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2. 2 Presented by John P. Monteverdi Professor of Meteorology Department of Geosciences San Francisco State University Research completed as part of appointments as Visiting Scientist Spring 2000 National Severe Storms Lab Norman, Oklahoma National Weather Service Forecast Office San Francisco Bay Area

3. 3 Great Plains, USA Tornado Alley

4. 4 Controls on “Wizard of Oz” (Supercell) Tornadic Thunderstorms Buoyancy (and forces that augment it) Strong shear (vertical change in wind direction and speed) which encourages rotation in updraft, strengthens updraft, and fosters “domino effect” (termed supercell cascade) to tornado

5. 5 What isVertical Shear? Is a measure of the change in wind direction and speed with height Is estimated visually best from a hodograph The dots represent the tips of the wind observations at each level. The length of the hodograph is proportional to the magnitude of the shear through the layer Arrows joining wind observations at various levels show the shear vector in the intervening layer. In this case, the wind and the wind shear vectors are veering with height This case shows a clockwise CURVED HODOGRAPH. Shear associated with a veering wind with height is called POSITIVE SHEAR. Positive Shear values are greatest in curved hodographs (in which the wind shear vectors also veer with height). Rotating thunderstorms (supercells)tend to develop in environments with large values of positive shear between the ground and 500 mb (termed 0-6 km positive shear). Rotating thunderstorms tend to become tornadic (tornadic supercells) when large values of positive shear are found in the inflow layer (this tends to be the 0-1 km layer)

6. 6 Importance of Shear Removes precipitation from updraft area and shunts it down wind (updraft is not suppressed and becomes more long-lived) Deep layer shear can create horizontal spin (vorticity) which can be tilted into the vertical by the updraft and transformed to vertical vorticity (storm scale rotation--mesocyclone) In certain configurations of low level positive shear, there are forces that augment the updraft by a factor of two to three times In certain configurations of positive shear the storm can be forced to “deviate” from motions of other storms

7. 7 Supercell Tornadic Storms: Cascade Paradigm Vertical Shear Allows Precipitation To Be Removed From Updraft Area Vertical Shear Sufficient To Generate Horizontal Rotation Which Is Tilted Into Vertical To Form Persistent Midlevel Mesocyclone If low level Shear Vector Veers Sufficiently (curved hodograph), Updraft And Rotation Will Be Augmented on Right Flank (with Respect to hodograph)

8. 8 Supercell Tornadic Storms: Cascade Paradigm Hook Echo Mean Wind Storm Motion

9. 9 Convective updraft converts 0-6 km shear into vertical vorticity (counterclockwise rotation) at midlevels (mesocyclone) Persistant mesocyclone causes precipitation hook to rear flank Rear flank downdraft (RFD) develops in association with hook Interaction of RFD with highly sheared inflow air (shear in 0-1 km layer) under upshear (usually northwest) side of mesocyclone causes tornado Supercell Tornadic Storms: Cascade Paradigm

10. 10 Sfc southeasterlies surmounted by mid and upper tropospheric southwesterlies creates favorable hodograph and shear favorable for tornadic supercells.

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14. 14 Central Valley: Great Plains West Tornado Alley West

15. 15 Birth of a Hypothesis Why do so many tornadoes occur in California’s Central Valley, and, to some extent, in the coastal valleys?

16. 16 Shear and Buoyancy Associated with 70 Tornadic and Non-Tornadic Thunderstorms in Northern and Central California, 1990-1994 John P. Monteverdi San Francisco State University Charles Doswell III National Severe Storms Laboratory Gary Lipari San Francisco State University

17. 17 Schematic Synoptic Pattern Central Valley Thunderstorms

18. 18 Combination of surface southeasterly flow and barrier- induced low level jet can yield strongly clockwise-curved hodographs in Sacramento and San Joaquin Valleys. Topographic channeling evident in coastal valleys as well.

19. 19 Example of Favorable Shear Profile Caused by Surface Southeasterly Flow Surmounted by Low Level Jet

20. 20 Low Level Jet

21. 21 An Example of Very Favorable Shear for A California Tornado Event This case shows a clockwise CURVED HODOGRAPH. It occurred at Hanford (Fresno) on the afternoon of two supercell tornadoes at nearby Lemoore.

