Eastern United States Synoptic Rain Event May 2014-Draft

1. Overview Eastern United States Synoptic Rain Event 15-16 May 2014-Draft heavy rain and flooding event By Richard H. Grumm National Weather Service State College, PA A deep 500 hpa trough with -4σ height anomalies over the central United States (Fig. 1) combined with a strong ridge with +2σ height anomalies over eastern Canada, produced strong southerly flow. The deep southerly flow brought a plume of high precipitable water (PW: Fig. 2) air into the region along with strong 850 hpa southerly flow (Fig. 3) resulting in heavy rainfall (Fig. 4) and isolated severe weather (Fig. 5). The heavy rainfall produced flooding in the Mid- Atlantic region from Virginia northward into New York State. The pattern was well predicted by the NCEP models and ensemble forecasts systems. The NCEP SREF will be used to illustrate the successful prediction of the larger scale pattern. The precipitation forecasts showed some uncertainty as to the amounts and locations of the heavy rainfall though all forecasts systems correctly predicted the potential for heavy rainfall in the general region where the rainfall was observed. The paper will provide an overview of the pattern, the resulting weather, and forecasts of the rainfall. The brief, local severe weather which affected central Pennsylvania is also examined including a mini-supercell which did not produce severe weather. The most significant impact on the region was heavy rainfall with areas where 50 to 150 mm (2-6 inches: Table 1) of rain was observed and the resulting flooding along streams and creeks. 2. Data and Methods The large scale pattern was reconstructed using the 00-hour forecast of the NCEP Global Forecast System as first guess at the verifying pattern. The standardized anomalies were computed in Hart and Grumm (2001). All data were displayed using GrADS (Doty and Kinter 1995). The focus on the larger scale pattern was to show the standardized anomalies and how they helped identify this event and the potential they have in anticipating similar high impact weather events (HIWE). The NCEP GFS, GEFS, and SREF were used to examine the forecasts of this event. As the pattern was generally well predicted the focus is on the quantitative precipitation forecast (QPF) produced by these systems. The stage-iv data were used to get a first guess at where, when, and how much precipitation was observed. These data were analyzed in planview with GrADS and in Python to make point

diagrams showing the timing of the rainfall. Python was used to plot all-time series and plume diagrams. Radar imagery was obtained from the local AWIPS in State College and QPE data was examine using the Stage-IV data. 3. Pattern and Weather i. pattern The large scale pattern (Figs. 1-3) showed a deep trough to the west and a strong ridge to the east. The resulting deep southerly flow allowed a surge of deep moisture and strong southerly flow into the eastern United States. A nearly textbook Maddox Synoptic rainfall (Maddox et al. 1979) event type. There was some shallow cold air damming and easterly flow which likely enhanced the lift over the frontal boundary in the Mid-Atlantic region contributing to the rainfall amounts in some areas in excess of 5 inches. The surface pattern, Figure 6 shows the surface pattern, which indicated a strong surface anticyclone off the East Coast and a large anticyclone over the central United States. Several relatively weak cyclones developed in the trough between the two anticyclones. Though not clearly indicated, the strong easterly flow at low-levels pushed a marine layer into central Pennsylvania on 14 May creating an enhanced frontal boundary over which the deep southerly flow (Fig. 3) moved over. It is beyond the scope of this study to show these mesoscale details. ii. rainfall The evolution of the precipitation shield as it moved over the eastern United States (Fig. 4).) shows the areas of 25 mm or more QPF. The first 3-panels show the 12-hour windows of the rapidly moving shield of heavy rain from 0000 UTC 15 May through 1200 UTC 16 May 2014. Figure 4d shows the 24-hour total, the window when most of the rain fell over Pennsylvania and the Mid-Atlantic region. The rain shield had an abrupt western edge and an embedded northsouth band of 50 mm with a 100 mm area within it over east-central Pennsylvania. It should be noted that two private rain gauges in Spring Mills, PA recorded around 150 mm ( 6inches) of rain just a few 10s of kilometers west of the 100 mm (4 inch) area outlined in the Stage-IV data. These data show the oft observed character of synoptic rainfall events including the sharp northsouth axes of heavy rainfall and the sharp edges of the heavier rainfall. iii. Pennsylvania severe weather The focus here is on three storms, two of storms which produced severe weather in central Pennsylvania and a mini-supercell. The mini-supercell, which developed near Lewistown, PA, and moved to the north-northeast shown near its peak (Fig. 7) was not associated with any known damage. The first severe storm was a bow-echo structure which produced damage north of Harrisburg, PA. The second severe storm was along a line of storms and heavy rain which

