FRONTS, FORECASTING TECHNIQUES AND UPPER AIR

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FRONTS, FORECASTING TECHNIQUES AND UPPER AIR

• MSC 243 Lecture 6, 10/1/09

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Heads-up Mid-Term next Thursday October 8th
Decoding short-range MOS forecasts Decoding METAR
code, plotting station models Drawing isobars and
isotherms (contours) from station
models Interpreting Satellite Imagery Interpreting
Radar Imagery Finding surface weather features
such as fronts, high and low pressure systems
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Reading assignment

• Vasquez Weather Forecasting Handbook
• 1.5 Pressure Coordinate System
• 3.1 Chart Analysis (esp. 3.1.3-3.1.8)
• 5.1-5.7 Air Masses and Fronts

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FRONTS
The interface or transition zone between two air
masses of different temperature.
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Air Masses
Air masses are large semi-permanent volumes of
air that exist in a specific area. Air masses are
named for their source region and
characteristics. Maritime or Continental Arctic,
Polar, or Tropical When air-masses collide,
fronts form.
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What is a front?

• The boundary between two air masses
• Named for the advancing air mass
• Cold Front
• Warm Front
• Stationary Front
• Occluded Front
• In any collision of air-masses, the denser air
  wins!
• Colder
• Drier

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Structure of a Cold Front
A lifting mechanism causes moist air to rise
and form clouds that produce precipitation. For a
cold front, moist warm air is forced by the
denser cold air to rise above the cold dome.
NW
SE
Structure of a Warm Front
North
South
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A Cold Frontal Passage

• Before a cold front approaches, winds are often
  from the S/SW/SE. Air is relatively warm and
  moist.
• As front approaches, pressure drops, it gets
  cloudier (cirrus then cumulus/cumulo-nimbus) and
  it often rains or snows heavily.
• After front has passed through, winds are from
  the N/NW, air is relatively cold and dry,
  pressure starts to increase as a high builds over
  the region. Sunny days and cold, calm nights.

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A Warm Frontal Passage

• Before a warm front approaches, air is relatively
  cool and dry.
• As front approaches, pressure does not change
  significantly, low-level stratus appears with
  accompanying drizzle / light rain or snow.
• After front has passed through, winds are from
  the S/SW/SE, air is relatively warm and moist,
  Cloudy, relatively humid days and mild nights due
  to clouds absorbing IR radiation from Earth.

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Stationary Front
Neither the cold nor warm air masses move much in
any direction. Layered, stratus-type clouds,
sometimes accompanied by light precipitation of
long duration
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Occluded Front
Cold Front catches up with Warm Front
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Types of fronts

• Cold Front
• Cold air advances
• Steep slope (vertically)
• Convective Clouds, sometimes heavy rain and snow
• Warm Front
• Cold air retreats
• Gradual Slope (vertically)
• Stratiform clouds, light rain or snow
• Stationary Front
• Neither air mass making much headway
• Occluded Front
• When a cold front catches a warm front
• Dry Line (not technically a front)
• A gradient in moisture, rather than temperature

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Characteristics of Fronts

• Can discern differences in
• Temperature
• Dew Point
• Low surface pressure
• Wind direction
• Clouds and precipitation

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Find the Front

• Fronts can be located by looking for
• A Wind Shift
• Temperature differences
• Dew Point Temperature differences
• Clouds or Precipitation
• Lower surface pressures

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Surface Weather Map

• All stations are not at the same elevation.
  Pressure decreases with height.
• Hence, all pressure readings at the surface are
  adjusted to sea level
• This allows accurate comparison of horizontal
  differences in pressure and eliminates vertical
  differences in pressures due to elevation.
• Isobar - line of equal pressure
• Units millibars or Pascals (Pa) (mb 100 Pa)
• Standard atmospheric pressure 1013.25 mb

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Surface Pressure and Winds
Surface charts show surface high and low pressure
centers, fronts, wind directions, and are good
for determining temperature advection.
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Surface Weather Advection
Red circle Warm advection. Winds bringing
warmer air from the south. Blue circle Cold
advection. Winds bringing cooler air from the
north.
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Surface Temperature Fronts
Cold front is easy to discern strong temperature
gradient with warmer temperatures to the south.
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Forecasting Summary Guide

• 1. NORTH AMERICA SURFACE MAP What are the major
  weather features? What is the big picture? What
  air masses will I be concerned with? Any pending
  frontal passages?
• 2. SATELLITE How are these major weather
  features moving? This gives you a quick
  impression of speed of these systems.
• 3. RADAR Where is precipitation occurring? How
  intense are these systems?

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Forecasting Summary Guide

• 4. METAR OBSERVATIONS What is happening
  locally? What happened last night and today? Link
  with what you see here with what you would expect
  to happen based on your conceptual models
  developed for the weather features you have
  identified in items 1,2, and 3.
• 5. CLIMATOLOGY What is normal? What are the
  extremes/records typically experienced? Will any
  of the weather features identified in the
  previous questions allow the approach of extremes
  or will they be typically normal?
• 6. LOCAL STATION MODEL OBSERVATIONS Begin to
  focus more on the small scale? How fast are
  nearby fronts moving? Any other interesting
  temperature, wind, or moisture patterns they will
  impact your forecast?

