Weather Analysis 2

Direct (natural) Circulation - Entrance Region

1. Cold air sinking, warm air rising (Weak static stability->more instability)

2. Thermal gradient is destroyed (warm air is forced to rise)

3. Cold air is forced to sink

Indirect (unnatural) Circulation - Exit Region

1. Warm air sinking, cold air rising (Strong static stability->less instability)

2. Thermal gradient is enhanced (warm air is forced to sink)

3. Cold air is forced to rise

Middle Atmosphere

1. Consists of the 500mb level

2. “Level of Non-Divergence” (LND) located at 550mb

3. Vorticity - spin around a vertical axis

Middle Atmosphere - Used to Identify

1. General flow of the atmosphere (mid level troughs and ridges, trough/ridge orientation)

2. Cyclonic/Positive Vorticity Advection (CVA/PVA)

3. Anticyclonic/Negative Vorticity Advection (AVA/NVA)

4. Shortwaves

Middle Atmosphere: Upper Air Plots

1. TT = temperature in C

2. DdDd = dewpoint depression in C (SPC gives actual dew point in C)

3. HHH = coded geopotential height

Middle Atmosphere: Upper Air Plots - HHH

1. 500mb and 400mb - thousands, hundreds, and tens of meters (Ex. 515, add a “0” after the last number)

2. pp = 12-hour height change

Middle Atmosphere: Trough - Deepening

1. Jet max is located behind the trough axis

2. Low-level cold air advection is occurring

Middle Atmosphere: Trough - Filling

1. Jet max is located ahead of the trough axis

2. Low-level warm air advection is occurring

3. Trough contains zonal flow near the rear

Middle Atmosphere: Ridge - Intensifying

1. Jet max is located behind the ridge axis

2. Low-level warm air advection is occurring

Middle Atmosphere: Ridge - Weakening

1. Jet max is located ahead of the ridge axis

2. Low-level cold air advection is occurring

3. Ridge contains zonal flow near the rear

Middle Atmosphere: Relative Vorticity

1. Curvature

2. Shear

Middle Atmosphere: Relative Vorticity - Curvature

Based on varying wind direction

Middle Atmosphere: Relative Vorticity - Shear

1. Based on varying wind speeds

2. Stronger winds alongside slower winds

Middle Atmosphere: Describing Vorticity

1. Positive / Cyclonic

2. Negative / Anticyclonic

3. Opposite in the Southern Hemisphere

Middle Atmosphere: Describing Vorticity - Positive / Cyclonic

1. Spins/rotates counterclockwise

2. Trough (curvature), faster winds to the south (shear)

Middle Atmosphere: Describing Vorticity - Negative / Anticyclonic

1. Spins/rotates clockwise

2. Ridge (curvature), faster winds to the north (shear)

Middle Atmosphere: Vorticity Advection

1. Understand wind flow

2. Positive / cyclonic vorticity advection → leads to divergence aloft (higher vorticity values are being advected into an area with lower vorticity values)

3. Negative / anticyclonic vorticity advection → leads to convergence aloft (lower vorticity values are being advected into an area with higher vorticity values)

Middle Atmosphere: Parcel and Vorticity - Entrance

1. As a parcel enters a vorticity field, the parcel begins to spin

2. Increased spin → vertical stretching

3. If the vorticity continues to increase, vertical stretching will continue

Middle Atmosphere: Parcel and Vorticity - Exit

1. As a parcel leaves a vorticity field, the parcel’s spin decreases

2. Decreasing vorticity → vertical compression

3. If the vorticity continues to decrease, vertical compression will continue

Middle Atmosphere: Locating PVA and NVA

1. PVA → upper-level divergence = rising air, clouds, and precipitation

2. NVA → upper-level convergence = sinking air, clear skies

Middle Atmosphere: Shortwaves

1. Identifiable at 500mb, but easier to identify at 700mb

2. An area of lower heights embedded within the large-scale flow

3. Related to thermal advection

4. Associated with colder temperatures

5. Contains vorticity due to cyclonic flow

6. Size = 1 to 40 in longitude, 100 to 1000 miles in amplitude

7. Upwards motion is found ahead of the shortwave (can contribute to rain and thunderstorms)

Middle Atmosphere: Shortwaves - Identifying

1. Cyclonic wind shift in the atmosphere

2. Thermal trough upstream

3. Highest 500mb vorticity is usually collocated with the location of a shortwave

Lower Middle Atmosphere: 700 mb

3,000 m (10,000ft)

Lower Middle Atmosphere: 700mb - Used to identify

1. Shortwaves

2. Depth of warm and cold air advection

3. Depth of moisture advection

4. Inversions

5. Shortwaves

Lower Middle Atmosphere: Upper Air Plots

1. TT = temperature in C

2. DdDd = dewpoint depression in C (SPC gives actual dew point in C)

3. HHH = coded geopotential height

4. pp = 12-hour height change

Lower Middle Atmosphere: Upper Air Plots - HHH

1. 700mb - hundreds, tens, and ones of meters

2. Thousands place is always given a value to “2” or “3”

3. Value should be close to 3,000m (Ex. 223)

Lower Middle Atmosphere: Overview

1. Weather systems typically have an open, broad pattern at this level

2. Fronts are found poleward at 700mb compared to fronts at 850mb, 925mb, and surface charts

3. Surface lows tend to move at 70% of the 700mb wind speeds

4. Most clouds in a frontal system form between 5,000’-15,000’ AGL

5. 700mb charts often provide isodrosotherms

6. Shortwaves are easily identifiable at this level

Lower Middle Atmosphere: Upward Motion

1. Relative humidity is a great measure of synoptic-scale lift and moisture

2. Relative humidity values of 70% or dewpoint depressions 5C or less tend to produce overcast conditions

Lower Middle Atmosphere: UVVs

1. Used to identify Upward Vertical Velocities (UVV)

2. Based on the Omega equation (Combination of thermal advection and vorticity advection)

3. WAA in the low-levels (850mb) and PVA in the mid-levels (500mb) (-omega or -UVV → rising air)

4. CAA in the low-levels (850mb) and NVA in the mid-levels (500mb) (+omega or +UVV → sinking air)

Lower Middle Atmosphere: Convective Forecasts

1. Flow tends to steer airmass thunderstorms

2. Can provide an inversion

3. Dew point depression of 5C or less at 700mb and 850mb indicates deep layer moisture (Heavy rain likely if a lifting mechanism is available)

Lower Middle Atmosphere: Convective Forecasts - Inversion

1. Temperature increasing with height causes a stable layer

2. Inhibits or delays convection based on temperature and whether the cap can be mixed out (Temperatures of 14C or greater will inhibit convection)

3. Originates from the southwestern U.S. and is advected eastward

Lower Middle Atmosphere: Subsidence Inversion

1. An inversion caused by warming from above (associated with downward motion from above)

2. Typically found around 700mb

3. It will take several hours for the inversion to “mix out” if the inversion is 2C or greater

Lower Middle Atmosphere: Radiation Inversion

1. An inversion caused by the cooling of the Earth’s surface

2. Found near the surface