Rules of Thumb for Weather ForecastingRules of Thumb for Weather Forecasting

from U. of Wisconsin AOS 452from U. of Wisconsin AOS 452

(used by permission of Prof. Martin)(used by permission of Prof. Martin)

Also contains common sense techniques which are more than Thumb Rules.

““Rule of thumb” definitionRule of thumb” definition

►A A rule of thumbrule of thumb is an easy-to-remember is an easy-to-remember guideline that is not necessarily a hard-guideline that is not necessarily a hard-and-fast rule or scientific formula but and-fast rule or scientific formula but more than just a dumb guessmore than just a dumb guess

►Oxford English Dictionary first finds an Oxford English Dictionary first finds an instance of the phrase in 1692instance of the phrase in 1692 ““What he doth, he doth by rule of Thumb, and What he doth, he doth by rule of Thumb, and

not by Art.” – Sir William Hope, not by Art.” – Sir William Hope, The compleat The compleat fencing-masterfencing-master

Forecasting temperatureForecasting temperature

► Cloud coverCloud cover► Wind speed and directionWind speed and direction

Advection of cold/warm airAdvection of cold/warm air MixingMixing Local effects (sea/lake breezes, downslope flow)Local effects (sea/lake breezes, downslope flow)

► Low-level moistureLow-level moisture With no air mass change, overnight low will be dewpoint at With no air mass change, overnight low will be dewpoint at

5 PM5 PM► Surface characteristicsSurface characteristics

VegetationVegetation WetnessWetness Snow coverSnow cover Urban vs. ruralUrban vs. rural

Thermal Wind/Thickness (dc)Thermal Wind/Thickness (dc) Really, geostrophic shearReally, geostrophic shear

Veering/Backing advection rules are a straightforward Veering/Backing advection rules are a straightforward application of this concept you learned in Dynamics – and application of this concept you learned in Dynamics – and they work. Backing with increasing height = CAA. Veering = they work. Backing with increasing height = CAA. Veering = WAA.WAA.

Thermal wind “blows” along thickness contours (cold Thermal wind “blows” along thickness contours (cold to the left)to the left)

Thickness is a useful representation of layer mean Thickness is a useful representation of layer mean temperature.temperature.

Forecasting clouds and Forecasting clouds and precipitationprecipitation

► MoistureMoisture Clouds: > 70% RH at 700 mbClouds: > 70% RH at 700 mb Precipitation: > 90% RH at 700 mbPrecipitation: > 90% RH at 700 mb Dew point depression less than 10°C indicates moisture Dew point depression less than 10°C indicates moisture

availabilityavailability► Lifting mechanismLifting mechanism

FrontsFronts Convergence zones (and boundariesConvergence zones (and boundaries

dc dc)) Daytime heatingDaytime heating OrographyOrography

► StabilityStability Amplitude modulatorAmplitude modulator No thunderstorms when 700 mb temperature > 12C and/or No thunderstorms when 700 mb temperature > 12C and/or

CIN CIN > 50 J/kg> 50 J/kg

Forecasting movement of Forecasting movement of weather systemsweather systems

► Front’s speed is 125% of ground-level cross-frontal Front’s speed is 125% of ground-level cross-frontal wind behind the frontwind behind the front Caution near topographyCaution near topography

► Isallobars provide direction of future movement of Isallobars provide direction of future movement of lows and highslows and highs Move in a line along max/min coupletMove in a line along max/min couplet

► Lows tend to travel in the general direction and at Lows tend to travel in the general direction and at ~70% of speed of the 700 mb wind~70% of speed of the 700 mb wind

► Lows move parallel to isobars in the warm sectorLows move parallel to isobars in the warm sector► Large disturbances tend to move more slowly than Large disturbances tend to move more slowly than

smaller disturbancessmaller disturbances► Adjacent lows tend to mergeAdjacent lows tend to merge

D(prog) / DtD(prog) / Dt

► Track the performance of models through Track the performance of models through timetime Is the model handling the situation well?Is the model handling the situation well?

► Extrapolation of forecast trends shown to have little forecast Extrapolation of forecast trends shown to have little forecast value value

(Hamill 2003, (Hamill 2003, Wea. ForecastingWea. Forecasting)) Used 2070 850 mb forecasts from January-March over 23-year Used 2070 850 mb forecasts from January-March over 23-year

periodperiod

Note that this is different from known model tendenciesNote that this is different from known model tendencies

(which should always be considered)(which should always be considered)

Forecasting methodsForecasting methods

► ClimatologyClimatology

Forecasting methodsForecasting methods

► ClimatologyClimatology► PersistencePersistence

Today equals Today equals tomorrowtomorrow

Forecasting methodsForecasting methods

► ClimatologyClimatology► PersistencePersistence► TrendTrend

Determining the Determining the speed and direction of speed and direction of movement for fronts, movement for fronts, high and low pressure high and low pressure centers, and areas of centers, and areas of clouds and clouds and precipitation to precipitation to predict where those predict where those features will be at features will be at some future time some future time

Forecasting methodsForecasting methods

► ClimatologyClimatology► PersistencePersistence► TrendTrend► AnalogAnalog

Pattern recognitionPattern recognition

Forecasting methodsForecasting methods

► ClimatologyClimatology► PersistencePersistence► TrendTrend► AnalogAnalog► NumericalNumerical

Model output statistics Model output statistics

(MOS)(MOS)

A forecaster on a rollgathers no MOS…

(Prof. Morgan)

Forecasting methodsForecasting methods

► ClimatologyClimatology► PersistencePersistence► TrendTrend► AnalogAnalog► NumericalNumerical► EnsembleEnsemble

Spaghetti plotsSpaghetti plots Probability density Probability density

functionsfunctions

Forecasting winter precipitation Forecasting winter precipitation typetype

► From From A Comprehensive Winter Weather Forecast A Comprehensive Winter Weather Forecast ChecklistChecklist by John Gordon (NWS-SGF) by John Gordon (NWS-SGF) http://www.crh.noaa.gov/sgf/papers/wwchklst.htmhttp://www.crh.noaa.gov/sgf/papers/wwchklst.htm

Not applicable to mountainous regionsNot applicable to mountainous regions

2560 m700-500 mb

4100 m850-500 mb1540 m850-700 mb

1300 m1000-850 mb

2840 m1000-700 mb5400 m1000-500 mb

Rain/snow lineCritical thickness