Weather Forecasting - I

1. Weather Forecasting - I

2. 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.

3. Another way of looking at it.

4. 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.

5. 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.

6. Importance of forecasting: air-travel

7. Importance of forecasting

8. Importance of forecasting

9. 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

10. 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

11. Organizational structure Carlos M. Gutierrez • 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) - Tropical Analysis and Forecast Branch - Technical Support Branch • Climate, Water and Weather Services • 122 Weather Forecast Offices (NWSFO) • 6 Regional Climate Centers (RCC) • 13 River Forecast Centers (RFC)

12. 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 Weather Warnings

13. 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! Weather Warnings

14. 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

15. Current Weather Conditions

16. Gainesville 10-day forecast Do you believe the 10th day forecast?

17. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. Probability of a “White Christmas”