Cold frontal Rainband and Mid-Atlantic Severe Weather Event 28 September 2006 by Richard H. Grumm And Ron Holmes 1. INTRODUCTION A strong cold front moved across central Pennsylvania and the Mid-Atlantic region during on the Thursday, 28 September 2006. The front was forecast to usher in cold air with 925 and 850 hpa temperatures forecast to be about -1 to -2 standard deviations below normal about 12-24 hours after the frontal passage. Ahead of the front, a strong low-level jet was forecast along with a modest amount of moisture and instability. The strong low-level jet, with strong southerly wind anomalies is an ever present signal in both Maddox Synoptic heavy rain events (Maddox et al. 1979) and with many strong severe weather events. Typically, the synoptic type heavy rain events also have above normal precipitable water (PW) anomalies (Grumm and Hart 2000) associated with them. In this event the PW values barely got above normal over most of Pennsylvania. Though not the focus of this study, decision trees for heavy rains indicated that the much above normal lowlevel jet had potential to produce heavy rains. In addition to heavy rains, strong southerly jets have been shown to be good indicator of severe weather in the Mid-Atlantic region. An unpublished study has shown that most F2 and greater tornadoes events and wide spread severe weather events are often associated with an anomalous southerly lowlevel jet. As will be shown, the convective available potential energy (CAPE) in this Figure 1 Storm Prediction center plot of severe weather, by time observed on 28 September 2006. event was above normal but not exceptionally high. Large CAPE, typically in excess of 1200JKG -1 is a requisite condition to produce large updrafts. Large updrafts, independent of the time of year require a significant amount of CAPE. The strong low-level winds likely produced strong shear, similar to CAPE shear is an important ingredient in the production of severe weather. The combination of CAPE and shear were likely sufficient to produce convection and as shown in Figure 1, severe weather was observed in the warm air ahead of the frontal zone during the afternoon hours of 28 September 2006. The Storm Prediction Center (SPC) plot shows that all 120 reports were associated with advancing cold front. In the Mid-
Atlantic region, all the reports were from high winds with 2 potential 1 tornado reports. This paper will examine the conditions associated with the Mid-Atlantic Severe weather event of 28 September 2006. The focus will be on the front and attendant weather and severe weather in southeastern Pennsylvania. 2. METHODS Satellite and radar data were retrieved from the National Weather Service office in State College AWIPS systems. These data were archived on made available on the local Weather Event Simulator. These data were used to get a perspective of the event, focused on specific damaging storms. The National Centers for Environment Prediction (NCEP) ensemble predictions system (EPS) data were retrieved and made available in near-real time from the NCEP data site. The NCEP short-range ensemble forecast (SREF:Du et al. 2004) data shown here are limited to the 15-member data are the primary EPS data shown here. Display focus on traditional ensemble display (Sivillo et al 1997). Minor modification will show ensemble means with the standardized anomaly, in standard deviations from normal (Grumm and Hart 2000). 3. RESULTS ii. large scale and forecasts 1 As of this time 1 tornado had been confirmed. Video evidence of a funnel cloud has been examined for the storm in Cumberland County, PA. The storm was surveyed on 29 September. Figure 2 Multi-sensor precipitation analysis valid for the 24 hours ending at 1200 UTC 29 September 2006. Values in inches. Courtesy of NWS Precipitation Analysis website(http://www.srh.noaa.gov/rfcshare/precip_analysis_new.php). Figure 2 shows the regional multi-sensor precipitation analysis. A general north-south band of heavy rain is shown form Virginia northward to northern New York. The band of rain has the same general orientation of the severe weather plots in Figure 1. Typical of the Maddox-Synoptic type event, severe weather often accompanies the heavy rain. This event was relatively fast moving but it still produced a significant amount of rain. A broad region from Virginia to New York saw 0.5 to 1.5 inches of rain (Green shades) with some embedded areas of 1.5 to 2.0 inches (yellows) and some 3 inch amounts (reds). Southeast Pennsylvania has a stripe of heavier rain, likely the result of the front tapping the warmer more unstable air which likely produced vigorous upright convection. The result was localized heavy rain and isolated severe weather. SREF forecasts focus on short-term forecast products to tell a brief story and paint the picture of the event. Figure 3 shows the confidence in the SREF 850 hpa temperature forecasts from