developed a break in the line and a strong rear flank downdraft near Lock Haven around 2300 UTC. This storm produced some wind damage and a short-lived tornado. The mini-supercell storm which produced wind damage over southern Clinton County and east of Lock Haven, PA is shows (Fig. 8) shows a hook in the reflectivity and a ZDR arc around mesocyclone (see black arc outlining the feature). The base winds (V) show the circulation associated with the low-level mesocyclone. This well-structured mini-supercell has not been associated with any wind damage, though it contained many features found in stronger and more meteorologically significant supercells. The bow echo developed over Cumberland County (Fig. 9) south of Newville and had over 40kts of wind associated with at 2212 UTC. The bow feature in the reflectivity became more pronounced as the feature moved eastward. The final image, at 2249 UTC shows the distinct bow feature well southwest of Amity Hall, which is just north of Duncannon where wind damage was reported. The second known severe storm developed in a line of showers and thunderstorms. It affected an area just west of the area affected by the mini-supercell about an hour later. At 2300 UTC (Fig. 10) the storm showed a classic broken-s pattern, a region where a mesocyclone often develops along a line of thunderstorms (orange arrow) and contains strong outbound velocities (white arrow) peaking over 50 kts. Not the low values of ZDR in the region of the strong velocities in the upper right panel. Higher values of ZDR were present to the east, near Loganton. Figure 11 shows the line as the mesocyclone at was still growing and the break in the line was not complete. The low ZDR region was present near the emergent mesocyclone, the outbound winds were weaker at this time and the break in the line, the broken-s was evident a few kilometers northwest of Mackeyville, PA. This storm produced wind damage and a brief tornado on the east side of Lock Haven, PA around 2300 UTC 15 May 2014. 4. Forecasts Forecasts of the larger scale pattern were generally good with about 3-5 days lead-time (not shown). This is revealed by the GEFS QPF (Fig. 12 ) which show the north-south oriented band of rainfall to exceed 50 mm. Forecasts issued prior to 0000 UTC 11 May had some issues with the location of the pattern and rainfall. Due to divergent solutions on the exact location and timing of the rainfall, forecasts issued on and after 1200 UTC 13 May showed the higher probability of 50 mm or more QPF. These data show the 24-hour window in which the heavy rainfall was observed. Higher QPF values were indicated when these GEFS forecasts we re viewed in 30 to 36 hour windows. The NCEP SREF (Fig. 13) showed a similar north-south axis of heavy rainfall from the central Appalachians into southern New York. The axis of the SREF heavy rainfall was west of that

produced in the GEFS and similar to the GEFS, showed the heavier rainfall with higher probabilities as the forecast horizon decreased. A comparison of select 0000 UTC GFS, and GEFS with 0300 UTC SREF data are shown in Figure. 14 and 1200 UTC GFS, GEFS with 0900 UTC SREF are shown in Figure 15. These data show similar forecasts by each system with regards to a north-south axis of heavy rainfall and the general area to be impacted by the rainfall. The GFS had higher QPF amounts than the SREF and GEFS though most of this could be attributed to averaging 21 members in the two EFSs. All of these data suggest a well predicted pattern, a synoptic pattern, though the details as to where the higher QPF would fall to have been more of a forecast issue. 5. Summary A strong frontal system ahead of a deep late season trough to the west and a strong ridge to the east brought a surge of deep moisture into the eastern United States resulting in heavy rain in the Mid-Atlantic region. Due to a lack of deep instability, severe weather was generally a low-level issue with this Synoptic-type heavy rainfall event. The heaviest rains fell in a 24 hour period between about 1800 UTC 15-16 May 2014 (Figs. 4 & 16) from Virginia into central Pennsylvania (Table 1). The heavy rainfall produced flooding along rivers and streams in the Mid-Atlantic region with approximately 34 points reaching or exceeding flood stage (Table 2). Similar to many synoptic events, the heavier rainfall fell in a general north-south oriented band in deeps southerly flow in a plume of air with above normal precipitable water. This is another case which demonstrates the value of anomaly based analysis and forecasting in identifying high impact weather events. This and other cases show the value of anomaly based forecasting in near, short, and medium range forecasting. In this event, the relatively accurate prediction of the anomalous plume of deep moisture and strong low-level southerly flow resulted in relatively useful and accurate QPF forecasts in the NCEP deterministic models and ensemble forecast systems. This relationship between the strong signal in the anomalies and the high QPFs should provide some modicum of confidence information to forecasters predicting heavy rainfall and potential flooding events. The NCEP guidance shown here implied a relatively impressive lead-time on the order of 3-6 days. The heavy rain produced rapid rises on several creeks and streams in central Pennsylvania such as Shermans and Penns Creek (Fig. 17). The black curves show the rapid rises on both creeks. Forecasts of the flooding were relatively slow in showing the flood potential. Similar to many previous events, when widespread rainfall over a basin exceeds 75 mm ( 3 inches), flooding along smaller streams and creeks often occurs. 6. Acknowledgements

