For stations in every climate zone except the Southwest, West, and Northwest and stations with elevation differences of less than 5,000 feet, a ratio of 50-150% between the first-order station and its nearest neighbor was used to determine if a station was valid. Stations were eliminated if they had three or more out of ten years that fell outside of the range. For stations in the Southwest, West, or Northwest, or stations with elevation differences of greater than 5,000 feet, a ratio of 35-170% was used. In addition to eliminating stations with inaccurate data, stations that reported wet or dry when compared to the first-order station for nine or ten of the years were also flagged. Additional analysis will be completed to determine the geographic region that has the most accurate hourly precipitation reports, and whether the nearest neighbor approach is most appropriate for this analysis.
Preliminary results indicated that approximately 25% of WMO stations would be eliminated using the thresholds previously discussed. If the ratio ranges were compressed to 75-125% for all climate zones, excluding the Southwest, Northwest, and West and a range of 60-145% was used for these stations, more than 65% of WMO stations would be eliminated. Additionally, over 35% of stations reported a dry bias during eight of the ten years, while only 4% of stations reported a wet bias for eight of the ten years.