Wind Storm of January 4th, 2008

1. Overview

            On the morning of January 4th, the low-pressure system was strengthening offshore of the Pacific Northwest coast.  A very strong “low-level jet” of southerly wind at around 10,000 feet MSL was crossing the Cascades into eastern Oregon .    At 10 am PST, a cold front was approaching the Oregon coast from the west; as depicted by the blue line on the NOAA Hydrometeorological Prediction Center surface analysis shown below.

2.  WFO Pendleton Forecast Area Wind Storm

            For Eastern Oregon and southeast Washington , the primary impact of the storm was damaging pre-frontal winds.  For a list of storm reports in the Pendleton Weather Forecast Office area, click here.

            A somewhat unusual aspect of this storm is that the strongest winds were not observed at higher elevations.  Observations in the WFO Pendleton forecast area at elevations above 5000 feet reported peak wind gusts in the 40 to 60 mph range.  (Round Mountain in central Oregon at elevation 5900 feet was the exception with a single gust to 70 mph.)  Instead, the strongest winds were observed at relatively lower elevations.  This is one indication that local terrain effects played an important role in the acceleration of wind during this event.  The following table shows selected peak wind gusts from January 4.

For additional observations from January 4, click here.

3.  Record Low Sea-Level Pressure

            Record and near record low sea-level pressures were observed on January 4.  At the Walla Walla Airport (KALW) sea-level pressure was reduced to a minimum of 28.93 inches of mercury at 9:43 am .  The previous record (since 1948), 28.95 inches of mercury, had occurred three times at KALW: on Oct 27, 1950 at 6 am, Dec. 5, 1951  between 1 and 3 am, and on Dec. 7, 1952 at 8 am.  The sea-level pressure at KALW fell an impressive 9.5 mb (0.28 inches of mercury) over the 3 hours leading up to the record, and 26 mb over the previous 24-hour period.  These low pressures along the foothills of the Blues were due to a combination of the low-pressure system offshore, a cold front moving across the interior Pacific Northwest, and the strong wind aloft moving across the Blue Mountains. 

4.  A Downslope Windstorm for the Northern Foothills of the Blues

             Terrain effects were especially important during this event, and for the northern foothills of the Blue Mountains, local wind acceleration was created by a downslope wind storm.  A zone of higher wind speed near the base of a mountain barrier on the downwind side characterizes Downslope windstorms.   These types of wind storms are more common along the front range of the Rocky Mountains, for example near Boulder, CO.  On January 4^th the greatest density of damage in the Pendleton forecast area was from a downslope wind storm over an area from Adams, Oregon northeast into the Walla Walla Valley . The Veteran's Affairs Medical Center in Walla Walla, WA reported 28 structures, 4 vehicles and 50 trees damaged just on their campus alone.

For additional photos from Walla Walla, WA, visit flickr.com. Please note some of these photographs may have copyright restrictions.

            A 6-hour computer model forecast (from the NAM WRF model), valid at 10 am PST on January 4, shows several important features of the storm, including a large change in sea-level pressure oriented along the foothills of the Blue Mountains.   (Sea-level pressure is depicted by the dashed yellow lines.) This 6-hour forecast of wind speed at 98 feet (30 meters) Above Ground Level (AGL) shows very strong winds over some of the areas that experienced damaging winds but also over other areas that did not.  This model forecast also correctly captures higher sea-level pressures and lower wind speeds in the northwest portion of the lower Columbia Basin where a pool of colder air provided protection during the morning hours from the very strong south-southeast winds. 

               At Juniper Dunes (JUFW1), 33 miles northwest of KALW, wind gusts were 30 to 40 mph less then at KALW.  And locations north and west of Juniper Dunes remained in a pool of cold air during the morning hours, which shielded them from the strong southeast winds.  For example, the Pasco, WA airport (KPSC), 42 miles west-northwest of KALW, experienced light westerly winds during the morning hours.  The table below shows the wind speeds at these three locations; note the colder temperatures at KPSC.

            5.  Summary

            In summary, the January 4 wind storm in eastern Oregon and southeast Washington is attributable to a powerful low-pressure system leading to record low sea-level pressures, along with the interaction of a very strong low-level jet and local complex terrain.