Mid Atlantic Severe Event of 1 May 2017 Central Pennsylvania QLCS event By Richard H. Grumm National Weather Service, State College, PA 16803

Transcription

1 1. Overview Mid Atlantic Severe Event of 1 May 2017 Central Pennsylvania QLCS event By Richard H. Grumm National Weather Service, State College, PA A strong upper-level wave (Fig.1) moving into a strong subtropical ridge over the western Atlantic produced a widespread severe weather event over the eastern United States on 1 May 2017 (Fig. 2). Most of the severe weather on 1 May was observed from southern Ohio northeastward into northeastern New York. It will be shown that as observed by radar this was a quasi-linear convective severe (QLCS) event. Thus most of the event was dominated by severe winds though there were some reports of hail and short-lived tornadoes. The deep southerly flow between the 500 hpa trough and ridge (Fig.1) brought a surge of moisture as shown by the precipitable water (PW: Fig. 3). The initial surge of high PW air was into the Mississippi Valley and southern Canada. Not surprisingly, there was a heavy rainfall event (Fig. 4) and flooding in portions of the Mid-Mississippi Valley (MMV). At the surface (Fig. 5) a slow moving cyclone was associated with the 500 hpa low (Fig. 1) and this system slowly lifted into the Great Lakes. There was a strong low-level jet (Fig. 6) in the warm air ahead of the cold front. This strong LLJ moved across eastern Ohio and into eastern Pennsylvania and New York between 1200 UTC 1 May and 0000 UTC 2 May 2017 (Figs. 6b-e). The CFSR showed modest CAPE in the eastern United States head of the warm surge (not shown). The HRRR analysis with 1-hourly spatial and 3km temporal resolution showed slightly more favorable CAPE (Fig.7). The layer CAPE was used as it was slightly more favorable than the surface base CAPE. Clearly, what CAPE the HRRR detected maximized in the 1800 to 2100 UTC timeframe (Figs. 7b-c) when CAPE values peaked at 1200 to 1600 JKg-1 over portions of central Pennsylvania. After 2100 UTC CAPE values were generally 600 to 800 JKg-1. This was a good setup for a low CAPE/high shear event and the shear somewhat favored a QLCS event with the potential for rotation. The low-level shear was well over 30kts from the southwest (240 degrees). The setup for a severe event was favorable. Section 2 shows some of the forecast guidance which favored a severe weather event and section 3 shows some of the stronger and more damaging features which affected central Pennsylvania. 2. Forecasts i) GEFS pattern and QPF The GEFS 500 hpa forecasts valid at 1800 UTC 1 May 2017 (Fig. 8) showed the deep closed 500 hpa cyclone in all 6 forecasts. These data show the standardized anomalies emphasizing the deep low over the upper-midwest and the sharp ridge over the western Atlantic. They do not show the spread in the fields and the uncertainty with the location of the 500 hpa low. In the deep southerly flow between the trough and ridge, the GEFS forecast a strong LLJ as shown with the 850 hpa winds and v-wind anomalies (Fig. 9). These strong south-southwesterly winds indicated the potential for strong shear. With the 850 hpa winds forecast to be +3s above normal around 1800 UTC in eastern Ohio and western Pennsylvania. Accompanying the strong shear was a surge of high PW air with standardized anomalies on the order of 2-3s above normal

2 (Fig. 10). The PW field also indicated a strong north-south cold front (not shown) which is historically been used as an indicator of a lifting mechanism for triggering convection. Not surprisingly in the plume of deep moisture and high shear, the GEFS produced JKg-1 of CAPE (Fig. 11). The GEFS QPF (Fig. 12) showed that the lift and instability produce QPF in the mode from 1200 UTC 1 May through 0000 UTC 2 May. The implication here is that the model had a mechanism to potentially realize the instability. ii) HRRR The pattern was similar to threat produced by the GEFS and as shown Figure 7 the HRRR analysis and higher CAPE than the GEFS. For simplicity the HRRR simulated radar and QPF are shown (Fig. 13 & 14). These data summarize the impacts of the lift, instability, and shear. The HRRR reflectivity valid at 2100 UTC 1 May 2016 from 6-different HRRR forecast initialization times showed that the HRRR forecasts a QLCS from western New York southward across Pennsylvania and into West Virginia (Fig. 13). The line had several segments and implied bowing elements. The HRRR QPFs (Fig. 14) for the period implied that the strong cores or cells along the line would move to the northeast and produce stripes of QPF on the order of 32 mm or more of QPF in the 6-hour period. A few HRRR cycles initialized on and after 1300 UTC implied some of the longer lived elements could produce upwards of 48 mm of QPF and the 1500 UTC HRRR run suggested over 64 mm. 3. Radar The larger view from KCCX radar at around 2100 UTC (Fig. 15) shows the QLCS system moving across west-central Pennsylvania. Note the area of stratiform rain trailing the more intense echoes along the line implying a strong rear inflow jet into the line. Bowing segments and mesocyclones along the line produced swaths of damage as this larger system moved across New York and Pennsylvania. A strong mesoscale circulation developed in Elk County and moved into McKean County, PA. This system produced 2 verified tornadoes in northern Elk County 1. Figure 16 shows the 0.5 degree radar imagery including the base reflectivity, base velocity, storm relative velocity and the correlation coefficient. The circulation at this time was in McKean county about 10 miles northeast of the damage swaths in Elk County. No tornado damage was found in the forest of southeastern McKean County though trees and other minor damage was visible from Kane to the county border to the east. Though not shown this circulation along the line intensified in the next volume scan. Another area of intense damage was observed in Centre County. This occurred shortly after the gust front blew through the radar where the lowest elevation cut estimated winds over 65kts. The strongest outbound winds moved across north Centre and Clinton counties producing widespread wind damage (Fig. 17). The lower panel of Figure 17 shows the strong outbounds which produced widespread wind damage as noted above. The bowing segments farther south produced 1 The two intense damage swaths were within about 200 yards of each other. The eastern one was less than 1 mile in length the second one was about 2 miles in length. Mainly snapped and uprooted pine trees east of Twin Lakes in Elk County.

