Friday, 11 November 2016: 10:30 AM
Pavilion Ballroom (Hilton Portland )
This study objectively identifies severe weather events with low-predictive skill from 1980 to 2013 over the northeast U.S. Storm Prediction Center (SPC) slight risk areas are used to verify storm reports of hail, wind, and tornados aggregated over each 24-hour period, from 1200 UTC to 1200 UTC. Skill scores of probability of detection (POD), false alarm ratio (FAR), and critical success index (CSI) are calculated for each event. A linear regression about the 66th percentile of the severe report coverage area through the study period is done to focus results on high-impact events. Low-predictive skill events are then defined as having a POD in the lowest 25th percentile of high-impact events (low-POD).
Results using Climate Forecast System Reanalysis data (CFSR) indicate low-POD, high-impact events occur most often under westerly and southwesterly midlevel flow. However, severe events under less common northwesterly, northerly and southerly flow directions have lower CSI scores due to higher occurrence rates of low-POD events per high-impact event. High-impact events under low 1000–500-hPa shear (<16 ms–1) conditions have a median CSI of 0.137, while high-impact events under high 1000–500-hPa shear (≥16 ms–1) conditions have a median CSI of 0.186. Additionally, low-POD events are more common under low-shear conditions than high-predictive skill cases, defined as having a CSI in the upper 25th percentile of high-impact cases.
To supplement the CFSR findings, sounding analyses are conducted using rawindsonde data prior to the first severe report of the 1200 UTC to 1200 UTC day to investigate the preconvective environment of low-POD events between 1999–2013. Preliminary results for low-POD events suggest storms initiate over regions of relatively low LCL heights before propagating into environments with warmer, drier boundary layers and producing severe wind gusts. Recognizing convection that is propagating into an area of sufficient instability but anomalously high planetary boundary layer heights and high downdraft CAPE values may be important forecast parameters for a heightened severe wind threat on these low-predictive skill days.
Results using Climate Forecast System Reanalysis data (CFSR) indicate low-POD, high-impact events occur most often under westerly and southwesterly midlevel flow. However, severe events under less common northwesterly, northerly and southerly flow directions have lower CSI scores due to higher occurrence rates of low-POD events per high-impact event. High-impact events under low 1000–500-hPa shear (<16 ms–1) conditions have a median CSI of 0.137, while high-impact events under high 1000–500-hPa shear (≥16 ms–1) conditions have a median CSI of 0.186. Additionally, low-POD events are more common under low-shear conditions than high-predictive skill cases, defined as having a CSI in the upper 25th percentile of high-impact cases.
To supplement the CFSR findings, sounding analyses are conducted using rawindsonde data prior to the first severe report of the 1200 UTC to 1200 UTC day to investigate the preconvective environment of low-POD events between 1999–2013. Preliminary results for low-POD events suggest storms initiate over regions of relatively low LCL heights before propagating into environments with warmer, drier boundary layers and producing severe wind gusts. Recognizing convection that is propagating into an area of sufficient instability but anomalously high planetary boundary layer heights and high downdraft CAPE values may be important forecast parameters for a heightened severe wind threat on these low-predictive skill days.
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