Title: Weather Forecasting
1Chapter 9
2A dangerous Job?
- Francisco Arias Olivera was a popular TV
personality in a small Peruvian town (Sicuani)
Population (21K). One day Francisco forecast a
two-inch rainfall but instead they got 19 inches
of rain. The local river flooded the town
washing away 250 homes and killing 17. Outraged
citizens stormed the TV station, and lynched
Francisco. Six people charged with his murder
were released after pleading justifiable
homicide. Rumor has it that the TV station still
has not hired another Weather Forecaster.
3Why make a forecast?
- Forecasts are issued to save lives, save property
and crops and to tell us what to expect in our
atmospheric environment.
- Who among us does not look at the weather at some
time or other during the week?
4Science or Art?
- Weather forecasters are responsible for
predicting weather accurately so thousands (if
not millions) of people will know what to expect
in the weather. - But weather forecasting is not an exact
scienceevery now and then we fail to get it
right. There is hard science behind every
weather forecast primitive equations, computer
models, calibrated equipment etc. However, there
is an art in making a forecast, interpreting the
current and forecast conditions. The atmosphere
is not a Closed system. We cannot track every
short wave or minor perturbation in the
atmosphere. - When we are wrong we are subject to jokes and
ridicule and even anger.
- Oh but when we get it rightit can be the best
feeling in the world.
5Acquisition of Weather Information
- Over 130 nations
- Over 10,000 land sites and hundreds of ship
observations report each day
- All in a common format
- WMO is responsible for the international exchange
of weather data and certifies that observation
procedures do not vary among nations
- Not everyone plays ball all the time. From 1991
to the overthrow of Sadaam, Iraq did not transmit
weather data.
6- Weather info from all over the world is
transmitted electronically to NCEP located in
Camp Springs Maryland.
- NCEP relays the weather info to various public
and private agencies for use in preparing local,
regional and global forecasts.
7NCEP
- Comprised of 9 centers which provide a wide
variety of national and international weather
guidance products to National Weather Service
field offices, government agencies, emergency
managers, private sector meteorologists, and
meteorological organizations and societies
throughout the world. NCEP is a critical national
resource in national and global weather
prediction. NCEP is the starting point for nearly
all weather forecasts in the United States. - Office of the DirectorAviation Weather
CenterClimate Prediction CenterEnvironmental
Modeling CenterHydrometeorological Prediction
CenterNCEP Central OperationsOcean Prediction
CenterSpace Environment CenterStorm Prediction
CenterTropical Prediction Center
8Climate Prediction Center
9The Modern Forecaster
- Has access to hundreds of maps, charts, vertical
atmospheric profiles (soundings), satellite
images, Doppler Radar, etc.
- Many radio and TV stations hire private
meteorologists or meteorological companies to
make their own forecasts with the aid of NCEP
products. - Some stations use untrained announcers to simply
read forecasts from the National Weather Service
or private companies (Accu Weather)
10National Weather ServiceWatches, Warnings and
Advisories
- Watch indicates that atmospheric conditions are
favorable for hazardous weather to occur over a
particular region during a specified time
period. - Warning indicates that hazardous weather is
either imminent or actually occurring within a
specified forecast area.
- Advisories are issued to inform the public of
hazardous driving conditions caused by wind,
dust, fog, snow, sleet, rain or freezing rain.
11Types of Watches, Warnings and Advisories
- Wind Advisory sustained winds reach 25 to 39
mi/hr or gusts up to 57 mi/hr
- Wind Chill Advisory wind chill of -30F to
-35F
- Winter Storm Warning- expect snowfall of 4 or
more in 12 hours or 6 or more in 24 hours
- Flash flood watch/warning
- Snow advisory
- Severe thunderstorm watch/warning
- Blizzard Warning falling or blowing snow and
winds of at least 35 mi/hr frequently restrict
vsby to less than ¼ mile for several hours
- Others on page 233 of text
12Forecasting Methods and Tools
- In the 50s many short ranged forecasts where made
by moving the existing systems along at a steady
rate (up two over one)
- In late 70s and into the mid 80s many weather
maps and charts were still plotted and analyzed
by hand
- Modern Computers and present observing techniques
make todays forecasts much better than those of
the past.