3 extensive tree damage from State College 2 eastward and down the valley to the east. Wind damage was extensive with winds over 65 kts measured around Boalsburg. Extensive damage continued farther east through Centre Hall, Madisonburg, and Rebersburg. An EF1 tornado occurred near Rebersburg (see map). There was an area of wind damage from Boalsburg to Centre Hall (yellow arrow). East of Centre Hall the valley becomes more restricted as another ridge is present east of Penn s Cave. In the narrower valley the winds appear to have slowed on the north end of the bow (Fig. 17) and an EF1 tornado was verified east of the town Rebersburg. There was some focused wind damage near Spring Mills (purple arrow: See Figure 19). The wind damage, though not always as focused extended far to the east. There was also wind damage along route 64 north of the Mt Nittany which originates just east of State College in Figure 18. The 4 panel radar imagery at 2047 (Fig. 19) shows several notches along the edges of the bowing segments, front to rear winds behind the bow, and the low reflectivity (upper panel). The complex terrain and elongated valleys to the northeast likely played a role in the some of the focused damage. It should be noted that north of these mesoscale features there were very strong winds which produced widespread areas of wind damage. 4. Summary A strong frontal system with strong low-level winds in excess of 50 kts combined with moisture and instability to produce a QLCS dominated severe weather event from eastern Ohio across Pennsylvania and New York. The NCEP models did relatively well forecasting the larger scale pattern and the potential for severe weather. Radar data from KCCX in central Pennsylvania showed several mesocyclones along the line, bowing segments, and weak circulations along the several of the bowing segments. The NCE GEFS had the pattern with the strong south-southwesterly flow, instability, and lift to indicate the potential for a convective event with at least 5 days of lead-time. Old school pattern recognition and knowledge of severe weather provided a relatively good long range heads up for a significant severe weather event. From a pattern perspective the GEFS provide relatively good long term guidance (Figs. 8-11). As the event neared, the 3km NCEP HRRR clearly indicated a QLCS system with the potential for intense bows along the larger line of enhanced echoes (Fig. 12). The mode of convection was well predicted by the HRRR. The HRRR radar showed the potential for bowing segments along the line and longer-lived convective elements. This was clearly indicated by the HRRR QPFs which showed intense southwest to northeast oriented axis of moderate to heavy rainfall. Interestingly these stronger elements were aligned and moved with the strong southwesterly flow. A good rule for severe weather with QLCSs and an enhanced tornado threat often occurs 2 Power outage summaries indicated that Centre County had the most customers without power Monday night into Tuesday.

4 when the 0-1 and 0-3km shear is over 30kt and nearly orthogonal to the line. The HRRR appeared to forecast this to happen. KCCX radar was shown to highlight that overall this was a QLCS event and matched the HRRR s general forecast. Several radar images of 2 select areas of interest showed that a mesocyclone developed in northwestern Pennsylvania. This circulation did sporadic wind damage across southeastern McKean County and produced to focused damage swaths in Elk County. These swaths were rated as EF1 and indicated two parallel tornadic damage swaths. Farther south several bowing segments produced high winds. The focus here was on the bowing segment and vortices on the bows in Centre County mainly west of State College. There were several reports of tornadoes and possible gustnadoes on the cyclonic northern edge of these bows. One of these produced an EF1 tornado near Rebersburg. It should be noted a similar bowing segment affected northern McKean County during the event. It downed many pine trees at Penn Hills Golf Course near Custer, PA. There was some convergence in the trees on the ridge side of the valley indicating a weak vortex. The tree damage by the bow and the gustnadoe was rated in the EF0 to EF1 range..

