Historic Central Pennsylvania Flash Floods of 21 October 2016 by Richard H. Grumm National Weather Service State College, PA 16803

Transcription

1 1. Introduction Historic Central Pennsylvania Flash Floods of 21 October 2016 by Richard H. Grumm National Weather Service State College, PA A slow moving frontal system produced heavy rainfall in Mid-Atlantic region (Fig. 1) with a small are of extreme rainfall over central Pennsylvania (Fig. 1b). Most of the rain fell in a very brief period of time with spotter gages showing rainfall rates in excess of 3-6 inches in 1 hour and one untrained spotter near Milesburg, PA recorded over 10 inches of rainfall. The Stage-IV rainfall compared to the 6-hour 100 year average recurrence interval (ARI) showed rainfall rates from 0000 to 0600 UTC 21 October 2016 at 100 to 175% of the 100 year 6-hour ARI (Fig. 2). The 24-hour ARI showed two elongated southwest to northeast bands of over 4 inches of rainfall with 100 to 150% of the 24-hour 100 year ARI. More impressive and related to the flash flood nature of the event was the large area of 100 to 175% of the 6-hour ARI values in central Pennsylvania, just west of State College. This rainfall was quite remarkable and the Stage-IV showed higher 6 and 24 hour rainfall amounts than the Ellicott City, MD flood of 30 July 2016 (Grumm 2016) event where rainfall amounts were on the order of 150 mm. Gage data too supported rainfall amounts on the order of 150 mm of rain. The fifty dollar question is why is the Ellicott City flash flood Nationally Known relative to the central Pennsylvania flood with over 250 mm of rainfall? The obvious responses are related to urbanization and population density. The Ellicott City rainfall fell in a densely populated urban area affecting thousands of people in a major metropolitan area. The central Pennsylvania floods occurred in a rural low-population density region. The flood response in Ellicott City, MD captured on video showed the rapid rise in the water. The response in Centre County, PA was equally impressive as shown by the hydrograph along Bald Eagle Creek, PA. The creek had extremely low flow and responded in 1-2 hours with a rapid rise to and then over flood stage (Fig 3). The Bald Eagle Lake had an impressively rapid rise in lake level during the event. The extreme rainfall, rapid rise along the creek, and the lake level resulted in loss of life but did resulted in 33.2 million of dollars in infrastructure damage in Centre, Bradford, Lycoming, and Sullivan Counties (CDT 2016). The heavy rainfall and flash flooding along Bald Eagle ridge resulted in over 2 million dollars to critical infrastructure such as roads and bridges. The patterns in which significant convective rainfall events develop are relatively well known. However, forecasting these events often is a difficult challenge. Larger scale models which do not allow for convective process often grossly underestimate rainfall in their quantitative precipitation forecasts (QPF). Convective allowing models (CAMS) can and often due produce significantly higher QPFs when convection is forecast though they often suffer significant temporal and spatial errors. The evolution of high resolution CAMS has improved short range QPFS (Juanzhen Sun et al. 2014). The evolution of CAM ensembles is also improving high end and extreme QPF forecasts. Currently these ensembles are expensive to run and time lagged ensembles (TLE) are often used a proxy for a true ensemble. Currently, the Global Systems

