Weather Forecasting - I

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Weather Forecasting - I
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Review of Chapter 12

• The polar front model (Norwegian model) of a
  developing mid-latitude cyclonic storm represents
  a simplified but useful model of how an ideal
  storm progresses through the stages of birth,
  maturity and dissipation.
• Cyclogenesis, lee-side lows, northeasters, bombs.
• For a surface mid-latitude cyclonic storm to
  form, there must be an area of upper-level
  divergence above the surface low. For the surface
  storm to intensify, this region of upper level
  divergence must be greater than surface
  convergence.

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Another way of looking at it.
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Review of Chapter 12

• When the polar-front jet stream develops into a
  looping wave, it provides an area of upper-level
  divergence for the development of surface
  mid-latitude cyclonic storms.

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Review of Chapter 12

• The curving nature of the polar-front jet stream
  tends to direct surface mid-latitude cyclonic
  storms northeastward and surface anticyclones
  southeastward.
• Skip the sections Conveyor belt model of
  mid-latitude cyclones. A developing mid-latitude
  cyclone the March storm of 1993. Vorticity,
  divergence, and developing mid-latitude cyclones.
  Earth vorticity, relative vorticity and absolute
  vorticity. Putting it all together a monstrous
  snowstorm. Polar lows.

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Importance of forecasting air-travel
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Importance of forecasting
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Importance of forecasting
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Acquisition of Weather Information

• Surface measurements (surface weather stations)
• Land (more than 10,000 stations)
• Ocean (ships, buoys, drifting automatic stations)
• Upper air data radiosondes, aircraft,
  satellites

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Collection and Exchange of Weather Information

• The World Meteorological Organization (WMO) UN
  agency, 175 nations, standardization and exchange
  of data.
• World Meteorological Centers Melbourne, Moscow,
  Washington D.C.
• National Center for Environmental Prediction
  (NCEP)
• Camp Springs, MD.
• Data analysis, preparation of weather maps,
  prediction of the weather over the country.
• Weather Forecast Offices (WFO)
• Regional weather maps
• National Weather Service
• Advisories and warnings for severe weather
• Advisory less hazardous conditions due to wind,
  dust, fog, snow, sleet, freezing rain
• Watch atmospheric conditions favor hazardous
  weather, actual location and timing of occurrence
  is uncertain
• Warning hazardous weather is imminent or
  actually occurring

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Organizational structure

• US Department of Commerce
• National Oceanic and Atmospheric Administration
  (NOAA)
• National Ocean Service (NOS) and National
  Geodetic Survey (NGS)
• National Environmental Satellite, Data, and
  Information Service (NESDIS)
• National Marine Fisheries Service (NMFS)
• Program Planning and Integration (PPI)
• Office of Oceanic and Atmospheric Research (OAR)
• National Weather Service (NWS)
• National Centers for Environmental Prediction
  (NCEP)
• Aviation Weather Center (AWC)
• Climate Prediction Center (CPC)
• Environmental Modeling Center (EMC)
• Hydrometeorological Prediction Center (HPC)
• Ocean Prediction Center (OPC)
• NCEP Central Operations
• Space Environment Center (SEC)
• Storm Prediction Center (SPC)
• Tropical Prediction Center (TPC)
• - National Hurricane Center (NHC)

Gary Locke
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Weather Warnings

• Wind Advisory Winds between 25-39 mph
• High Wind Warnings Winds above 40 mph
• Wind Chill Advisory Wind-chill temperatures -30
  to -35F and below
• Heat Advisory/warning Daytime heat index reaches
  105F/115F or higher for 3 or more hours.
• Flash Flood Watch Heavy rain may result in
  flash flood in the area.
• Flash Flood Warning Flash flooding is occurring
  in the area.
• Severe Thunderstorm Watch Thunderstorm with
  winds above 57 mph are possible.
• Severe Thunderstorm Warning Thunderstorms with
  winds above 57 mph have been visually sighted

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Weather Warnings

• Tornado Watch/Warning Tornado may/has
  develop(ed) in the area
• Snow Advisory There is a substantial snowfall
  expected in the area 2 in. or more in 12 hrs or
  3 in. or more in 24 hrs.
• Blizzard Warning Snow with winds exceeding 35
  mph, visibility less than ¼ mi for several hours.
• Small Craft Advisories Winds from 21-39 mph can
  be expected.
• Gale Warnings Winds range from 39-54 mph in the
  area.
• Hurricane Watch A hurricane or a tropical storm
  is a threat to a costal line. Be prepared!
• Hurricane Warning The storm appears to be
  striking the area within 24 h. It is time to get
  out!

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Forecasting Methods

• Weather prediction by hand
• Charts and maps have been drawn and analyzed by
  hand.
• The upper air motion determines the general
  direction of movement of the weather systems and
  the likely location for the development of
  storms.
• Relatively accurate short term forecasts.
• Impractical for long term forecasts.
• Numerical weather prediction
• An atmospheric numerical model is used to
  describe the atmosphere set of equations that
  describe how the atmospheric temperature,
  pressure, winds, moisture will change with time.
• The area of interest is covered by a
  3-dimensional grid, the equations are solved at
  the grid points (4 km or more apart)
• The current atm. conditions are used to calculate
  the atm. properties after some time (time step).
• The process is repeated as the calculated atm
  parameters are used to make a new calculation

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Current Weather Conditions
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Gainesville 10-day forecast
Do you believe the 10th day forecast?
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Why the forecasts are not perfect?

• Short term forecasts (3-5 days) are usually
  rather accurate.
• Long term forecasts have large uncertainties
• Sources for errors
• Observations
• Inaccuracy in the measurements
• Uneven distribution of the observing stations
• The coverage of the Earth is not complete
• The theoretical models
• The models are somewhat simplified.
• Not all the physics is well understood
• The computer simulations
• Round-off errors
• Model grid resolution
• Small scale turbulence and chaotic processes are
  rather typical for the atmosphere and this makes
  predictions difficult.

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Prognostic charts (progs) from two different
models

• Somewhat different results but the trend is the
  same!

Upper level maps of pressure at constant height
surfaces.
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Forecasting Tools

• AWIPS Advanced Weather Interactive Processing
  System
• Meteogram collection of observations at a given
  station over some time
• Sounding vertical profile of the temperature,
  due point T and wind.

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Forecasting Methods (1)

• Persistent forecast (short term forecast) There
  is no reason for the weather conditions to
  change.
• Steady state forecast (trend method). If we know
  the speed and the direction of the weather
  system, we can extrapolate its future location.
• Analogue forecast (pattern recognition) I have
  seen this atmospheric conditions before and based
  on what happened back then, I can predict the
  weather today or tomorrow.
• Statistical forecast the forecast is made based
  on past computer simulations that are weighted
  for the actual humidity, cloud cover, wind
  direction.

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Forecasting Methods (2)

• Probability forecast based on historical data.
  What is the chance to snow in Gainesville on
  Christmas?
• Weather type. Uses general criteria such as the
  position of the subtropical highs, upper-level
  flow, prevailing storm track
• Climatological forecast Based on information for
  the typical weather conditions at a given
  location for a given season.

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Probability of a White Christmas