Transcript CHAPTER – 3

METEOROLOGY
GEL-1370
Chapter Nine
Weather Forecasting
Goal for this Chapter
We are going to learn answers to the following questions:
• Is weather prediction an exact science? If not, why not?
• What are the different methods of data collection?
• How does weather watch differs from weather warning?
• What is a prog and how does it differ from analysis?
• How are computer-generated weather forecasts prepared?
• What is ensemble forecasting?
• What are the different methods used to predict movement
of a surface mid-latitude cyclonic storm?
Acquisition of weather information
• Weather forecasting is not an exact science
• Over 10,000 land-based stations and hundreds of ships
provide surface weather info 4 times/day; major airports
conditions hourly
• WMO: World Meteorological Organization; over 130
nations; responsible for the international exchange of
weather data
• NCEP:National Center for Environmental Prediction
(NCEP) formerly called National Meteorological Center,
in Camp Springs, MD – all meteorological data is
analyzed, prepare weather maps and charts and
prediction of weather on a global and national scale
begins (this transmitted to private & public agencies)
Weather Information & forecasting tools
• Watches, Warning and Advisories: NCEP issues advisories in the
form of watches & warnings; issued to the public of hazardous
driving conditions caused by wind, dust, fog, snow, sleet or
freezing rain
• Watch: Indicates that atmospheric conditions favor hazardous
weather occurring during a specified time period
• Warning: Indicates hazardous weather is either imminent or
actually occurring within the specified forecast area
• Key Warnings: Wind advisory, Wind-chill advisory, flash-flood
watch and warning, severe thunderstorm watch and warning,
tornado watch and warning, hurricane watch and warning, snow
advisory, Winter storm warning, blizzard warning, dense fog
advisory
Watches, warnings & Advisories
• Wind Advisory: Issued when sustained winds reach 25
to 39 mi/hr or when wind gusts are up to 57 mi/hr
• High wind warning: Sustained winds are >40 mi/hr or
wind gusts >57 mi/hr
• Wind-chill advisory: When wind-chill temp < -30 to 35°F
• Flash-flood watch & warning: May result in flash
flooding (warning) & is occurring or is imminent in the
specified area (watch)
• Severe thunderstorm watch & warning: winds >57
mi/hr and/or hail >3/4 inch diameter & have been
visually sighted or indicated by radar
Watches, warnings & advisories – contd.
Tornado watch & warning: May develop & has been spotted
either visually or by radar; take shelter immediately
Snow advisory: Expect Snowfall of 2 inch or more in 12 hours or 3
inch or more in 24 hrs expected
Winter storm warning (or heavy snow warning): Expect
Snowfall of 4 inch or more in 12 hours or 6 inch or more in 24
hrs expected
Blizzard warning: When falling or blowing snow & winds of >35
mi/hr; restricts visibility to <1/4 mile
Dense fog advisory: visibility < ¼ mile
Storm warning: Winds > 54 mi/hr expected
Hurricane watch and warning: When a tropical storm or
hurricane becomes a threat to a coastal area & when it appears
that the storm will strike an area within 24 hrs with wind speeds
> 74 mi/hr
Flags indicating advisories and warnings in
maritime areas
Computer and Weather Forecasting: Numerical weather
prediction
• Analysis: The drawing & interpretation of the pattern of various
weather elements on a surface or upper-air chart
• Numerical weather prediction: Daily forecasting of weather
based upon the solutions of mathematical equations by high-speed
computers
• Atmospheric models: Simulation of the atmosphere’s behavior
by mathematical equations or by physical models
• Method of solution: dozens of equations describe how
atmospheric parameters (temp, pressure, humidity, wind speed &
direction) change with time ---each equation is solved a future Dt
& for a large number of locations – “grid points” --- Each equation
is solved for many (50) levels in the atmosphere; calculated values
are put back in to those equations and calculated for the next
interval of time and so on for 12, 24, 36, 48, 120 hours & so on.
Computer & Weather forecasting – contd.
• Prognostic Chart (Prog): A chart showing expected or
forecasted atmospheric conditions (pressure patterns,
frontal positions, and so on); forecasters use prog as a
guide to predict the weather
• Variability in weather prediction: Variety of models,
each produce slightly different interpretation of the
weather mainly due to the way the models are used,
what model is used, distance of grid points, etc. Some
models are better than others for a specific task (such as
predicting the position of the trough on upper-level
chart, position of surface lows, etc)
Why sometimes forecasts don’t work??
• Factors that affect the prediction of weather (such as
surface temp, winds, precipitation) forecasts:
• 1) Each models idealize the atmosphere and the
assumptions made about the atmosphere may be on
target on some times, but not all the times
• 2) Many models are not global in their coverage and
thus, the model’s assumed boundaries could lead to
errors (e.g., air masses entering from western Pacific in
to North America, if the boundary is set as the west coast
& not including the western Pacific); global model
would involve longer computation time
• 3) Sparness of data: Data from oceans and high latitudes
are sparse; data from satellites have helped; computer’s
forecast is as good as the data fed in to it
Why sometimes forecasts don’t work?? – contd.
