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Weather Forecasting
Rain, rain go away…but come again
another day…
Weather: What is it?
* Weather: Conditions of the atmosphere/air in a particular
* These could be forecast (predicted) based on the
movements of the air masses on Earth’s surface.
* The study of weather and the atmosphere is called
Synoptic Meteorology
•Synoptic meteorology is
primarily concerned with largescale weather systems.
Synoptic means "view
together" or "view at a
common point".
•The forecast weather map is
for a common point in time,
and each of the many different
elements that create our
weather (e.g. the high and low
pressure systems, fronts, and
precipitation areas) can be
viewed together.
What information is involved in a weather
forecast for a particular region?
•Temperature highs and lows
•Air pressure differences
•Clouds—types and amounts
•Precipitation chances or amounts/type
•Winds—types, speed and direction
•Moisture—dew points or humidity
•Severe Weather chances—watches or
•Visibility—due to fog, haze, etc.
So, what are all of these weather conditions?
• Horizontal movements at surface
• Names from WHERE it came from…
• not where it is going!!!
Pressure Cells:
High – In and Up
Converge at
Ascend in center
Diverge Aloft
Low – Down and Out
Converge aloft
Descend in
Diverge at
Air has Different Temperatures +
Different Pressures
These differences cause a
Pressure Gradient Force=Winds!
• Difference in pressure
over a given distance--between isobars (areas
of similar air pressure)
 Close together = steep
pressure gradient
 STRONG winds
 Far apart = gentle
pressure gradient
 Light winds
Clouds and their Formation
Warm air rises (less
dense), and will
gradually cool. The air
will begin condensing
(when the temperature
and dew point—
temperature where
water will form from
air-- become closer
together), and water
droplets bond onto
condensation nuclei.
These nuclei will collide
with other nuclei,
eventually forming a
of water falling out of the air
Two basic ways precipitation forms:
•“Collision” process (warm clouds)
“Ice Crystal” process (cold clouds)
Easier for water vapor to deposit directly onto ice
crystals. Crystals then grow heavy enough to start
Rain or Snow??
• Usually expressed as distance that an object can be
viewed clearly through the surrounding air.
• Moisture and particles in the air both contribute to a
decrease in visibility
• Related to the air quality based on smoke/pollen or
other particles that may be inhaled by people.
How are these weather conditions
• Various weather services provide a forecast
(prediction) of the weather conditions for a
particular area or region on the Earth’s surface.
• Data must be collected on all current conditions over
• An analysis of this data is conducted
• A variety of models are used to help predict the
Weather Services
• In the United States, the governmental agency
responsible for gathering and disseminating
weather related information is the National
Weather Service (NWS).
• Perhaps the most important services provided by
the the NWS are forecasts and warnings of
hazardous weather including:
winter weather,
and extreme heat.
• The process of providing weather forecasts and
warnings throughout the United States occurs in
three stages.
Stages of weather forecasting
• First, data is collected and analyzed on a global
• Second, a variety of techniques are used to
establish the future state of the atmosphere; a
process called weather forecasting.
• Finally, forecasts are disseminated to the public,
mainly through the private sector.
Data Collection
• Weather is observed throughout the world
• and the data is distributed in real time.
• Many types of data and networks, including:
– Surface observations from many sources
– Radiosondes and radar profilers
– Fixed and drifting buoys
– Ship observations
– Aircraft observations
– Satellite soundings
– Cloud and water vapor track winds
– Radar and satellite imagery
• Satellite data is now the dominant data
source (perhaps 90%)
• Huge increases in the numbers of surface
stations and aircraft reports.
What does this weather symbol indicate
about the weather?
Quality Control
• Automated algorithms and
manual intervention to detect,
correct, and remove errors in
observed data.
• Examples:
– Range check
– Buddy check (comparison to
nearby stations)
– Comparison to first guess fields
from previous model run
– Hydrostatic and vertical
consistency checks for soundings.
• A very important issue for the
forecaster--sometimes good
data is rejected and vice versa.
Bad Observation
Objective Analysis/Data Assimilation
• Often starts with a “first guess”, usually the gridded
forecast from an earlier run (frequently a run starting
6 hr earlier). Often called the “Nowcast” model
• This first guess is then modified by the observations.
• Adjustments are made to insure proper balance.
• Objective Analysis/Data Assimilation produces what
is known as the model initialization, the starting
point of the numerical simulation.
Crunching weather data
There are 3 basic methods of
• Persistence
• Experience
• Computer Modeling
•Not much is going to change.
•Tomorrow will be like today.
•Works great in summer.
•Not so good the rest of the year.
•Forecast what was seen before to
•This is good for 1 to 2 day forecasts.
•Works great a lot of the time.
•Problem when something new
•They are better than people past 3 days.
•Works great most of the time.
•Problem when bad data gets put in or if
something really new occurs.
Major U.S. Forecast Models
• Global Forecast System Model (GFS).
Uses spectral representation rather than
grids in the horizontal. Global,
resolution equivalent to 25 km grid
model. Run out to 384 hr, four times per
• Weather Research and Forecasting
Model (WRF). WRF is a mesoscale
modeling system that is used by the
NWS and the university/research
community. Two versions (different ways
of representing the dynamics): WRFNMM and WRF-ARW. Universities use
WRF-ARW. The NWS runs WRF-NMM at
12-km grid spacing, four times a day to
84h. AFWA is also using WRF (ARW).
Forecasting Models
• MM5 (Penn. State/NCAR Mesoscale Model
Version 5). Had been the dominant model in
the research community. Run here at the UW
(36, 12 resolution).
• COAMPS (Navy). The Navy mesoscale
model..similar to MM5.
• There are many others—European models,
• Forecasters often have 6-10 different models
to look at. Such diversity can provide valuable
Ensemble Forecasting
• All of the individual model forecasts
reflect a deterministic approach.
• This means that we do the best job
we can for a single forecast and do
not consider uncertainties in the
model, initial conditions, or the
very nature of the atmosphere.
These uncertainties are often very
• Traditionally, deterministic
prediction has been the way
forecasting was done, but this is
Uncertainty--A More Fundamental Issue
• The work of Lorenz (1963, 1965,
1968) demonstrated that the
atmosphere is a chaotic system, in
which small differences in the
initialization…well within
observational error… can have
large impacts on the forecasts,
particularly for longer forecasts.
• Similarly, uncertainty in model
physics can result in large forecast
differences and errors.
• Not unlike a pinball game….
• Often referred to as the “butterfly
Probabilistic-Ensemble Forecasting
• There are several ways to
produce probabilistic
information but the most
viable and popular is
ensemble prediction.
• Instead of running one
forecast, run a collection
(ensemble) of forecasts, each
starting from a different
initial state or with different
• The variations in the
resulting forecasts can be
used to estimate the
uncertainty of the prediction.
• The ensemble mean is on
average more skillful than
any individual member.
Ensemble Forecast Example:
Human Interpretation
• Once all the numerical
simulations and postprocessing are done, humans
still play an important role:
– Evaluating the model output
– Making adjustments if needed
– Attempting to consider
features the model can’t
– Communicating to the public
and other users.
For Current Weather Information:
NOAA weather radio
Online links:
Weather Bug
National Weather Service
The Weather Channel
WRAL TV Weather
WTVD TV Weather
Unisys Weather (European Weather Model)