Weather Forecasting - I
Review of Chapter 12
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polar front model (Norwegian model) of a developing midlatitude cyclonic storm represents a simplified but useful model of
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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.
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.
Review of Chapter 12
• The
curving nature of the polar-front jet stream tends to direct
surface mid-latitude cyclonic storms northeastward and surface
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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.
Importance of forecasting: air-travel
Importance of forecasting
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
Collection and Exchange of Weather Information
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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):
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♦ 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)
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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)
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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)
Gary Locke
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
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!
Forecasting Methods
• Weather prediction “by hand”:
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♦ 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
Current Weather Conditions
Gainesville 10-day forecast
Do you believe the
10th day forecast?
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.
Forecasting Tools
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• temperature, due point T and wind.
AWIPS – Advanced Weather
Interactive Processing System
Meteogram – collection of observations
at a given station over some time
Sounding: vertical profile of the
Forecasting Methods (1)
• no reason for the weather conditions to change.
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Persistent forecast: (short term forecast) There is
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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.
Forecasting Methods (2)
• is the chance to snow in Gainesville on Christmas?”
Probability forecast: based on historical data. “What
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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.
Probability of a “White Christmas”