Orographically enhanced heavy rainfall of 23 May 2010 By Richard H. Grumm National Weather Service Office State College, PA 16803

Transcription

1 Orographically enhanced heavy rainfall of 23 May 2010 By Richard H. Grumm National Weather Service Office State College, PA INTRODUCTION During the overnight hours and into the early morning of 23 May 2010 a north south oriented line of showers brought heavy rain from northern Virginia into southern Pennsylvania. Two areas of precipitation affected Pennsylvania. The precipitation was focused along to generally southwest to northeast geographically features, Blue Mountain and South Mountain. The heaviest rain Figure 1. The unified precipitation data showing total rainfall from 1200 UTC 22 to 1200 UTC 23 May 2010.

2 fell in Pennsylvania fell over the westernmost features, Blue Mountain. One report of 6 inches of rainfall was reported near St Thomas near Blue Mountain. Figure 1 shows the unified precipitation data (UPD) analysis of rainfall for the event. These relatively course data do not represent the overall distribution of the rainfall well. These data are heavily biased toward the coarse National Weather Service (NWS) COOP sites and often fail to capture unique events such as this. Higher resolution precipitation data (Fig. 2) will be shown to illustrate the limitations of the UPD. The UPD is good for a first guess and for synoptically forced events. It is severely limited in value when truly mesoscale events occur. Even the higher resolution data shown in Figure 2 show only a few areas of 3-4 inches of rainfall where there were 2 reports of 5-6 inches of rainfall. This locally heavy rainfall produced flash flooding and flooding along streams and creeks in southern Pennsylvania and Maryland. As shown in Figure 2, the heavy rainfall fell over a limited area. Forecasts of this event from the larger scale models and ensemble forecast systems grossly underestimated the rainfall produced by this event. Not surprisingly, finer resolution models, such as the NCEP 4km NMM did remarkably well focusing the potential for additional rainfall focused near the terrain features. This case may illustrate the value of higher resolution models at shorter forecast ranges and the limitations of larger scale systems to forecast truly mesoscale events. Figure 2. As in Figure 1 except for high resolution mult-sensor rainfall data for the 24 hour period ending at 1200 UTC 23 May 2010.

3 This event shared many characteristics of a subtle heavy rainfall event (SHARS:Spayd and Scofield 1983). The shallow cool air and frontal boundary, combined with flow into the mountains were characteristics that this event shared in common with the destructive Madison County Virginia Flood of 27 June 1995 (Pontrelli et al. 1999). In this event and the Madison County event, upslope flow played a dominant role in the flooding. This paper will attempt to document the heavy rainfall and flooding event of 23 May Focus is on the pattern and short-term forecasting issues. A mix of model, radar, and satellite data is used to portray this truly mesoscale event. 2. METHODS Reports on the flooding and heavy rainfall amounts were retrieved from offices local storm reports (LSR) which are both collected and disseminated in real-time. These data are verified and quality controlled by the local NWS office. The pattern was reconstructed used the NCEP GFS and NAM and were possible the JMA 1.25x1.25 data (Onogi et al. 2007). All data were plotted in GrADS (Doty and Kinter 1995). The severe weather data was overlaid on the JRA data. The higher resolution NCEP NAM is used to show the conditions during the event. The anomalies were computed from the NCEP/NCAR re-analysis data (Kalnay et al 1996) as describe by Hart and Grumm 2001 and Grumm and Hart Unless otherwise stated, the base data was the NAM and the means and standard deviations were computed by comparing the NAM to the NCEP/NCAR 30-year climatological values. Radar data and satellite data were retrieved from the NWS-State College AWIPS system.. For brevity times are referred to in the format of 23/0600 for 0600 UTC 23 May 2300 UTC. 3. RESULTS i. The pattern The 500 hpa height and height anomalies are shown in Figure 3. The dominant large scale features over eastern North America was the building 500 hpa ridge with positive height anomalies. Between 22/1800 and 23/0600 UTC a short wave trough moving through the large scale ridge was evident over the Mid-Atlantic region. The conditions over the Mid-Atlantic region (Figs. 4-8) show a surge of modestly high precipitable water (PW) with PW anomalies of +1SD above normal, around 36mm in value, moving over the affected region. The PW feature became more pronounced when it moved offshore (Fig. 4a). The surface pressure field (Fig. 5) showed a strong anticyclone offshore and a surface cyclone well to the west. This latter feature was of no impact to the region. The anticyclone implied shallow cold air damming and low-level southeasterly flow over the region, which was evident at 850 hpa (Fig. 6). The 850 hpa u-winds showed -1 to -2SD

