A Diagnostic Analysis of a Difficult-to-Forecast Cutoff Cyclone from

the 2008 Warm Season

Matthew A. Scalora, Lance F. Bosart, Daniel KeyserDepartment of Earth and Atmospheric Sciences

University at Albany/SUNY

Neil A. Stuart and Thomas A. WasulaNOAA/NWS, Albany, NY

6 November 2008

NOAA/CSTAR Grant NA07NWS4680001

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

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

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

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

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

SPC Storm Reports:23 July 2008

TornadoWindHail

NEXRAD:1218 UTC 23 July

NEXRAD:1804 UTC 23 July

Surface Observations:0100 UTC 24 July

NEXRAD:0121 UTC 24 July

Temp (°C), Dewpoint (°C), SLP (hPa)

1800 UTC 23 July 0000 UTC 24 July

1800 UTC 23 July 0000 UTC 24 July

1800 UTC 23 July 0000 UTC 24 July

1800 UTC 23 July 0000 UTC 24 July

1800 UTC 23 July 0000 UTC 24 July

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 θe ridge provide focus for severe weather

SPC Storm Reports:24 July 2008

TornadoWindHail

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

1200 UTC 24 July 1800 UTC 24 July

1200 UTC 24 July 1800 UTC 24 July

1200 UTC 24 July 1800 UTC 24 July

1200 UTC 24 July 1800 UTC 24 July

1200 UTC 24 July 1800 UTC 24 July

1200 UTC 24 July 1800 UTC 24 July

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 θe region provide focus for severe weather

• Hail reports concentrated near cold pool in a region of steep midlevel lapse rates

Conclusions

• Location and orientation of upper-level jets, low-level convergence, and 850 hPa θe 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

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