18 September Synoptic Heavy Rainfall Event-Draft

Transcription

1 1. Overview 18 September Synoptic Heavy Rainfall Event-Draft By Richard H. Grumm National Weather Service State College, PA Contributions by Craig Evanego A strong frontal system brought heavy rain (Fig. 1) the eastern United States on 18 September The event was textbook example of a Maddox-Synoptic rainfall event (Maddox et al. 1979) including a deep trough (Fig. 2) with strong southerly flow ahead of the frontal system. The 500 hpa pattern suggested the interaction of a strong northern stream wave with a southern stream wave. This resulted in a deep trough, with -1 to -2σ height anomalies, over the eastern United States (Fig. 2). The strong southerly flow ahead of the trough brought a plume of subtropical moisture (Fig. 3) into the eastern United States. The deep moisture associated with the southern stream wave produced precipitable water anomalies on the order of +2 to +4s above normal in the eastern United States. Heavy rainfall in the eastern United States often falls with deep southerly flow (Bodner et al 2012; Grumm 2011) and above normal PW anomalies. The value of using standardized anomalies (Hart and Grumm 2001) to identify significant weather events was first demonstrated by Hart and Grumm (2011). Graham and Grumm (2010) demonstrated the value of standardized anomalies to identify high impact weather events in the western United States. Studies related specifically to heavy rainfall demonstrated the value of precipitable water anomalies and 850 hpa u- and v-wind anomalies (Grumm and Hart 2001a; Stuart and Grumm 2007). Junker et al. (2008) showed how standardized anomalies along with forecast guidance can aid in better identifying potential for rare heavy rainfall events. This paper will document the heavy rainfall event of 18 September The focus is on the pattern and the anomalies associated with the event. Forecasts are used to show how the NCEP models were capable of predicting the pattern and thus produced relatively high quantitative precipitation forecasts. 2. Data and Methods Model data used to reconstruct the conditions are from the NCEP models. The GFS 00- hour forecasts were used to show the verifying analysis used to identify the patterns. The standardized anomalies were computed using the GFS 00-hour data as explained in Hart and Grumm (2001). All model and anomaly data were plotted using GrADS (Doty and Kinter 1995). The Stage-IV precipitation data were used to plot the observed precipitation data. These data were archived in 6-hour increments allowing the computation and evaluation of precipitation in 6,12,18,24 and 36 hour periods. 3. Pattern over the region

2 The large scale pattern over the United States from 17/0000 through 19/1200 UTC featured a northern stream wave (Fig. 2a) and southern stream wave. These two features merged on 17 September (Fig. 2a-c) producing a deep trough over the central United States by 1800 UTC 18 September 2012 (Fig. 2d). The 500 hpa height anomalies in the trough were in the -2 to -3σ range at this time. The height gradient implied strong southerly flow ahead of the trough. The deep low-level southerly flow at 850 hpa over the eastern United States from 18/0000 through 19/0600 UTC (Fig. 4) showed the development of a strong low-level jet over the southeast (Fig. 4b-c) with 50 to 75kts winds by 18/1800 UTC associate with +6σ 850 hpa total wind anomalies. The south-to-north flow (Fig. 4) and the surge of high PW air from the south (Fig. 3) showed the classic Maddox-Synoptic rainfall pattern, with an implied north-tosouth oriented frontal boundary approaching to the west. The 850 hpa temperature field (Fig. 5) showed a strong frontal zone with -1 to -2σ temperature anomalies on the cold side of the front and around 1σ above normal temperature anomalies ahead of the front. The strong low-level flow and deep anomalous moisture resulted in extreme values of 850 hpa moisture flux (Fig. 6). The 850 hpa moisture flux anomalies peaked over 6σ above normal over Virginia (Fig. 6c), Pennsylvania and New York (Fig. 6d) and along the coastal plain into New England (Fig. 6e). Extreme rainfall events are often associated with 850 hpa moisture flux values in the 4 to 6σ range. Fortunately, the frontal system was moving rapidly to the east and the period of heavy rainfall was limited to about 6-hours in most locations. Note that there was little overlap in the areas affected by 5-6σ 850 hpa moisture flux anomalies. Despite this, there was an extensive area of 50 to 100 mm (2-4 inches) of rainfall observed from northern Maryland into New England (Fig. 1; Table 1). The strong southerly flow brought moisture and some instability into the eastern United States. The analyzed convective available potential energy (CAPE: Fig. 7) was relatively low with 400JKg -1 reaching into New Jersey (Fig. 7d). CAPE over 800 and 1200JKg -1 was confined to the coastal plain and CAPE over 400JKg -1 reached as far north as southern New Jersey. The relatively low CAPE did not support deep updrafts and limited the severe thunderstorm potential, despite deep and strong shear in the layers from 10m to 850,700, and 500 hpa. The shear in the 700 hpa layers (Fig. 8) implies deep shear over an approximate 3km layer. The period of heavy rain and the north-south bands on radar (not shown) were observed during periods where the 3km shear was in excess of 40 and at time 60ms Observations (await final data to improve QPE images) Locations which received in excess of 87.5 mm (3.5 inches) of rainfall are listed in Table 1 and Table 2. The second table is based on WFO local storm reports. The heaviest rainfall was observed primarily in eastern Pennsylvania into New York State. This is somewhat confirmed by the estimated rainfall using a mix of gauges and radar data (Fig. 1).

