PAC NW Heavy Precip Event: 19-21 November 2012

Models are coming into agreement that a heavy precipitation episode will impact the Pacific Northwest during Thanksgiving week. This is due to a quasi-stationary midlevel trough over the Gulf of Alaska and a strong low-level impulse driven by a 70 kts low-level jet and relatively broad atmospheric river (AR).

The strength of the low-level jet collocated with the AR result in a very focused moisture flux anomaly across Washington and Oregon (lower right panel in above image). Values that exceed 5 sigma tend to occur less than 0.001% of the time based on the climatological record in that location. The similar historical analogs to this forecast (20121115/0000F120) are also indicating a high-impact heavy precipitation event. The following two images show the mean of the top 15 analogs and associated GFS forecast for 850-mb isotachs and precipitable water. The analogs and GFS forecasts for these fields are both similar in orientation and strength which gives confidence in the impact guidance based on these similar historical events.

Not surprisingly, these historical events have a strong signal for heavy rain along the coast and heavy snow in the Cascades. Median 72-h precipitation amounts exceed 3" over a large portion of the PAC NW with 30-40% probabilities for greater than 5" extending into northern California.

In addition to the heavy rainfall, heavy snow is very possible in the Cascades. Mean 72-h COOP snowfall amounts exceed well over 12" and high probabilities of over two feet of snow are common in favored mountain regions.

Although the historical precipitation guidance above may be similar to deterministic and ensemble model output, it gives the forecaster an avenue to gauge confidence in model QPF. However, what model output cannot give are impacts associated with forecast QPF. This requires forecaster interpretation and historical knowledge of his or her geographically area of responsibility...but not all forecasters are created equal and this is where the analogs can help. In one of the most similar historical events to the forecast, 7 December 1996, here are some of the highlights from NCDC Storm Data for the event (all available here).

California - WFO EKA:

Widespread flooding of creeks and streams in Del Norte and Humboldt Counties. More than $1.3M in damage in Ferndale when a muddy torrent flowed through downtown at 2pm on the 8th. Many road closures around Crescent City, Eureka and Arcata. All routes into NW California were cut at one time. Rainfall at the NWS in Eureka - 24 hour total 5.04 inches - new all time record. Hiouchi (7NW Crescent City) received 9.44 inches on the 8th.

High winds downed numerous trees and power lines, especially from Garberville north to Orick. A falling tree damaged a car in Blue Lake. Some peak speeds: 45 mph 6E Garberville, 84 mph Cooskie Mtn, 74 mph Maple Creek, 51 mph 1.5N Arcata, 54 mph 1W Orick.

Oregon - WFO MFR:

A series of storm systems dumped impressive amounts of rain over Southern Oregon Dec 7 through Dec 10. 24 hour rainfall between 4-8 inches fell along the coast and 2-6 inches inland. Most major rivers and many of the small streams in southwest Oregon flooded. Mudslides and flooding closed many roads and highways. About 11,000 home were with out power in Curry County then power transmission line were down by the storms.

Over the coming days as model guidance continues to hone in on the forecast, the top analogs may fluctuate. Before that occurs, by extracting historical impacts to the upcoming forecast the analog guidance can help in the NWS Hazardous Weather Outlook phase of the forecast.

The analog run that this blog post was written from is available here.