The NOAA Forecast Systems Laboratory (FSL) has developed Global Positioning System (GPS) technology to measure integrated precipitable water (IPW), with sensors installed at numerous sites around the country, including one recently at Blacksburg, Virginia. The data are generated at a temporal resolution of 30 minutes, and available in near-realtime (15 to 20 minute latency). Data were collected for this study during the summer months of 2000, in order to correlate these GPS-measured IPW values and trends at Blacksburg, VA with WSR-88D parameters indicative of heavy rainfall (primarily Base Reflectivity and VIL) within 80km of the GPS site (covering much of southwest Virginia and parts of southeast West Virginia). Correlations are also made with rainfall rates as measured by automated rain gages with 15 minute resolution across southwest Virginia.

The study first suggests that a threshold value for IPW may be effective in distinguishing the days when heavy rainfall rates are most likely. Correlations between GPS IPW trends and measurements of heavy rainfall rates are made for each event (and we focus mainly on two long-duration convective events in this study so that there are enough data points to construct meaningful correlations). Preliminary evidence shows that trends observed in the IPW signal may provide some amount of lead time before maximum observed rainfall rates occur (as measured by radar). Subjective evaluation by operational forecasters also suggests that upward trends in GPS IPW at Blacksburg occurred during the onset of convection over southwest Virginia, sometimes reaching a peak with a short lead time before convection formed or moved over the region, and other times peaking at essentially the same time or even after convection first formed.