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

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

Severe Weather Part II: The Synoptic Scale Sets the Table Red – e (unstable) purple – e (conditional) blue – e (stable) Black - d and green m Profiles:

Severe Weather Part II: The Synoptic Scale Sets the Table Static Stability: S = d – e = Hydrostatic: when greater (smaller) than, air parcels will descend (ascend).

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

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.

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. Its very simple to calculate. Empirically includes stability and bouyancy. This index was popularized by the Air Force.

Severe Weather Part II: The Synoptic Scale Sets the Table The K index: In 1971 The K index was incorporated into the NMCs (now NCEPs) regular data stream. Georges K Index (J.J. George) Weather Forecasting for Aeronautics (1960).

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 indexs performance to include it. K = (T850 – T500) + Td850 – (T700 – Td700) K = (T850 – T500) + Td850 – DD700

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.

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

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

Severe Weather Part II: The Synoptic Scale Sets the Table The SweaT (Severe Weather Threat ) Index SW = 12 * D + 20 (TT – 49) + 2f8 + f (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)

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.

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

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

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

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

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.

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

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