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Forecasting
Lecture 14
Weather and Climate Online
If you know where the low [or the
high] is now
• You should be able to guess your future
weather
• You should be able to guess your future
temperature
A Low is Approaching
• It moves to your • It moves to your
north or west south or east
– Winds will shift to the – Winds will shift from
south
easterly to northerly
• Any frozen to northwesterly
precipitation will
• Frozen precipitation
likely turn to rain or
drizzle will continue
• By the time the cold • Cold air will arrive
air arrives, moisture
is gone
evolve with time,
Problem: Steering Flow will this storm move? changing wind
Which way
Where will low move?
Problems:
• Low may change directions, or slow
down, especially when it‟s occluding
• Low may leave, but clouds stay behind
• Upper levels affect surface (dry slots)
Solution
• Computer simulations can do a great
job of describing the future evolution of
the weather
Why are forecasts important?
Weather forecasts play a
role in the everyday
decisions that an
average person makes.
Who employs Weather
Forecasters?
• Government
• Military
• Private Forecasting Companies
• Energy Companies
• TV stations
• Insurance Companies Many entities need
• Airlines accurate weather
• Recreational Industries information
• Agricultural Companies
• And many more!
How does a weather
forecaster make a forecast?
The Current Weather
• Before a weather forecast can be made,
the forecaster has to know the current
state of the atmosphere.
– You can‟t predict if you don‟t know your
starting point!!
• How do we know the current state of the
atmosphere?
Distribution of Observations
Distribution of Observations
Distribution of Observations
Distribution of Observations
The individual satellite paths are obvious – one is circled
Distribution of Observations
Comments on previous slides:
• Not many observations over the ocean
– What does this mean if you live just west of
an ocean? Will you know the structure of
the feature that is bearing down on you?
• Fewer „obs‟ in southern Hemisphere
compared to northern hemisphere
– Partly because there‟s more ocean there
– Also fewer aircraft flights
Even if the storm is over the ocean, sometimes you can
infer a lot from its structures on a satellite image
(This storm is likely extremely strong with strong winds)
Still…
• Knowledge of the present structure
won‟t help make an accurate forecast
• It does give you an accurate start
• But the state of the atmosphere will
change in ways you might not be able to
predict given the initial conditions
• Must turn to Numerical Weather
Prediction
Numerical Weather Prediction
• Approximate
weather (wind,
temperature,
relative humidity) at
points on the globe
• Use mathematics to
describe the
evolution of the
weather
• Solve equations on
a computer
Note the grid – meteorological values are defined at every point on the grid
Lewis Fry Richardson
• Visionary Meteorologist who devised a
scheme to create forecasts numerically
during WWI (he was an ambulance driver)
• Published a book describing the methods
• Abandoned meteorology when War
Departments became too interested in his
work
Why use computers?
• Computers excel at repetitive calculations
• It would take a team of mathematicians or
meteorologists years to produce the millions
of calculations that go into a single forecast!
Air Flow is governed by equations
• Equations can be approximated by
computations on grid points in a model
• Step the model forward in time -- you
produce a forecast
• Different computer models have
different assumptions and will yield
different results
Two Models Valid at the Same Time
Different models make different assumptions about aspects
of the weather. Or, they have different resolutions. Result:
different forecast outcomes. Overall agreement is usually okay
How does forecast change with time?
If sequential model runs show similar results, you should
forecast that event with more confidence
What if there are big
differences between two
different models or between
sequential runs of the same
model?
Don‟t forecast with a lot of confidence in that case:
Emphasize what you know, and mention the possibilities
How Can A Person Improve
Numerical Model Forecasts?
The Human Element
• Humans have great pattern recognition skills
• Forecaster will identify the deterioration of a
forecast long before the model simulation
does
• Compare model forecast to satellite images
and current surface observations
• Adjust for known model biases
Why Do Numerical Forecasts
Go Awry?
MANY reasons
• Small scale features poorly resolved in
models and must be parameterized
• Large scale features are not reproduced
with 100% accuracy
• Initial fields of model contain significant
errors in horizontal and vertical
What is the high on the east coast is actually
stronger than analyzed? How will that affect the
evolution of errors in the subsequent forecast?
Sometimes, the computer
simulation dies (or is obviously
wrong), or it cannot be
transmitted
If there are problems with
numerical weather prediction
models, what other methods
can a forecaster use?
Persistence Forecast
Persistence assumes tomorrow is a carbon copy of today
Steady State or Trend Method
Things are changing at a fixed rate that doesn‟t change with time
Meteogram (Meteorogram)
You could use the
information on this
chart to predict
weather on the
20th or 21st by
assuming that the
evolution of the
weather will
resemble the
previous days‟
evolutions.
Trend Forecasting
• If today is a bit warmer than yesterday,
then tomorrow will be a bit warmer than
today
• If the pressure is falling now, the
pressure will continue to fall tomorrow
• Changes just keep going out in time
without alteration
Example!