forecasts initialized at 0900 UTC 27 September 2006. The red areas shows were the current SREF forecasts had larger spread than the mean spread at that point over the past 15 days. Thus, with the approaching cold front, uncertainty with the timing and the tight thermal gradient acted to produce large spread and thus indicated high uncertainty with timing of the frontal passage. The uncertainty was higher than what would have been expected based on recent SREF performance. The same information is displayed on a plan view map in Figure 4 from forecast initialized 18 hours later (0300 UTC 28 September 2006). The upper panel shows the high uncertainty with the front, the hot colors show the spread is larger in the forecast than in the 15-day mean spread. Thus, timing the rain and severe weather was an issue. The lower panel shows the mean and the deviation in SDs from normal. The salient point being the cold, if not unseasonably cold air behind the front, implying a strong front. Figure 5 shows the SREF probability of CAPE greater than 600 JKG -1 and the shear. These data imply a surge of relatively high CAPE into southeastern Pennsylvania between 1800 and 0000 UTC. This image show shows the mid-point. The data also show the high shear predicted by the SREF. The high shear was directly related to the strong low-level. The SREF and NAM both forecast an anomalous low-level 850 hpa jet. For brevity the NAM 3-hour forecasts are shown valid at 28/2100 UTC. A broad Current forecast 15-day spread Figure 3 SREF forecasts initialized at 0900 UTC 27 September 2006 showing the uncertainty at a point near State College. Black lines shows the spread of the 850 hpa temperatures from the SREF forecast and the blue line shows the spread from the past 15-days of SREF forecasts. Red areas show low confidence where the forecast spread is larger than the 15-day averaged spread (the expected spread). southerly jet was present with 850 hpa V- wind anomalies in the 2 to 2.5 SD above normal range. Figure 7 shows the CAPE and precipitation forecasts at a point near Harrisburg, in close proximity of the heavy rainfall area in Figure 2 and the southeastern Pennsylvania severe weather shown in Figure 1. These data clearly show the mean CAPE forecast to be around 800 JKG -1 with some members forecasting CAPE as high as 1600 JKG -1. The higher end amounts would have supported large updrafts, exceeding that critical 1200 JKG -1 threshold. Analysis data (not shown) indicated CAPE on the order of 1000 to 1200 JKG -1 did enter southeastern Pennsylvania after 1900 UTC. CAPE and shear were both present.
Figure 5 As in Figure 4 except SREF forecasts valid at 1800 UTC 28 September showing a) probability of CAPE greater than 600JKG-1 and the mean CAPE every 600JKG-1 and b) the 1.5km shear and the probability of the shear exceeding 3x10-3 s-1. Figure 5 SREF forecasts initialized at 0300 UTC 28 September 2006 showing the ensemble mean 850 hpa temperatures (C) and a) confidence (forecast spread expected spread)/expected spread, and b) the departure of the mean in standard deviations from normal. The precipitation plume clearly shows the potential for around 1 inch of rain with up to 2 inches. Most of the rain was forecast to arrive around between 28/2100 and 29/0300 UTC. The frontal uncertainty contributed to the timing differences. Experience suggests most of the rain with these systems lasts 3-6 hours. ii. radar imagery The two significant and most damaging storms of the day appeared to interact with the terrain. The western storm, which was part of a larger line of storms, came off the higher terrain, a broad feature known as Kittatinny Mountain in western Franklin County and interacted with lone storm ahead of this line. Which was moving up the Great Valley from the south-southwest (Fig. 9). Great Valley is the low bounded by Kittatinney Mountain to the west and South Mountain (of Civil War fame) to the southeast. The result was a severe thunderstorm in the valley in the Chambersburg area.
