Weather ForecastingRain, rain go away…but come again

another day…

* Weather: Conditions of the atmosphere/air in a particular region. * 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 Meteorology.

Weather: What is it?

•Synoptic meteorology is primarily concerned with large-scale 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.

Synoptic Meteorology

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 warnings•Visibility—due to fog, haze, etc.

So, what are all of these weather conditions?

Winds Winds • Horizontal movements at surface• Names from WHEREWHERE it came from…• not where it is going!!!

Pressure Cells: High – In and Up

Converge at surfaceAscend in centerDiverge Aloft

Low – Down and OutConverge aloftDescend in centerDiverge at surface

Air has Different Temperatures + Air has Different Temperatures + Different Pressures Different Pressures

CoolAir

WarmAir

Denser

MorePressure

Less Dense

LessPressure

Low High

These differences cause a These differences cause a Pressure Gradient Force=Winds!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 cloud.

PRECIPITATION—a form of water falling out of the air

Two basic ways precipitation forms:

•““Collision” process (warm clouds)Collision” process (warm clouds)

““Ice Crystal” process (cold clouds)Ice Crystal” process (cold clouds)

Easier for water vapor to deposit directly onto ice crystals. Crystals then grow heavy enough to start falling.

Rain or Snow??

Visibility

• 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 predicted?

• 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 time.

• An analysis of this data is conducted• A variety of models are used to help predict the

weather.

Weather Services• In the United States, the governmental agency

responsible for gathering and disseminating weather related information is the National National Weather ServiceWeather Service (NWS).

• Perhaps the most important services provided by the the NWS are forecasts and warnings of hazardous weather including:– thunderstorms, – flooding, – hurricanes, – tornadoes, – 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

scale. • Second, a variety of techniques are used to

establish the future state of the atmosphere; a process called weather forecastingweather 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.

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

FORECASTINGFORECASTING

• PersistencePersistence

• ExperienceExperience

• Computer ModelingComputer Modeling

There are 3 basic methods of There are 3 basic methods of forecasting:forecasting:

•Not much is going to change.Not much is going to change.

•Tomorrow will be like today.Tomorrow will be like today.

•Works great in summer.Works great in summer.

•Not so good the rest of the year.Not so good the rest of the year.

PERSISTENCE:PERSISTENCE:

•Forecast what was seen before to Forecast what was seen before to repeat.repeat.

•This is good for 1 to 2 day forecasts.This is good for 1 to 2 day forecasts.

•Works great a lot of the time.Works great a lot of the time.

•Problem when something new Problem when something new happens.happens.

EXPERIENCE:EXPERIENCE:

•They are better than people past 3 days.They are better than people past 3 days.

•Works great most of the time.Works great most of the time.

•Problem when bad data gets put in or if Problem when bad data gets put in or if something really new occurs.something really new occurs.

MODELS:MODELS:

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 day.

• 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): WRF-NMM 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, etc.

• Forecasters often have 6-10 different models to look at. Such diversity can provide valuable information.

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 significant.

• Traditionally, deterministic prediction has been the way forecasting was done, but this is changing.

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

effect”

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 physics.

• 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.

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Analysis Region

48h forecast Region

12hforecast

36hforecast

24hforecast

Ensemble Forecast Example:

Human Interpretation• Once all the numerical

simulations and post-processing are done, humans still play an important role:– Evaluating the model output– Making adjustments if needed– Attempting to consider

features the model can’t handle

– Communicating to the public and other users.

For Current Weather Information:For Current Weather Information:

NOAA weather radioOnline links:Weather BugNational Weather ServiceIntellicastThe Weather ChannelWRAL TV WeatherWTVD TV WeatherUnisys Weather (European Weather Model)Accuweather