2 Divisions (GSD) High Resolution Rapid Refresh (HRRR: Benjamin et al. 2016) is used to produce a TLE known as the HRRR-TLE. The future of warn-on-forecast for flash floods will likely be related to the evolution of improved CAMS and CAM ensembles. This event is a good test of the value of the HRRR in forecasting heavy rainfall and flash flooding. The patterns in which significant convectively enhanced heavy rainfall events occur are often recognized by forecasters and generally well predicted by global models. The models and associated ensemble forecast systems can and often correctly forecast the patterns and general areas of rainfall. Though these models are challenged in their ability to produce the extreme QPF amounts verse the relatively observed extreme rainfall. It will be shown that in this event the NCEP GFS/GEFS forecast heavy rainfall and near record rainfall based on the GEFS internal model QPF climatology (M-Climate) in portions of northwestern Pennsylvania and southwestern New York. Several GEFS runs actually produced an extreme rainfall event relative to the model climatology. However, the QPF amounts were at best 30% of the higher end rainfall observations and the locations were poorly forecast. The GEFS and other models forecast heavy because they forecast the pattern associated with a slow moving frontal system with strong southerly flow on the warm side of the boundary. This is very similar to a Maddox Synoptic (Maddox al. 1979) heavy rain and flood event type. This paper will present the pattern, verification, and forecasts of the historic and devastating central Pennsylvania Flash Floods 21 October This paper will also examine the forecast issues in the NCEP models and several more advanced CAMs.. 2. Methods and data The climate forecast system re-analysis (CFSR) data was used to reconstruct the pattern and the standardized anomalies associated with the event. The CFSR is used to show the pattern, which forecasters often use to gain confidence in a potential significant weather event. These same patterns, when forecast may produce high end QPF which may reinforce confidence in the forecast. The Stage-IV rainfall data (Seo 1998) was used to estimate the rainfall over several 6, 12 and 24 hour periods on 20 to 22 October The rainfall pattern also reveals some interesting information about position and movement of the cold front, which clearly stalled for several hours in central Pennsylvania. Data was produced locally using archived GRIB files from the CFSRV2, GFS, GEFS, and HRRR. Other data from CAMS were provided by NCEP, GSD, and NCAR. The Average Recurrence interval images for the GFS and Stage-IV data were produced using NOAA14 ARI data in GRIB format. 3. Results a. The Large scale pattern and frontal boundary The large pattern of the eastern United States (Fig. 4) showed a sharp trough over the Mid- Mississippi Valley (Fig. 4a) with a sharp downstream ridge and positive height anomalies over

3 New England. The 850 hpa temperatures (Fig. 4b) and the precipitable water (PW: Fig. 4c) showed a strong frontal system with warm moist air on the warm side of the generally southwest to northeast oriented frontal boundary. The PW anomalies just ahead of the implied cold front were 40 to 50 mm which was about 1.5 to 2.0σ above normal. The CFS showed an area of low pressure over central Pennsylvania with -1s pressure anomalies (Fig. 4d) with bagginess and an implied secondary low along the frontal boundary. This secondary low likely contributed to the delayed eastern progress of the frontal boundary over central Pennsylvania. By 0600 UTC (Fig. 5) the frontal boundary was still hung up in central Pennsylvania (Fig. 5b & 5c). The PW anomalies were on the order of 2 to 4σ above normal with PW values on the order of 40 to 50 mm. The period from 0000 UTC through 0600 UTC encompasses the main period of heavy rainfall and flash flooding. The 3km HRRR 00-hour forecasts of the PW field (Fig. 6) show the slow moving frontal boundary and high PW values over central Pennsylvania from 0000 to 0600 UTC. Note how the PW values diminish from 0900 UTC onward. The split in the PW field and lowering of the values after 0600 UTC is directly related to the arrival and eventual passage of the second wave along the frontal boundary (Fig. 7) as shown in the HRRR 3-hour analysis of the mean sea-level field. Though not shown, the HRRR and CFS showed strong south-southeasterly flow in the warm air and westerly flow in the cooler drier air to the west. The key point in these data was the waves along the frontal boundary stalling the frontal boundary allowing for a prolonged period of heavy rainfall. It will be shown that the NCEP models and the HRRR correctly stalled the frontal boundary. The convection along the boundary showed the heavy rainfall in the narrow band of intense returns from near State College to Wellsboro at 0158 UTC. By 0401 UTC the bands began to rapidly move off to the east (Fig. 8). These large scale images do not show the detailed mesoscale feature which evolved and are best viewed in radar loops. These data revealed the strong echoes reached Bellefonte around 0128 UTC (Fig. 9). A strong bow echo developed south of Bellefonte and moved eastward producing wind damage along its path south of Howard (pink winds at 0205 UTC). An outflow boundary pushed south from this feature producing a boundary near and south of State College. This boundary served to focus the development of new convective elements which moved up the quasi-stationary line. Rapid looping showed several interesting features just moving over and just west of Bellefonte. These features are worthy of deeper investigation though using static images a few key features can be addressed. A key feature in Fig. 9c is the area of divergence south of Bellefonte with a small region of inbounds (green) west of the outbounds (red) and the broader area of converge along the zero isodop from south of State College northward from State College through Centre Hall which becomes less descript close to Howard. The heaviest rainfall fell over the dome of outflow just west of the road (Route 220). b. GEFS forecasts

