Weather Forecasting - II

Review The forecasting of weather by high-speed computers is known as numerical weather prediction. Mathematical models that describe how atmospheric temperature, pressure, winds and moisture will change with time are programmed into the computer. The computer then draws surface and upper-air charts, and produces a variety of forecasts called progs. After a number of days, flaws in the computer models – atmospheric chaos and small errors in the data – greatly limit the accuracy of weather forecasts. Available to the forecaster are a number of tools that can be used when making a forecast, including surface and upper-air maps, computer progs, meteograms, soundings, and satellite information. Ensemble forecasting is a technique based on running several forecast models (or different versions of a single model), each beginning with slightly different weather information to reflect the errors in the measurements (similar to BCS computer averages).

Ensemble forecasting A technique to improve medium-range forecasts ♦ Run different models ♦ Run the same model with slightly different initial conditions Superimpose all results on the same graph ♦ “Spaghetti plot” The most common outcome is probably the most likely one

Forecasting Tools AWIPS – Advanced Weather Interactive Processing System Meteogram – collection of observations at a given station over some time Sounding: vertical profile of the temperature, due point T and wind.

Forecasting Methods (1) Persistent forecast: (short term forecast) when there is no reason for the weather conditions to change. Steady state forecast (trend method). If we know the speed and the direction of the weather system, we can extrapolate to its future location assuming same speed. ♦ Example: a cold front, located 90 km due west from here, is moving eastward at 30 km/hr. Prediction: a cold front will pass through in exactly 3 hours. Analogue forecast (pattern recognition): “I have seen these atmospheric conditions before and based on what happened back then, I can predict tomorrow’s weather”. Statistical forecast: the forecast is made based on ♦ Numerical model’s forecast for weather elements (humidity, cloud cover, wind direction, temperature). ♦ Statistically weighted analogue forecasts based on those

“The chance of rain is 60%” It will rain over 60 % of the forecasted area. NO There is a 60% chance that it will rain somewhere within the forecasted area. NO It will rain 60% of the time. NO There is 60% chance that any random location will receive a measurable amount of rain. YES If you stay at one place there is a 60% chance that you will see rain. YES

Forecasting Methods (2) Climatological forecast: Based on information for the typical weather conditions at a given location for a given season. Probability forecast: based on historical data. “What is the chance to snow in Gainesville on Christmas?”

Forecasting Methods (3) Weather type. Uses general criteria such as the position of the subtropical highs, upper-level flow, prevailing storm track… In general: For a forecast to show skill, it must be better than a persistence forecast or a climatological forecast

The weather over the weekend

Type of Forecasts Very short-range forecasts (up to 6 h): based on satellite images and Doppler radar. Not likely to include numerical models. Short-range forecast: (2-3 days) variety of techniques - satellite images, Doppler radar, wind and pressure maps of the surface and aloft … also numerical models – usually beyond 12 hrs. Medium range forecast: (3-8 days) Mostly based on computer generated weather predictions. Long-range forecast (up to 16 days). It is not very accurate. Gives mostly the trends of the weather and the global expectations. Extended forecast – one that goes beyond 3 days Outlook – how the average weather conditions for a particular month or a season compare to normal ones.

90-day outlook for (a) precipitation and (b) temperature Below normal Above normal Below normal

Predicting the weather from local signs. Signs of an approaching cold front: hot and humid air, pressure quickly dropping. Signs of an approaching warm front: cool weather, high clouds that are thickening and lowering, decreasing pressure, precipitation starting with snow (if cold enough) changing to sleet, freezing rain and rain. Signs of an approaching tropical storm: decreasing pressure, increased cloud cover, the direction and the speed of the cloud motion (winds).

Weather folklore When halo rings the moon or sun, rain's approaching on the run When windows won't open, and the salt clogs the shaker, The weather will favor the umbrella maker! A cow with its tail to the West makes the weather best, A cow with its tail to the East makes the weather least Cats and dogs eat grass before a rain. TRUE FALSE

The Station Model – Appendix B Temperature=77 F Dewpoint =68 F Pressure (mbar) = 998. Pressure tendency the last 3 h. The pressure here has fallen.3 mb the last 3 hours. Cloud cover: it's mostly black showing that this station is mostly cloudy. Wind barb: it points to where the wind is coming from. The two ticks represent 20 knot winds. Present weather: a thunderstorm is occurring at the station. Pressure tendency: the 1st hour the pressure was steady, then fell the last two hours. Past weather (one hour ago). In this case it was a light rain shower.

Example

Using surface maps to predict the weather. Pressure Tendency Upper level map Current weather Predicted front position

What actually happened Skip the last sections “A forecast for six cities” and “A meteorologist makes a prediction”