NATS 101 Lecture 25 Weather Forecasting I

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NATS 101Lecture 25Weather Forecasting I

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Review ET CyclonesIngredients for
Intensification

• Strong Temperature Contrast
• Jet Stream Overhead
• S/W Trough to West
• UL Divergence over Surface Low
• If UL Divergence exceeds LL Inflow, Cyclone
  Deepens
• Similar Life Cycles

deepening
filling
Ahrens, Meteorology Today, 5th Ed.
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Reasons to Forecast Weather Climate

• Should I bring my umbrella to work today?
• Should Miami be evacuated for a hurricane?
• How much heating oil should a refinery process
  for the upcoming winter?
• Will the average temperature change if CO2 levels
  double during the next 100 years?
• How much to charge for flood insurance?
• How much water will be available for agriculture
  population in 30 years
• These questions require weather-climate forecasts
  for today, a few days, months, years, decades

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Forecasting Questions

• How are weather forecasts made?
• How accurate are current weather forecasts?
• How accurate can weather forecasts be?
• We will emphasize mid-latitude forecasts out to
  15 days where most progress has been made.

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Types of Forecasts

• Persistence - forecast the future atmospheric
  state to be the same as current state
• -Raining today, so forecast rain tomorrow
• -Useful for few hours to couple days

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Types of Forecasts

• Trend - add past change to current condition to
  obtain forecast for the future state
• -Useful for few hours to couple days

10 am 11 am 12 pm
59? F 63? F 67? F
Past Now Future
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Types of Forecasts

• Analog - find past state that is most similar to
  current state, then forecast same evolution
• -Difficulty is that no two states exactly alike
• -Useful for forecasts up to one or two days

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Types of Forecasts

• Climatology - forecast future state to be same as
  climatology or average of past weather for date
• -Forecast July 4th MAX for Tucson to be 100 F
• -Most accurate for long forecast projections,
  forecasts longer that 30 days

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Types of Forecasts

• Numerical Weather Prediction (NWP) - use
  mathematical models of physics principles to
  forecast future state from current conditions.
• Process involves three major phases
• Analysis Phase (estimate present conditions)
• Prediction Phase (computer modeling)
• Post-Processing Phase (use of products)
• To justify NWP cost, it must beat forecasts of
  persistence, trend, analog and climatology

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Analysis Phase

• Purpose Estimate the current weather conditions
  to use to initialize the weather forecast
• Implementation Because observations are always
  incomplete, the Analysis is accomplished by
  combining observations and the most recent
  forecast

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Analysis Phase

• Current weather conditions are observed around
  the global (surface data, radar, weather
  balloons, satellites, aircraft).
• Millions of observations are transmitted via the
  Global Telecommunication System (GTS) to the
  various weather centers.
• U.S. center is in D.C. and is named National
  Centers for Environmental Prediction (NCEP)

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Analysis Phase

• The operational weather centers sort, archive,
  and quality control the observations.
• Computers then analyze the data and draw maps to
  help us interpret weather patterns.
• Procedure is called Objective Analysis.
• Final chart is referred to as an Analysis.
• Computer models at weather centers make global or
  national weather forecast maps

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Surface Data
Sparse data over oceans and Southern Hemisphere
Courtesy ECMWF
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Surface Buoy Reports
Some buoy data over Southern Hemisphere
Courtesy ECMWF
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Radiosonde Coverage
Little data over oceans and Southern Hemisphere
Courtesy ECMWF
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Aircraft Reports
Little data over oceans and Southern Hemisphere
Courtesy ECMWF
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Weather Satellites

• Satellite observations fill data void
  regions
• Geostationary Satellites
• High temporal sampling
• Low spatial resolution
• Polar Orbiting Satellites
• Low temporal sampling
• High spatial resolution

Ahrens, Figs. 9.5 9.6
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Obs from Geostationary Satellites
Courtesy ECMWF
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Temperature from Polar Satellites
Courtesy ECMWF
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Atmospheric Models

• Weather models are based on mathematical
  equations that retain the most important aspects
  of atmospheric behavior
• - Newton's 2nd Law (density, press, wind)
• - Conservation of mass (density, wind)
• - Conservation of energy (temp, wind)
• - Equation of state (density, press, temp)
• Governing equations relate time changes of fields
  to spatial distributions of the fields
• e.g. warm to south southerly winds ? warming

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Prediction Phase

• Analysis of the current atmospheric state (wind,
  temp, press, moisture) are used to start the
  model equations running forward in time
• Equations are solved for a short time period
  (5 minutes) over a large number (107 to 108) of
  discrete locations called grid points
• Grid spacing is 2 km to 50 km horizontally
  and 100 m to 500 m vertically

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Model Grid Boxes
100-500 m
10-20 km
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A Lot Happens Inside a Grid Box(Tom Hamill,
CDC/NOAA)
Rocky Mountains

• Approximate Size of One Grid Box for NCEP Global
  Ensemble Model
• Note Variability in Elevation, Ground Cover, Land
  Use

Denver
Source www.aaccessmaps.co
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13 km Model Terrain
Big mountain ranges, like the Sierra Nevada
Range, are resolved. But isolated peaks, like the
Catalinas, are not evident!
100 m contour
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40 km Model Grid and Terrain
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Post-Processing Phase

• Computer draws maps of projected state to help
  humans interpret weather forecast
• Observations, analyses and forecasts are
  disseminated to private and public agencies,
  such as the local NWS Forecast Office and UA
• Forecasters use the computer maps, along with
  knowledge of local weather phenomena and model
  performance to issue regional forecasts
• News media broadcast these forecasts to public

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Suite of Official NWS Forecasts
CPC Predictions Page
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NWP Forecasts

• Next lecture, we will show more analyses and
  forecasts from the current suite of NCEP forecast
  products

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Summary Key Concepts

• Forecasts are needed by many users
• There are several types of forecasts
• Numerical Weather Prediction (NWP)
• Use computer models to forecast weather
• -Analysis Phase
• -Prediction Phase
• -Post-Processing Phase
• Humans modify computer forecasts

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Summary Key Concepts

• National Center for Environment Prediction (NCEP)
  issues operational forecasts for
• El Nino tropical SST anomalies
• Seasonal outlooks
• 10 to 15 day weather forecasts
• 2 to 3 day fine scale forecasts

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Assignment for Next Lecture

• Topic - Weather Forecasting Part II
• Reading - Ahrens pg 249-254
• Problems - 9.11, 9.15, 9.18