Kari Murray

 This article is extending on a 10-year climatological study done by Rose et al.  Rose et al. found that tornadoes most commonly occur in the left exit region of 250 hPa jet streaks and least commonly in the left entrance region  Clark et al. include hail and wind reports in their study and have factored in jet streak dynamics

 For this study a jet streak was defined as an enclosed area of wind speeds exceeding 25 ms -1  Jet cores were defined as points within the area of maximum wind where the acceleration was zero  Jet ends defined as areas upwind and downwind of the jet core when the acceleration reached 0 again  Only 0000 UTC jet streaks with 1 or more storms recorded 3 hours before or after their occurrences were used  Tornado reports were recorded based on their point of origin  The study time period was March-September

 Enhance vertical wind shear leading to long- lived convection  Decrease static stability  Release CAPE via cooling of a layer of air to saturation  Enhance moisture convergence at low levels  Increase CAPE in jet exit regions  Differential temperature and moisture advection with height

The familiar 4-cell model is used to represent linear jet streaks This is not a typical representation of upper level jet streaks most of the time because jet streaks tend to experience curvature Curved flow will enhance divergence/convergence to the east of a trough/ridge when there Is a jet streak present in the base/top of either Curvature results in a 2-cell model Cyclonic and Anticyclonic Curvature 4-cell model

 May is the peak month for jet-related storm reports  June is the peak month for all storm reports  A decrease in jet-related storm reports in the warm season attributed to  Stronger insolation/relative humidity  Northward shift of the jet stream  Decrease in baroclinicity  Upper level cyclone associated with the North American Monsoon

 In March and April the majority of storm reports occurred in the left exit region and shifted to the right entrance region after April  This shift is due to the northward movement of the jet stream  In March and April the jet stream’s position in the southern U.S. leaves the right entrance region to our south and after April the northern position leaves the left exit region to the north of the U.S.

 Entrance region: storm report maximum in the right entrance  Region of intense upper level divergence  Exit region: storm report maximum in right exit  Attributed to increased CAPE and SRH due to underlying warm sector  Low level convergence more reliable for diagnosing the location of storms than upper level divergence  Composites of mean sea level pressure showed low pressure centers in different areas along the streaks

 Determined using change in jet streak direction along the major jet axis  Used the distance between the midpoints of entrance and exit regions to estimate the radius of curvature (R)  A curvature parameter (R*) used R to assign degrees of curvature to jet streaks  R*>=5: Cyclonic curvature  R*<=-5: Anticyclonic Curvature  -1.5<=R*<=1.5: Linear

 Tornado, wind, and hail reports were more frequent in the entrance/exit regions of anticyclonically/cyclonically curved jets  Linear jet streaks had more evenly distributed tornado and hail reports but wind reports closely resembling those of anticyclonic streaks  Storm report maxima coincide with each jet streak’s strongest area of vertical velocity

 Tornado, hail, and wind reports were maximized in left entrance and exit regions  This is attributed to the strongest divergence/convergence on the cyclonically sheared side of the jet streak  An area of strong low-level convergence is found in conjunction with a surface closed low along the jet axis in the center of the exit region  This area coincides with the highest number of storm reports  The low pressure is in an area where quasi-geostrophic processes contribute to lowering the surface pressure, leading to low level shear and CAPE

 Tornado, wind, and hail reports were maximized in the right entrance regions  A low pressure center is present in the right entrance region of this jet but is not as strong as the low found in the cyclonically curved jet  Recorded the strongest divergence/convergences out of the 3 jet classes and also the strongest vertical velocities

 Tornado and hail reports were evenly distributed but a maximum in wind reports was found in the right entrance  reflecting occurrences of long-lived Mesoscale Convective Systems  The divergence and convergence distribution is generally consistent with the 4-cell model  The strongest vertical velocities were found in the left exit region but were still weaker than velocities recorded in either the cyclonic or anticyclonic streak

 The results related to jet streak curvature coincide with a 2-cell model suggested for curved jet streaks  Overall, “maxima in storm report frequency for each class of curvature correspond to the region where the strongest vertical velocities are predicted” (Clark et al 2008). ams.confex.com/ams/pdfpapers/ pdf

 In the study years WSW jet streaks were the most prominent  Storm reports were most common in the right entrance region of SSW streaks with increased frequency in SW and WSW streaks in all quadrants  Due to overlap between large CAPE and SRH values in SW and WSW streaks  With clockwise turning of streak direction storm reports decreased but those that were reported were more frequent in exit regions  Divergent areas shrink while convergent areas intensify and extend into other regions ams.confex.com/ams/pdfpapers/ pdf

 Clark et al studied tornado, wind, and hail reports relative to upper level jet streaks  Climatology: Tornado reports concentrated in exit regions, hail reports were evenly distributed between the 4 quadrants, and wind reports were most common in the right entrance  Composites: MSLP, low-level convergence, CAPE, and SRH were favorable for severe weather in the right exit region

 Curvature: Storm report maxima occurred in quadrants where vertical velocities were highest  Cyclonic: left entrance/exit  Anticyclonic: right entrance/exit  Linear: even distribution  Direction: SSW jet streaks had the strongest upward vertical velocities  Storm report maxima associated with SW and WSW jet streaks