1. Case Study: Using GIS to Analyze NWS Warnings
Ken Waters
NWS Phoenix
June 5, 2008

2. Outline
Background
Process Used
Applications

3. Background---the issue
Warngen-created warnings used in NWS offices since late 1990’s
Included polygon definition latitude/longitude pairs
Polygon information largely ignored by the agency
Polygon very often simply replicated the county boundary(ies) for the affected counties
The Challenge: begin collecting these polygons and use the information within a GIS

4. Background Feb 2003: began collecting text weather warnings
Jan 2004: AMS poster session in Seattle
First display of NWS warning polygons in historical context (11 months of warnings overlayed)
Jun 2004: NWS Polygon Warning Team formed to implement polygon warnings
Late 2004: Set up real-time system to ingest warnings and create GIS shapefiles of warnings and post on PRH website

5. Background 2005: Project submitted as an NWS experimental project
2005: Experimental ArcIMS project started using Internet to dynamically display warnings with LSRs
Had problems, though, as ArcIMS wasn’t able to handle the rapidly changing shapefiles without the use of ArcSDE ($$$$)

6. Background
Oct 2005: Polygon Team meeting with first look at polygon warning verification
2006: Shapefile server application migrated to national server
Oct 2007: Storm-Based Warnings (aka “Polygon Warnings”) became operational
2008: Project to be declared operational by OCWWS (decision made in April)

7. Polygon Warnings

8. Polygon Warnings
7 days’ worth of all NWS Warngen warnings, May 29-June 4, 2008

9. Process Run on a Linux server; all code done in Perl with freeware
Text warnings delivered to NWS HQ Server
Each minute warnings are collected and database updated (new warnings added, expired warnings removed)

10. Process
Employ the open source “Shapelib” library available for a number of operating systems
Has advantage of being able to create GIS (ESRI) Shapefiles without having to purchase any proprietary software
Step 1: Create polygon from lat/lon points
Step 2: Load data attribute information

11. Process
Create Web graphics using another public domain library, “GD”

12. Applications---Database of historical warnings
Historical “Climatology” of Warngen warnings and official StormData reports
Warning files composed of separate shapefiles for TOR, FFW, SVR, SMW, and EWW

13. Applications---Local Storm Reports
Processing of the Local Storm Reports (caveat emptor!)
Preliminary reports
Potential errors in time and space
Multiple reports for same phenomenon

14. Applications---LSR Processing
Allows quick analysis of event shortly after it occurs
Cautions: potential pitfalls with using LSRs
Source:

15. Applications---Polygon Verification
CAR: County Area Reduction (aka, reduction in falsely alarmed area)
Ranges from 0 to 1 with high values indicating huge reduction in area, near 0 indicating nearly same area as aggregate county area
Early studies indicate a typical CAR of 0.7, or 70% reduction in area
Calculated using a VBA custom code which looks up the county area for each affected county(ies) in a CSV table and compares with polygon area
Source:

16. Applications---Polygon Verification
Initial studies of 2005 warnings and Storm Data used to estimate potential GPRA numbers for legacy verification statistics (PoD, LT, FAR) and further to set standards for these measures
Source:

17. Applications---Polygon Verification
Use of Visual Basic for Applications (VBA) custom code to compute equivalent
Allowed batch processing of warnings and Storm Data reports and computing PoD by WFO
Important Note: Bringing these data into a GIS and doing geoprocessing operations uncovered interesting errors in the existing official Storm Data database. For example, tornadoes reported in one county and CWA actually occurring (and scored for) another forecast office’s CWA

18. Applications---Monitoring Individual Polygon Warning Shapes
7-day running archive of all individual warning shapes
Warngen modifying polygon to follow land/water boundary using coarse AWIPS shapefiles (including “illegal” polygons crossing over itself)
Polygon contorted due to Warngen rule not allowing SVR over water
Source:

19. Applications---Vertex Tracking
Number of vertices of warnings can vary from 3 up to the recently-imposed limit of 20
Tracking numbers of vertices allows analysis of trends in polygon shapes

20. Questions? WEBSITE: http://weather.gov/regsci/gis Ken Waters
Phoenix WCM