"Modern" Windstorms 1995-2006:
Adjusting Peak 5-Second Wind
to Better Fit With Past Instant Gust Measures

compiled by

Wolf Read

Introduction

After a recent storm that looked like a potential high-wind producer flopped, a storm-watcher friend wrote and suggested not to worry, for it's been eight years since that last major event (December 12, 1995), and that it was about time for the next. That comment got me thinking. Was December 12, 1995 really the last big storm? Or had the new 5-second peak gust measured by the National Weather Service's relatively new Automated Surface Observation System (ASOS) "masked" some of the storms since 1995? In other words, could a major storm have struck and not produced the expected peak gust signature owing to the difference in measure?

For some these may be trivial questions, but to a climatologist they are not. The five-second gust is a completely different measure than the peak instant gust that was formerly noted on official surface observation forms. This issue has been covered in a number of webpages on this site, including the case studies for the December 12, 1995 and January 16, 2000 windstorms. This essay will examine the climatological problems that result from a 5-second wind in a more systematic manner. This is important, for the new gust measure literally invalidates comparison between storms of history with those of today.

There a new complication in the works: As the NWS switches from cup anemometers to sonic versions in the mid-2000s, the wind measure is changing again, this time to a 3-second gust, which is the international standard, and used for building codes. A 3-second gust is somewhat different from a 5-second one. When time permits, the 3-second gust will be examined more thoroughly.

Discussion

A 5-second "gust" is something entirely different from an instant gust. The former is an average, so technically it should be called a 5-second "wind" (and usually is by the NWS and NCDC). An instant gust is a direct reading. A specific value for a 5-second wind can contain much higher instant readings within its duration. To see this, take a look at Figures 1 through 3, below.

Figure 1: Peak gust, Oregon City, OR, February 7, 2002 cyclone. Peak instant gust 27 mph, peak 5-second wind 21 mph. Same wind, different measure.

Figure 2: Peak gust, Albany, OR, December 5, 2003 open wave. Peak instant gust 38 mph, peak 5-second wind 30 mph.

Figure 3: Peak gust, Albany, OR, January 29, 2004 cyclone. Peak instant gust 44 mph, peak 5-second wind 37 mph.

The above figures focus on short time periods around the peak gusts of various storms, and show 0.5-second samples from a Maximum Vigilant anemometer and various average velocities calculated from the samples. The February 7, 2002 South Valley windstorm, which only gave my location a glancing blow, produced a gust of 27 mph, with a peak 5-second wind of just 21 mph. The December 5, 2003 open wave brought an instant gust of 38 mph at my home, yet the peak 5-second average only managed 30 mph. The January 29, 2004 cyclone landed a gust to 44 mph at my home, which resulted in a peak 5-second wind of 37 mph. These differences are significant, amounting to a factor of about 1.20 to 1.30. It is clear that the 5-second average smoothed out the spikes--peak instant gust is lost in the longer-term measure. The values and actual ratios are listed below in Table 1. Official readings taken during the December 12, 1995 windstorm are included. It is fortunate that the old direct-reading anemometer was still in operation then, and that the peak gust was noted by NWS personnel.

Table 1: Summary of known peak gust to 5-second wind measures. The NWS readings were from two different systems, the old direct-reading equipment and the ASOS sensor, a fact that confounds the estimation.

Maximum Vigilant Values (Unofficial)
  Peak Velocities, mph
Date of Event Instant 5-second Ratio
07-Feb-2002 27 21 1.29
05-Dec-2003 38 30 1.27
29-Jan-2004 44 37 1.21
       
NWS Measures (Official, from Portland, OR)
Date of Event Instant 5-second Ratio
12-Dec-1995 74 62 1.19
       
    Average 1.24

The information in the above table can be used to make an upper-bounds instant-peak-gust estimation for windstorms that occurred during modern times. There's some evidence of a trend in the above data where the ratio decreases as the peak gust increases, suggesting that the higher the gust, the longer its duration. If this is true, using the straight average of the ratios would result in overestimation of peak gusts at the high end. For the following calculations, the 1.19 ratio from the official data will be used for the peak gust adjustments. At some point a sliding scale might be employed for finer-tuned estimations--this probably won't happen until more data is added to the above list, especially for faster than 27, 38 and 44 mph peak gust readings.

