> 40 dBZ) overhead.  Nearby WeatherFlow observing stations, Tolly Point and Greenbury Point, recorded wind gusts of 44 knots and 46 knots, respectively, at 1520 UTC.  The concurrent peak wind gust and passage of the heaviest rainfall core over the observing stations suggest that higher momentum from the mid-tropospheric dry air layer was being transported to the surface by heavy precipitation within the squall line.  The physical process in downburst generation described with this event exemplifies the highly dynamic environment that is typical for cold-season severe convective storms.

Figure 4.  GOES-13 WV-IR BTD image at 1515 UTC 1 December 2010 with overlying radar reflectivity from Dover, DE NEXRAD.

Strong November Storms over Mid-Atlantic Region

During the late evening of 16 November and early morning of 17 November 2010, lines of strong convective storms developed and tracked eastward over Pennsylvania, Maryland and Virginia.  The line of convective storms that developed along a cold front boundary produced strong downburst winds over north-central Maryland, especially eastern Frederick County, as well as over the Tidal Potomac region.  Several downbursts occurred over eastern Frederick County between 0545 and 0625 UTC.  Sterling, Virginia NEXRAD indicated winds between 35 and 40 knots associated with the downbursts.  

      

Figure 1.  GOES-13 WV-IR BTD image at 0615 UTC with overlying radar reflectivity from Sterling, VA NEXRAD.

Figure 2.  GOES-13 WV-IR BTD image at 0615 UTC with overlying radar velocity from Sterling, Virginia NEXRAD.

Figures 1 and 2 show a downburst in progress over eastern Frederick County near the location of the white cross.  Strong winds near 39 knots were measured by Doppler radar at the time of downburst occurrence between 0605 and 0610 UTC.  The strong downburst occurred immediately downstream of a mid-tropospheric dry-air channel, marked by the white line pointing toward eastern Frederick County. The dry-air channel, pointing into the rear flank of the convective storm line, fostered downburst generation by injecting drier (unsaturated) air into the precipitation core of the convective storm.  The resulting evaporational cooling and generation of negative buoyancy accelerated the convective storm downdraft toward the surface, producing strong winds on impact.
The Tidal Potomac Region was also effected by the squall line between 0545 and 0630 UTC, as evidenced by high winds recorded by NOAA data buoys and WeatherFlow observing stations.  During this time, several dry-air channels became apparent in BTD imagery on the western flank of the convective line.  Figures 3 and 4 show that three channels over western Virginia were coming in line with rear-inflow notches as identified in radar imagery.

Figure 3.  GOES-13 WV-IR BTD image at 0532 UTC with overlying radar reflectivity from Sterling, VA NEXRAD.

Figure 4.  GOES-13 WV-IR BTD image at 0632 UTC with overlying radar reflectivity from  Sterling, VA NEXRAD.

Figure 3 shows the squall line moving into the Washington, DC metropolitan area with the apex of a bow echo moving into southern Maryland.  A severe wind gust of 50 knots was recorded by the Upper Potomac River buoy (white cross in Figure 3) between 0540 and 0550 UTC.  Note that a dry-air notch, as apparent in the GOES BTD image, was apparent on the western flank of the squall line pointing east-southeastward toward the Upper Potomac buoy.  At this time, the dry-air notch was in phase with rear-inflow notch (RIN) as evident in NEXRAD imagery.  By 0602 UTC, as shown in Figure 4, two dry-air notches were pointing toward the location of downburst occurrence at Potomac Light 33 station where a wind gust of 43 knots was recorded.  About 20  minutes later, near 0625 UTC, a stronger downburst wind gust of 45 knots was recorded  by Cobb Point station.  At this time, as displayed in Figure 5,  a dry-air notch had come in phase with a bow echo moving over the lower Potomac River.  It is evident that the injection of mid-tropospheric dry air into the heavy precipitation core within the convective storm line was providing a large amount of downdraft energy to realize strong downburst winds at the surface.  

Figure 5. GOES-13 WV-IR BTD image at 0632 UTC with overlying radar reflectivity from Sterling, VA NEXRAD.

Near this time, as a bow echo was tracking over the Chesapeake Bay, a wind gust of 46 knots was recorded at Thomas Point Lighthouse Coastal-Marine Automated Network (C-MAN) station.  The downburst wind gust was associated with the passage of the apex of the bow echo that tracked directly over Thomas Point Light.  Figure 6 shows the passage of the bow echo over Thomas Point Light.

Figure 6.  GOES-13 WV-IR BTD image at 0632 UTC with overlying radar reflectivity from Dover, DE NEXRAD.

The GOES BTD image in Figure 6 shows that a dry-channel (white line) was pointing toward Thomas Point Lighthouse (white cross) in phase with a (RIN) as apparent in radar imagery. Again, the channeling of dry air into the rear flank of the bow echo was a major forcing factor for downburst winds that were observed at the C-MAN station.  It is noteworthy that one of the strongest wind gusts of the event occurred as the dry-air notch, RIN, and bow echo apex were in phase as the convective storm line passed over the Chesapeake Bay.

New Results of Microburst Product Comparison Study

A cross-comparison of the Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) to the Haines Index that characterizes the potential impact of dry, unstable air on wildfire behavior and growth is currently in progress. During the afternoon of 23 June 2010, strong convective storms developed ahead of an upper-level disturbance over New Mexico and then tracked eastward into the western Texas Panhandle region. Convective storms produced numerous downbursts over western Texas during the evening hours of 23 June. Both the 2300 UTC GOES Microburst Windspeed Potential Index (MWPI) and Rapid Update Cycle (RUC) model-derived Haines Index products effectively indicated the potential for strong downbursts about three to four hours prior to each event. Large convective available potential energy (CAPE) and a steep temperature lapse rate, especially below the 700-mb level, were forcing factors for strong convective downdrafts.

Figure 1. Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) product image at 2300 UTC 23 June 2010.

Figure 2. Haines Index product based on the Rapid Update Cycle (RUC) model analysis valid at 2300 UTC with overlying radar reflectivity at 0224 UTC 24 June (bottom). White crosses mark the location of Friona (FAS) and Dimmitt (DMS) West Texas Mesonet stations.

Figure 3. Geostationary Operational Environmental Satellite (GOES) sounding profile from Eunice, Texas (near Dimmitt) at 2300 UTC 23 June 2010.

Figure 1 shows that the 2300 UTC Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) product image indicated a local maximum in values downstream of developing convective storms over New Mexico that would eventually track into western Texas after 0000 UTC 24 June. The corresponding GOES sounding profile in Figure 3 at Eunice, Texas displays a favorable classic “inverted V” profile that prevailed over western Texas near the maximum in MWPI values. A deep dry adiabatic lapse rate (DALR) layer below the 700-mb level and a dry subcloud layer fostered intense downdraft development due to evaporational cooling and the resulting downburst winds. The Rapid Update Cycle (RUC) model-derived Haines Index product in Figure 2 was generated and visualized by Man computer Interactive Data Access System (McIDAS)-V. A vertical temperature difference (between 850 and 700-mb levels) greater than 15°C (dark orange shading) and a dewpoint depression greater than 25°C yielded a Haines Index value of six (6) in the vicinity of Dimmitt and Friona, Texas, the highest for the index. Consequently, this index echoed favorable conditions for strong downbursts as presented in the MWPI image. Downburst wind gusts of 40 knots and 54 knots were recorded at Dimmitt and Friona West Texas Mesonet station, respectively, between 0200 and 0300 UTC 24 June. This case shows tremendous potential for the Haines Index, originally conceived as a wildfire threat product, in downburst potential assessment.