National Weather Service-Pennsylvania State University Weather Events

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1 National Weather Service-Pennsylvania State University Weather Events Abstract: Tropical Storm Debbie, standardized anomalies and heavy rainfall by Richard H. Grumm National Weather Service State College PA Heavy rainfall impacted the State of Florida from June 2012 as the circulation associated with tropical storm Debby lumbered up the West Coast of Florida. Rain from the system affected southern Florida on the 23 rd moving into central Florida on the 24 th. Rainfall rates in excess of 96mm in 6 hours affected central and northern Florida from 1800 UTC on 24 through 1200 UTC on 26 June The heaviest rain fell in the Florida panhandle on strong easterly winds. Rainfall in the Florida panhandle exceed 300 mm and there over 250 mm of rain fell between 1800 UTC 25 to 0000 UTC 26 June This rainfall event was an excellent example on the use of standardized anomalies to identify the potential for heavy rainfall. Forecasts from NCEP models and ensemble prediction systems showed the potential for heavy rainfall in the quantitative precipitation forecasts. These systems also showed the surge of high precipitable water air into the threat regions along with above normal wind anomalies. The signals in the mass fields reinforced the information contained in the model quantitative precipitation forecasts.

2 1. INTRODUCTION The flow around tropical storm Debby brought heavy rainfall to Florida and extreme southern Georgia from 23 to 26 June 2012 (Fig. 1). The heaviest rain was observed in a quasi-east west band which extended across northern Florida and into the Florida panhandle. Total rainfalls within the band exceeded 400 mm (16 inches). The extremely heavy rainfall in northern Florida fell between 1200 UTC 25 June through 1200 UTC 26 June 2012 (not shown) in strong easterly flow north of Debby (Fig. 2). At the height of the event the stage-iv estimates indicated up to 250 mm of rainfall in 6- hours. It is important to document the pattern associated with all high impact weather events (HIWE). The patterns in which HIWE occur often have signals in which key parameters depart significantly from climatology. Standardized anomalies (Hart and Grumm 2001;Grumm and Hart 2001) can aid in visualizing regions were key fields depart significantly from normal. This technique can be employed using re-analysis data (Grumm 2011; Grumm and Hart 2001;Graham and Grumm 2010;Junker et. al. 2008; Grumm 2011a,Grumm 2011b;Galarneau 2012;Durkee 2012;) in a case study mode or to gain insights from an event. The technique can also be applied to model and ensemble guidance output (Junker et al. 2009; Stuart and Grumm 2006; Grumm 2011a;Grumm 2011b). Previous studies related to heavy rainfall (Grumm 2011b;Junker et al 2008;Junker et al. 2009) have shown the strong low-level southerly winds, precipitable water, moisture flux all aid in identifying heavy rainfall pattern. These patterns and the anomalies of the critical fields when used together with the probabilities of heavy rainfall can aid in forecasting high end and extreme heavy rainfall events. The use of the anomalies in the synoptic pattern and the probabilities in an ensemble forecast system (EFS) can aid in better predicting extreme rainfall events. This paper will document the pattern and anomalies associated with the Florida heavy rainfall event of June The focus is on using the synoptic pattern and anomalies to classify the event and the probabilities from EFS to better predict future high impact heavy rainfall events. 2. Methods and Data The NCEP GFS is used to re-produce the conditions associated with the event including large scale pattern. The pattern and standardized anomalies followed the methods outlined in Hart and Grumm (2001) and the GFS 00-hour forecasts were used to establish the pattern and standardized anomalies. The term R-Climate is used in reference to the use re-analysis climate data to diagnose or forecast the departures from normal. Forecasts of quantitative precipitation were limited to those produced by the 27.5km NCEP GFS and GEFS. In addition to the QPFs, the synoptic pattern and probabilities of key anomalies within the pattern were derived using the climatic dataset and the forecast system output.