The Pennsylvania State University for data access and providing students for research and case study projects. Ron Holmes for processing river stages and forecasts. Charles Ross for consultation and editing Elyse Colbert for summarizing observations and rainfall observations. Charlie Chillag for data on river flooding and stages. 7. References Bodner, M. J., N. W. Junker, R. H. Grumm, and R. S. Schumacher, 2011: Comparison of atmospheric circulation patterns during the 2008 and 1993 historic Midwest floods. Natl. Wea. Dig., 35, 103-119. Doswell,C.A.,III, H.E Brooks and R.A. Maddox, 1996: Flash flood forecasting: ingredients based approach. Wea. Forecasting, 11, 560-581. Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS. Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H. Bredekamp, NASA, Washington, D.C., 209-219. Durkee, JD., L Campbell, K Berry, D Jordan, G Goodrich, R Mahmood, S Foster, 2012: A Synoptic Perspective of the Record 1-2 May 2010 Mid-South Heavy Precipitation Event. Bull. Amer. Meteor. Soc., 93, 611 620. doi: http://dx.doi.org/10.1175/bams-d-11-00076.1 Junker, N. W., R. S. Schneider and S. L. Fauver, 1999: Study of heavy rainfall events during the Great Midwest Flood of 1993. Wea. Forecasting, 14, 701-712. Maddox,R.A., C.F Chappell, and L.R. Hoxit. 1979: Synoptic and meso-alpha aspects of flood events. Bull. Amer. Meteor. Soc., 60, 115-123. flash Stuart, N., Grumm, R. (2009) The Use of Ensemble and Anomaly Data to Anticipate Extreme Flood Events in the Northeastern United States. Nat. Wea. Digest 33: 185-202.

Figure 1. GFS 00-hour forecasts of 500 hpa heights (m) and 500 hpa height anomalies (standard deviations) in 12 hour increments from a) 0000 UTC 14 May 2014 through f) 1200 UTC 16 May 2014. Return to text.

Figure 2. As in Figure 1 except for precipitable water (mm) and precipitable water anomalies in 6-hour increments from 0600 UTC 15 May through 1200 UTC 16 May 2014. Return to text.

Figure 3. As in Figure 2 except for 850 hpa winds and wind anomalies. Return to text.

Figure 4. Estimated rainfall for amounts of 25mm or greater from the stage-iv rainfall data for the 3 12-hour periods ending at a) 1200 UTC 15 May, b) 0000 UTC 16 May, and c) 1200 UTC 16 May 2014 and d) the 24-hour accumulated rainfall over the period ending at 1800 UTC 16 May 2014. The 24 hour period covers the heavy rainfall over Pennsylvania and the Mid-Atlantic region.. Return to text. Return to rainfall portion.

Figure 5. Storm reports from Storm Prediction Center. Colored by type. Return to text.

Figure 6. As in Figure 3 except for MSLP. Return to text. NWS State College Case Examples

Figure 7. Base reflectivity (Z) and base velocity (V) focused over Centre and southern Clinton counties showing the evolving storm at (top) 2018 UTC, 2123 UTC and 2137 UTC. The hook echo is clear in the latter two images and at least weak rotation is evident in each image. Return to text.

Figure 8. KCCX 4 panel valid at 2137 UTC 15 May 2014 showing (clockwise from upper left) base reflectivity (Z), differential reflectivity (ZDR), correlation coefficient (CC), specific differential phase (KDP). Note ZDR arc in the upper right panel. The ZDR arch is drawn to the left of the feature to highlight it. Return to text.

Figure 9. As in Figure. except for 2212, 2239, and 2249 UTC. Return to text.

Figure 10. KCCX radar showing 0.5 dgree base reflectivity, ZDR, CC, and winds at 2303 UTC 15 May 2014. THe orange arrow show the break or broken- S in the reflectivity data, the pink arrow shows the ZDR minimum in the region of strong velocities deonted by the white arrow in the lower left panel. Return to text.

Figure 11. As in Figure 10 except valid at 2253 UTC and the CC is replaced by the storm relative velocity. Return to text.

Figure 12. GEFS forecasts showing the probability of 50 mm or more QPF in the 24-hour window ending at 1800 UTC 16 May from GEFS initialized at a) 00000 UTC 10 May, b) 0000 UTC 11 May, c) 1200 UTC 11 May, d) 1200 UTC 12 May 2014, e) 1200 UTC 13 May, and f) 1200 UTC 14 May 20144. Data include the probability of exceeding 50 mm and contours in 25 mm increments. Return to text.