• 4) Inadequate representation of many pertinent
processes (e.g., interactions of water, ice & local terrain
on weather systems, such as radiation budget locally,
etc.)
• Some models take large geographic features (oceans,
mountain chains) neglecting small scale features (hills
and lakes) and could have a marked influence on local
weather
• 5) Inherent chaotic behavior within the atmosphere:
Several small, unpredictable atmospheric fluctuations
and disturbances (and the uncertainties) get amplified
with time in projecting the weather farther into the
future
500-mb progs for 7 PM EST, May 4, 1999 – 48 hrs in to
the future; Prog (a): MRF Model; Prog (b): ETA Model;
solid lines: height contours; 552 = 5520 m
Tools for Forecasting the Weather
• High-speed data modeling systems are used
• AWIPS (Advanced Weather Interactive Processing
System) processes information received from the
Doppler radar system and the Automated Surface
Observing System (ASOS) operated in airports; ASOS
systems provide continuous information on wind, temp,
pressure, cloud-base height, & runway visibility
• Soundings: A two-dimensional vertical profile of
temperature, dew point & winds
• Several meteorological indexes can aid in determining
the likelihood of thunderstorms, tornadoes & hail
The AWIPS work station provides various weather
maps
Doppler Radar data during a severe hailstorm in Orlando,
FL; 100% probability that thunderstorm was producing
hail; algorithm estimated hail size > 3”
Satellites & Weather Forecasting
• Satellites provide photographs of areas where there are no groundbased observations
• Geostationary Satellites (Geosynchronous satellites): Orbit the
equator at the same rate as the earth spins (remains at 36,000 km
above a fixed spot on earth’s surface); allows continuous
monitoring of a specific region
• Pictures received from Geostationary satellites to determine the
cloud movement, dissipation, or development associated with
weather fronts & storms --- useful to forecast the progress of large
weather systems
• Polar-orbiting satellites: Pass over the north and south polar
regions on each revolution; provide sharp pictures in polar
regions; circle the earth at ~850 km above the earth & provides
detailed photographic information about violent storms & cloud
systems
Geostationary satellite – monitors one area constantly,
as it remains stationary with respect to an observer on
earth’s surface
Polar Orbiting satellites – scan from north to south
Satellites – contd.
• Satellites use radiometers to observe clouds both day &
night (by detecting radiation from top of the clouds)
• Imager: Advanced radiometer that satellite pictures
with much better resolution
• Sounder: Advanced radiometer that provides a more
accurate profile of temp, moisture at different levels in
the atmosphere
• Infrared satellite picture can distinguish warm, low
clouds from cold, high clouds (warm objects radiate
more energy than cold objects)
Infrared satellite picture can distinguish warm, low (Gray)
clouds from cold, high (white) clouds
A visible image (left) and an infrared image (right) of the
eastern Pacific taken at the same time
Satellites – contd.
• Clouds appear white in the visible image; in the IR image, clouds
appear to have many shades of gray
• Along the elongated band of clouds associated with the occluded
front, the clouds appear white and bright in both pictures,
indicating thick & heavy clouds
• Time-series photographs provide info on the movement of the
clouds and hence are useful to predict arrival of clouds and storms
and the passage of weather fronts
• Other Forecasting Methods:
• Persistent Forecast: A prediction that future weather will be the
same as present weather
• Steady-state or trend method: Surface weather systems tend to
move in the same direction and at the same speed as they have
been moving
Other forecasting methods – contd.
• Nowcasting: Extension of trend method from minutes to a few
hours
• Analog Method: Existing features on a weather chart may
strongly resemble features that produced certain weather
conditions
• Weather types can be used to long-range weather forecasting; do
not adequately predict specific weather elements
• Ensemble forecasting: A technique based on running several
forecast models (or different versions of a single model), each
beginning with slightly different weather info to reflect errors in
the measurements – agreement – forecasters can place a high
degree of confidence in the forecast; low degree of confidence
means that the models do not agree
Forecasting – contd.
• Climatological Forecast: Forecast based on the
climatology of a particular region; e.g., in LA, rainfall
amounts more than a trace occur once in 90 days (~1%
of the time); we can predict with 99% confidence of
what can happen!
• What does it mean 70% chance of rainfall?
– 60% of the forecast area, it will rain??
– 60% chance it will rain within the forecast area??
– 60% chance that any random place in the forecast area will
receive measurable rainfall
Forecasting words used by the National
Weather Service (precipitation > 0.01”)
%
probability
of• preci.
.
Forecast wording
for steady precipi.
Forecast wording
for showery
precipi.
