1. A Diagnostic Analysis of a Difficult- to-Forecast Cutoff Cyclone from the 2008 Warm Season Matthew A. Scalora, Lance F. Bosart, Daniel Keyser Department of Earth and Atmospheric Sciences University at Albany/SUNY Neil A. Stuart and Thomas A. Wasula NOAA/NWS, Albany, NY 6 November 2008 NOAA/CSTAR Grant NA07NWS4680001

2. Motivation Forecasting heavy precipitation and severe weather associated with slow-moving 500 hPa cutoff cyclones is a challenge Models often have trouble predicting evolution of cutoffs and accurate precipitation amounts Complex terrain over the Northeast U.S. plays a significant role in modulating the precipitation distributions in cutoffs

3. Objectives Investigate the 23–24 July 2008 cutoff cyclone that produced heavy precipitation and severe weather over the Northeast Composite common synoptic and dynamical features throughout the troposphere Focus on selected parameters used in convective weather forecasting

4. Datasets and Methodology 0.5° GFS analysis 2.5° NCEP–NCAR reanalysis to create climatologies of July 1992–2007 averaged fields of: - lapse rate - jet strength - wind shear - PWAT Standardized anomalies of these fields from the 0.5° GFS with respect to climatology created from NCEP–NCAR reanalysis 6-h precipitation amounts from the NWS National Precipitation Verification Unit

5. 23–24 July 2008 Cutoff Cyclone Cutoff forms from preexisting trough over eastern Canada Dominant precipitation modes: convective lines/bow echoes, HP supercells, stratiform rain regions Widespread flash flooding and rain amounts of 7–9 cm along north–south oriented bands Numerous wind and hail reports, including two tornadoes

6. 22–25 July 2008 mean 500 hPa Z (dam), track of 500 hPa cutoff low center

7. SPC Storm Reports: 23 July 2008 Tornado Wind Hail

8. NEXRAD: 1218 UTC 23 July NEXRAD: 1804 UTC 23 July

9. Surface Observations: 0100 UTC 24 July NEXRAD: 0121 UTC 24 July Temp (°C), Dewpoint (°C), SLP (hPa)

10. 1800 UTC 23 July0000 UTC 24 July

11. 1800 UTC 23 July0000 UTC 24 July

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14. 1800 UTC 23 July0000 UTC 24 July

15. 23 July 2008 Summary Cutoff exhibits deep cyclonic flow with embedded vorticity maxima and corridor of strong 925–700 hPa wind shear Right-entrance region of upper-level jet streak provides favorable QG forcing for ascent Convergence of Gulf of Mexico and western North Atlantic moisture contributes to heavy precipitation over the Northeast Surface boundary and 850 hPa θ e ridge provide focus for severe weather

16. SPC Storm Reports: 24 July 2008 Tornado Wind Hail

18. Surface Observations: 1500 UTC 24 July NEXRAD: 1549 UTC 24 July Pot. Temp (°C, contoured) Temp (°C), Dewpoint (°C), SLP (hPa) Tornado touchdown at 1538 UTC

19. 1200 UTC 24 July1800 UTC 24 July

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23. 1200 UTC 24 July1800 UTC 24 July

24. 1200 UTC 24 July1800 UTC 24 July

25. 24 July 2008 Summary Cutoff orientation pivots from neutral to negative tilt Upper-level jet dynamics remain favorable Low-level boundaries and associated convergence maintain rainband over eastern New England Strong low-level jet and high θ e region provide focus for severe weather Hail reports concentrated near cold pool in a region of steep midlevel lapse rates

26. Conclusions Location and orientation of upper-level jets, low-level convergence, and 850 hPa θ e maxima important for determining precipitation distribution Surface boundaries enhance preexisting convection Steep lapse rates important for convective weather when dynamics are weak Anomalously high PWAT values contribute to widespread heavy rainfall

27. Future Work Perform additional case studies (e.g., 11–13 May 2008 and 16–19 June 2008) Focus on cases that illustrate various problems with forecasting heavy precipitation and severe weather associated with cutoffs Determine thresholds of selected dynamic/thermodynamic parameters to forecast convective and stratiform precipitation Develop forecast methodologies and conceptual models that can be used in operations