Weather Forecasting

Overview: • Weather forecasting methods o Numerical weather prediction o Why do forecasts go awry o Other forecasting methods o Predicting weather from local signs • Weather forecasting using surface charts o Determining the movement of weather systems o Making a local weather forecast • Watches, Warnings, and Advisories

Weather Forecasting Methods

Numerical Weather Prediction

Numerical weather prediction (NWP): Forecasting the weather using computer models (known as NWP models) based on the mathematical equations that describe the behavior of the atmosphere.

What is the basis for the equations used in NWP models?

Du 1 ∂p & ∂ 2u ∂ 2u ∂ 2u) = − + ν( 2 + 2 + 2 + + (2Ωvsin φ − 2Ωwcosφ) Dt ρ ∂x ' ∂x ∂y ∂z * Dv 1 ∂p & ∂ 2v ∂ 2v ∂ 2v) = − + ν( 2 + 2 + 2 + − (2Ωusin φ) Dt ρ ∂y ' ∂x ∂y ∂z * Dw 1 ∂p & ∂ 2w ∂ 2w ∂ 2w) = −g − + ν( 2 + 2 + 2 + + (2Ωucosφ) Dt ρ ∂z ' ∂x ∂y ∂z *

How do NWP models forecast the weather? € The equations that are the basis of NWP models predict the state of the atmosphere at a large number of locations, known as grid points.

The distance between grid points in the NWP model is referred to as the model resolution. The model has both a horizontal grid and a vertical grid.

These predictions are made over short increments of time known as the model time step. This short forecast is then used as the basis for making another short forecast. This process is repeated until the desired length forecast is made.

What aspects of the atmosphere are NWP models best able to predict?

How far into the future do NWP models reasonably predict the weather? Why do forecasts go awry?

Human forecasters use the forecasts from multiple models as well as their own analysis of current weather observations and knowledge of local weather to make a weather forecast.

• Inaccuracies in the NWP model equations and representation of atmospheric processes • Uncertain observations of the initial atmospheric state • Errors in lateral boundary conditions in regional models • Features that are not resolved by the model resolution • Inherently unpredictable small atmospheric fluctuations known as chaotic behavior or chaos

Ensemble forecasting: Creating multiple NWP forecasts using slightly different NWP models or initial atmospheric conditions to represent the uncertainty in the model representation of the atmosphere and the uncertainty in the initial state of the atmosphere.

Spaghetti plot: A forecast chart showing an ensemble forecast of the atmospheric state

How would a forecaster determine how much confidence to place in a forecast based on a spaghetti plot?

Other Forecasting Methods

Persistence forecast: The future weather will be the same as the current weather.

Climatological forecast: A forecast based on the long term climate of a given location.

Steady state (trend) forecast: Assume that the weather feature of interest will continue to move in the same direction and at the same speed as previously observed.

Weather Forecasting Using Surface Charts

Rules of thumb for forecasting the movement of surface pressure systems and fronts

• Low pressure systems tend to move in a direction that parallels the isobars in the warm sector of the storm • Lows tend to move towards the region of largest pressure drop • Highs tend to move towards the region of largest pressure rise • Surface pressure systems tend to move in the direction of the 500 mb winds but at half the speed of these winds • Surface pressure systems and fronts tend to move in the same direction and at the same speed as they did in the previous 6 hours

Isallobars: Contour lines of pressure tendency

Based on the isallobars shown below, in what direction will the surface low and high pressure systems shown on the weather map on the previous page move?

Based on the 500 mb map below, in what direction will the surface low and high pressure systems shown on the weather map on the previous page move?

Making a Local Weather Forecast Assuming steady state movement the position of the surface fronts and pressure centers for the next 24 hours are shown below.

How will the weather at the six cities shown on the weather map below change in the next 24 hours?

How will the temperature change at the forecast city? Will the temperatures warm or cool over the next day?

Factors that will influence the temperature include:

• What wind direction and temperature advection is expected at each city? • Will a front approach or pass the forecast cities? • Will clouds impact the daytime or nighttime temperatures?

Will clouds or precipitation occur at the forecast city?

If precipitation is forecast, what type of precipitation is expected?

Factors that will influence the formation of clouds and precipitation include:

• Are clouds and precipitation expected based on the movement of the fronts and low and high pressure centers? o The idealized depiction of the weather associated with fronts, discussed in chapter 12, can be very helpful in predicting changes in clouds and precipitation. • Is rising motion expected at the forecast city? o Upper level divergence (associated with troughs or jet streaks) or near surface convergence (associated with low pressure centers) will result in rising motion o Where relative to an upper level trough will rising motion occur? o In what quadrants of a jet streak will rising motion occur? • Will clear skies occur? o What weather conditions favor clear skies? o Upper level convergence (associated with ridges or jet streaks) or near surface divergence (associated with high pressure centers) will result in sinking motion o Where relative to an upper level ridge will sinking motion occur? o In what quadrants of a jet streak will sinking motion occur? • How will local factors like mountains or large bodies of water impact cloud formation or dissipation?

Watch: Issued when atmospheric conditions favor hazardous weather occurring over a particular region during a specified period of time but the actual location and timing is uncertain.

Warning: Issued when hazardous weather is either imminent or occurring within the specified forecast area.

Advisory: Issued to inform the public of less hazardous conditions.