Zone-Based Snowfall Verification
Download
Report
Transcript Zone-Based Snowfall Verification
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 PNS
Contains 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 ArcMap
Can 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
underforecast
Conclusions
This Gridded GIS-based verification
method yields a more representative
depiction of snowfall spatially across
a region
Aides in computing legacy zonebased 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 biases
Can 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?
[email protected]
[email protected]
[email protected]