VERIFICATION - PAC NW Heavy Precip Event: 19-21 November 2012

A week and a half ago, a blog post that illustrated how historical events can be used in the forecast process (located here) highlighted the potential for a high-impact heavy-precipitation event for the Pacific Northwest for early Thanksgiving week. The precipitation map for the 72-h period ending 21 November 2012 at 1200 UTC shows that coastal areas of Oregon and Washington received widespread >5"(with some locations greater than 7") of precipitation.

The median 72-h precipitation utilizing the top 15 most similar historical events based on the GFS 120-h forecast (first image below) highlights the potential for widespread areas of >4" of precipitation. In addition, the precipitation signal is primarily north of the San Francisco Bay area...this gradient is also present in the observed precipitation (above). Probabilities of >5" of precipitation also highlight coastal areas from northern California north to the U.S./Canadian border.

The GFS 72-h precipitation forecast (shown below) indeed shows the potential for the heavy precipitation event as well. Using deterministic and ensemble model-derived QPF with the analog guidance can give forecasters greater than average confidence for high-impact events when the signals are similar. This confidence can allow forecasters to provide strongly-worded decision support to NWS partners well in advance of the upcoming event.