13Numerical Weather Prediction
- Each day many thousands of observations
transmitted to NCEP are fed into high speed
computers that are used to prepare Numerical
Weather Predictions. - Computers analyze data and use it to predict the
weather.
- Routine daily forecasting of weather by the
computer has come to be known as Numerical
Weather Prediction
- Atmospheric models simulation of the
atmospheres behavior by mathematical equations
14SOME HISTORY
- 1755 Euler develops first equation of fluid
mechanics.
- 1827 and 1845 Navier-Stokes add molecular
viscosity terms to fluid equations.
- 1904 Bjerknes proposes a paradigm shift from
empirical forecasts to ones applying basic laws
of physics.
- 1922 L. F. Richardson calculates using crude
method first numerical forecast using the
primitive equations (PE).
- 6 weeks to crank out a 6 hour forecast
- Pressure calc an order of magnitude in error
15THE COMPUTER AGE
- Computers arrive in 1940s
- Von Neumann, Charney, Rossby, Eliassen, and
Platzman attempt to use computer resources to
solve PE.
- Realized need to simplify due to scarce computer
resources.
- 1950 first successful numerical forecast
- Took 24 hours to crank out a 24 hour forecast
- 1952 Computer technology advances cut 24 hour
forecast time to 5 minutes
- 1954 Services for JNWPU. Attempt baroclinic
model run and not successful (no skill over
empirical methods)
- 1963 First successful baroclinic model 6-level
PE.
- 1976 Limited Fine Mesh model started up. Some
skill, phased out in 1994.
16THE COMPUTER REVOLUTION
- 1980 12-level Global Spectral Model
- Consists of 2 parts
- Aviation forecasts in support of aircraft out
to 5 days
- MRF Medium Range Forecast out to 15 days
- 14 minutes runtime/day 3.5 hours for 15 day
forecast
- 1990 Nested Grid Model (NGM) introduced
- Early 90s ETA Model introduced
- 32 km resolution and 45 layers
- 1999 Meso-ETA introduced
- 10 km horiz grid, 60 layers
17ECMWF European Center for Medium Range Weather Fo
recast
Reading England
18OPTIMUM INTERPOLATION
X
X
X
X
X
Each ob must be corrected for
Time Location Quality Compared to those around
it and to previous forecasts This is done for ove
r 6 million obs per day
O Model Grid Points
X Observations
19(No Transcript)
20Pronostic Chart (PROG)
- NWP provides a series of charts or numerical
output for a specified period, 12, 24, 36, 48, 72
hours etc out to 7 to 10 days in some cases
longer 16-30 days. - The final forecast chart representing the
atmosphere for a specified time in the future is
called a PROG
21Two surface pressure and precipitation progs for
2000 EST, September 29, 2003 48 hours into the
future. Prog on left is Navy Operational Global
Atmospheric Prediction System (NOGAPS) model,
whereas prog on right is the Global Forecast
System (GFS) model from NCEP
22Why Forecasts go Wrong_at_!!
- Flaws in computer models limit the accuracy of
the forecast
- Models idealize the atmosphere, making certain
assumptions that may be correct in some
instances, but wrong in others.
- Data sparse regions of the world leave gaps in
our observations
- Models cannot accurately interpret many factors
that influence weather, may not handle terrain
features well, may not have the resolution to
get the local weather right - Chaos Theory a butterfly flaps his wings in a
rainforest in South America and your forecast in
Asheville goes to pot. Small disturbances in the
atmosphere, not picked up by the model, can ruin
your forecast
23Tools for Forecasting the Weather
24Advanced Weather Interactive Processing System -
AWIPS
Can process DOPPLER, ASOS, ingest models, etc.