3 The rainfall plots were from the gridded stage-iv data. The 6-hourly data were used to produce the images shown. These data were summed over 6-,12-,24- and storm total time periods to arrive at estimates of the precipitation and to identify periods of extreme rainfall. For brevity times are presented as day and hour in the format 25/1200 UTC and 26/0000 UTC which would be 1200 UTC 25 June and 0000 UTC 26 June Results i. The large and regional pattern The large scale pattern, centered over the western Atlantic in 24 hour increments from 0000 UTC June 2012 (Fig. 3) shows the evolution of the weak upper-level height anomaly west of Cuba on 22/0000 UTC. This feature, a weak sub-tropical upper tropospheric trough appears as a weakness between two larger ridges. The western ridge was associated with the North American heat wave of June 2012 and the weaker eastern ridge was the subtropical ridge over the Atlantic basin. The surge of high precipitable water air into the Caribbean and the eastern Gulf of Mexico was associated with the general easterly flow beneath and on the western flanks of the Atlantic subtropical ridge (Fig. 4). A strong short-wave and cold frontal system moved over the eastern United States on 25 June 2012 and the negative height anomalies with this system interacted with the height anomalies with the tropical system (Fig. 3e). At low-levels came over the ridge a frontal system moved into the Gulf States and northern Florida. This is implied in the precipitable (PW) field (Fig. 5). The PW values ranged from 55 to 60 mm when impinged upon Florida and the PW anomalies were on the order of 3 to 5σ above normal. The high PW values combined with the strong flow about Debby produced both 925 and 850 hpa moisture flux values over 240 g m kg-1s-1. This translated into 6σ moisture flux anomalies over the region. These data show that a key parameters, including above normal moisture, strong winds, and high moisture flux impacted Florida resulting in an extreme rainfall event. Other features, such as the mean-sea level pressure fields indicated pressure anomalies in the -6σ range. ii. Forecasts The GEFS QPFS showing the probability of 100 mm or more QPF in 48 hours are shown in Figures 7 & 8. The forecasts from 1200 UTC 20, 21 and 22 June (Fig. 7) showed the higher potential for heavy rainfall along the Florida coast line. The system generated an odd dipole looking pattern, with some potential for heavy rainfall over the peninsula, well south of where the heavier rainfall was observed. The forecasts from 23/0000, 23/1200 and 24/0000 UTC (Fig. 8) did not show the dipole like pattern and converged on forecast showing an northern east-west axis of heavy rainfall and a more north-south oriented axis of heavy rainfall along the West Coast of Florida. Though difficult to see, the ensemble means QPF was over 150 mm in the band of higher QPF. The SREF QPFs are shown closer to the event and focus over the period of extreme rainfall (Figs. 9 &10). The forecasts from the 23/0900, 23/1500 and 24/0300 UT

4 SREF (Fig. 9) showed a similar to the GEFS, the SREF tended to produce the higher probability of the higher QPFs along the coastal regions of Florida. Shorter range forecasts from 24/0900, 24/1500 and 24/2100 UTC showed improved forecasts related to the axis of heavy rainfall and closer to the location in northern Florida. These later SREF cycles also showed the potential for over 150 mm of QPF (Fig. 11). The experimental 16km SREF (Fig. 12) produced a similar QPF field relative to the operational pseudo 32km SREF. However, it produced a higher probability of over 100 mm and moved the axis of heavy rainfall deeper into the Florida panhandle. Additionally, it had a higher probability of 150mm of QPF (Fig. 13) and had several members predicting in excess of 200 mm of QPF in the 36 hour period (Fig. 14). For comparison purposes 4 NCEP GFS QPFs are provided (Fig. 15). The GFS was used to determine the larger scale pattern and its forecasts got the concept of the general pattern relatively well. However GFS too had difficulty with the location of the higher QPFs. Longer range forecast (Fig. 15a) had the rainfall too far south over the Florida peninsula. It was a slow evolution to bring the QPF shield to the north. The 48 hour accumulations showed the same general trends (Fig. 16). iii. Observations The accumulated rainfall in 6-hour increments on for 24, 25 and 26 June are shown in Figures These data show the rain moving up the West Coast of Florida and the heavier bands which impacted western and then the Florida panhandle on 25 June. The purple colors are in excess of 96 mm of precipitation in 6-hours. The heaviest rainfall was between 1800 UTC 25 and 0000 UTC 26 June where over 256mm of rainfall was estimated in the Florida panhandle. 4. Conclusions Tropical storm Debby developed on the western edges of the subtropical anticyclone in the Atlantic. It was associated with a plume of above normal PW which moved over the Caribbean then up the West Coast of Florida. This relatively modest storm produced extremely heavy rainfall in Florida with well over 300mm of rainfall in the Florida panhandle. The fields typically associated with heavy rainfall were present in the NCEP models and model analysis fields. PW, low-level winds and moisture flux values were all well over 3s above normal. Signals often associated with heavy rainfall events. The moisture flux standardized anomalies have been shown to be a good indicator of heavy rainfall (Bodner 2011). Bodner (2012) showed composites of moisture flux over the central United States during two periods of record rainfall in June-July 1993 and June of This event had incredible moisture flux values, well over 6s above normal. Due to the high moisture and well predicted pattern the NCEP models and ensemble prediction systems indicated the potential for a heavy rainfall event. However, initial forecasts were too far south, indicative of uncertainty with the location of the forcing and movement of tropical storm Debby. The GEFS also had issues with where the rain would fall, keeping most of it off the Florida coasts, creating an odd dipole like pattern to the QPFs. Producing heavy rain over Florida was difficult relative to the QPF produced over the oceanic regions.

