A classic west-jet extratropical cyclone, the December 21, 2015 windstorm tracked across the northeast Pacific almost without an deviation from a nearly due east path until landfall in the vicinity of Long Beach, Washington (Figure 1.1). Also in classic fashion for this track type, referred to here as the "Storm King" path, the highest winds were focused on the immediate south side of the low-pressure center. Sheltered stations along the Oregon coast received wind gusts of 45 to 60 mph (70 to 95 km/h), with exposed sites reporting speeds of 60 to 75 mph (95 to 120 km/h). This is in sharp contrast to much of the Washington coast where peak gust speeds at the more sheltered locations were generally in the range of 20 to 40 mph (30 to 65 km/h), with some exposed locations reporting gusts approaching 50 mph (80 km/h).

For the interior sections, the strongest winds were focused on the Willamette Valley, where peak gusts ranged from 40 to 55 mph (65 to 90 km/h), with the highest speeds being concentrated in the vicinity of Portland. In southwest Oregon, interior gusts climbed relatively high for the narrow, often wind-sheltered valleys, roughly 25 to 35 mph (40 to 55 km/h), marking a fairly vigorous frontal passage. The Washington and British Columbia interior generally received gusts of 20 to 25 mph (30 to 40 km/h), save for the extreme southern section where Kelso and Vancouver reported gusts around 45 mph (72 km/h). Over the interior waterways, gusts tended to be 30 to 45 mph (50 to 70 km/h).

The peaks at most stations at the north end of the Willamette Valley were associated with the passage of a strong and well-defined bent-back front, and had a somewhat unusual SW to WSW direction. Areas typically sheltered during more common southerly windstorms were in some cases more directly hit by the December 21, 2015 event, toppling vulnerable trees and catching some people by surprise. A near-surface wind direction counter to the mean orientation of the terrain (south to north) likely reduced the peak wind speed potential.

West-jet storms with a similar track include January 7-8, 1990 and January 11, 1988. These earlier extratropical cyclones produced higher interior wind gusts than the December 21, 2015 windstorm, with speeds reaching and exceeding 60 mph (95 km/h). Indeed, the 2015 storm ended up being a marginal high-wind event on official anemometers, with gusts topping out at 55 mph (90 km/h). This is below the official 58 mph (93 km/h) minimum for high wind gusts, but is right at the cutoff used here on The Storm King for windstorm climatological studies. When considering gust speeds of 50 mph (80 km/h) or more, the spatial distribution is quite similar to other Storm King events from history. In this regard, the storm had the outcome for its track class.

The December 21, 2015 windstorm developed over the open Pacific Ocean far to the west and tracked nearly due east (Figure 2.1), carried on a strong jet stream. The system entered a rapid deepening phase just outside of 135ºW, and the intensification continued for the next 15 h, with the central pressure falling a solid 18 hPa (0.53" Hg) to 980 hPa (28.94" Hg) by 1000 PDT on December 21st. The central pressure began to rise just before landfall, climbing to an estimated 982 hPa (29.00" Hg) by the time the storm center moved over Long Beach, Washington, this estimate being based on a cyclostrophic wind model. Landfall occurred with the storm at near peak intensity. Filling continued as the low progressed inland, with the central pressure climbing to 984 hPa (29.06" Hg) as the low tracked due north of Portland, Oregon. The low continued weakening as it moved inland, and lost much definition as it crossed the rugged Cascades.

While over the open ocean, the extratropical cyclone moved quickly, approaching 60 mph (95 km/h) for a time. As the low neared landfall, its forward motion slowed dramatically, in classic fashion. The storm moved ashore with a speed around 25 mph (40 km/h) and continued at this rate across southwest Washington and into the Cascades. This had the effect of prolonging the period of strong pressure gradient over northwest Oregon and provided time for near-surface winds to accelerate to the gradient's potential.

The December 21, 2015 extratropical cyclone developed under the left-exit region of a strong 300 hPa jet stream maximum (Figure 2.2). The jet stream had a strongly zonal configuration, with a west to west-northwest airflow. The surface low also developed in the base of a broad and shallow 500 hPa trough, quite typical for a "west-jet" scenario, with the low tracking nearly due east. Deeper, more U-shaped troughs tend to support storm tracks with a northeast to north direction. An example of a more pronounced trough is included in the analysis for the December 11, 2014 classic windstorm. For the December 21, 2015 storm, there is clear warm advection ahead of the system, but cold advection wrapping around the base of the low appears to have been fairly weak.

Satellite imagery reveals the strong jet stream racing to the east-southeast that supported the formation of the December 21, 2015 extratropical cyclone (Figure 2.3), as described in section 2.2. A clearly defined dry slot suggests strong development of the low (dark gray and orange), located just off of Astoria at the time of the 4-km resolution photos. The system had a well-defined bent-back front, the tip of which swept right into northwest Oregon, seen wrapping around the low in the 1-km visible image.