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Forecasting Summary Guide

• 7. (LECTURES 7 and 8) UPPER LEVEL FLOW Now we
  need to link the upper air weather flow pattern
  to the surface weather features? Is the upper air
  flow zonal or meridional? Is your location
  downstream or upstream of a upper air trof or
  ridge? Is it likely that there will be upper air
  divergence or convergence? What does all this
  mean for the strengthening or weakening of any
  surface features? Moreover, how will this
  intensity change, if any, alter the magnitude and
  timing of the surface forecast parameters?
  Identify large scale temperature and moisture
  advection? Jet stream position?

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Forecasting Summary Guide

• 8. LOCAL EFFECTS How will topography affect this
  forecast? Near mountains, big lakes, the ocean,
  in a valley, downtown? Downsloping vs Upsloping?
• 9. NUMERICAL MODEL OUTPUT Now that you have an
  idea of the future weather for a forecast region
  after looking at a number of tools, what do the
  numerical models say will happen? Are you close?
  If so, great! If not, decide what you are missing
  by focusing on each forecasted parameter that is
  in question and what factors in turn affect that
  parameter. After thoroughly checking your
  forecast, if in your own mind you still can not
  rationalize the difference, stick to your guns!
  Models are often wrong!
• 10. MOS NUMBERS Look at more specific model
  information from MOS and grid point data. Adjust
  these numbers depending on how your assessment of
  things compared with what the models had to say.

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High Temperature Forecasting

• Three primary factors
• ADVECTION
• Warm advection results in temperature rises
• Cold advection results in temperature falls
• Even advection above the surface can affect
  surface temperatures.
• ADIABATIC WARMING / COOLING
• (will leave until later)
• DIABATIC WARMING / COOLING

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Diabatic Effects

• Factors affecting incoming solar radiation
• Cloud cover
• Type (thickness)
• Duration
• Time of Day
• Ground Moisture / Vegetation
• If dew points are lower than the air temperature,
  falling precipitation will cool temperatures to
  the wet-bulb temperature (in between temperature
  and dewpoint)

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Low Temperature Forecasting

• Factors that promote cool minima
• Clear Skies
• Enhanced radiational cooling
• Light Winds
• Surface decoupling
• Snow Cover
• Enhanced radiational cooling / insulates surface
  from ground below (reduces heat flux below it)
• Low Dew Points
• Water Vapor good absorber of IR radiation, i.e.
  less radiation is absorbed in the atmosphere if
  dew points are low

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Low Temperature Forecasting

• Factors that promote warm minima
• Clouds / Fog
• Absorb IR radiation emitted from ground etc.
• Strong Winds
• Keep the boundary layer mixed
• Urban Heat Island
• High heat capacity of city versus country
• High Dew Points
• Water Vapor good absorber of infrared radiation

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Upper Levels

• So far, we have only looked at surface weather
  features.
• However, the upper levels are crucially important
  for the development of weather systems, and hence
  their forecasts.

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Pressure Levels

• Pressure is the force exerted on an object by all
  air molecules that impinge on a surface area in
  general, the weight of a column of air per unit
  area
• Pressure decreases with height.
• Meteorologists concentrate on a few standard
  pressure levels, plus the surface
• Each of these levels are important in weather
  forecasting for different reasons

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Upper Level Weather Maps

• Upper level weather maps are plotted on a
  constant pressure surface
• Contours of equal geopotential height are plotted
  (height in meters of the "x" mb pressure surface)

Thickness determines temperature of the layer
(e.g. 1000 - 500 mb). Thickness is useful in
determining precipitation type
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Aloft Ridges and Troughs

• Mountains and valleys of warm and cool air
• The height of the pressure level depends on the
  relative temperature of the column

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Height of Pressure Surfaces

• Pressure Surface Average Height
• 850 mb 1460 m / 5000 feet
• 700 mb 3000 m / 10000 feet
• 500 mb 5600 m / 16000 feet
• 300 mb 9180 m / 30000 feet
• 200 mb 11800 m / 35000 feet

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850 mb Chart
The 850 mb chart is good for identifying warm and
cold advection, estimating surface temperatures,
low level moisture, and determining precipitation
type (rain, snow, sleet).
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700 mb chart
The 700 mb chart is used to determine cloud cover
or rainfall, using the relative humidity field
and the vertical motion field.
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700 mb chart
The 700 mb chart is also used to determine
short-wave disturbances via the geopotential
height field.
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500 mb geopotential height
The 500 mb chart is the forecasters favourite
for depicting the motion of weather systems.
It shows the large-scale flow (long waves) and
jet streams, and also the small-scale flow
(short-waves, low level storm systems)
RIDGE
TROUGH
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250 mb Chart
The 250 mb chart is used to locate the jet
stream. Strong upper- level winds help develop
surface low pressure in mid-latitudes.