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

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A Synoptic Cookbook Approach

First let’s 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).

pT

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 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. 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 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 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 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 + 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 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?

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