Handles and displays obs
and weather maps.
25WSR-88 Doppler Radar
- Doppler radar data from Melbourne, Florida, on
March 25, 1992, during a severe hailstorm that
caused 60 million in damages in the Orlando
area. In the table near the top of the display,
the hail algorithm determined that there was 100
percent probability that the storm was producing
hail and severe hail. The algorithm also
estimated the maximum size of the hailstones to
be greater than 3 inches. A forecaster can
project the movement of the storm and adequately
warn those areas in the immediate path of severe
weather.
26NEXRAD
27Meteogram
- Meteogram illustrating predicted weather elements
at Buffalo, New York, from 1 P.M. January 18,
1999, to 7 P.M. January 20, 1999. Notice that at
7 A.M. on January 19, the weather is projected to
be sea-level pressure 1007 mb cloud height 2000
ft. southwest winds at 15 knots visibility 10
miles light snow air temperature 33F dew
point 24F and the minimum temperature for the
day should be 30F. The forecast is derived from
the model output statistics (MOS) of the Nested
Grid Model (NGM).
28Profiler winds for California
29Upper air sounding
- A sounding of air temperature, dew point, and
winds at Pittsburgh, PA, on January 14, 1999.
Looking at this sounding, a forecaster would see
that saturated air extends up to about 820 mb.
The forecaster would also observe that
below-freezing temperatures only exist in a
shallow layer near the surface and that the
freezing rain presently falling over the
Pittsburgh area would continue or possibly change
to rain, as cold easterly surface winds are
swinging around to warmer southwesterly winds
aloft
30Major Types of Satellites
Polar Orbiting
Very important for covering vast areas of globe.
70 of earth is watervery few surface
observations over the oceans
31GOES (Geostationary Operational Environmental
Satellite)
- Orbital period exactly matches the rotation of
the earth
- Altitude of (35,800 km) or 22,000 miles
- Appears to "hover" over one spot on the Earth's
equator.
32Global GOES Coverage
GOES-10
33GOES-EAST (GOES-8)
GOES-WEST (GOES-10)
GOES-CENTRAL (GOES-11)
34GOES Images
35POES
- Altitudes usually range from 700 to 800 km
- Satellites in this category include NOAA, DMSP,
Landsat, and SPOT
- Slightly more than 14 orbits in a single day.
36POES ORBIT COVERAGE
37Defense Meteorological Satellite Program (DMSP)
38Meteorological Uses
- Cloud cover, types, obstructions to visibility
- Tropical Cyclones
- Low pressure systems at sea
- Winds velocity
- Rain rates
- Sea heights
39Other Forecasting Methods
- Persistence Forecast easiest way to make a
forecast. Simply predict future weather will be
the same as present weather.
- Steady-state or trend Forecast uses the
principle that surface weather systems tend to
move in the same direction and at approximately
the same speed as they have been moving. Today
using this method for very short duration
forecasts is called nowcasting. - Analogue Method relies on the fact that the
existing features on a weather chart (or series
of charts) may strongly resemble features that
produced certain forecast conditions in the past.
Ive seen this situation before, and this is
what happened
40Other Forecasting Methods
- Ensemble forecasting approach based on running
several forecast models or different versions
(simulations) of a single model each beginning
with slightly different weather information to
reflect errors inherent in the measurements. If
at the end of a specified time, the models match
each other fairly well, the forecaster can issue
a forecast with high confidenceif they dont
matchwell
41Other Forecasting Methods
- Climatological forecast a forecast based on the
climatology (average weather) of a particular
region. I.e. It rarely ever rains in Los
Angeles in July and August. Stats from all past
years indicated it is not prudent to forecast
much rain in these months. - Probability forecast a forecast of the
probability of occurrence of one or more of a
mutually exclusive set of weather conditions.
