NWS State College Case Examplescms.met.psu.edu/sref/severe/2015/09Oct2015.pdf · A modest surge of high PW and high CAPE with a modest LLJ produced severe weather in southeast Pennsylvania

NWS State College Case Examples

Eastern Pennsylvania Severe Event of 9 October 2015 By

Richard H. Grumm and

Shane Kearns National Weather Service State College, PA

1. Overview

A line of thunderstorms with several bowing segments produced severe weather across southeastern Pennsylvania in the afternoon and early evening hours of 9 October 2015 (Fig. 1). A list of the storm reports is included in Table 1. These data showed that numerous locations in both Dauphin and Lebanon Counties had damage despite 1 entry in the table.

The large scale pattern as diagnosed by the Climate forecast system reanalysis data (CFSR: Fig. 2) at 1800 UTC 9 October 2015 showed a surge of high precipitable water air into the Pennsylvania ahead of an approaching cold front (Fig. 2a). The high PW ahead of the front was associated with modestly strong low-level 850 hPa flow. The CAPE at 1800 UTC (Fig. 2d) implied the highest CAPE was to the east with near 1200JKg-1 of CAPE over New Jersey. The CAPE over eastern Pennsylvania was around 600JKg-1. There was virtually no cape prior to and after 1800 UTC in the CFSR data.

The 3km HRRR 00-hour analysis (Fig. 3-4) showed a surge of CAPE over 1200 JKg-1 moving northward out of Maryland at 1800 UTC (not shown) and extending into southeastern Pennsylvania by 1900 UTC (Fig. 3d). The high CAPE was associated with high LCL levels and modestly strong 10m to 850 hPa shear (Fig. 3a). By 2000 UTC the high CAPE had shifted eastward (Fig. 4) and began to rapidly lower. The HRRR CAPE verse the CFSR CAPE is interesting and telling about model resolution of features critical in forecasting mesoscale features. The surge of high CAPE for a brief 2-3 hour window as an interesting forecast issue.

From a forecast perspective, the 3km HRRR did forecast a line of showers and thunderstorms to move across eastern Pennsylvania (Fig. 5). The 1800 UTC HRRR actually forecast the potential for bowing segments along the line to include the strong feature simulated at 2100 UTC in extreme eastern Pennsylvania. The convective mode was correctly predicted with some expected

Figure 1. Storm reports color coded by type from the Storm Prediction Center for the 24 hour period ending at 1200 UTC 10 October 2015.


issues related to the exact location of timing. Though not shown HRRR forecasts from 1500 to 2000 UTC all showed the potential for a line of enhanced showers with bowing segments in southeastern Pennsylvania.

The following section shows the key radar images associated with the observed severe weather, all of which was in the form of wind and wind damage.

2. Quasi-linear convection wind damage

The 0-2 km hourly low-level rotation tracks (Fig. 6) were aligned with the severe weather paths observed on 9 October. The two images show the tracks and intensity of the low-level rotation tracks. The white lines parallel these two enduring features. Along the Pennsylvania and Maryland border there were sporadic tracks which were not as persistent and focused.

Figure 2. CFSR data valid at 1800 UTC 9 October 2015 showing a) 850 hPa temperatures ( C) and temperature anomalies, b) 850 hPa winds and u-wind anomalies, c precipitable water and anomalies, and d) Convective available potential energy. Return to text.


The two paths shown in Figure 6 were aligned with strong outbound winds and bowing segments along the larger line of showers and thunderstorms (Fig. 7 lower panel).

The 0.5 degree base velocity data at 1908 UTC (Fig. 8a) showed the strong outbound winds with the bowing portion of the line and first track in the MRMS data. This clean appearance of the winds verse the noisy reflectivity data is often seen in Z verse V data in multi-cellular convective systems. By 1920 UTC the strong outbounds had pushed to the east and there was strong convergence head of the line (Fig. 8b). By 1935 UTC (Fig. 8c) where were two protruding velocity maximum in the line. Each one was associated with wind damage reports. By 1940 UTC the two protrusions were still visible and producing reports of wind damage. The system reached its maximum outbounds and strongest convergence along the line as seen on KCCX by 1945 UTC (Fig. 8e). The protrusion of “spearhead” echo moved out of the KCTP warning area by 2013 UTC (Fig. 8f). It would continue to produce wind damage to the east.

3. Summary

A modest surge of high PW and high CAPE with a modest LLJ produced severe weather in southeast Pennsylvania during the afternoon hours of 9 October 2015. The potential for convection 2 to 3 hour convective event with bowing segments was forecast by the 3km NCEP HRRR with 4-6 hours of lead-time.

The radar data showed clear and focused wind maximums which lined up well with the reports of damage during the event. As in many bow echo cases with embedded spearhead echoes, the focus of the damage was close to these features. The main bow and protruding velocity features accounted for nearly all the reported wind damage. The MRMS rotation track data appeared to provide some clues to the more persistent features long the larger line and may add value in warning in future QLCS events.

4. Acknowledgements

NSSL MRMS page and NSSL on demand page for access to MRMS data to compare to the AWIPS displays.

5. References

http://mrms.ou.edu/applications/mrms_2d_maps_main.html

http://ondemand.nssl.noaa.gov/


Figure 3. 3km HRRR 00-hour forecasts valid at 1900 UTC 9 October 2015 showing a) 10m to 850 hPa shear (ms-1), b) 850 hPa lapse rates, c) lifting condensation level heights, and d) CAPE. Return to text.


Figure 4. As in Figure 3 except valid at 2000 UTC. Return to text.


Figure 5. NCEP 3km HRRR forecasts of composite reflectivity form the HRRR initialized at 1800 UTC showing forecasts valid at 2000 and 2100 UTC. Return to text.


Figure 6. MRMS low-level circulation tracks for the 0-2km layer ending at 1934 and 2018. Other data include warning areas when time matched. White lines show the key features which produced severe weather. Return to text.


Figure 7. KCCX 0.5 degree reflectivity at 1901 and 1942 UTC. The data show the line of storms and the area of enhanced bowing, as indicated by the white arrow, along the line clearly defined at 1942 UTC. Return to text.


a) 1908

Figure 8. Series of base 0.5 degree velocity date for the specified times. The white arrow is used to show key features alluded to in the text... Return to text.


b) 1920


c) 1935


d) 1940


e) 1945


f) 2013










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NWS State College Case Examplescms.met.psu.edu/sref/severe/2015/09Oct2015.pdf · A modest surge of high PW and high CAPE with a modest LLJ produced severe weather in southeast Pennsylvania