5 This case had considerable uncertainty and the use of probabilistic tools worked well along with the pattern alerting forecasters to the potential for heavy rainfall. Due to the uncertainty the lead-time for locations was quite elusive. However the signals for heavy rainfall were well in place. On a brighter note, the 16km SREF showed some promise in its ability to predict higher end QPF amounts, over 150 and 200mm that the 32km SREF has less ability to produce. 5. Acknowledgements NCEP for access to the 16km SREF in near real-time. 6. References Bodner, Michael J., Norman W. Junker, Richard H. Grumm and Russ S. Schumacher, 2011: Comparison of Atmospheric Circulation Patterns during the 2008 and 1993 Historic Midwest Floods. Natl. Wea. Dig., 35:2, Durkee, JD., L Campbell, K Berry, D Jordan, G Goodrich, R Mahmood, and S Foster, 2012: A Synoptic Perspective of the Record 1-2 May 2010 Mid-South Heavy Precipitation Event. Bull. Amer. Meteor. Soc., 93, doi: Galarneau T.J. Jr., TM. Hamill, RM. Dole, J. Perlwitz. (2012) A Multiscale Analysis of the Extreme Weather Events over Western Russia and Northern Pakistan during July Monthly Weather Review 140:5, Graham, Randall A., Richard H. Grumm, 2010: Utilizing Normalized Anomalies to Assess Synoptic-Scale Weather Events in the Western United States. Wea. Forecasting, 25, Grumm, Richard H., 2011a: The Central European and Russian Heat Event of July August Bull. Amer. Meteor. Soc., 92, doi: Grumm, R.H. 2011b: New England Record Maker rain event of March NWA,Electronic Journal of Operational Meteorology,EJ4. Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. and Fore., 16, 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, M.J.Brennan, F. Pereira,M.J.Bodner,and R.H. Grumm, 2009:Assessing the Potential for Rare Precipitation Events with Standardized Anomalies and Ensemble Guidance at the Hydrometeorological Prediction Center. Bulletin of the American Meteorological Society,4 Article: pp 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, Lalaurette, F. 2003: Early Detection of abnormal weather conditions using a probabilistic extreme forecast index. QJRMS,129, Lau, W., K. M., K-M Kim, 2012: The 2010 Pakistan Flood and Russian Heat Wave: Teleconnection of Hydrometeorological Extremes. J. Hydrometeor, 13, doi:

