A Gridded Snowfall Verification Method using ArcGIS: Zone-Based

Verification & Bias Maps

Joe Villani

Ian Lee

Vasil Koleci

NWS Albany, NY

Motivation

Utilize Geographic Information Systems (GIS) technology for a gridded and more representative snowfall verification method

Legacy method (adding reports and dividing by # of reports) has become antiquated:Does not factor in spatial variability

between data points

Methodology

Use ArcGIS 10.2 software ArcCatalog and ArcMap

Create contoured snowfall maps and zonal statistics based on observations

Methodology

Careful QC of snowfall observations is necessary (from trained spotters, public, social media, etc.)Summary of reports issued as Public

Information Statement (PNS)

A Local Hydrometeorological Data Message (RRM) file is generated automatically from PNSContains location name, snowfall amount,

LAT/LON

Methodology

Important to have snowfall reports outside forecast area Allows for more representative interpolation

Gather RRM files from surrounding offices

Python script collects snowfall reports from RRM files and compiles them to a CSV file

Import CSV file into a spreadsheet

RRM File Example

Relevant Data Extracted from

RRM File

Creation of Gridded Snowfall Map

Generate GIS shape file of snowfall reports from spreadsheet using ArcCatalog

Import into ArcMap

Creation of Gridded Snowfall Map

Run an Inverse Distance Weighting (IDW) function in ArcMapCan try other interpolation schemes such

as Natural Neighbor or Spline

Creates a gridded (raster) dataset and contoured snowfall map

Color scheme can be matched to ER standardized forecast snowfall ranges

Gridded (IDW) Snowfall Map

IDW takes into account

distance between points &

interpolates

Zonal Statistics

Run Zonal Statistics (including mean) on

the Gridded

Map

Image source: http://help.arcgis.com

Verification by Forecast Zone

A verification map is

created using ranges

corresponding to local office

advisory & warning criteria

Verification by Forecast Zone

Run Zonal Statistics as a table for statistical values

Output for Mean Snowfall and other stats within each forecast zone

Verification

Table is based on statistics computed from the Gridded (IDW) snowfall map

Mean snowfall used for verification by forecast zone

An Argument for the Gridded Method…

Legacy method would result in mean snowfall skewed towards higher totals in eastern part of the zone where the snowfall reports are concentrated

Gridded method more representative based on incorporating lower amounts west of the zone

Creation of Forecast Bias Maps

Preceding an event, export Forecast Snowfall from GFE to a netCDF file

Script converts netCDF file to a shape file,

Import into ArcMap and convert shape file to raster

Creation of Forecast Bias Maps

Create Gridded snowfall map from observations after event:Subtract

Forecast – Observed to compute bias

Creation of Forecast Bias Maps

Forecast – Observed Snowfall = Bias

Warm colors indicate over-forecast

Cold colors indicate under-forecast

Conclusions

This Gridded GIS-based verification method yields a more representative depiction of snowfall spatially across a region

Aides in computing legacy zone-based verification in a more comprehensive manner

Conclusions Used for official snow verification at ALY

during the 2013-2014 winter season and ALY staff has given positive feedback

Can be used in Social Media posts to give our users a graphical representation of snowfall after an event

Conclusions

Bias Maps show how well or poor a gridded forecast verified spatially

Bias maps of snowfall events can be compiled over entire winter seasons to compute potential positive or negative biasesCan be valuable for determining if

certain areas consistently show a bias

More Examples

More Examples

Acknowledgements

John Quinlan – NWS Albany, providing conceptual ideas for computing zone average statistics

Steve Welch – NWS Buffalo, method for creating shape file for snowfall based on reports and LAT/LON

Luigi Meccariello – NWS Albany, assisting in creating observed snowfall maps during the 2013-2014 winter season

Questions/Comments?

Joe.Villani@noaa.gov

Ian.Lee@noaa.gov

Vasil.Koleci@noaa.gov