Note that other gusts are included in some of the charts in Figures 1 through 3. The January 29, 2004 cyclone produced several that were stronger than the other storms. One could take those gusts and establish larger set of data from which to extrapolate ratios. Out of curiosity, I did this for the January 29, 2004 event, counting 15 gusts ("gust," in my study equaled any increase in 5-second wind of 2 mph or more, which resulted in 15 samples during the 3 minutes and 40 seconds). The result was an average ratio of 1.15, with a minimum of 1.06 and a maximum of 1.28. I don't use this information directly in this study for one main reason: I'm interested in how the 5-second wind has changed the record of peak gusts in windstorms. There could be something unique about peak gust that makes it different from all the others. So, to reduce possible confounders (there are plenty!), this study is limited to the peak gust for each individual storm. With some patience, a good dataset will be established. This paragraph is provided to provide the reader with a possible range of variation until more data are acquired.

Using the 1.19 adjustment factor on some wind events from 1995-2006, several interesting results occur. But before discussion, here's the table:

Table 2: Peak Gust Adjustments for "Modern" Storms, 1995-2006, mph.
High wind warning criteria gusts (58 mph or more) are shown in red.
Storm
ACV
OTH
AST
UIL
MFR
EUG
SLE
PDX
OLM
SEA
BLI
Avg
12Dec1995*
Official
58
86
62
61
45
49
59
62
57
60
76
61.4
Adjusted
58
86
72
61
54
58
70
74
57
60
76
66.1
01Jan1997
Official
46
64
56
47
25
45
41
51
46
39
55
46.8
Adjusted
55
76
67
56
30
54
49
61
55
46
65
55.7
05Feb1999
Official
15
52
49
43
13
35
39
43
38
40
55
38.4
Adjusted
18
62
58
51
15
42
46
51
45
48
65
45.7
06Feb1999
Official
33
54
55
31
40
46
43
40
37
45
40
42.2
Adjusted
39
64
65
37
48
55
51
48
44
54
48
50.2
03Mar1999
Official
40
49
66
55
38
52
46
51
47
60
63
51.5
Adjusted
48
58
79
65
45
62
55
61
56
71
75
61.3
16Jan2000
Official
47
51
66
45
39
39
60
59
54
52
66
52.5
Adjusted
56
61
79
54
46
46
71
70
64
62
79
62.5
13Dec2001
Official
43
58
49
50
26
36
45
38
40
40
41
42.4
Adjusted
51
69
58
60
31
43
54
45
48
48
49
50.4
07Feb2002
Official
39
53
33
10
36
70
31
31
16
21
21
32.8
Adjusted
46
63
39
12
43
83
37
37
19
25
25
39.1
27Dec2002
Official
32
61
59
32
30
39
37
39
40
52
22
40.3
Adjusted
38
73
70
38
36
46
44
46
48
62
26
47.9
05Dec2003
Official
24
45
49
36
15
30
33
31
37
32
43
34.1
Adjusted
29
54
58
43
18
36
39
37
44
38
51
40.6
29Jan2004
Official
36
48
47
47
47
39
44
41
37
40
43
42.6
Adjusted
43
57
56
56
56
46
52
49
44
48
51
50.7
25Dec2005
Official 35 55 54 36 30 37 35 46 41 38 48 41.3
Adjusted 42 65 64 43 36 44 42 55 49 45 57 49.1
01Jan2006
Official 45 43 46 51 35 41 45 44 45 49 53 45.2
Adjusted 54 51 55 61 42 49 54 52 54 58 63 53.8
04Feb2006
Official
39
51
59
53
32
46
39
44
43
47
62
46.8
Adjusted 46 61 70 63 38 55 46 52 51 56 74 55.7
14Dec2006**
Official 36 48 69 59 47 54 53 53 53 69 55 54.2
Adjusted 43 57 82 70 56 64 63 63 63 82 65 64.5
ACV OTH AST UIL MFR EUG SLE PDX OLM SEA BLI Avg
* Dec 1995: Not all stations had switched to ASOS; these values are left unadjusted.
** Dec 2006: Gusts for OTH and OLM may be low due to data interruptions. Gusts are highest reported.
Numbers in italics are values extrapolated from peak 2-minute wind by applying a standard 1.3 gust factor.

Before going into the discussion, it should be noted what the adjusted values in Table 2 aren't. Likely, the adjusted values aren't the actual peak instant gust from the sample storms. The actual instant peak gust could have been lower, or even higher. The number simply represents a realistic upper boundary for the peak instant gust. The official value provides the lower boundary. Most likely, the real value lies between the two numbers.

That said, we can do some comparisons. The December 12, 1995 storm still stands on top of everything that has occurred during the ASOS era. However, two storms, when adjusted, become very close in average gust strength to the 1995 event: March 3, 1999 and January 16, 2000.

By my own breakdown, storms that produce a peak gust average of 55.0 or greater at the 11 stations are considered "major" events. The adjustments elevated the two abovementioned storms well into this category, from positions that were still a strong showing, but not of "epic" proportions. Storm that rank in the 60s are very rare; the adjustments reveal that possibly two happened within less than a year in 1999-2000. But we don't know for certain--that's the problem with the change in wind measure brought by ASOS. A few details could help with assessing just how powerful the March 3, 1999 and January 16, 2000 events were.