Figure 6. NAM 3-hour forecast valid at 2100 UTC showing the 850 hpa winds and a) the 850 hpa U-wind anomalies and b) the 850 hpa V-wind anomalies. Figure 7. SREF plumes from forecasts initialized at 0900 UTC 28 September 2006. Upper panel shows each members CAPE forecast and the lower panel shows the accumulated rainfall (green) and 3-hour precipitation from each member. The second storm, produced a tornado in Cumberland County about 2 miles south of Kittatinney Mountain near Interstate 81. The funnel may have been captured on video. Both storms occurred shortly before 2100 UTC on the 28 th of the September. Figures 9 & 10 show the Franklin County storm. The terrain features suggest the storm was confined to the valley location. KLWX (Sterling Virginia) storm relative velocity data show this was a broad circulation over the region, with implied divergence. The 0.5 degree base velocity data (not shown) indicated 35-45KTS of outbounds near Chambersburg at about 5000FT above the surface. This storm produced the most widespread damage. The tornadic storm near Wertzville in Cumberland County is shown in Figure 11. The county line is close to the ridge line. Werztzville is a valley location about 2 miles south of the ridge line. The storm formed to the south and intensified as it moved off of South Mountain and moved north-northeastward to the region. The storm had enhanced echoes north of the town, along the ridge line reaching 70 dbz at 2053 and 2057 UTC. The SRM data
Figure 9 KCCX 0.5 degree reflectivity and terrain valid at 2042 UTC 28 September. Dashed blue lines show the approximate ridge lines of Kittatinny and South Mountains. Most of the towns between the two ridges lie in Great Valley. showed not distinct or strong signatures until about 2057 UTC and there clear was some rotation by 2101 UTC (Fig. 12). The time of the tornado is a subject of mystery. Radar imagery indicates it was may have been a few minutes either side of 2100 UTC. The first call a 911 center was 2106 UTC though other reports suggested it was 2130 UTC. Thus, the 2100 UTC time is likely a good first and reasonable guess. This storm lost it s structure rapidly after this time period. 4. CONCLUSIONS A strong cold front brought rain, and locally heavy rain to the Mid-Atlantic region during the afternoon hours of 28 September 2006. Southeastern Pennsylvania and Maryland had areas of over 1 inch of rainfall and severe weather. At least 1 tornado was confirmed in Pennsylvania near the town of Wertzville. The SREF showed that severe weather, based on shear and instability was a possibility in southeast Pennsylvania and Maryland. CAPE likely exceed 1200JKG-1 in some areas in the Mid-Atlantic region. The strong shear, instability, and terrain
Figure 10 KLWX 0.5 degree reflectivity and 0.5 degree storm relative velocity valid at 2042 UTC 28 September. White arrow highlight the Chambersburg area. facilitated the development of short-lived tornado. The signal for a quick moving Maddox synoptic type event (Maddox et al 1979) was well forecast by the SREF and other NCEP EPS. The strong southerly jet is good signal for heavy rain and severe weather. This event lived up to both possibilities. Experience suggests most of the rain with these systems lasts 3-6 hours. This was a good approximation with this event. The large scale conditions, and the uncertainty with the frontal passage were both well forecast and depicted by the SREF. The details of the severe weather to expect can and often are elusive. 5. Acknowledgements WFO BGM for backup during part of this event due to a fire alarm. All State College Staff who worked the event and helped with verification efforts. Finally, the spotters, 911 centers and Emergency management offices that helped identify the storms and the damage. 6. REFERENCES Du, J., and Coauthors, 2004: The NOAA/NWS/NCEP Short-Range Ensemble Forecast (SREF) system: Evaluation of an initial condition vs multiple model physics ensemble approach. Preprints, 16th Conf. on Numerical Weather Prediction, Seattle, WA, Amer. Meteor. Soc., CD-ROM, 21.3. Grumm, R.H. and R. Hart, 2000: Anticipating heavy rainfall events: Climatological aspects. Preprints, Symposium on Precipitation Extremes: Prediction, Impacts, and Responses, Alburquerque, New Mexico, Amer. Meteor. Soc. 66-70. Hart R., and R.H. Grumm, 2000: Anticipating heavy rainfall events: Forecast aspects. Preprints, Symposium
Figure 11. KCCX 0.5 degree reflectivity and SRM data valid at (upper) 2053 and (lower) 2157 UTC. The blue line shows the approximate ridge line and the white arrow depicts Wertzville. on Precipitation Extremes: Prediction, Impacts, and Responses, Alburquerque, New Mexico, Amer. Meteor. Soc. 271-275. Maddox,R.A., C.F Chappell, and L.R. Hoxit. 1979: Synoptic and meso-alpha aspects of flash flood events. Bull. Amer. Meteor. Soc.,60,115-123. Sivillo,J.K, J.E Ahlquist, and Z. Toth: 1997: An ensemble primer. Wea. Forecasting.12,809-818, Stuart, N. A and R.H. Grumm, 2006: Using wind anomalies to forecast East Coast Winter Storms. Wea. Forecasting,228,(in press)
Figure 12. As in Figure 11 except valid at 2101 UTC. White arrows show the rotating couplet near Wertzville.