As is typical for a west-jet storm, only a few stations received the most extreme pressure minima--those stations close to the track of the low-pressure center (Table 3.1). Astoria, very close to the landfall location of the center, reported the lowest pressure among the listed coastal stations. The low took an east-northeast jog during its trek across sourthwest Washington and passed fairly close to Olympia, thus bringing the lowest pressure to this inland station. Not shown in the table, Chehalis reported a minimum altimeter reading of 29.06" Hg (984.1 hPa) at 1235 PDT. The exact center appears to have tracked just south of this station, and north of Kelso.

As the December 21, 2015 storm moved east to east-northeast on its inland route over southwest Washington, barometers well to the south and north barely felt the compact system (Figures 3.1 and 3.2). The pattern of sea-level pressure readings on the coast is simply classic for this kind of storm. Astoria shows an inverted shark-fin form--this is the mark of the passage of the strong low just to the north, followed by the arrival of the bent-back front and its associated extreme pressure rises, sometimes called a pressure surge.

An important feature in both of the above charts is the tendency for higher pressure to reside to the south of incoming extratropical cyclones (Figure 3.3). The pressure over northern California remained far higher than over northern Vancouver Island as the extratropical cyclone moved inland. A mature 982 hPa (29.00" Hg) low resided over the coastal waters northwest of Haida Gwaii, with a long trough extending to the December 21, 2015 storm. This, too, is a classic pattern for Northwest windstorms, with a key result being a tendency for lower pressure gradients to the north of the low. This helps prevent intense northerly winds--north of the storm track--during many Cascadia windstorms.

South-to-north cross sections of sea-level pressure reveal a well-developed extratropical cyclone at landfall around noon on December 21, 2015 (Figure 3.4). There is evidence of a funnel-shaped profile, especially on the south side of the low, an indication of a strong system near peak intensity. As the low tracked inland, encountering the frictional effects of the rugged terrain, the pressure gradients relaxed rather quickly (Figure 3.5). The cross-section at 1300 indicates a trend to a more bowl-like form. This likely mitigated the potential for high winds at inland locations, including the Willamette Valley. The low center tracked between Kelso and Chehalis, with the result being strong S winds at the former and light N winds at the latter. The central pressur at this time was around 984 hPa (29.06" Hg).

As the surface pressure trends in section 3.1 indicate, the strongest pressure tendencies occurred from Newport to Hoquiam on the coast, with Astoria right at the center of the action, and Portland to Olympia in the interior (Table 3.2). The readings at Astoria indicate a vigorous storm. However, they are well below the tendencies of a number of major landfalling windstorm events, including -7.7 hPa/hr and +16.6 hPa/hr at North Bend during the November 10, 1975 storm and -7.1 hPa/hr and +13.9 hPa/hr at Hoquiam on November 3, 1958. These earlier windstorms brought much higher peak wind and gust speeds to their respective strike zones than the December 21, 2015 event.

The coincidence of pressure tendency extremes at stations near the landfall point of the storm center (Figures 3.6 and 3.7) is classic for a west-jet storm. This is, of course, due to the geometry of the situation, with the stations lined approximately south to north up to Buoy 46087 and the low tracking inland perpendicular to the line of observation sites. Earlier peaks at the southern stations, such as Eugene, are likely the result of the leading front arriving ahead of the low.

The standard (1-dimensional) pressure gradients for the December 21, 2015 windstorm were moderate in general, with the strong values occurring very close to the storm center on the south side (Table 3.3). A number of extratropical cyclones from history have brought more extreme values than the +10.2 hPa for the EUG-PDX measure. This includes "Storm King" type events on February 5, 1965, October 2, 1967 and January 7-8, 1990. These three historic storms produced higher wind gusts in the Willamette Valley than the December 21, 2015 windstorm, including 78 mph (130 km/h) at Portland in 1967. The lower pressure gradients associated with the December 21, 2015 extratropical cyclone is likely the most significant reason that winds were not stronger in the Willamette Valley.

Time series plots of the coastal pressure gradients reveal a classic positive and negative shark-fin pattern (Figure 3.8). This is the mark of a strong low-pressure center tracking right between key station pairs, OTH-AST and AST-UIL, allowing for a near-perfect signal. For a strikingly different pattern, compare these charts to the results of the December 11, 2014 windstorm that tracked north-northeast up the coast.

As is typical, the signal is softened at interior stations (Figure 3.9), the mark of weakening pressure gradients. However, some of the less-sharp signal has to do with the low center tracking right through the middle of the PDX-SEA measure, a fact that would prevent a very strong gradient from being captured. A finer resolution would probably capture more detail over the southwest Washington interior.