22. 22 November 22, 1996 Upper and mid- tropospheric jet Sfc leeside trough Sfc southeasterlies Curved hodograph-- favorable deep layer shear

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24. 24 Purposes of Study  To extend previous study (LM) by determining if buoyancy and shear played a significant role in distinguishing between tornadic and non-tornadic thunderstorms in the study period.  To determine if the data array and the statistical analyses of the results suggested possible “threshold values” to be used operationally in the forecasting of tornadic thunderstorms.

25. 25 Analysis Techniques As in LM, Used soundings from OAK (mostly 00Z) (one VBG, one MFR), modified by surface conditions at site closest to event Considered 3 different event types for period 1990-1994, inclusive –NULL cases … all cases in which thunder observed at SAC or FAT but no observed tornadoes in California –F0 tornado cases (from L M, suspectmost non-supercells) –F1+ tornado cases (from LM, suspect many/most supercells)

26. 26 Buoyancy calculated via “SHARP” program, updated with obs from nearest surface site Shears calculated two ways: –Positive shear calculated by SHARP (portion of hodograph in which wind veers or there is neutral directional shear) –as vector differences between top and bottom of the layers (0-1, 0- 2, 0-3, and 0-6 km … all AGL), updated with surface observations

27. 27 Northern and Central California Tornadoes 1990-94

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29. 29 An Unusual Supercell Sunnyvale, CA May 4, 1998

30. 30 An Unusual Supercell

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33. 33 Buoyancy Associated With California Thunderstorms is typically “low” (SBCAPE ~ 2000 J/kg in Plains) this relatively low (when compared to warm season Great Plains values) CAPE was and is used by many forecasters as a reason to discount tornado risk in the state

34. 34 Dispensing With Outmoded Notions How to prove that tornado occurrence is unrelated to buoyancy (strength of convection)?

35. 35 Average buoyancy was less than 500 J/kg for non- tornadic thunderstorms, thunderstorms with F0 tornadoes, and thunderstorms with F1/F2 tornadoes There were no statistically-significant differences between the case bin buoyancies. Buoyancy magnitude could not be used as a discriminator between non- tornadic thunderstorm, F0 and F1/F2 events.

36. 36 Shear Associated With California Thunderstorms deep layer shear (0-6 km) can be very large when thunderstorms occur in association with cool season patterns low-level shear (0-1 km) is very large in association with cool season thunderstorm patterns due to topographic channeling, particularly in the Central Valley and many coastal valleys, and to the development of a low-level barrier jet in the Sacramento Valley

37. 37 Results of Study Mean shear magnitudes for F1/F2 bin are significantly larger than those observed for either the Non-tornadic (NULLS) and F0 bins

38. 38 There was a statistically significant Difference between 0-1 km shear for F1/F2 tornadoes and that for F0 tornadoes There was a statistically significant Difference between 0-6 km shear for F1/F2 tornadoes and that for F0 tornadoes There was no statistically significant Differences between the shear magnitudes For the Null and F0 Bins

39. 39 With Much Caution Warranted Due to Small Sample Size Some Thresholds Are Suggested

40. 40 The data groupings suggest That 0-1 km Positive Shear Was a discriminator for the F1/F2 events and…. ….that shear thresholds can be defined that might be of operational use in anticipat- ing F1/F2 Events …and of some operational use in anticipating tornado events in general, though with significant FAR

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43. 43 Implications for Forecasting Buoyancy unimportant in distinguishing risk for tornadic thunderstorms from risk from general thunderstorms Results suggest that shear values can aid forecasters in anticipating F1/F2 events (probably supercellular ) Results suggest that shear values alone cannot be used absolutely to distinguish between non-tornadic and F0-producing thunderstorms

44. 44 Current Directions of Research Expansion of California data set in two phases: 1995-present and 1950-1989 (with C. Doswell III) Comparison with low-buoyancy high-shear cases in Australia (with C. Doswell III and B. Hanstrum, Australian Meteorological Services)

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