3 In addition to the heavy rain there were reports of downed trees and wires during the event (Fig. 9). High winds downed trees and wires from Virginia into southern New Hampshire. A review of satellite and lightning data indicated no lightning north of northern New Jersey. Though convection was responsible in transporting high winds to the surface, there was a general lack of thunder in many of the linear convective elements which produced wind damage Forecasts Overall, all the models predicted the pattern quite well and thus the models were able to predict a region of relatively heavy rainfall over the eastern United States. The GFS and NAM both forecast a broad region of 25 to 50 mm or QPF over the region affected by the heavy rainfall (Figs ). Both models suffered some forecast errors with location. The NAM initially (Fig. 11) predicted the axis of the heaviest rainfall too far west of the verifying location. Similar errors were evident in the GFS. The GEFS did not predict any areas of 100mm or more QPF in the 36 hour period examined and predicted only small areas of 75 mm or more QPF in the 36 hour period ending at 1200 UTC 19 September. The focus was on eastern Pennsylvania and the Catskill region of New York (Fig. 12). The probability of 50 mm or more of QPF indicated a broader area of heavy rainfall and showed a high probability of 50 mm or more QPF from Pennsylvania into New York. The SREF was a bit more robust with forecasting 75 mm or more QPF (Fig. 14) and like the GEFS lacked areas in excess of 100mm north of West Virginia. The SREF had a broad region of relatively high probability of exceeding 50 mm of QPF (Fig. 15). A moderate rainfall event was well predicted by both the SREF and GEFS with the potential for over 75mm these forecasts likely implied a high end rainfall event. Though not shown, the SREF predicted 850 hpa moisture flux values in excess of 6σ above normal too. 6. Summary A Maddox-Synoptic pattern produced heavy rainfall over the eastern United States on 18 September A widespread 25 to 50 mm rain event was observed over a large portion of the eastern United States. Portions of northeastern Pennsylvania and New York State received 100 to 150 inches of rainfall with locally heavier amounts exceeding 175 mm. The strong low-level flow with over 2σ above normal PW anomalies and over 6σ 850 hpa moisture flux anomalies were good clues in the forecast guidance for a higher end heavy rainfall event. The overall pattern was relatively well predicted by the NCEP models including both the GFS and NAM. The pattern was also well predicted by the both NCEP ensemble forecast systems to including both the SREF and GEFS. The SREF showed potential for areas of 75 to 100 mm of QPF and the GEFS showed some potential for around 100 mm of QPF. Overall the