Figure 13. As in Figure 11 except for SREF QPF probabilities initialized at a) 0300 UTC 13 May, b) 2100 UTC 13 May, c) 0300 UTC 14 May 2014, d) 0900 UTC 14 May, e) 1500 UTC 14 May, and f) 0300 UTC 15 May 20l14. Return to text.

Figure 14. As in Figure 12 except showing 24 hour accumulations (GFS) and probabilities of 50 mm or more QPF in the GEFS and SREF from a) GFS initialized at 0000 UTC 14 May, b) SREF initialized at 0300 UTC 14 May, c) GEFS initialized at 0000 UTC 14 May, and c) GFS initialized at 0000 UTC 15 May, b) SREF initialized at 0300 UTC 15 May, c) GEFS initialized at 0000 UTC 15 May. Contours are every 25 mm. Return to text.

Figure 15. As in Figure 14 except for 1200 UTC GFS and GEFS and 1500 UTC SREF on 14 and 15 May 2014. Return to text.

LOCATION COUNTY STATE AMOUN T Spring Mills Centre PA 6.00 2 WSW DEER RUN PENDLETON WEST VIRGINIA 5.50 1 SSE CHARLOTTESVILL CITY OF CHARLOTTESVILLE VIRGINIA 5.22 WINTERGREEN NELSON VIRGINIA 5.19 NEWPORT PERRY PENNSYLVANIA 5.00 2 E MYERSVILLE FREDERICK MARYLAND 4.65 1 NW STERLING PARK LOUDOUN VIRGINIA 4.50 CASHTOWN 1S ADAMS PENNSYLVANIA 4.41 2 WSW POTOMAC MONTGOMERY MARYLAND 4.40 SOUTH MOUNTAIN FRANKLIN PENNSYLVANIA 4.28 8 W CHARLOTTESVILLE ALBEMARLE VIRGINIA 4.28 PURCELLVILLE LOUDOUN VIRGINIA 4.25 1 NNE WASHINGTON GRO MONTGOMERY MARYLAND 4.25 1 SSE WEST MCLEAN FARIFAX VIRGINIA 4.23 1 WSW CAVETOWN WASHINGTON MARYLAND 4.22 WILLIAMSPORT LYCOMING PENNSYLVANIA 4.20 WEST SPRINGFIELD FARIFAX VIRGINIA 4.20 CASHTOWN ADAMS PENNSYLVANIA 4.20 2 N DARNESTOWN MONTGOMERY MARYLAND 4.18 E OAKTON FAIRFAX VIRGINIA 4.16 2 W CHARLOTTESVILLE ALBEMARLE VIRGINIA 4.16 3 NNE SMITHSBURG WASHINGTON MARYLAND 4.12 SUMMERDALE CUMBERLAND PENNSYLVANIA 4.10 COUNTRYSIDE LOUDOUN VIRGINIA 4.10 BURKE FARIFAX VIRGINIA 4.10 2 N WOLFSVILLE FREDERICK MARYLAND 4.10 3 SW FAIRPLAY WASHINGTON MARYLAND 4.08 2 WNW STANARDSVILLE GREENE VIRGINIA 4.05 2 SE LYDIA GREENE VIRGINIA 4.05 1 SSW ORANGE ORANGE VIRGINIA 4.02 FRONT ROYAL WARREN VIRGINIA 4.00 7 NNW MADISON MADISON VIRGINIA 4.00 Table 1. List of locations, counties, and States where point data equal to or exceeding 4.00 inches (100 mm) of total rainfall. There were too many reports, over 120, with 3.00 to 3.99 inches (75mm) to record easily. The Spring Mills reports came from an individual and a Newspaper report. Return to text.

Figure 16. Stage-IV data for a point near Spring Mills, PA and a point to the east from the Stage-IV data. The heaviest rainfall in the data was just east of this point. Return to text.

Crest Statistics Flooding and Crests - 05/16/2014-05/20/2014 First flood of 1 - May, 2014 and Seventh flood in 2014 Number of MARFC Forecast Points Flooding - 33 + 1 point flooding more than once - 34 Number of Floods Cresting in Minor Range - 21 Number of Floods Cresting in Moderate Range - 12 Number of Floods Cresting in Major Range - 1 Number of Floods Cresting in Missing Range - 0 MARFC Power Ranking - 91 (Minor = 1 - Moderate = 5 - Major = 10 - Missing =1) E = Estimated Table 2. Summary of Flooding in the Mid-Atlantic Region. Data provided by the Mid-Atlantic River Forecast Center (MARFC). Return to text.

Figure 17. River Stages (black) and forecasts for a point on Sherman s Creek near Sharman s Dale and Penns Creek, at Penns Creek. Forecasts are in colored lines. Return to text.