20%
Slight chance of
precipitation
Chance of
precipitation
Precipitation likely
Widely scattered
showers
Scattered showers
30 to 50%
60 to 70%
>80%
Precipitation, rain,
snow
Numerous
showers
Showers
Probability of a ‘white christmas’ – one inch or
more snow on the ground (30 yr average)
Accuracy and skill in weather forecasting
• Forecasts made for 12-24 hrs are usually accurate
• 1-3 day forecasts are reasonably good – not perfect;
better than 50%
• With longer time, the accuracy decreases and >7 days,
accuracy falls off rapidly
• The forecast must be better than current weather or the
normal (climatology) weather; in LA example
• Teleconnection prediction: Warming of the equatorial
tropical Pacific affects weather in different regions of the
world; the interactions, where a warmer tropical Pacific
can influence rainfall in CA are called ‘teleconnections’
– aids to predict seasonal changes, but not diurnal
Predicting weather from local signs
• A halo around the moon portends rain – Why?
– Halo is caused by by the bending of light as it passes through
ice crystals and that ice crystals-type clouds (cirrostratus) are
often the forerunners of an approaching storm
Few forecasting rules that may be applied when
making a short-range local weather forecast
Observation
Indication
Local weather forecast
Surface winds from
Possible cool
Possible
S or SW; clouds
front & thundershowers;
Building to the west;
storms approaching possibly,
Warm (hot) & humid
from the west
turning cooler;
windy
Forecasting from weather signs
•Observation
.
Winternight
Clear, relatively calm
with low humidity
If clear, relatively
calm, low humidity,
snow on the ground
Cloudy, relatively calm
with low humidity
Summer night:
Clear, hot, humid
Indication
Local weather
forecast
Rapid radiational
cooling
A very cold night
Rapid radiational
cooling
A very cold night
Clouds will absorb
and radiate IR to
surface
Strong
absorption/emission
of IR
Not as cold when
the sky is clear
High minimum
temp
Contd.
Observation
Indication
Local weather
forecast
Clear and relatively dry
More rapid radiational
cooling
Lower min temp
If surface winds are from
N and become NE, then
E, then SE
Surface high pressure
area moving to E; low
pressure approaching
from W
Increasing clouds
with the possibility of
precipitation within
24 hours
If surface winds are from
NE then N, then NW
Surface low pressure
Possible showers or
area is moving to the
thunderstorms with
east, surface high press. gusty winds
Approaching from W
Afternoon cumulus
clouds with flat bases,
and tops at just about the
same level
Stable layer above the
clouds
Partly cloud, with no
precipitation
Weather forecasting using surface charts
• Thumb Rules for forecasting:
• 1) Storms & fronts tend to move in the same direction
and speed during the previous 6 hrs, over a short-time
intervals
• 2) Lows tend to move in a direction that parallels the
isobars in the warm air ahead of the cold front
• 3) Lows tend to move toward the region of greatest
surface pressure drop, whereas highs tend to move
toward the region of greatest surface pressure rise
• 4) Surface pressure systems tend to move in the same
direction as the wind at 500 mb level. Speed at the
surface ~ ½ the speed at aloft
Forecast example
• Projection of pressure systems, fronts and current
weather in to the future by assuming steady-state
conditions (12- 24 hours projection)
• The storm center over the Central Plains and should
move northeast; the 500-mb chart suggests that the
surface low should move northeast at a speed of 25
knots
• Six Cities:
• Augusta (GA), Washington D.C., Chicago, Memphis,
Dallas, and Denver
Surface weather map for 6:00 AM Tuesday. Dashed
lines: positions of weather features 6 hrs ago; shaded
green: precipitation
500-mb chart at the same time; L: position of
the surface low
Weather Forecast
• Augusta, GA: cP associated with a high pressure center
brought freezing temp and fair weather; southerly winds
on the western side bring warmer & moist air – clear
and cold in the morning, increasing high clouds, wind
direction will be light; barometric pressure falling
slowly
• -Reality – fog was found --- temp of the ground was not
considered; warm, moist air over the cold surface --- fog
• Washington D.C.: Storm approaches from the west;
slowly increasing cloudiness;
Weather prediction – contd.
Weather prediction – contd.
• Sleet was found – not predicted – Why?? Intensification
of the storm; influx of warmer air from the ocean slowly
raised the surface temp and the sleet soon became rain
• Chicago: Major snowstorm – total snowfall is 12 inches
– actual snowfall is 13 inch – problem storm system
slowed as it became occluded
• Memphis, TN: Predicted thunderstorm did not
materialize; winds were not strong enough to mix the
cold, moist air that had settled in the valleys with the
warm air above
Weather prediction – contd.
• Dallas: Gulf of Mexico effect
• Denver: Rocky mountain effect
• Problems associated with the prediction
Surface weather map for 6:00 AM wednesday
Summary – Chapter -9
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Warning and Watch
Problem with modern day weather prediction
Prog, numerical weather prediction
Geosynchronous satellites
IR images vs visible images
Persistent forecast, Steady-state forecast, nowcasting,
analogue weather prediction
• Ensemble weather forecasting & probability forecasting
• Problems associated with forecasting