4 potential for heavy rainfall was well predicted. The difficult issue was where the heavy rain would fall. The ensemble forecasts show that the probability of 50 mm or more QPF was relatively well predicted and there were forecasts showing low probabilities, in a pattern conducive for heavy rainfall, of amounts in excess of 75 mm. What is not known is how extreme an event a 50mm or 75 mm QPF is in the SREF and GEFS over a 36 hour period in this region of the forecast domain in September. Model internal phase space information is critically lacking. In addition to the threat of heavy rain, the strong winds and strong wind shear caused some concern for high synoptic scale winds, severe thunderstorms, and tornadoes. In Pennsylvania there were sporadic reports of trees down, most of this occurred during the period of heavy rainfall. Typically, strong southerly winds do not produce widespread wind damage and synoptic wind events typically are observed near and behind cold fronts when the static stability favors transporting strong winds the surface. Most of the wind damage in Pennsylvania and New York occurred near enhanced shallow convection which lacked cloud-to-ground lightning. In southeastern Pennsylvania and farther southeast there was some lightning associated with the rainbands. The weak CAPE did not support large updrafts and the strong but nearly linear shear did not support supercells and supercell tornadoes. Despite several tornado watches, to date no tornadoes have been observed in the eastern United States. An examination of the rainfall data reported from gages and by the public showed that the majority of reports were in the 1 to 2.5 inch range. Many of the amounts in excess of 3 inches are near terrain features and most of the extreme amounts, in excess of 5 inches were in terrain areas favoring upslope. Figure 16 shows a zoomed in plot over the eastern Catskills of New York. Lower elevations generally had 1.5 to 3 inches of rainfall. However, long the southsoutheastern exposed regions of the higher terrain, rainfall amounts ranged from 4.78 to 7.37 inches of rain. The strong low-level southerly jet relative to the terrain clearly played a significant role in the heavier rainfall totals. Note a low lying observation of 1.73 inches in close proximity to several higher elevation reports of 6.21, 6.67, and 7.37 inches. Note that the 8 inch value in Pennsylvania (Table 2) was not used. Quality control of the 8 inch report in northeastern Pennsylvania indicated it was suspect and it was not used in these plots. 7. Acknowledgements The Mid-Atlantic River Forecast Center provided river flood data and rainfall data over their region of responsibility. Craig Evanego of the National Weather Service Office in State College provided tables of rainfall data. 8. References Doswell,C.A.,III, H.E Brooks and R.A. Maddox, 1996: Flash flood forecasting: An ingredients based approach. Wea. Forecasting, 11,

5 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, 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, R.H. 2011: New England Record Maker rain event of March NWA,Electronic Journal of Operational Meteorology,EJ4. Grumm, R.H., and R. Hart, 2001a: Anticipating Heavy Rainfall: Forecast Aspects. Preprints, Symposium on Precipitation Extremes, Albuquerque, NM, Amer. Meteor. Soc., Grumm, R.H. and R. Hart. 2001b: 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 Maddox,R.A., C.F Chappell, and L.R. Hoxit. 1979: Synoptic and meso-alpha aspects of flash flood events. Bull. Amer. Meteor. Soc., 60, 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., Stuart, N. and R. Grumm 2009, "The Use of Ensemble and Anomaly Data to Anticipate Extreme Flood Events in the Northeastern United States",NWA Digest,33,

6 Figure 1. Total rainfall over the eastern United States for the period of 0800 UTC September Units in inches. Return to text. Data being processed to replace this image.

7 Figure 2. NCEP GFS 00-hour forecasts showing 500 hpa heights (m) and 500 hpa height standardized anomalies (standard deviations) in 12-hour intevals from a) 0000 UTC 17 September through f) 1200 UTC 19 September Contours every 60 m. Return to text.

8 Figure 3. As in Figure 2 except for precipitable water (mm) and precipitable water standardized anomalies in 6-hour periods from a) 0000 UTC 18 September through f) 0600 UTC 19 September Precipitable water contours every 5mm. Return to text.

9 Figure 4. As in Figure 3 except for 850 hpa winds (kts) and wind anomalies. Return to text.

10 Figure 5. As in Figure 3 except for 850 hpa temperature ( C) and temperature anomalies. Isotherms every 2C. Return to text.

11 Figure 6. As in Figure 3 except for 850 hpa moisture flux. Return to text. NWS State College Case Examples

12 Figure 7. As in Figure 3 except for CAPE (JKg-1). Contours every 400JKg-1 and shading is for 800, 1200, 1500, and 2400 JKg-1. Return to text.

13 Figure 8. As in Figure 3 except for shear in the layer from 10m to 700 hpa in ms-1. Return to text.

14 Figure 9. Storm Prediction Center plot of severe weather reports color coded by type for the 24 hour period ending at 1200 UTC 19 September Return to text.

15 Figure 10. NCEP GFS forecasts of precipitation for the 36 hour period ending at 1200 UTC 12 September 2012 from forecasts initialized at a) 0000 UTC 16 September, b) 1200 UTC 16 September, c) 0000 UTC 17 September, and d) 1200 UTC 17 September Return to text.

16 Figure 11. As in Figure 10 except for the NAM 12km NAM. Return to text.

17 Figure 12. NCEP GEFS forecasts of QPF for the 36 hour period ending at 1200 UTC 19 September Upper panels show the probability of 75 mm or more QPF and lower panels show the mean QPF (mm) and each members 75 mm contour from NCEP GEFS initialized at a-d) 0000 UTC 15 September 2012, b-e) 0000 UTC 16 September 2012, and c-f) 0000 UTC 17 September Return to text.

18 Figure 13. As in Figure 12 except for GEFS forecasts for 50 mm more QPF. Return to text.

19 Figure 14. As in Figure 12 except for the SREF initialized at a-d) 0900 UTC 16 September, b-e) September and c-f) 2100 UTC 17 September Return to text.

