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

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