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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

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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

• First let’s review stability – the atmospheric resistivity to lift.

### 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 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 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

• 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

• 1) includes upper air dynamics as well as thermodynamics.

• 2) empirical number

### Severe Weather Part II: The Synoptic Scale Sets the Table

• 1) Not a coverall index.

• 2)more complex, cannot calculate relatively quickly.

### Severe Weather Part II: The Synoptic Scale Sets the Table

• Interpretation

• 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.

• The end!

• Questions?