Weather Forecasting

1
Weather Forecasting
2
Forecasting Goal With Respect to Severe Weather
is to Minimize ImpactReduce Loss of Life and
Limit Property Damage
3
General Forecasting Notes

• Four day forecasts today are about as accurate as
  two day forecasts in the 1970s
• Improved measurement systems are part of the
  reason
• Improvements in numerical modeling techniques are
  part of the reason
• Speed and memory increases in computers are part
  of the reason

4
Forecast Methods

• Persistence (what happened today will continue
  tomorrow)
• Trend (temperature lowered 5 degrees in last 24
  hours, will continue tomorrow)
• Climatology (never rains in July, wont tomorrow)
• Interpolation (60 F at SFO, 80 at SJC so 70 at
  SQL (San Carlos))
• Analogue
• Weather Types (jet stream pattern looks like
  storm of Dec 95)
• 30-90 Day Outlooks
• Numerical Weather Prediction (NWP) Models

5
Numerical Weather Prediction

• Initial Conditions
• Surface Observations
• Airports
• Auto Remote Sites
• Buoys
• Ships
• Upper Air
• Radiosondes
• Satellite Sounders
• Parameters
• Temperature
• Moisture
• Wind
• Pressure

6
Numerical Weather Prediction

• Grids
• Horizontal
• Vertical
• Other Inputs
• Topography
• Date/Season
• Ground Cover
• Numerical Models
• Time Step Forward
• 3, 6, 12, 24, 36, 48384hrs

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Scales of Weather Phenomenon
8
Initializing and Running a Model

• Atmosphere is three dimensional, so both vertical
  and horizontal grids are needed
• Vertical grids are developed on constant pressure
  surfaces (Fig 4.5)
• Notice how grid lines are not equally spaced in
  the verticalWhy? Answer pressure does not
  decrease with height in equal proportionsgeometri
  c decrease not arithmetic
• Horizontal grids are typically equally spaced
• NAM (formerly called ETA) model has 60 vertical
  layers and a grid spacing of 12 km (_at_ 8 miles)
• Can severe weather phenomena be resolved using
  such a grid? See exercise 4.1 (Noteweather
  phenomena smaller than the grid spacing can not
  be resolvedfor examplea 12 km grid can not
  resolve a 1 km tornado

9
How Do Models Work

• Initialization Process
• Measured data are interpolated to gridpoints
• First guess (prior 12 hour forecast) data are put
  on gridpoints
• Observations are used to adjust first guess to
  reality
• Final mathematical adjustments are made to insure
  that data on the model grid satisfy the equations
  that govern the atmosphere

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After Initialization Computational Work Begins

• Rates of change of variables (T,WS,P) are
  evaluated from the equations and calculated over
  a period of time (dx/dt)
• Timesteps are usually five minutes
• Change in quantity of each variable is added to
  the initial conditions, and equations are run
  again
• At the end of an output timestep, say 12 hours,
  gridpoint solutions are filed and a mapping
  process is initiated

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Examples of Model Development DifficultiesSnowco
ver

• Snow ability to radiate energy (albedo) affects
  nighttime temperaturesclear skies and snow cover
  will result in lower minimums
• Model must be initialized with proper snow cover
  area and depth, and changes in snow cover must be
  calculated
• In reality, snow cover is derived from
  satellites, snow depth is estimated (guessed)

12
ALBEDO

• Percent of sunlight reflected from clouds and
  earth surfaces
• Earth average albedo 30

Snow fresh Snow old Ice Water high sun Water low sun Bare earth Forest Green Crops Cities Clouds opaque Clouds - thin 75-95 40-60 70 3-5 10-50 15 5-10 15-25 14-18 50-85 5-50
General rule the lighter the surface the lower
the albedo
High albedo more sunlight reflected Low albedo
more sunlight absorbed
13
Examples of Model Development DifficultiesTopogr
aphy

• Mountains have a tremendous effect on weather
  patterns, and coarse grid spacing is not
  desirable
• Gridding makes terrain features look like lego
  structures
• Even with refined resolution, local weather for
  mountains and valleys can not be resolved

14
Examples of Model Development DifficultiesTopogr
aphy

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Forecast Model Limitations

• Errors in Initial Conditions
• Measurements not made at exact gridpoints
• Satellite derived temperatures have errors no
  RAOBS near the poles
• Small errors grow over time and contaminate
  forecasts

16
Forecast Model Limitations

• Inadequate Resolution
• Narrow topographical features will be
  overlookedlocal rainfall forecasts are affected
• Thunderstorms lake effect snowfalls and local sea
  breeze conditions may be missed

17
Watches and Warnings

• Advisories
• Less Severe
• Dense Fog, Wind, Urban Flooding
• Watches
• Possible Severe Weather (could happen)
• Tornadoes, Severe Thunderstorms, Flash Flooding
• Warnings
• Probable Severe Weather (are happening)
• Tornadoes, Severe Thunderstorms, Flash Flooding