4 The GEFS was able to forecast the surge of high PW air into Pennsylvania and New York and it properly depicted a stalled frontal boundary for several periods. Thus the GEFS was able to forecast heavy rainfall in Pennsylvania in New York. However, the model forecast the higher QPF amounts north and west of the verifying position. The rainfall was focused too deep into the cold air and of course the model grossly under forecast the higher end QPF amounts (Figs. 10 & 11). The 24-hour GEFS QPF forecasts of 50 mm or more QPF show the large swath of 50 mm or more QPF in northern Pennsylvania into New York (Fig. 10). The mean forecasts and each member s 50 mm contour showed forecast from some members in excess of 75mm of QPF (not shown). Most of the QPF was forecast to fall in a 12 hour window from UTC 21 October 2016 and the ensemble mean and each member s forecasts of 50 mm or more QPF is shown in Figure 11. Though not shown, the highest QPF amounts and rates were actually focused in the period from 0600 to 1200 UTC. The GEFS PW forecasts are shown in Figure 12. These data show the frontal boundary and the plume of warm moist air east of this boundary. An examination of the 0600 UTC data showed that the GEFS stalled the boundary out, thus the low QPF amounts in southeastern Pennsylvania. The GEFS also (not shown) had a strong southerly jet at 850 hpa in the warm air and at least 2 implied waves along the front. Given the QPF shield some of this larger scale forcing should be self-evident. c. GFS forecasts For brevity 6 comparative GFS QPF forecasts are presented in Figure 13. These data are focused on the period of heaviest rainfall which over Pennsylvania was in the 0000 to 1200 UTC window on 21 October Similar to the GEFS the GFS showed the higher QPF amounts west of the observed location with the heaviest rainfall in New York State. Shorter range forecasts showed enhanced QPF in central Pennsylvania with an axis of 50 mm extending into central Pennsylvania. The pattern in the GFS was similar to the observed pattern and the pattern forecast in the GEFS and is not shown. d. Convective allowing model (CAMS) forecasts the 3km HRRR The NCEP 3km HRRR QPFs from 6 select forecast cycles are shown in Figure 14. These data show that the HRRR forecast a generally intense and narrow swath of high QPF across central Pennsylvania. The axis of heaviest rainfall was west of the verifying axis of heavy precipitation. But the axis was also east of the axis of heavy rainfall produced by the GEFS/GFS forecasts. Another interesting observation in the HRRR was the decrease in the QPF in the 2 more recent forecasts. This implies that the HRRR has a spin-up issue related to QPF. The HRRR simulated radar from 6 HRRR forecast cycles valid at 0400 UTC (Fig. 15) and 16 times from 0000 to 0500 UTC 21 October 2016 (Fig. 16) from the 2200 UTC 20 October HRRR show the band of intense convection in central Pennsylvania into New York. These forecasts

