17 August 2007 Severe Weather Episode: Fingerprints of a Northwest-Flow Event

1. 17 August 2007 Severe Weather Episode: Fingerprints of a Northwest-Flow Event Matthew Steinbugl – NOAA/NWS/CTP 9th Northeast Regional Operational Workshop (NROW)November 7-8, 2007

2. Overview • Briefly Compare/Contrast Synoptic Patterns and some Meteorological Parameters associated with NWF events • Examine Large Scale Forcing/Jet Dynamics • Thermodynamic environment/Mesoscale boundaries • Early Forecasts/Event Evolution • Display Model Data/Radar Imagery • Potential Near-Miss for 2007 LLWS…WHAT IFs

3. NWF Synoptic Climatology Highlights (Johns 1984) • SW flow (SWF) vs. NW flow (NWF) – seasonal frequencies, jet streams, baroclinicity • NWF events usually characterized by strong thermodynamic forcing in vicinity of surface boundaries • Mid/Upper jet typically plays an indirect role in determining outbreak location • Influence of the 500mb short wave trough generally weak • Differences from the basic (composite) pattern occur frequently • Large directional and small speed contributions to the wind shear vector

4. 17 August 2007 Event Highlights • Not a classic NWF case (digging short wave, anomalous cyclonically curved upper jet, location vs. composite) • Primary foci for vigorous ascent and rapid convective development included coupled upper level jet streaks superimposed within region of maximum PVA (divergence) aloft • Relatively “Early” event time (16-18Z) versus severe weather climatology • Similar to NWF synoptic set-up in that outbreak occurred • In the convergence zone near a sfc low center and attendant warm front. • South of the 500mb jet (anticyclonic-shear side). • Other similarities noted as well.

5. 17 August 2007 Event Overview • 38/216 reports from PA • Occurred between 12-3pm • Nickel to baseball size hail • Rotating supercells with TVSs • Wind damage but no tornadoes

6. 500hPa event location comparison

7. Anomalous 500 hPa trough and upper jet 500 hPa heights 250 hPa u and v winds

8. Coupled jets and corresponding UVM • Primary mechanism for rapid intensification of convection by enhancing 700-500mb UVVELs RUC 300mb isotachs (fill) ageostrophic wind and 700-500mb omega valid at 1700/1900 UTC. UVM increases from -6 to -12 microbars/sec.

9. Jet Streak Analysis/Vertical Circulations

10. - Vorticity Advection – An approximation of divergence aloft • PVA (divergence) superimposed within coupled jet circulation induced significant large scale ascent • This was associated with the explosive convection over Central PA X

11. Evolution of Surface/mesoscale boundaries and the release of conditional instability 12Z 15Z 00Z 18Z

12. Initial slight risk/general thunder areas where south and east of where severe convection developed. Forecast was amended around 1245 pm. Local and Regional WFO forecasts did not indicate the potential for thunderstorms. A collaboration issue serving as the primary motivation for this case study. Early Forecasts

13. SVR Tstorm Watch – 135pm

14. Certainly not Cut and Dry… MSLP 15Z MSLP 18Z PWAT 18Z

15. 1700 UTC IPT LAPS Sounding • TT = 50 • FzgLvl= 11.8 • Stm Motion = 294/25kts • 0-3km SRH = 223 m2s2 • LI = -6.1 • CAPE = 1700 J/kg • CIN = 0 J/kg • 70-80KT midlvl speed max • Lolvl wnd backed

16. High resolution NAM did very well 15Z 21Z SB CAPE Conv Pcpn Conv Pcpn SB CAPE Showalter LI LI Showalter

17. SREF 6hr Prob of 0.05” and IPT CAPE Plume Mean ~500 Jkg-1 A few members over 1000 Jkg-1 18 to 00z

18. Water Vapor/300mb isotachs Loop • 1515-1630Z loop • Convection develops over Central PA in coupled jet • “X” denotes S/W crossing Lake Erie providing additional large scale lift/PVA • Little to no moisture in the mid to upper levels X

19. Rapid initiation, intensification IPT storm splits, right mover becomes storm of interest SVR issued 1610Z TOR issued 1656Z First Hail report approx 1700Z Radar Loop 1528-1726Z IPT

20. Radar Loop continued 1730-1820Z • Storm continues ESE (rightmover) • Several large hail reports • Wind damage • No reports of tornadoes • Lancaster county storm was also severe (hail/damaging winds)

21. 0.5° Reflectivity/SRM Close-up1652Z 70+ dBZ TVS IPT

22. 0.5° Reflectivity/SRM Close-up1704Z…12 minutes later 70+ dBZ TVS

23. Four panel display 1.3 = 73dBZ, 12kft 2.4 = 72dBZ, 19kft 3.1 = 69dBZ, 24kft 4.0 = 66dBZ, 30kft Height of: 0°C = 12kft, -20°C= 22kft -60-70dBZ depth of 30kft - Deep Well-defined, persistent mesocyclone (not shown) Lost Archive Level II data feed ( NO GR2) 4-Panel View 1.3°, 2.4°, 3.1°, 4.0°

24. Storm Photos/Damage Williamson

25. Summary and Conclusions • NWF event awareness – watch out! • Sometimes difficult to forecast – large forcing, subtle features, rapid evolution = need high situational awareness • Coupled jet circulation juxtaposed in max PVA/divergence collocated in moist/unstable warm sector = vigorous UVM • Favorable shear profiles INVOF sfc warm front = supercell environment • Unique NWF event - but many similarities or “fingerprints” to synoptic climatology/parameters • Near-miss worst case scenario for the 2007 LLWS

26. 2007 LLWS WHAT Ifs! • Pre-Event Awareness Low (SPC Convective Outlook/CTP HWO) – “Sudden” • Had a tornado struck the stadium (capacity of around 9,000 with room for an additional 30-35,000 on the terraced hills beyond the outfield • Storm hit just prior to the first pitch of the 2007 LLWS – time for evacuations? Where? Shelter for approx. 40K +? • Attendees could have been subjected to large hail/damaging winds – numerous injuries/possible fatalities? • Lightning safety – considering the venue

27. Questions Matthew.Steinbugl@noaa.gov http://nws.met.psu.edu/severe/2007/17Aug2007.pdf