Challenges of Forecasting in the West - NOAA

Challenges of Forecastingin the WestBrad Colman, Sc.D.NOAA/National Weather ServiceSeattle, WA

Weather Forecasting:The process of inferring sensible weatherfrom a blend of data, understanding,climatology, and solutions of thegoverning equations.

Overview of human-based forecastingI. Forecasters require an analysis of the current situationand the forming of an hypothesis about how the currentweather came to be. This is an absolute necessity.II.Forecasters also require a set of physical models withwhich to build their hypothesis. These arephenomenological conceptual models, which are:a. concepts of atmospheric processes, and how they aremanifest in the operational data stream, andb. physical models formed through a mixture of theory,experience, and climatology.GOAL: Maximize human capacity for perception ofprocesses, and minimize incorrect judgment.

REMOTE SENSINGSATELLITERADAROTHERS….UPPER AIRDATASURFACE DATASUBJECTIVEANALYSISOBJECTIVEANALYSISPHYSICALMODELSINITIALIZATIONDIAGNOSISNUMERICALMODELSMETHODS FOR TRENDPERSISTENCECLIMATOLOGYPATTERN RECOGNITIONDECISION TREESMODEL OUTPUTMOSMOSIFPS/GFE

Historically in the West these havebeen the mesoscale structures notcaptured by synoptic-scale models• Unique to the West are many forced mesoscalecirculations.• Most local forecast adjustments are done in alinear fashion. I.e., take the synoptic forcing andthen add the mesoscale structure.• Models are beginning to capture many of thelarger mesoscale features – both good and bad.

Mesoscale Meteorology“….the study of more or less ubiquitousenergy transfer processes and the specialevents that occur locally and intermittentlyas a result of topographic forcing andmesoscale instabilities.”(Emanuel, 1986)

Categorize mesoscale circulationsinto two types:1. Free (internal) circulations/disturbances:• Occur as a result of an instability inherent within the fluid (someprecip bands, MCCs, jetstreaks, polar lows, squall lines,hurricanes)• To predict, look at “background” flow to determine what instabilitiesmay be present2. Forced circulations/disturbances:• Owe their existence, at least in part, to an external boundary of thesystem• External forcing controls location and influences the scale of thesystem• To predict, need to be familiar with local physiographic featuresand complicated mesoscale dynamics

1. Free (internal) circulations/disturbances:• Precipitation bands (CSI, etc.)• Convective complexes• Polar lows• Squall lines• Gravity waves2. Forced circulations/disturbances:• Blocked flow events• Lake-effect snowstorms• Terrain-modulated wind and convergence bands• Gravity waves No consistent approximation to the full primitive equations covers allmesoscale phenomena (as quasigeostrophy does for synoptic-scalemotions)

Simple Geostrophic Flow10341030102610221018L RL R = hNf

Basic Forecasting ApproachesI. Objective techniques:a. Numerical models – perfect prog approachb. Statistical approachesII.Subjective (human-based) techniques:a. Decision treesb. Empirical rulesc. Synoptic climatologies

Objective techniques:a. Numerical models – perfect prog approach:a. Critically dependent upon grid resolutionb. Will play a greater role in IFPS/GFE environmentc. Have complicated biases and limitationsb. Statistical approaches:a. MOS is our best known exampleb. Simple spatial corrections can be developedc. Can capture sub-grid processesd. Powerful tool that should be getting more attentione. Loses context of physical model

Decision trees:• Organize the mental processes of a forecaster• Represent a combination of subjectivity andobjectivity• Often use parameter values, or pattern recognition(Our example: Enumclaw windstorms)

Empirical techniques:• Typically result from a local study• Should be physics-based (although often are not)• Need to be re-evaluated on a regular basis(Our example: gap winds)

Synoptic climatologies:• A specific type of local study• Represents a composite of a large number ofcases (at least ~20)• Helps identify salient features• Must be combined with phenomenologicalconceptual models to ensure correct physics anddynamics(Our example: Taku winds)

Forecasting in the West is:• Focusing on scale interactions and processes• Dealing with free and forced mesoscale circulations thatinfluence:– Precipitation distribution and type– Temperature extremes– Wind direction and speed– Cloud characteristics• NOT linear and requires conceptual models that arephysics based• Requires a continual re-evaluation of tools and modelsas improvements in NWP gradually resolve theimportant mesoscale structure• And, this must all be done in an environment withinferior upstream initial conditions.