Observed High Observed Low Climatological High Climatological Low
(Sept 15) 61 38 72 50
(Sept 16) 57 49 71 50
(Sept 17) 78 55 71 49
Consider this hypothetical example from 15-17 Sept
in Madison…If you‟re making a climatological forecast
on 16 September for 17 September, you would forecast
a high of 71 and a low of 49. If you‟re making a persistence
forecast on 16 September for 17 September, you would
forecast a high of 57 and a low of 49. If you‟re making
a trend forecast on 16 September for 17 September, you
would forecast a high of 53 (61 to 57 to 53).
Analogue Method
• Used when a weather map depicts
features similar to an event observed in
the past.
• Also called “pattern recognition”.
• While weather systems may look similar
they are never exactly the same.
Similar paths, but
Different strengths
Still -- use „old‟ path to
forecast weather during
„new‟ storm
Ensemble Forecasting
• Use one set of computer code (One model)
• Start the forecast at the same time, but add
small perturbations to the initial field, and see
what impact those perturbations have on the
subsequent forecast
• This tells you something about the
predictability of the atmosphere…You might
be in a region where no matter how the fields
are perturbed, the forecast remains the
same…or you may be somewhere where
forecasts change dramatically
This is the focus of most current advances in NWP
Ensemble Forecasts
• Statistical approach to forecasting: what forecast
scenario is most likely?
• Use one numerical model, perturb initial conditions,
and see what happens.
• In the maps on the next page, you see two 500-mb
height contours; close together at 00h (initial time),
but they diverge some by 3.5 days (84h) and diverge
even more by 240h (10 days).
– Still – are there regions where even at 10 days the different
model simulations suggest similar weather?
Are you forecasting for a place where the forecast spread is high or low?
If you’re forecasting for someplace that’s not predictable,
you change the definitiveness of your wording!
Ensemble
Forecasting
You can also use
different models,
with the same
initial fields, and
see how each
model describes
the evolution of
the atmosphere
Climatological Forecasting
Think about Phoenix, AZ.......if you had to
make a forecast for tomorrow, would
you forecast rain or sunshine?
A climatological forecast is based solely
on what the climate records indicate for
that location.
Probability Forecasting
• Relies heavily on climatological data.
Chance of a White Christmas
Forecast for Today
• Persistence: (A repeat of Yesterday)
• Climatological: Based on „normals‟
• Trend: Compare Yesterday and the day
before, and extrapolate to today
• Think about the assumptions in this type of
forecast: are they valid?
– How often is a day „normal‟ at your location?
– (This means you have to know what normal is!)
– Will be changes continue from day to day?
How accurate are model
forecasts?
• 12 to 24 hour forecasts are usually quite
accurate
• 1 to 3 days is pretty good
• 3 to 5 days better than flipping a coin
– 3.5 days is where 1 day was 30+ years ago
• beyond 7 days relies mostly on climatology
• For a forecast to have “skill” it must be better
than a forecast of persistence or climatology
Rules of Thumb
• Experience is the best tool a forecaster
has!
• Thus many forecasters rely on “rules of
thumb”.
What is a “rule of thumb”?
Meteorological Rules of
Thumb
• Will it rain/snow?
– look at the -5o C isotherm on 850mb chart
– look at the 540 „thickness‟ line
• Will it be cloudy or clear?
– On 700mb chart look for relative humidities
>70%
• What will the overnight low be?
– take the 5pm dew point temperature
Meteorological Rules of
Thumb
• Where will the low pressure system
move?
– Low pressure systems tend to move in a
direction parallel to the isobars in the warm
sector.
Determining Movement of
Weather Systems
• Surface pressure systems tend to move
in the same direction as the wind at 500
mb.
• Surface systems move at a speed half
the speed of winds at 500 mb.
Making a forecast for
temperature
Clouds affect the overnight low –
cloudy nights are usually warmer
Making a forecast for
temperature
Cloudy days are usually cooler than clear days
Making a forecast for
temperature
High to your west? Cooler
High to your east? Warmer
Making a forecast for
temperature?
Are south winds moving in warmer air?
Making a forecast for
temperature
Snow cover leads to cooler high temperatures
Weather Forecasting
• What is involved and what do you need
to know?
• Forecasting
– Minimum Temperature
– Maximum Temperature
– Sky condition/Weather Type
What do you need to know
and what is involved?
• Current Information
• Constraints
• Exceptions to Constraints
• Rules/Processes
• Supplemental Information
– National Weather Service Forecast
– Numerical Weather
Prediction/Computer Model Forecast
Forecasting
• Minimum (Low) Temperature
• Maximum (High) Temperature
• Sky Condition/Weather Type
– Cloudy, Partly cloudy, clear, etc.
– Rain, Snow, etc.