42Probability Forecast
- Probability of a "White Christmas" - one inch or
more of snow on the ground - based on a 30-year
average. The probabilities do not include the
mountainous areas in the western United States.
43Accuracy and skill in forecasting
- 12-24 hour forecasts are quite accurate
- 1-3 day forecasts are fairly good
- 7-10 days are getting better but still often fall
into the smoke and mirrors category.
- What qualifies as a right or wrong forecast? If I
forecast 65oF for a max temp tomorrow and we get
66oF did I blow it??
- We say a forecast shows skill when it is more
accurate than a forecast based only on
persistence or climatology.
- Today we often accurately predict large scale
weather events many days in advance (major snow
storms, blizzards, hurricane strikes)
44Book of Signs
- Theophrastus, student of Aristotle, 300 BC
compiled all sorts of weather indicators in his
Book of Signs
- Ways to foretell the weather by examining natural
signs shape of clouds, intensity at which a fly
bites.
- Our own folklore halo around the moon portends
rain
- See Table 9.2 on page 248 of book
- Discuss how knowing model of a mid latitude low
can help foretell weather. SW winds before
front, NW after, etc.
45Weather forecasting using surface charts
- Surface weather map for 600 A.M. Tuesday. Dashed
lines indicate positions of weather features six
hours ago. Areas shaded green are receiving
precipitation.
46Determining the Movement of Weather Systems
(Rules of Thumb)
- For short intervals, storms and fronts tend to
move in the same direction and at the approx.
same speed as they did during the previous 6
hours unless there is reason to indicate
otherwise. - Lows tend to move in the direction that
parallels the isobars in the warm air ahead of
the cold front
- Lows tend to move toward the region of greatest
surface pressure drop, highs tend to move toward
the region of greatest surface pressure rise.
- Surface pressure systems tend to move in the same
direction as the wind at 5500 m (18,000 ft) the
500 mb level. The speed at which the surface
systems move is about ½ the speed of the winds at
this level.
47Lows move toward pressure falls
- Isallobars - lines of equal 3-hour pressure
change - for 600 A.M. Tuesday. The "F"
represents the region of greatest pressure fall,
while the "R" shows the region of greatest
pressure rise. A plus 2 indicates a rise of 2 mb.
48Movement based on 500 mb
- A 500-mb chart for 600 A.M. Tuesday, showing
wind flow. The light red L represents the
position of the surface low. The winds aloft tend
to steer surface pressure systems along and,
therefore, indicate that the surface low should
move northeastward at about half the speed of the
winds at this level, or 25 knots.
49Rain of snow for DC on Wednesday?
- Projected 12- and 24-hour movement of fronts,
pressure systems, and precipitation (shaded green
area) from 600 A.M. Tuesday until 600 A.M.
Wednesday. Tuesdays weather chart on right.
Increasing clouds today and continued cold. Snow
beginning by early Wednesday Morning possibly cha
nging to rain. Winds will be out of the
southeast. Pressures will fall
50Chicago on Wednesday?
- Projected 12- and 24-hour movement of fronts,
pressure systems, and precipitation (shaded green
area) from 600 A.M. Tuesday until 600 A.M.
Wednesday. Tuesdays weather chart on right.
Cloudy and cold with light snow beginning by noon
becoming heavy by evening and
Ending by Wednesday morning. Total accumulations
will range between six and ten
Inches. Winds will become strong and gusty out
of the east or northeast today,
Becoming northerly tonight and northwesterly by
Wednesday morning. Pressures will
Fall sharply today and rise tomorrow.
51How about Denver?
- Projected 12- and 24-hour movement of fronts,
pressure systems, and precipitation (shaded green
area) from 600 A.M. Tuesday until 600 A.M.
Wednesday. Tuesdays weather chart on right.
Clear and cold tomorrow. Northerly winds today
becoming light and variable by
Tonight. Low temperatures tomorrow morning will
be near Zero. Pressure will Rise.
52Surface weather map for 600 A.M. Wednesday.