5 show the dearth of returns in southeastern Pennsylvania and Maryland. These radar and QPFs also show the less intense QPF farther west deep in the cold air as implied by the NCEP GFS and GEFS. e. Convective allowing model ensembles The GSD HRRR 1 time lagged ensemble (HRRR-TLE) showing the probability of 3 inches or more QPF for the 6 hour period ending at 0600 UTC 21 October (Fig. 17) showed a high probability of over 3 inches of QPF in north-central Pennsylvania into western New York. These forecasts relative to observations were just a bit to the north and west of the verifying QPF. Useful guidance and in line with the individual runs shown in Figures The 3km NCAR ensemble showed a high potential for over 6 inches of rainfall (Fig. 18) from the forecasts initialized at 0000 UTC 20 October However the accumulations over 6 inches required an additional 6 hours of accumulation time to reach the threshold. The 3 inch probabilities were even higher but suffered the same temporal issues. The 0000 UTC 21 October 2016 run of the NCAR EFS is shown in Figure 19. These data show the lower probabilities and a displacement to the north. These data would not have been available by 0600 UTC though they provide little additional information. Like the HRRR these forecasts suffered from a spin-up error missing the heavy rainfall prior to 0600 UTC. 4. Conclusions A slow moving frontal system produced heavy rainfall in Mid-Atlantic region (Fig. 1) on 21 October The heaviest rain fell over central Pennsylvania over a 2-4 hour window. The intense rainfall produced considerable damage to infrastructure to include roads and bridges. The Stage-IV data showed areas of around 10 inches of rainfall. These data compared to the 6- hour 100 year average recurrence interval (ARI) showed rainfall rates from 0000 to 0600 UTC 21 October 2016 at 100 to 175% of the 100 year 6-hour ARI (Fig. 2). The 24-hour ARI showed two elongated southwest to northeast bands of over 4 inches of rainfall with 100 to 150% of the 24-hour 100 year ARI. The intense short duration rainfall led to flash flooding in rural areas of central Pennsylvania. This event impacted mainly rural areas. Thus despite the intense rainfall rates and the high rainfall amounts the event did not gain National attention and the flooding only damaged property and infrastructure. Thus, despite higher amounts and higher intensity of the rainfall rates, this event was not as well-known as the Ellicott City, Maryland event. The key feature of this event was the sharp south-to-north frontal boundary. This boundary had +2 to +3σ PW values on the warm side of the boundary co-located with strong southerly flow and convective instability. The deep plume of moisture and the waves along the stalled the frontal boundary in central Pennsylvania and New York. Locally in central Pennsylvania radar showed the north south line of intense convection and a Mesohigh with a stalled outflow 1 During this time the base GSD HRRR was the same as the NCEP HRRR. However GSD makes its own runs and uses them in the HRRR-TLE (Time lagged ensemble).

6 boundary which allowed fresh convection to move over the same region for about 2-3 hours. This intense convection led to the localized heavy rainfall along a southwest to northeast axis. The NCEP GEFS and GFS were able to forecast the pattern relatively well. However, both systems forecast the heavy rainfall too far north and west of the verifying location. The higher resolution GFS had higher QPF than the GEFS and the shorter range forecasts actually attempted to depict convectively enhanced rainfall in central Pennsylvania (Fig. 13e & f). Despite the concept of a heavy rainfall event and the stalled frontal boundary, neither system could get the timing or location of the heavy rainfall correct. Both systems had the higher QPF amounts too deep into the cold air. Similar to the NCAR EFS the GFS and GEFS peak rainfall period in central Pennsylvania was actually after the period of heaviest rain. This event is a good test of the value of the HRRR in forecasting heavy rainfall and flash flooding. The HRRR had some encouraging signals including correctly placing the heavy rainfall farther to the east in the warm air where the heaviest rain was observed. However the HRRR still had some location and intensity issues. Interestingly, the shorter range HRRR forecasts appeared to reduce the total QPF implying some kind of spin-up issues in the model. Despite the use of radar and proxies for latent heat the HRRR clearly had signals implying a rainfall spin-up issue. The HRRR-TLE showed some promise in forecasting the potential threat for 3 or more inches of QPF. Shorter range HRRR-TLE forecasts (not shown) had issues related to QPF spin-up issues associated with HRRR cycles issued closer to when the rain began. The NCAR 3km ensemble issued at 0000 UTC 20 October had the best forecasts of the potential for both 3 and 6 inches of QPF. Using the neighborhood probabilities within a 25-mile radius the NCAR EFS showed the high risk for over 3 inches of rainfall and reasonable risk for over 6 inches of QPF. However, the model produced the most QPF in the 6-hour period after most of the rain had fallen. The 0000 UTC 21 October NCAR EFS had considerably lower QPF implying a reduced threat. However, these data were not available in real-time seem to imply a QPF spin-up issue in the NCAR ensemble. It would have been interesting to see a 1200 UTC 20 October 2016 NCAR ensemble forecast. The data shown here imply that larger scale models and ensembles have some useful clues related to heavy rainfall. However, they appear unable to get the location and they appear biased toward producing too much rainfall too far into the cold air. They do well with the general pattern by not on the details. Temporal and spatial errors are quite large. The high resolution data appear to better simulate convection and thus produce more QPF in the warm air. Both the HRRR and the NCAR ensemble had more realistic amounts and shown smaller spatial errors relative to the GFS and GEFS. However, the NCAR EFS suffered similar temporal issues as those in the GFS/GEFS with the period of heavy rainfall forecast 1-6 hours after the actual 6-hour period of heaviest rainfall. Both the NCAR EFS and HRRR showed some issues spinning up QPF. Thus the QPF values dropped off as the forecast lengths decreased. Something operational users of these data should be mindful of. A drop in QPF near onset time may be real or may be a model spin-up issue.