Maximum standard pressure gradients show the classic pattern of higher values south of the low-pressure center (Figures 3.10). North of the low, peak pressure gradients fall off sharply near the low center, then then continue downward more gradually, reaching quite modest levels. The short measures capture the pattern quite strongly (Figure 3.11). Note that the interior value at 42ºN, MHS-MFR, is in question. The station MHS is at quite a high elevation, 3,540 feet (1,079 m). There are known issues with reducing pressure to sea level from high-elevation stations, and the error is likely showing up in the steep MHS-MFR pressure gradient. RBL-MHS would also be affected by the error, in the opposite direction, meaning the reported value is probably too low.

Maximum absolute (2-dimensional) pressure gradients tell a story similar to the standard 1-dimensional gradients in section 3.3 (Table 3.4). The highest gradients occurred on the immediate south side of the low, with more modest maxima to the north. Pressure gradient orientations (pressure slope) are interesting. Often with landfalling storms of this nature, the maximum gradients--in those regions immediately to the south of the storm center--tend to have a south-southwest to southwest orientation. In the case of the December 21, 2015 windstorm, the gradient had a southeasterly orientation at peak. This suggests that peak gradient occurred ahead of the low, probably near the time of landfall, instead of when the low tracked due north of places such as the Willamette Valley in what would be a more classic response. The difference in pressure slope may have mitigated peak gust speeds to some extent. Nevertheless, pressure gradient maxima agree quite well with the observed near-surface wind response, as indicated by the estimated peak wind and gust for different regions. The December 21, 2015 extratropical cyclone only provided a modest "taste" of what a Storm King type event can deliver.

Two regions were affected by strong pressure gradients approaching 7.5 hPa/100 km during the December 21, 2015 windstorm: the north Oregon Coast and Willamette Valley (Figures 3.12 and 3.13). Areas further south experienced moderately strong gradients of around 5.0 hPa/100 km. As is typical of this kind of event, regions north of the track tended to have the lowest pressure gradients. The area of peak gradient around the storm center does not stand out very strongly relative to the background, or surrounding regions (Figure 3.14). Compare this outcome to some other recent storms that produced much stronger differences in peak pressure gradient, such as December 11, 2014 and October 15, 2016. The somewhat muted profile and the given pressure gradient magnitudes fits well with wind speeds that in general did not quite reach official high wind criteria in most areas.

Pressure gradient profiles reveal a trend of pressure gradients relaxing to the north and south of the storm track as the December 21, 2015 windstorm neared the coast (Figure 3.15). At the same time, gradients near the track escalated. For southern sections, pressure gradients were at their strongest in the warm sector during the early morning, with the low still offshore. At the same time, northern regions were under a relatively strong easterly gradient in the storm's northeast quadrant. As the low tracked inland around noon, pressure gradients to the south relaxed after the cold front swept through. To the north, gradients also relaxed as the long trough stretching from a mature Gulf of Alaska low moved inland with the December 21, 2015 extratropical cyclone, depressing barometric readings over a large area. Strong gradients only resided near the low-pressure center, reflecting a compact core.

While pressure gradients escalated gradually on the coast, the interior sections experienced a more sudden onset with a sharp jump between 0800 and 1000 PST (Figure 3.16). It appears that pressure gradients climbed rapidly after the leading warm front swept through the Willamette Valley, with temperatures escalating from roughly 5ºC (40ºF) to 10ºC (50ºF). The pressure slope also shifted from easterly to southeasterly. With the cold surface layer scoured out by invading warm air, and the pressure slope orientation becoming better aligned, the warm front opened the gates for strong south winds. Over the next three hours, the southerly winds shifted to southwest and escalated to peak values as the bent-back front neared, this despite a slight relaxation of the pressure gradient over the Valley as a whole. What appears to have happened is that the pressure gradient over the north half of the Valley intensified as the low center neared while over the south Valley the gradient may have already begun diminishing, with the effect being a slight decrease in the overall measure. This is reflected in the 1-D pressure gradient short measures (Table 3.3), with a much sharper peak for SLE-PDX, 6.7 hPa/100 km at 1300 PST, compared to EUG-SLE, 4.6 hPa/100 km at 1100 PST. The difference from south to north across the Valley at 1300 PST can also be seen in Figure 3.5. The numbers provide another reflection of a storm with a very compact core and an approximate funnel-shaped pressure profile.

Data Sources

Surface observations are from the National Climatic Data Center, the National Data Buoy Center, Environment Canada and the University of Washington. Surface maps used for storm track determination are from the US. Weather Prediction Center. Upper-air analysis is based on maps from the US. National Center for Environmental Prediction. Satellite photos are from the US. National Weather Service. Upper-air sounding data are from the University of Wyoming Department of Atmospheric Science.