The March 3, 1999 storm caused some incredible wind gusts unofficially, and much damage. It is considered by forecasters as being one of the most intense storms in recent memory. The March 1999 Storm Data publication shows unofficial gusts of 120 mph at Depoe Bay, 105 at Cannon Beach, 92 at Tillamook along the Oregon coast, and 77 mph at Sandy and 75 mph at Sheridan in the Willamette Valley. Instrument exposure and calibration notwithstanding, these values suggest that my 1.19 factor may underestimate the power of the March 3, 1999 event. This storm looks every bit as bad as the December 12, 1995 windstorm. For another example, in Washington, the Evergreen Point Floating Bridge was closed during the March 3, 1999 gale--an eventuality that hadn't happened since the 1995 storm.

The January 16, 2000 sou'wester also appears quite intense. The January 2000 Storm Data shows unofficial gusts of 115 mph at Cannon Beach, 87 at Newport (officially 60 mph) and 80 mph at the Newport Jetty. No unofficial observations for the Willamette Valley are available, but the data suggest a storm of similar magnitude to March 3, 1999. Instant gusts into the low 70s are not out of line for the Salem to Portland region. The Storm Data publication also notes the closure of the Evergreen Point Floating Bridge during the January 16, 2000 event, suggesting similar conditions to the big 1999 and 1995 storms.

So, was December 12, 1995 the last truly major storm? The evidence suggests, "No." Both March 3, 1999 and January 16, 2000 appear to belong in this "esteemed" category. With two major events in less than a year, plus some solid storms in February 1999, it seems that these cyclones were the last gifts of a stormy period that began in late 1994. Since the January 16, 2000 windstorm, the Pacific Northwest has been subjected to periods of drought, sometimes quite extreme, and generally weaker windstorm activity, with one significant exception: February 7, 2002.

It is noteworthy that not a single ASOS station managed to show a peak gust of 70 mph or more in Table 2, save for Eugene during the February 7, 2002 windstorm. Not even the better-located coastal stations have recorded gusts as high [Footnote 1]. This serves to emphasize the significance of that 70 mph wind gust at Eugene, and the true power of the February 7, 2002 windstorm. Considering the damage wrought by this mesoscale event, which included swathing of trees, physical damage to structures by raw wind force, and lines of powerpoles being toppled--relatively new poles that were rated at 112 mph!--the 83 mph instant gust estimate at Eugene seems an appropriate estimate, if a bit low! It appears that Eugene, and the South Willamette Valley, were visited by wind speeds comparable to the great Columbus Day Storm of 1962. Eugene's peak instant gust during the "mother of all windstorms" was 86 mph, highest ever recorded at the station. Just for the sake of examination, the average of the gust adjustment ratios in Table 1 yields an estimated peak gust of 88 mph at Eugene; higher than the peak for the 1962 Big Blow.

Had storm watchers received their long awaited Columbus-Day winds, or did the February 7, 2002 windstorm fall short? We will never truly know if 83, 86 or 88 mph was the case for February 7, 2002, or if the highest gust was simply the 70 mph shown by the ASOS. Unfortunately, unofficial instant-gust readings for this storm's main strike zone are absent from the record (such as Storm Data). All we have are the estimates in Table 2, and the visible storm damage to suggest that, indeed, winds of Columbus Day strength visited the Willamette Valley in 2002.

This brings up another way to look at what ASOS has done to peak wind comparisons. We can do the calculations in inverse to estimate what the maximum 5-second wind might have been for two major storms in history, the Columbus Day Storm of 1962 and November 14, 1981. Here are the results:

Table 3: Peak Gust Adjusted to 5-Second Wind for Two Major Storms of History
High wind warning criteria gusts (58 mph or more) are shown in red.
Storm
ACV
OTH
AST
UIL
MFR
EUG
SLE
PDX
OLM
SEA
BLI
Avg
12Oct1962*
Official
58
81
96
78
58
86
90
104
78
58
98
80.5
Adjusted
49
68
81
66
49
72
76
87
66
49
82
67.6
14Nov1981
Official
60
92
68
48
62
58
71
71
64
67
64
65.9
Adjusted
50
77
57
40
52
49
60
60
54
56
54
55.4
* For the 1962 storm, TTI is substituded for UIL.