7 Comparing the rainfall with this event relative to the Ellicott City event suggests that where it rain matters. Urban areas sustain more damage from lower rainfall amounts and due to the population densities they get more attention in the media than rural events. These raises questions as to how many extreme rainfall events occur in rural areas which are never known and thus studied? 5. Acknowledgements The Pennsylvania State University for real-time data access. Matt Pyle (NCEP/EMC) for HREF forecasts in image format, Curtiss Alexander for information on the GSD HRRR and facilitating use of the GSD HRRR-TLE website. Thanks to NCAR for access to their high resolution ensemble. The Albany MAP and Lance Bosart for insights related to heavy rainfall events 6. References Benjamin, S.G, and contributors: 2016: A North American Hourly Assimilation and Model Forecast Cycle: The Rapid Refresh. MWR,144, CNN, 2016: West Virginia floods devastate 1,200 homes, many lives, 28 June 2016 and similar stories. Ebert, E. E., 2001: Ability of a poor man s ensemble to predict the probability and distribution of precipitation. Mon. Wea. Rev., 129, Galarneau, TJ,Jr. and LF Bosart: 2006: Ridge Roller: Mesoscale disturbances on the periphery of cutoff anticyclones. Preprint, Symp. on Challenges of Severe Convective storms, Atlanta, GA, American Meteor Soc., P1.11. Galarneau, T. J., Jr., L. F. Bosart, and A. R. Aiyyer, 2008: Closed anticyclones of the subtropics and midlatitudes: A 54-yr climatology ( ) and three case studies. Synoptic Dynamic Meteorology and Weather Analysis and Forecasting: A Tribute to Fred Sanders, Meteor. Monogr., No. 55, Amer. Meteor. Soc., Juanzhen Sun, Ming Xue, James W. Wilson, Isztar Zawadzki, Sue P. Ballard, Jeanette Onvlee- Hooimeyer, Paul Joe, Dale M. Barker, Ping-Wah Li, Brian Golding, Mei Xu, and James Pinto, 2014: Use of NWP for Nowcasting Convective Precipitation: Recent Progress and Challenges. Bull. Amer. Meteor. Soc., 95, Maddox,R. A,, C. F. Chappell, and L. R. Hoxit, 1979: Synoptic and Meso-α Scale Aspects of Flash Flood Events, Bull Amer. Meteor. Soc., 60, , DOI: Doswell, C.A, H. E. Brooks, and R.A. Maddox, 1996: Flash Flood Forecasting: An Ingredients-Based Methodology, Wea. Forecasting, 11, :DOI:

8 WP 2016: West Virginia flood was one in a thousand year event, Weather Service says; more heavy rain forecast. Washington Post June Weaver, S. C., and S. Nigam, 2008: Variability of the Great Plains low level jet: Large scale circulation context and hydroclimate impacts. J. Climate,21,

9 b Figure 1. Total observed estimated rainfall (QPE) for the period of 2100 UTC 20 October 2016 through 0000 UTC 22 October Lower panel is a zoomed in view with contours every 25 mm to accent the extreme rainfall amounts. Return to text.

10 Figure 2. The 6- and 24-hour Stage-IV rainfall compared to the 6-hr and 24-hour 100 year ARI data. Contours are rainfall in inches and shading is the ratio of the ARI to the QPE in the respective interval expressed as a percentage. Return to text.

11 Figure 3. The discharge at a point on Bald Eagle Creek showing the rapid rise due to the extreme rainfall along the Creek on October A point along this Creek observed over 10 inches of rainfall. Return to text.

