Severe Weather Part II: The Synoptic Scale Sets the Table

1. Severe Weather Part II: The Synoptic Scale Sets the Table Lab 11 Anthony R. Lupo Atms 4310 / 7310

2. Severe Weather Part II: The Synoptic Scale Sets the Table • Severe Weather Part II: The Synoptic Scale Sets the Table • -or- • A Synoptic Cookbook Approach • First let’s review stability – the atmospheric resistivity to lift.

3. Severe Weather Part II: The Synoptic Scale Sets the Table • Red – Ge (unstable) purple – Ge (conditional) blue – Ge (stable) Black - Gd and green Gm • Profiles:

4. Severe Weather Part II: The Synoptic Scale Sets the Table • Static Stability: S = Gd – Ge = • Hydrostatic: • when r’ greater (smaller) than r, air parcels will descend (ascend).

5. Severe Weather Part II: The Synoptic Scale Sets the Table • Stability: • We also have examined SI, LI, and CAPE. •  Now we’ll look at more empirical information. •  These indicies were drawn up by looking at many case studies and stating something general about the stability conditions.

6. Severe Weather Part II: The Synoptic Scale Sets the Table •  Usually we look at the 500 hPa 850 hPa lapse rate. Some of these take moisture conditions, or attempt to include moisture empirically also. • Some indicies used: • 1) Vertical Totals (VT): T850 – T500 •  this is based on the 850 hPa 500 hPa lapse rate, or stability. We are looking for values in excess of 26 as a good indicator of severe weather.

7. Severe Weather Part II: The Synoptic Scale Sets the Table • 2) Cross Totals (CT): Td 850 – T 500 •  cross totals because we are mixing dewpoint temperatures with ambient temperatures. This is a crude measure of bouyancy. • 3) Total Totals (TT) = CT + VT = T850 + Td 850 – (2T500) •  This is an index that is very popular to use. It’s very simple to calculate. •  Empirically includes stability and bouyancy. This index was popularized by the Air Force.

8. Severe Weather Part II: The Synoptic Scale Sets the Table • The “K” index: •  In 1971 The K index was incorporated into the NMC’s (now NCEP’s) regular data stream. • George’s K Index (J.J. George) “Weather Forecasting for Aeronautics’ (1960).

9. Severe Weather Part II: The Synoptic Scale Sets the Table • The K-Index appears in the NCEP composite moisture chart (Upper Left-hand corner under the lifted Index). They were impressed enough with the index’s performance to include it. • K = (T850 – T500) + Td850 – (T700 – Td700) • K = (T850 – T500) + Td850 – DD700

10. Severe Weather Part II: The Synoptic Scale Sets the Table •  Takes into account stability, moisture content of lower troposphere. The DD700 tries to account for “deep layer” moisture assuming that a deeper moisture layer means more fuel for the fire. •  The K-Index turns out to be a better indicator of airmass type-thunderstorms as opposed to baroclinic type severe weather.

11. Severe Weather Part II: The Synoptic Scale Sets the Table • The “K” index also forecasts thunderstorm probability • Advantages to all these indicies: • 1) Quick, one only need look at two maps and have a pen and paper handy, (or calculator) • 2. Rooted in thermodynamic principles

12. Severe Weather Part II: The Synoptic Scale Sets the Table • Disadvantages: • 1) can differ in differing regions, seasons, and situations. • 2) Not “coverall” indicies • 3) We look at lower troposphere only. • 4) excludes upper air dynamics

13. Severe Weather Part II: The Synoptic Scale Sets the Table • The SweaT (Severe Weather Threat ) Index • SW = 12 * D + 20 (TT – 49) + 2f8 + f5 + 125(S + 0.2) • This index tries to take into account everything we learn in “cookbook” approaches. • Plenty of Lowlevel moisture: • D = 850 hPa dewpoint (Use 0 if Td < = 0 C)

14. Severe Weather Part II: The Synoptic Scale Sets the Table • Stability and bouyancy: • TT – 49 : Total Totals index – 49. • The higher, the better! • TT – 49 = 0 if TT < = 49, there are negative numbers used here.

15. Severe Weather Part II: The Synoptic Scale Sets the Table • Strong moist, low level winds: • 2*f8 = twice the 850 hPa wind speed. • Strong vertical speed shear (strong jet stream): • F5 = wind speed at 500 hPa

16. Severe Weather Part II: The Synoptic Scale Sets the Table • Strong baroclinicity or Directional shear • S = sine of the (500 hPa – 850 hPa) wind direction. • Set S equal to zero if the following are not met: • a) 850  wind direction 130 – 250 • b) 500  wind direction 210 – 310 • c) 500dir – 850dir  < = 0

17. Severe Weather Part II: The Synoptic Scale Sets the Table • Advantanges: • 1) includes upper air dynamics as well as thermodynamics. • 2) empirical number

18. Severe Weather Part II: The Synoptic Scale Sets the Table • Disadvantages: • 1) Not a coverall index. • 2) more complex, cannot calculate relatively quickly.

19. Severe Weather Part II: The Synoptic Scale Sets the Table • Interpretation • Studied 189 tornado cases • Studied 402 severe thunderstorm cases • Summary: • SW > 400 necessary for severe weather, Severe weather not a threat untill SW > 500, tonadoes not a threat until SW > 600.

20. Severe Weather Part II: The Synoptic Scale Sets the Table • The end!

21. Severe Weather Part II: The Synoptic Scale Sets the Table • Questions? • Comments? • Criticisms?