4 easterly wind anomalies. These data implied enhanced low-level flow into the terrain. The cold air damming implied in Figure 4 suggested that the air was stable and the terrain impacts were likely increased due to the increased static stability. showers and rainfall over southern Pennsylvania in close proximity to where the locally heavy rainfall developed. However, the simulated radar also showed strong echoes and thus the potential for heavy rainfall in northern New Jersey which did not verify. ii. Forecasts iii. Satellite and radar echoes Figures 7 & 8 show the NCEP GFS and NAM QPFs valid at 23/1200 UTC. The GFS with effective resolution in the 35 km range varied potential for 16 to 32mm of QPF (Fig 7a-f). The shorter range forecasts began to focus the QPF with the upslope flow along the Appalachian mountains (Figs. 7d-e) though the GFS under forecast the maximum QPF and had the rainfall over a broad area, reflecting the horizontal resolution issues and the inability to sense the varied terrain. The NAM QPFs (Fig.8) showed more focused forecasts linked to the terrain. However, these data are 12km NAM forecasts posted on a coarse 40km grid. These data suggest that the NAM under forecast the potential heavier rainfall near the key terrain features. The NCEP NAM4km is shown in Figure 9. These data show the QPF in 3-hourly increments. At 4km resolution, the NAM could focus the rainfall on the terrain features and in fact did so quite well relative to the GFS and NAM. Though the focus along the mountains of southern Pennsylvania looked good, the heavy rainfall in New Jersey was not so good. The hourly reflectivity from the NAM4km is shown in Figure 10. These data show the evolution of the potential Figures 11 &12 show the cold cloud tops aligned along a north-south axis. These IR images showed enhanced elements with convection in them over Virginia which than moved northward. There was little if any lightning associated with rainfall over southern Pennsylvania. The enhanced elements move from south-to-north as did the radar elements (Figs ). The N-S motion suggested training and repeat echoes moving over the region where the heavy rain was observed. Clearly, the high cloud produced by the system streamed farther north than did the deep clouds and rainfall. The cloud tops began to warm markedly after 0800 UTC. As shown in Figure 12, by 1025 UTC there were relatively warm cloud tops over the area where the rain was still falling. At 23/0009 UTC the radar showed (Fig. 13-upper) what appeared to be a mesoscale circulation over Somerset county and rain bands feeding into this feature. Franklin had received only light rainfall and the STP showed that no rain had yet fallen over Adams County. Three hours later, 23/0303 UTC the vortex was over northern Huntington County and north-south rainbands were over Franklin County. Adams County was still relatively rain free (Fig. 13- lower).

5 The radar showed widespread rainfall over Adams County by 0549 UTC (Fig. 14-upper) and enhanced echoes over Franklin County. Radar rainfall estimates showed 2-3 inches of rainfall over portions of Franklin County by this time. A flood advisory was issued about this time due to the rainfall and training. By 0648 UTC the echoes appeared less organized on radar. But bands to the south and the cold cloud tops over Maryland implied more organized rainfall could move over the region. Which did occur, and by 1016 UTC there was still some enhanced rain over the mountains of western Franklin County and radar rainfall estimates showed 3-4 inches of rain (yellows) and some inch amounts (dark orangeyellow). About within a few minutes of the time of this image a spotter from St. Thomas called in with around 6 inches of rainfall. 5. Acknowledgements. 6. References Doty, B. E., and J. L. Kinter III, 1995: Geophysical data and visualization using GrADS. Visualization Techniques Space and Atmospheric Sciences, E. P. Szuszczewicz and Bredekamp, Eds., NASA, Doty, B. E., and J. L. Kinter III, 1995: Geophysical data and visualization using GrADS. Visualization Techniques Space and Atmospheric Sciences, E. P. Szuszczewicz and Bredekamp, Eds., NASA, Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. and Fore., 16, CONCLUSIONS