12 Figure 4. CFSR analysis of a) 500 hpa heights (m), b) 850 hpa temperatures (C ), c) precipitable water (mm), and d) mean sea-level pressure (hpa). Each field is also shown with the standardized anomalies associated with each field. The black dot is the approximate location of State College, PA. Return to text.

13 Figure 5. As in Figure 4 except for valid at 0600 UTC. Return to text.

14 Figure 6. NCEP 3km HRRR-V2 00-hour forecasts of precipitable water (mm) and precipitable water anomalies in 3-hour increments from a) 0000 UTC 21 to f) 1500 UTC 21 October Return to text.

15 Figure 7. As in Figure 6 except for HRRR mean-sea level pressure (hpa) and anomalies. Return to text.

16 Figure 8. KCCX radar showing the echoes during the heavy rainfall at 0158 UTC and as the bands were moving to the east at 0400 UTC. Return to text.

17 Figure 9a. KCCX radar at 0128 UTC showing the base reflectivity (right) and base velocity (left). Key features relate to the text. The yellow arrow is the bow echo which would move east and produce multiple reports of severe weather south of Lock Haven. The RIN can be seen in the reflectivity. The dashed line shows the stalled larger scale boundary shown in Figure 8. Return to text.

18 Figure 9b. As in 9 except for 0205 UTC. The RIN can be seen near the red highway which is I80 and the arrow shows the strong outflow with the strongest outbounds (pink) just ahead of the arrow. The line has not moved much and the outflow boundary is evident in the zero isodop near and south of State College. A red arrow denotes the southern extent of this feature.. Return to text.

19 C Figure 9c. As in 9b except for 0253 UTC and the depiction of the broader feature showing divergence near Bellefonte and convergence to the south and east. Return to text.

20 Figure 10. GEFS forecasts of 24 hour QPF of equal to or greater than 50 mm in the 24 hour period ending at 0000 UTC 22 October Forecasts initialized at a) 1200 UTC 18 October, b) 0000 UTC 19 October, c) 1200 UTC 19 October, d) 1200 UTC 20 October, e) 0000 UTC 20 October, e) 1200 UTC 20 October, and f) 1800 UTC 20 October Return to text.

21 Figure 11. As in Figure 11 except for ensemble mean and each members 50 mm contour for the 12 hour period ending at 1200 UTC 22 October Return to text.

22 Figure 12. As in Figure 11 except for the GEFS forecasts of precipitable water and the precipitable water anomalies valid at 0000 UTC 21 October Return to text.

23 Figure 13. NCEP GFS forecasts of QPF for the 12 hour window from 0000 to 1200 UTC 21 October GFS forecasts initialized at a) 1200 UTC 18 October, b) 0000 UTC 19 October, c) 1200 UTC 19 October, d) 1200 UTC 20 October, e) 0000 UTC 20 October, e) 1200 UTC 20 October, and f) 1800 UTC 20 October Return to text.

24 Figure 14. NCEP 3km HRRR QPFs for the period ending 0600 UTC 21 October HRRR forecasts issued at every hour from a) 1800 to f) 2300 UTC 20 October Values in mm as per the color bar and contours show 75, 100, and 150 mm values. Return to text.

25 Figure 15. As in Figure 14 except for HRRR simulated reflectivity valid at 0400 UTC 21 October. Return to text.

26 Figure 16. HRRR simulated radar from the 2100 UTC valid in 1 hour increments a) 0000 UTC through b) 0500 UTC 21 October Return to text.

27 Figure 17. GSD HRRR-time lagged ensemble (HRRR-TLE) showing the probability of greater than 3 inches of rainfall in the 6-hour period ending at 0600 UTC 21 October Return to text.

28 Figure 18. NCAR 3km ensemble forecasts of the probability of 6 and 3 inches of rainfall over the entire forecast period ending at 1200 UTC 21 October EFS initialized at 0000 UTC 20 October Return to text.

29 Figure 19. As in Figure 18 except for the 0000 UTC 21 October 2016 NCAR EFS showing the probability of 3 inches or more QPF for the total time of the forecasts ending at 0600 and 1200 UTC. Return to text.

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