For boundary layer gradient winds the equations have a quadratic form, and there is a square root. Thus, theoretically, when the pressure gradient is doubled, the wind speed does not double, but increases with the square root of 2, or 1.41 times. Another way of looking at this is that, because the wind force increases with the square of velocity, when the pressure gradient is doubled, the wind force, not speed, doubles.
For the intense landfalling extratropical cyclone scenario outlined above and shown in Figure 3.1, this means that expected peak winds would not be around 1.66 times the CDS, but closer to sqrt(1.66) = 1.29 times. This is still a phenomenal 75-100 mph (120-160 km/h), or in other words category I to II hurricane wind speeds. Gusts would be higher, potentially approaching 140 mph (225 km/h) in places.
The outcome for the Valley would be catastrophic. A large percentage of trees would be broken or uprooted, many windows would be shattered and structures would be unroofed or demolished completely, resulting in much flying debris, which can cause damage to other structures including electrical substations. Indeed, 140 mph gusts may push or exceed the design limits of steel-framed high-rises in downtown Portland and also critical infrastructure like transmission towers. Such destruction would devastate the power grid--to a level outside the experience of many people working at the region's electrical utilities today, save for those that have been involved with post-hurricane recovery. Most roads would be literally buried under toppled trees and shattered structures, limiting the ability of first-responders and repair crews to reach the afflicted region. Many people, indeed communities in the Valley are probably not prepared for such a windstorm.
It is important to keep in mind that the suggested scenario has an extremely low probability of occurring. This is perhaps a 5,000-year storm for the Willamette Valley, which means a rather slim 1 in 5,000 chance each year. Civilizations can rise and fall without such an event occurring.
A number of factors influence measured surface wind speeds aside from pressure gradients, including storm track direction, storm speed and upper-air support. Also, hourly observations may not exactly capture the peak gradient. As evidenced by Table 1, and the peak winds for the listed windstorms (not shown), maximum pressure gradients do not have a perfect relationship with peak wind speeds (e.g. 02 Oct 1967 vs. 08 Jan 1990). Thus, the peak wind prediction for the above extreme windstorm scenario has some error and the actual outcome could be lower than the basic calculations would suggest. Peak winds near the center of 12 Mar 2012 extratropical cyclone as it landed on northern Vancouver Island were about on par with the CDS, in the range of 60-67 mph (96-107 km/h) for 2-min averages with gusts upwards of 81-88 mph (131-143 km/h), and did not exceed the 1962 windstorm. However, weather stations were sparse in the landfall region, were not necessarily in the best locations to capture the strongest S to SE winds and observations were intermittent at some reporting sites, leaving the possibility that the highest winds associated with the 2012 storm were not captured. Being further north than the Willamette Valley, wind speeds would also be a little lower for an equivalent pressure gradient--see the difference in calculated geostrophic wind magnitude above (7.1%). In any event, the above peak wind and gust estimates for the hypothetical storm can only be considered rough approximations.
Although the actual peak wind speeds may not be as strong as indicated by the basic calculations done here (they could in fact be stronger), the probability that such a storm scenario would result in higher winds than the CDS is nonzero. Does it seem likely that such a large exceedance in peak surface pressure gradient would result in slower winds than the CDS, especially given a similar track direction, overall storm speed and upper support?
In the scenario, the low center is moving inland at a shallow angle to the coastline, thus the storm would be interacting closely with the terrain well before it reached Astoria. Perhaps there would be some reduction of the strong pressure gradient in the southeast quadrant of the low due to ageostrophic surface flow causing rapid filling. How much weakening depends on the proximity of jet-stream support for maintaining a deep surface low.
A more likely scenario for bringing such a strong pressure gradient inland is a more zonal--northeast to east--storm track. With such tracks, the upper-air flow tends to be more westerly, and therefore the upper wind support for southerly near-surface winds is weaker, mitigating wind speeds to some extent. However, with a 30.9 hPa/169 km gradient, record wind speeds may be possible even with a more zonal track. On 10 Nov 1975 a 97.5 kPa (28.79" Hg) extratropical cyclone on an east-northeast path brought a 24 hPa/161 km (100 miles) gradient inland, which resulted in perhaps the highest gust ever recorded at Roseburg, 75 mph (120 km/h), this in a very narrow north-south valley that is a fairly wind-sheltered location.
Peak measured 1-minute winds during the 02 Oct 1967 extratropical cyclone that tracked through northwest Oregon reached 48 mph (78 km/h) at Salem. A fastest mile of 70 mph (113 km/h), equivalent to a 51-sec average, occurred at Portland. The storm brought a peak pressure gradient of 16.2 hPa/169 km, with a pressure slope of 228° indicating a low center well inland. Now bring in the 12 Mar 2012 low: sqrt(30.9/16.2) = 1.38. This suggests the potential for a 66 mph (106 km/h) peak 1-min wind at Salem, and a stunning 97 mph (156 km/h) 51-sec wind for Portland, speeds higher than the CDS.
Some things to consider about the scenarios presented here: They are modeled from a real storm that had a minimum central pressure of around 96.1 kPa (28.40" Hg). Deeper central pressures have occurred with a number of Pacific Northwest windstorms. The 09 Jan 1880 storm may have had a central pressure of 95.5 kPa (28.20" Hg) before landfall and tracked across extreme northwest Oregon to a position due north of Portland, this with a central pressure still deep enough to depress the barometric indicator to an all-time record low of 967.0 hPa (28.56" Hg) in the Rose City. The 14 Nov 1981 major classic windstorm had a minimum central pressure of at least 94.7 kPa (27.96" Hg) (Reed and Albright 1986). A number of extratropical cyclones have tracked in the vicinity of Astoria, including the 1880 storm, the CDS, and 05 Feb 1965. A strong extratropical cyclone on 27 Mar 1963 not only tracked inland very close to Astoria, but was also on a recurving path shifting from northeast to north-northeast at the time of closest approach. There is at least one historical storm that produced a stronger pressure gradient than the 12 Mar 2012 windstorm: The 03 Nov 1958 extratropical cyclone brought a truly phenomenal 19.8 hPa /100 km, or 33.4 hPa/169 km, pressure gradient into southwest Washington. In summary, the key elements in the presented scenarios are well within the realm of possibility.
There does appear to be a relationship between windstorm central pressure and peak gust speeds (Read 2015b), this despite some noise due to the magnitude of the background pressure field having an influence on the absolute central pressure value. For example, a 96.0 kPa storm in broad region of already low pressure may be equivalent--have the same total pressure difference from the edge of the storm to the center--to a 98.0 kPa storm in a region with higher overall background pressure. Regardless of the noise, the observed tendency for faster gusts around deeper lows likely reflects a trend of steepening pressure gradients as storm central pressure drops. Thus, it seems possible that an even more extreme windstorm than in the proposed scenarios could happen. A 94.7 kPa (27.96" Hg) extratropical cyclone such as the 14 Nov 1981 event tracking over Astoria as in Figure 3.1 could potentially have steeper gradients than the 12 Mar 2012 windstorm, and therefore generate even faster winds than the already extreme values presented above.
Focus has been on the Willamette Valley mainly because this is where the CDS brought the strongest winds to the interior lowlands. Scenarios that suggest wind speeds in excess of the CDS in the Willamette Valley also point to the possibility of exceedence in other regions affected by the CDS, such as the Puget Lowlands and the Lower Mainland.