5 Figure 1. CFSR 500 hpa heights and height anomalies in 12-hour increments from a) 0000 UTC 24 April through f) 1200 UTC 26 April Contours every 60m shading in standard deviations from normal. Black dot is the location of Raleigh, NC. Return to text.

6 Figure 2. Storm reports by type (color key in lower left) for the period ending at 1200 UTC 2 May Return to text.

7 Figure 3. As in Figure 6 except for the precipitable water (mm) and PW anomalies from a) 0000 UTC 1 May through f) 0600 UTC 2 May Return to text.

8 Figure 4. Stage-IV QPE in 24 hour increments from a) 1200 UTC 30 April to c) 1200 UTC 1 May 2017 and d) total QPE for the 3 day period. Return to text.

9 Figure 5. As in Figure 3 except for mean sea-level pressure and pressure anomalies. Return to text.

10 Figure 6. As in Figure 5 except for 850 hpa winds and v-wind anomalies. Return to text.

11 Figure. 7. NCEP 3km HRRR 0-hour forecasts of CAPE valid in 3-hour increments from a) 1500 UTC 1 May through f) 0600 UTC 2 May Return to text

12 Figure 8. NCEP 21-member GEFS valid at 1800 UTC 01 May 2017 showing the 500 hpa heights and height anomalies from GEFS members initialized at 0000 UTC a) 27 April, b) 28 April, c) 29 April, d) 30 April, e) 01 May and f) 1200 UTC 01 May Contours every 60 m and anomalies in standard deviations as in the color bar. Return to text.

13 Figure 9. As in Figure 8 except for GEFS 850 hpa winds (ms-1) and v-wind standardized anomalies. Return to text.

14 Figure 10. As in Figure 9 except for GEFS precipitable water (mm) and standardized anomalies. Return to text.

15 Figure 11. As in Figure 10 except GEFS CAPE and spread about the mean. Return to text.

16 Figure 12. As in Figure 11 except for the QPFs valid for the period of 1200 UTC 1 to 0000 UTC 2 May. Shading is the ensemble mean QPF (mm) and contours show each member 25 mm contour. Return to text.

17 Figure 13. NCEP 3km HRRR forecasts of simulated reflectivity valid at 2100 UTC 1 May The 6 panels show HRRR forecasts initialized at a) 0600, b) 0900, c) 1200, d) 1300, e)1400 and f)1500 UTC. Return to text.

18 Figure 14. As in Figure 13 except for the HRRR QPF for the period of 1800 UTC 1 May through 0000 UTC 2 May Return to text.

19 Figure 15.KCCX 0.5 degree reflectivity at 2059 UTC 1 May Return to text.

20 Figure 16. KCCX 0.5 degree radar at 2009 UTC 1 May showing base reflectivity, SRM, V, and CC. The yellow arrow in the SRM panels shows the mesocyclone. The yellow arrow in the base velocity images shows the approximate track of the mesocyclones and the general area of Elk County affected by the mesocyclone along the line.. Return to text.

21 Figure 17, As in Figure 16 except valid at 22:45 UTC. The yellow arrow shows one of the notches where the winds slowed due to the terrain along the north edge of the bowing segment. The bow caused extensive damage and the notched feature(s) were near short-lived tornadoes and possible gustnadoes between Madisonburg and Rebersburg, PA. Another notch to the south may have produced a gustnadoe near Spring Mills. Return to text.

22 Figure 18. Map of the bow echo and key damage areas mainly west of State College. The Yellow arrow extends from Boalsburg to Centre Hall, the orange arrow extends from Centre Hall to Madisonburg, and the red arrow Madisonburg to Rebersburg. The purple arrow was an area of damage and an likely gustnadoe near Spring Mills. Return to text.

23 Figure 19. As in Figure 17 except valid at 2247 UTC. The two red arrows in the SRM data show the notches likely due to the terrain. The northern notch, near Rebersburg was associated with an EF1 tornado. The notch to the south was closer to Spring Mills. The blue arrow shows the strong outbounds and the notch in the rear associated to the lower reflectivity. The weak reflectivity notch is shown by the back arrow in the upper panel. And the Light green arrow shows the strong 60-70kt outbounds north of the bow echo. Return to text.

24 Twin Lakes Wilcox Figure A1 Approximate damage swath to pine trees near Twin Lakes, PA. Wilcox was farther southeast down route 321 and Twin Lakes was northwest of the focused tree damage. Tree damage was mainly snapped pines 8 to 14 feet up the trunks and uprooted pines with root balls. Return to text.