20 Figure 15. As in Figure 14 except SREF forecasts of 50 mm or more QPF. Return to text.

21 Figure 16. Plots of gage and spotter reported storm totally rainfall (inches) with terrain. Higher amounts were in the higher elevations of the Catskills of southeastern New York State. Return to text.

22 State Town Rainfall (in) NY Denning 7.37 NY Phoenicia 6.67 NY Tannersvil 6.6 NY Slide Moun 6.21 NY East Jewit 5.83 NY Claryville 5.8 PA Lehighton 5.56 PA Lehighton 5.22 PA Beltzville 5.05 PA Jim Thorpe 4.92 PA Tobyhanna 4.9 PA Lehighton 4.87 NY Neversink 4.78 PA Mount Poco 4.6 NY Roscoe 2S 4.5 PA Hamburg F/ 4.3 PA Hamburg Ob 4.2 PA Blakeslee 4.11 PA Myerstown 4.1 PA Kresgevill 3.89 VA Round Hill 3.8 PA County Tow 3.78 VA Mt Weather 3.78 MD Thurmont PA Milanville 3.68 PA Berne 3.59 PA Landingvil 3.59 NY Big Flats 3.58 MD Emmitsburg 3.55 PA York Haven 3.54 PA Hawley 1E 3.52 PA Weatherly 3.52 Table 1. State, Location and rainfall (inches) for locations with over 3.5 inches. There were 9 locations with over 5 inches, 19 with over 4 inches, and 50 locations reported over 3 inches of rainfall. Data courtesy of the Mid-Atlantic River Forecast Center. Return to text. NWS State College Case Examples

23 Location Amount (inches) Report Day Type State ALBRIGHTSVILLE Sep DAILY TOTAL PENNSYLVANIA DENNING Sep NEVERSINK NEAR DENNING NEW YORK STONE RIDGE Sep SPOTTER NEW YORK HUNTER Sep REPORT FROM WNYT NEW YORK PHOENICIA Sep CO-OP OBSERVER NEW YORK TANNERSVILLE Sep MESONET NEW YORK WILLOW Sep TRAINED SPOTTER NEW YORK LATHAM Sep WEATHERNET6 NEW YORK EAST JEWETT Sep CO-OP OBSERVER NEW YORK WEST SHOKAN Sep WEATHERNET6 NEW YORK LEHIGHTON Sep MESONET PENNSYLVANIA BELTZVILLE Sep MESONET PENNSYLVANIA GUILFORD Sep TRAINED SPOTTER VERMONT FREEHOLD Sep WEATHERNET6 NEW YORK 2 NNE VOORHEESVILLE Sep NWS EMPLOYEE NEW YORK HAMBURG Sep MESONET PENNSYLVANIA MAPLECREST Sep WEATHERNET6 NEW YORK 4 WNW LITTLESTOWN Sep COCORAHS PENNSYLVANIA CAIRO Sep WEATHERNET6 NEW YORK HEBRON Sep WEATHERNET6 NEW YORK KRESGEVILLE Sep MESONET PENNSYLVANIA VOORHEESVILLE Sep WEATHERNET6 NEW YORK BLAKESLEE Sep MESONET PENNSYLVANIA 1 SSE ABBOTTSTOWN Sep COCORAHS PENNSYLVANIA GUILDERLAND CENTER Sep WEATHERNET6 NEW YORK LANDGROVE Sep WEATHERNET6 VERMONT 2 SSE MILANVILLE Sep COCORAHS PENNSYLVANIA BALLENGER CREEK Sep TRAINED SPOTTER MARYLAND 3 W DINGMANS FERRY Sep COCORAHS PENNSYLVANIA 3 E FREEHOLD Sep COCORAHS NEW YORK 5 SE FAIRFIELD Sep COCORAHS PENNSYLVANIA 2 NNE HAWLEY Sep COCORAHS PENNSYLVANIA 4 WSW DOVER Sep COCORAHS PENNSYLVANIA COLONIE Sep WEATHERNET6 NEW YORK 7 SSE GETTYSBURG Sep COCORAHS PENNSYLVANIA 6 SSW SCHENECTADY Sep COCORAHS NEW YORK BERNE Sep MESONET PENNSYLVANIA 3 NE HILLSBORO Sep TRAINED SPOTTER VIRGINIA SCHUYLERVILLE Sep REPORT THROUGH WNYT NEW YORK DURHAM Sep WEATHERNET6 NEW YORK BLUEMONT Sep NWS EMPLOYEE VIRGINIA