But some companies might want specific
things forecast: When will the RH exceed
90%? What is the wind direction?
Information that forecasters have
• Past 24 hours of observations
– Text
– Meteorogram (Time series)
• Climatology
• Satellite Loop
• Weather Map
• National Weather Service Forecast
• Numerical Weather Prediction
Model Forecast
Minimum Temperature
• Rules/Processes
• Constraints – Trends/rate of
change
– Low will be greater than
or equal to the dewpoint – Moving in of warmer
temperature or colder air masses
– Low will be less than – Radiation effects of
today‟s high clouds (blanket
temperature effect)
• Exceptions to Constraints
– Almost never forecast – Dewpoint may change
record temperatures over night
– So, you should know – Temperature may go up
the records for the day! over night
– It may be that a record low
is going to occur
Maximum Temperature
• Rules/Processes
• Constraints – Trends/rate of
change
– High will be no
higher than weather – Moving in of warmer
balloon forecast or colder air masses
method – Radiation effects of
– High will be more clouds &
than tomorrow‟s low precipitation
• Exceptions to Constraints
temperature – Weather balloon method only
– Almost never works on dry/windless days
forecast a record – Temperature may go down
high temperature during the day
– It may be that a record high
is going to occur
Sky Condition/Weather Type
Clouds Weather Type
• Rules/Processes: • Rules/Processes:
– Clouds --Could be weather
– Existing clouds moving in? with them
• Fronts? Other cloud – Consider trends/persistence
masses? especially of pressure
– New clouds forming?
• New front/low pressure • Constraint/Exception:
system? Other? – Temperature:
– Consider trends/persistence • <32 F Snow
especially of pressure • >32 F Rain
– Stability? – Clouds - Might not have
weather associated with
them
• Constraint/Exception: – Non-cloud related weather
– Too dry for clouds? (Fog, blowing snow, etc.)
The Forecaster‟s Excuse
• Take a large, almost round, rotating sphere 8000
miles in diameter,
• Surround it with a murky, viscous atmosphere of
gases mixed with water vapor,
• Tilt its axis so it wobbles back and forth with respect
to a source of heat and light,
• Freeze it at both ends and roast in the middle,
• Cover most of its surface with liquid that constantly
feeds vapor into the atmosphere as the sphere
tosses billions of gallons up and down to the rhythmic
pulling of a captive satellite and sun,
The Forecaster‟s Excuse
THEN TRY TO PREDICT THE
CONDITIONS OF THAT
ATMOSPHERE OVER A SMALL AREA
WITHIN A 5 MILE RADIUS FOR A
PERIOD OF ONE TO FIVE DAYS IN
ADVANCE!!
What you see
What the
meteorologist
sees!
How do we get weather
information from around the
world?
• World Meteorological Organization (WMO) Weather
Weather Watch (WWW) Program
– United Nations (UN) agency
– Each country has one representative
– Global Telecommunications System (GTS)
• WMO monitors procedures so that data is comparable
• In The US, data from the WMO is sent to the National
Oceanic and Atmospheric Administration (NOAA)
– National Weather Service
– National Centers for Environmental Prediction
– Others..
Who makes forecasts in the US?
• National Weather Service (NWS)
• Private Companies
• TV stations
• Special national laboratories
– Storm Prediction Center
– Hurricane Prediction Center
All of the above get the same data from
NOAA/NCEP! That’s why many forecasts
sound the same!
Watch vs. Warning
• A watch means that conditions are right
for hazardous weather.
• A warning means hazardous weather is
imminent or already occurring.
• Advisories are issued to warn the public
of hazardous driving (boating)
conditions due to weather.
Long-Range Forecasting
• Forecasts made a month or more in advance
– used in recent years in a local pool advertisement
• These forecasts tend to give general information
rather than specifics
• Climate Prediction Center
– 6 to 10 day forecast
– 30 day outlook
– 90 day seasonal outlook
Extended Outlook for this coming winter
• Precipitation: • Temperature
– above normal –near normal
Typical for La Nina Year
Forecasts for Several Cities
• Augusta, GA
• Will start with clear
skies, and begin to
get clouds.
• Temperatures will
warm
• As warm humid air
moves over cold
ground, fog may
form
Forecasts for Several Cities
• Dallas, TX
• Will get a cold
wave
• Possible showers
associated with the
front
• Winds will switch
from sw to nw
Forecasts for Several Cities
• Denver, CO
• Cold but clear
• Will be dominated
by high pressure
• Rising pressure
Forecasts for Several Cities
• Chicago, IL
• Will continue to be
in the cold air
• Could experience
quite the snow
storm
Forecasts for Several Cities
• Memphis, TN
• Within the next 24
hours there will be a
passage of both a
warm and cold front
• Wind will shift from
SE to SW to NW
• Pressure will fall, level
off, and then fall
again.
