a gridded snowfall verification method using arcgis: zone-based verification & bias maps joe...
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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