6 Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, Junker, N. W., R. H. Grumm, R. Hart, L. F. Bosart, K. M. Bell, and F. J. Pereira, 2008: Use of standardized anomaly fields to anticipate extreme rainfall in the mountains of northern California. Wea. Forecasting,23, Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40- Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, Pontrelli, M. D., G. Bryan, and J. M. Fritsch, 1999:The Madison County, VA Flash Figure 3. NAM 00-hour forecasts of 500 hpa heights (m) and height anomalies (standard deviations) valid at a) 1200 UTC 22 May, b) 1800 UTC 22 May, c) 0000 UTC 23 May, d) 0600 UTC 23 May, e) 1200 UTC 23 May and f) 1800 UTC 23 May Flood of 27 June Wea.

7 Forecasting, 14, Onogi, K., J. Tsutsui, H. Koide, M. Sakamoto, S. Kobayashi, H. Hatsushika, T. Matsumoto, N. Yamazaki, H. Kamahori, K. Takahashi, S. Kadokura, K. Wada, K. Kato, R. Oyama, T. Ose, N. Mannoji and R. Taira (2007) : The JRA-25 Reanalysis. J. Meteor. Soc. Japan,85, Lin, Y. and K. E. Mitchell, 2005: The NCEP Stage II/IV hourly precipitation analyses: development and applications. Preprints, 19th Conf. on Hydrology, American Meteorological Society, San Diego, CA, 9-13 January 2005, Paper 1.2. Spayd L.E. Jr. and R.A. Scofield, 1983: Operationally Detecting Flash Flood Producing Thunderstorms which have Subtle Heavy Rainfall Signatures in GOES Imagery. Preprints, 5 th Conf. on Hydrometeorology, Tulsa, OK,

8 Figure 4. As in Figure 3 except precipitable water (mm) and precipitable water anomalies.

9 Figure 5. As in Figure 3 except for mean sea level pressure (hpa) and pressure anomalies.

10 Figure 6. As in Figure 3 except for 850 hpa winds (kts) and u-wind anomalies.

11 Figure 6. As in Figure 5 except showing v-wind anomalies.

12 Figure 7. NCEP GFS forecasts of QPF (mm) valid at 1200 UTC 23 May 2010 from forecasts initialized at a) 0000 UTC 22 May, b) 0600 UTC 22 May, c) 1200 UTC 22 May, d) 1800 UTC 22 May 2010, e) 0000 UTC 23 May 2010 and f) 0600 UTC 23 May

13 Figure 8. As in Figure 7 except for NAM QPF.

14 Figure 9. NAM4km QPF from the 0000 UTC 23 May run showing total QPF (mm) in 3-hour increments.

15 Figure 10. As in Figure 9 but showing NAM4km run showing model simulated radar (dbz) valid at a) 0100 UTC, b) 0200, c) 0300, d) 0400, e) 0500, f) 0600 UTC 23 May 2010.

16 Figure 11. GOES IR images with surface observations focused over Pennsylvania at (left) 0425 and (right) 0625 UTC 23 May 2010.

17 Figure 12. As in Figure 11 except fo 0725 and 1025 UTC.

18 Figure 13. KCCX radar showing composite reflectivity and storm total rainfall valid at (upper) 0009 UTC and (lower) 0309 UTC. Rainfall is in inches.

19 Figure 14. As in Figure 13 except for valid 0549 and 0648 UTC

20 Figure 15. As in Figure 13 except valid at 1016 UTC.

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