Severe weather part ii the synoptic scale sets the table
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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. Severe Weather Part II: The Synoptic Scale Sets the Table -or- A Synoptic Cookbook Approach

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Severe weather part ii the synoptic scale sets the table

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 table1

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.


Severe weather part ii the synoptic scale sets the table2

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:


Severe weather part ii the synoptic scale sets the table3

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).


Severe weather part ii the synoptic scale sets the table4

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.


Severe weather part ii the synoptic scale sets the table5

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 table6

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.


Severe weather part ii the synoptic scale sets the table7

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).


Severe weather part ii the synoptic scale sets the table8

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


Severe weather part ii the synoptic scale sets the table9

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 table10

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 table11

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 table12

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)


Severe weather part ii the synoptic scale sets the table13

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 table14

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 table15

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 table16

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 table17

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 table18

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 table19

Severe Weather Part II: The Synoptic Scale Sets the Table

  • The end!


Severe weather part ii the synoptic scale sets the table20

Severe Weather Part II: The Synoptic Scale Sets the Table

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