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. 

Downburst Applications of the Haines Index

During the afternoon of 5 August 2010, strong convective storms developed over western Maryland and Virginia ahead of a cold front and produced a severe downburst that was observed at Washington, DC National Airport. The 1900 UTC Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) product on 5 August effectively indicated the potential for strong downbursts over the greater Washington, DC metropolitan area during the late afternoon. The MWPI product indicated wind gust potential of 35 to 49 knots where wind gusts of 42 to 51 knots were recorded in the Washington, DC area between 1940 and 2000 UTC. The pre-convective environment over the Washington area was characterized by an "inverted-V" profile with strong lower atmospheric instability that resulted from intense solar heating during the afternoon hours. Large convective available potential energy (CAPE) and a steep temperature lapse rate, especially below the 850-mb level, were forcing factors for strong convective downdrafts.

It has been found recently that the Haines Index (Haines 1988) can provide beneficial information for assessing downburst potential. The 5 August downburst event very effectively highlighted the application of the Haines Index especially when compared to corresponding MWPI product imagery and RUC sounding profiles. The Haines Index characterizes the potential impact of dry, unstable air on wildfire behavior and growth (Haines 1988) and is driven by classic atmospheric stability indicators such as temperature lapse rate (DT) and near-surface dewpoint depression (DD). Similar to the MWPI, the Haines Index is composed of both a stability (A) and moisture (B) component. The A component represents the environmental lapse rate, while the B component is the dewpoint depression for a specific pressure level. For both components, the calculated temperature and dewpoint depression are categorized into three groups that are assigned an ordinal value of 1, 2, or 3, and then summed. The resulting index has a range from 2 (very low risk) to 6 (high risk). A Haines Index product that consists of composite of the vertical temperature difference and dewpoint depression has been implemented in Man computer Interactive Data Access System (McIDAS)-V.

Figure 1. RUC sounding at 1900 UTC 5 August 2010 (top) compared to Haines Index product based on the RUC one-hour forecast valid at 1900 UTC (bottom). White cross marks the location of Washington National Airport and the above sounding. Note that the severe downburst, indicated by the spearhead echo apparent in radar imagery, occurred in a region of maximum temperature lapse rate and dewpoint depression.

The Haines Index product in Figure 1 was generated and visualized by McIDAS-V. Figure 1 shows that the severe downburst, associated with a wind gust of 51 knots, recorded at Washington National Airport at 1952 UTC occurred in a region characterized by strong low-level instability with maxima in temperature lapse rate and dewpoint depression:

DT (950-850mb): 9C

DD (950mb): 13C

A value: 3

B value: 3

Haines Index: 6 (high)

The index value of six is the highest for the Haines Index. Consequently, this index value indicated that sub-cloud evaporational cooling in a highly unstable lower atmospheric layer fostered intense downdraft generation that resulted in the observed downburst winds. The high index value corresponded well with the unstable, "inverted-V" sounding profile displayed in Figure 1. This case shows tremendous potential for the Haines Index, originally conceived as a wildfire threat product, in downburst potential assessment.

References

Haines, D.A. 1988. A lower atmospheric severity index for wildland fire. National Weather Digest, 13, 23-27.