Post on 21-Dec-2015
transcript
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
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
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
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