Storm watchers, pay close attention to the modified readings for the Columbus Day Storm. That's the new signature of a "storm-of-the-century" event. The extreme instant gust values are reduced considerably, with the storm's average falling close to the unmodified value for the November 14, 1981 windstorm. The Columbus Day Storm stands out somewhat less. Gusts in the 60s and 70s don't seem as spectacular as 80s, 90s, and 100s. Nevertheless, even with the reduction, some stations still have a strong showing, including Portland, OR, with an 87 mph 5-second wind. Of course, the 104 mph was an estimate--a necessity as the storm knocked out power early and shut down the direct-reading wind equipment. Using the measured value of 106 mph at Troutdale, we get 89 mph, which does stand out, and is a close match to an 88 mph "fastest mile" (88 mph 41-second wind) at Portland that was recorded near the beginning of the storm. In any event, a Columbus Day type of event apparently will show up less strongly under the 5-second wind regime, but, owing to some stations being subjected to particularly strong winds, will still stand out as something unusual.

For a major, but not Columbus-Day-Storm-extreme, event like November 14, 1981, the story is different. Save for at North Bend, the peak wind values don't stand out much. Many storms in history have produced 50 to 60 mph gusts. The average of 55.4 is still in major category, but is right at the borderline. This clearly demonstrates how the new peak 5-second wind measure can "mask" a rare windstorm. This table suggests that the new official signature for a top-end major event is 50 to 60 mph gusts, not 60 to 75.

There's another twist to the story being examined here: What happens to the frequency of high wind warnings between the pre-ASOS and ASOS eras? Did more than one standard for a high wind event, by NWS definition, change?

High Wind Warnings in the New ASOS Era

National Weather Service criteria for a high wind warning (HWW) event is for gusts of 50 knots, or 58 mph, to occur within the warning area. Gusts meeting this criterion are shown in red in Tables 2 and 3. This definition has been around some time; minimum HWW criteria during the pre-ASOS era certainly wasn't higher. I recall a few high wind warnings for 55 mph gusts in the Seattle area during the 1980s.

If the NWS bases its wind forecasts on how the official stations respond, then, quite possibly the frequency of HWW might have decreased since the inception of ASOS. Note that three storms in Table 2 did not produce HWW criteria "gusts" under the official peak 5-second wind record of ASOS: February 5, 1999, February 6, 1999 and December 3, 2003. Yet, based on the 1.19 adjustment factor, it appears that these three events could have achieved gusts of 58 mph or higher at coastal stations. For the last storm, this point is kind of moot, for a HWW was issued for the coast (one that was verified by stations not included in the table).

High wind warnings for the interior sections, like the Willamette Valley, are rare. The difference between peak 5-second wind and peak instant gust is probably more significant than on the coast, where gusts to 60 and 70 mph are regular occurrences during the winter season. The January 1, 1997, March 3, 1999 and December 27, 2002 windstorms, which generally didn't have a strong showing in the interior for 5-second wind measures (save SEA and BLI in 1999), meet HWW criteria with the peak instant gust adjustment, either at one point, or over a wider region.

Another way of looking at this is with Table 3, especially the November 14, 1981 windstorm. The number of stations meeting HWW criteria drops from 10 to 3 when the peak instant gusts are adjusted to 5-second wind! The storm appears to barely meet the high wind warning standard, yet it was one of the biggest in history.

It seems that the standard for high wind was automatically changed by employing a new peak 5-second "gust" at official stations. Instant gusts of 69 mph are a reasonable expectation when 5-second winds are approaching 58. So, the new ASOS era appears to have changed the HWW criteria to approximately a minimum of 70 mph for instant gust. The other way to look at this is that peak 5-second winds of 48 to 50 mph (implying potential instant gusts of 57 to 60 mph) at ASOS stations would have likely met the minumum HWW critera pre-ASOS. It seems that 5-second "gusts" of 43 knots, approximately 50 mph, should be the new standard for high wind warnings, certainly for interior sections of the Pacific Northwest (there's good reason to distinquish between coastal and interior regions when providing wind advisories and warnings, a topic in its own right).

Since it's doubtful that 58 mph instant gusts have changed in force after the installation of ASOS stations, it's interesting that the NWS has kept the 58 mph minimum for HWW when using a 5-second wind to "verify" warnings. Skywarn spotter reports probably reduce this issue to some extent. A number of personal anemometers still report instant gust in analog format (Maximum equipment being one example), though digital stations that show short-duration averages, like 2.5-seconds on Davis systems, are also common, watering down the effect.

The bottom line: We're in a new era, with a new kind of wind measure. This should be kept in mind when trying assess the strength of a particular wind event.

Footnotes

[Footnote 1] Certain stations, like Cape Arago and Cape Blanco, of course, have recorded significantly faster 5-second winds, but these are among the most wind-favored locations in the Northwest and aren't included in Table 2.

Last Modified: September 23, 2007
Page Created: December 9, 2003

You can reach Wolf via e-mail by clicking here.

| Back |