24 TAMAQUA Sep COOP PENNSYLVANIA STILLWATER Sep AMATEUR RADIO NEW YORK NISKAYUNA Sep SPOTTER NEW YORK HAWLEY Sep CO-OP OBSERVER PENNSYLVANIA 2 W ORWIGSBURG Sep COCORAHS PENNSYLVANIA QUEENSBURY Sep WEATHERNET6 NEW YORK PRESTON HOLLOW Sep WEATHERNET6 NEW YORK GERMANTOWN Sep WEATHERNET6 NEW YORK FEURA BUSH Sep WEATHERNET6 NEW YORK HANOVER Sep COOP PENNSYLVANIA 2 NNE GETTYSBURG Sep COCORAHS PENNSYLVANIA 2 N ABBOTTSTOWN Sep COCORAHS PENNSYLVANIA 3 WSW BIGLERVILLE Sep COCORAHS PENNSYLVANIA EARLTON Sep WEATHERNET6 NEW YORK WELLS Sep WEATHERNET6 NEW YORK 3 SSE PALMYRA Sep COCORAHS PENNSYLVANIA KEFFERS Sep IFLOWS PENNSYLVANIA RAVENA Sep WEATHERNET6 NEW YORK ESOPUS Sep WEATHERNET6 NEW YORK 3 S ROCK HILL Sep TRAINED SPOTTER NEW YORK 4 NE YORK Sep COCORAHS PENNSYLVANIA 1 E WESTMINSTER WEST Sep COCORAHS VERMONT 3 NW BIGLERVILLE Sep COCORAHS PENNSYLVANIA 1 WNW MOUNT CARMEL Sep COCORAHS PENNSYLVANIA BETHEL Sep MESONET PENNSYLVANIA MAHANOY CITY Sep COOP PENNSYLVANIA CLIFTON PARK Sep NWS EMPLOYEE NEW YORK 2 ENE LOVETTSVILLE Sep TRAINED SPOTTER VIRGINIA 4 WSW GLENS FALLS Sep COCORAHS NEW YORK CASHTOWN 1S Sep COOP PENNSYLVANIA 5 SW GETTYSBURG Sep COCORAHS PENNSYLVANIA ALBANY AP Sep CO-OP OBSERVER NEW YORK FLATBROOKVILLE Sep MESONET NEW JERSEY EAST BERNE Sep WEATHERNET6 NEW YORK DEHART DAM Sep COOP PENNSYLVANIA BLAIRSTOWN Sep EVENT TOTAL NEW JERSEY WEST PENN TWP Sep IFLOWS PENNSYLVANIA COUNTY AIRPORT Sep IFLOWS PENNSYLVANIA 3 NNW YORK Sep COCORAHS PENNSYLVANIA RED OAKS MILL Sep SPOTTER NEW YORK 1 NE LEHIGH Sep COCORAHS PENNSYLVANIA 4 S EVERETT Sep COCORAHS PENNSYLVANIA POUGHKEEPSIE Sep SPOTTER NEW YORK

25 FORKS TWP Sep TRAINED SPOTTER PENNSYLVANIA MALTA Sep WEATHERNET6 NEW YORK 2 NNW MARLBORO Sep COCORAHS NEW YORK BEAVERTOWN Sep TRAINED SPOTTER PENNSYLVANIA HALCOTT CENTER Sep WEATHERNET6 NEW YORK TORRINGTON Sep TRAINED SPOTTER CONNECTICUT 6 WSW GETTYSBURG Sep COCORAHS PENNSYLVANIA 6 SSW MECHANICSBURG Sep COCORAHS PENNSYLVANIA PARK PLACE Sep IFLOWS PENNSYLVANIA CHATHAM Sep WEATHERNET6 NEW YORK 2 E WINCHESTER Sep AMATEUR RADIO VIRGINIA GLENMOORE Sep PUBLIC PENNSYLVANIA 1 SE YORK SPRINGS Sep COCORAHS PENNSYLVANIA 2 N ELIZABETHVILLE Sep TRAINED SPOTTER PENNSYLVANIA SWIFTWATER Sep PUBLIC PENNSYLVANIA JOHNSTOWN Sep WEATHERNET6 NEW YORK SE WEST HALIFAX Sep COCORAHS VERMONT CASHTOWN Sep TRAINED SPOTTER PENNSYLVANIA HANOVER Sep CO-OP OBSERVER PENNSYLVANIA Table 2. WFO reports of rainfall data by town, amount (inches) date of observation, type of observation, and State. The 8 inch report was filtered by the MARFC as potentially erroneous. Return to text.