GPS integrated precipitable water (GPS-IPW) data have been available at FSL for more than five years. A long-term impact study using the original 60-km RUC with and without GPS data has been ongoing since November 1997. Verification of the analysis and forecasts from the two parallel runs has been performed against RAOBs at 0000 and 1200 UTC daily, resulting in a longterm, data-rich sample. These statistics show a consistent, modest impact in relative humidity forecasts at 850 hPa, 700 hPa and 500 hPa, important levels in diagnosing severe weather potential. The impact increases with the density of the data.

Individual cases have also been assessed, both with the 60-km RUC and the new 20-km RUC. A 20-km case study has been run for 24 May 2000. This case includes a widespread severe weather outbreak in the south central US, where most of the GPS receivers are located. Assimilation of GPS-IPW in this case resulted in drier (more unstable) conditions aloft and an increase in precipitation in this area.