6 Neiman, P. J., F. M. Ralph, G. A. Wick, J. D. Lundquist, and M. D. Dettinger, 2008: Meteorological characteristics and overland precipitation impacts of atmospheric rivers affecting the west coast of NorthAmerica based on eight years of SSMI/satellite observations. J. Hydrometeor., 9, Neiman, P. J., F. M. Ralph, G. A. Wick, J. D. Lundquist, and M. D. Dettinger, 2008: Meteorological characteristics and overland precipitation impacts of atmospheric rivers affecting the west coast of North America based on eight years of SSMI/satellite observations. J. Hydrometeor., 9, Ralph,F.M. and M.D Dettinger, 2012: Historical and national perspectives on extreme west-coast precipitation associated with atmospheric rivers during December,BAMS, 92,NEED PAGES. Stuart, N.A and R.H. Grumm 2006: Using Wind Anomalies to Forecast East Coast Winter Storms. Wea. and Forecasting,21, 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,

7 Figure 1. Total estimate precipitation (mm) from the Stage-IV rainfall data set for the period of 0000 UTC 23 to 1200 UTC 26 June Shading as in the color scale contours are every 50 mm. Return to text.

8 Figure 2. GFS 00-hour forecasts of 925 hpa winds (kts) and 925 hpa u-wind standardized anomalies.

9 Figure 3. As in Figure 2 except for GFS 00-hour forecasts of 500 hpa heights (m) and height anomalies in 24 hour periods from a) 0000 UTC 22 June through f) 0000 UTC 27 June Return to text.

10 Figure 4. As in Figure 3 except for NCEP GFS precipitable water in 24 hour increments from a) 0000 UTC 19 June through f) 0000 UTC 24 June Contours ever 10 mm and shading is in mm as per the color bar at the bottom of the images. Return to text.

11 Figure 5. As in Figure 4 except for precipitable water and precipitable water anomalies in 6-hour increments from a) 0000 UTC 25 through f) 0600 UTC 26 June Precipitable water contours every 5mm. Return to text.

12 Figure 6. As in Figure 5 except for 850 hpa moisture flux and moisture flux anomalies. Return to text.

13 Figure 7. NCEP GEFS forecasts showing total accumulated QPF and the probability of 100 mm or more QPF for the 48 hour period ending at 1200 UTC 26 June 2012 from GEFS cycles initialized at a&d) 1200 UTC 20 June, b&e) 1200 UTC 21 June, and c &f) 1200 UTC 22 June Upper panels show the probability of 100mm and the ensemble mean 100mm contour. Lower panels show the ensemble mean QPF and each members 100mm contour.. Return to text.

14 Figure 8. As in Figure 7 except for forecsats initialized at 0000 UTC 23 June, 1200 UTC 23 June and 0000 UTC 24 June Return to text.

15 Figure 9. As in Figure 7 except for NCEP SREF showing 100mm of QPF for the 36-hour period ending at 1200 UTC 26 June Forecasts from a&d) 0900 UTC 23 June, b&e) 1500 UTC 23 June, and c&f) 0300 UTC 24 June Return to text.

16 Figure 10. As in Figure 9 except for SREFs a&d) 0900 UTC 24 June, b&e) 1500 UTC 24 June, and c&f) 2100 UTC 24 June 2012 Return to text.

17 Figure 11. As in Figure 10 except for 150 mm probability thresholds. Return to text.

18 Figure 12. As in Figure 11 except NCEP 16km SREF forecasts of 100mm or more QPF. Return to text.

19 Figure 13. As in Figure 12 except for 16 km SREF forecasts of 150mm or more QPF. Return to text.

20 Figure 14. As in Figure 12 except for 200 mm threshold. Return to text.

21 Figure 15. NCEP GFS forecasts of total QPF for the period of 1200 UTC 25 June through 1200 UTC 26 June 2012 from the GFS initialized at a) 0000 UTC 21 June, b) 0000 UTC 22 June, c) 0000 UTC 23 June and d) 0000 UTC 24 June Return to text.

22 Figure 16. As in Figure 15 except for 48 hour accumulations ending at 1200 UTC 26 June Return to text.

23 Figure 17. As in Figure 1 except for total 6-hour precipitation ending at a) 0000 UTC, b) 0600 UTDC, c) 1200 UTC, and d) 1800 UTC 24 June Return to text.

24 Figure 18. As in Figure 17 except for a) 0000 UTC, b) 0600 UTDC, c) 1200 UTC, and d) 1800 UTC 25 June Return to text.

25 Figure 19. As in Figure 18 except for 26 June Return to text.