#### Transcript Severe Weather Part II: The Synoptic Scale Sets the Table Lab 11

```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
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Severe Weather Part II: The Synoptic
Scale Sets the Table
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-or-
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A Synoptic Cookbook Approach
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First let’s review stability – the atmospheric
resistivity to lift.
Severe Weather Part II: The
Synoptic Scale Sets the Table
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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
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Static Stability: S = Gd – Ge =
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Hydrostatic:
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T 

 p
r  r
when r’ greater (smaller) than r, air parcels
will descend (ascend).
Severe Weather Part II: The
Synoptic Scale Sets the Table
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Stability:
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We also have examined SI, LI, and CAPE.
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 Now we’ll look at more empirical information.
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 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
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 Usually we look at the 500 hPa 850 hPa lapse
rate. Some of these take moisture conditions, or
attempt to include moisture empirically also.
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Some indicies used:
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1) Vertical Totals (VT): T850 – T500
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 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
Cross Totals (CT): Td 850 – T 500
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2)
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 cross totals because we are mixing dewpoint temperatures
with ambient temperatures. This is a crude measure of bouyancy.
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3)
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 This is an index that is very popular to use. It’s very simple to
calculate.
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 Empirically includes stability and bouyancy. This index was
popularized by the Air Force.
Total Totals (TT) = CT + VT = T850 + Td 850 – (2T500)
Severe Weather Part II: The
Synoptic Scale Sets the Table
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The “K” index:
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 In 1971 The K index was incorporated into
the NMC’s (now NCEP’s) regular data
stream.
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George’s K Index (J.J. George) “Weather
Forecasting for Aeronautics’ (1960).
Severe Weather Part II: The
Synoptic Scale Sets the Table
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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.
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K = (T850 – T500) + Td850 – (T700 – Td700)
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K = (T850 – T500) + Td850 – DD700
Severe Weather Part II: The
Synoptic Scale Sets the Table
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 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.
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 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
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The “K” index also forecasts thunderstorm
probability
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1) Quick, one only need look at two maps and have
a pen and paper handy, (or calculator)
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2.
Rooted in thermodynamic principles
Severe Weather Part II: The
Synoptic Scale Sets the Table
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1) can differ in differing regions, seasons, and
situations.
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2) Not “coverall” indicies
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3) We look at lower troposphere only.
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4) excludes upper air dynamics
Severe Weather Part II: The
Synoptic Scale Sets the Table
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The SweaT (Severe Weather Threat ) Index
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SW = 12 * D + 20 (TT – 49) + 2f8 + f5 + 125(S +
0.2)
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This index tries to take into account everything we
learn in “cookbook” approaches.
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Plenty of Lowlevel moisture:
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D = 850 hPa dewpoint (Use 0 if Td < = 0 C)
Severe Weather Part II: The
Synoptic Scale Sets the Table
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Stability and bouyancy:
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TT – 49 : Total Totals index – 49.
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The higher, the better!
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TT – 49 = 0 if TT < = 49, there are negative
numbers used here.
Severe Weather Part II: The
Synoptic Scale Sets the Table
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Strong moist, low level winds:
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2*f8 = twice the 850 hPa wind speed.
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Strong vertical speed shear (strong jet
stream):
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F5 = wind speed at 500 hPa
Severe Weather Part II: The
Synoptic Scale Sets the Table
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Strong baroclinicity or Directional shear
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S = sine of the (500 hPa – 850 hPa) wind direction.
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Set S equal to zero if the following are not met:
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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
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1) includes upper air dynamics as well as
thermodynamics.
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2) empirical number
Severe Weather Part II: The
Synoptic Scale Sets the Table
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1) Not a coverall index.
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2) more complex, cannot calculate relatively
quickly.
Severe Weather Part II: The
Synoptic Scale Sets the Table
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Interpretation
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Studied 402 severe thunderstorm cases
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Summary:
SW > 400 necessary for severe weather, Severe
weather not a threat untill SW > 500, tonadoes not a
threat until SW > 600.
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Severe Weather Part II: The
Synoptic Scale Sets the Table
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The end!
Severe Weather Part II: The
Synoptic Scale Sets the Table
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Questions?
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