786 JOURNAL OF APPLIED VOLUME 41

The 2000 Fire Season: -Caused Fires

MIRIAM L. RORIG AND SUE A. FERGUSON Paci®c Northwest Research Station, USDA Forest Service, Seattle, Washington

(Manuscript received 22 May 2001, in ®nal form 9 February 2002)

ABSTRACT A large number of lightning-caused ®res burned across the western United States during the summer of 2000. In a previous study, the authors determined that a simple index of low-level moisture (85-kPa dewpoint depression) and instability (85±50-kPa difference) from the Spokane, Washington, upper-air soundings was very useful for indicating the likelihood of ``dry'' lightning (occurring without signi®cant concurrent rainfall) in the Paci®c Northwest. This same method was applied to the summer-2000 ®re season in the Paci®c Northwest and northern Rockies. The mean 85-kPa dewpoint depression at Spokane from 1 May through 20 September was 17.7ЊC on days when lightning-caused ®res occurred and was 12.3ЊC on days with no lightning-caused ®res. Likewise, the mean temperature difference between 85 and 50 kPa was 31.3ЊC on lightning-®re days, as compared with 28.9ЊC on non-lightning-®re days. The number of lightning-caused ®res corresponded more closely to high instability and high dewpoint depression than to the total number of lightning strikes in the region.

1. Introduction storms. The Haines index contains a moisture factor in combination with a stability factor to produce a single The summer of 2000 was notable for the number of categorical index used to quantify the risk of wild®re large wild®res that burned across the western United growth. The unstable, dry conditions identi®ed by the States. In 2000, more than 122 000 wild®res ignited, Haines index also occur in conjunction with high-based burning more than 3.2 ϫ 106 ha. This is in comparison (see below) that produce ®re-igniting with the annual average for the previous 10 years of lightning strikes (Werth and Ochoa 1993), but the use- about 100 000 ®res and 1.5 ϫ 106 ha. Lightning causes most wild®res in the western mountainous states, in- fulness of this index for our purpose is limited because cluding Washington, Oregon, California, Nevada, Idaho, the categories are too coarse. Montana, Wyoming, and Utah. A database compiled by The problem of estimating the risk of ®re from dry the U.S. Department of Agriculture (USDA) Forest Ser- lightning is a complicated one. Whether a lightning vice, including the period of 1986±96 and consisting of strike will result in an ignition depends on many factors, locations, dates, and causes of ®res, reveals that light- including fuel moisture conditions, atmospheric mois- ning starts almost 60% of ®res on public land in these ture conditions, concurrent rainfall amounts and dura- states (Schmidt et al. 2002). Therefore, there is a need tion, and ®re suppression efforts. In unusually dry years, for better forecasts of the dry, unstable conditions that such as 2000, ®res may start despite signi®cant rainfall give rise to lightning that ignites ®res. because fuel moistures are extremely low. In addition, In addition to , low-level mois- a simple de®nition of dry lightning itself is somewhat ture de®cit is a key ingredient for ``dry'' lightning (light- elusive. Lightning can occur without signi®cant precip- ning that strikes the ground with little or no accompa- itation if a is high based, with rainfall nying rainfall). Fire-weather forecasters try to anticipate evaporating before it reaches the ground; it can occur episodes of dry lightning using existing tools such as outside the rain shaft of a ``wet'' thunderstorm; or it traditional stability indices (e.g., and K in- can occur in conjunction with a fast-moving thunder- dex) and the Haines index, which is an indicator of ®re storm when signi®cant rainfall amounts do not accu- growth (Haines 1988). The stability indices are effective mulate at any one location. Nevertheless, the importance in predicting periods of but were not de- of dry lightning strikes has long been recognized by the signed to distinguish between dry and wet thunder- ®re-research community. For example, the lightning ac- tivity level, a component of the National Fire Danger Rating System, includes a special case for high-based Corresponding author address: Miriam L. Rorig, Paci®c Northwest thunderstorms that produce no at the Research Station, USDA Forest Service, 4043 Roosevelt Way NE, Seattle, WA 98105. ground (Fuquay et al. 1979). E-mail: [email protected] The National Centers for Environmental Prediction JULY 2002 RORIG AND FERGUSON 787

(NCEP) Storm Prediction Center recently began fore- TABLE 1. Mean values of sounding variables (ЊC) at Spokane, WA, casting the potential for dry thunderstorms (those that for ®re and other days, 1 May±20 Sep 2000 (DD is dewpoint de- pression; TD is temperature difference). produce less than 2.5 mm of rain) as part of its Fire Weather Program (Naden 2001). These forecast prod- 85-kPa DD 85±50-kPa TD ucts include maps that depict convective available po- Fire days 17.7 31.3 tential energy and the average relative in the Other days 12.3 28.9 lowest 6 kPa of the (about 600 m above the t* 5.34 4.25 t ** 3.97 3.12 surface). This product indicates where thunderstorms crit(0.95,140) are expected and where there is a moisture de®cit, but * t ϭ value of the t statistic. there is no indication of how dry and unstable the con- ** tcrit(0.95,140) ϭ critical t value for the 95% signi®cance level with ditions must be for dry lightning to be of concern to sample size of 140. ®re-weather forecasters and there is no estimation of risk. Lightning-strike locations alone cannot be used to and 86 other days. No large ®re ignitions were reported estimate risk of ®re. Lightning-strike densities do not after 20 September. Lightning occurred on both ®re days correlate well with ®re start locations for several rea- and other days (lightning was recorded on 134 of the sons, including fuel type, fuel condition, and atmo- 143 days somewhere in the four-state region). Therefore, spheric conditions (Rorig and Ferguson 1999). This pre- it is important to discriminate those days on which the vious study showed an improvement in the ability to atmosphere is particularly unstable and dry, leading to estimate risk of ®re starts by using separate indicators a higher risk of ®re starts from lightning. of stability and moisture from upper-air soundings to Because Rorig and Ferguson (1999) found the 85- discriminate between dry and wet lightning days in the kPa dewpoint depression and 85±50-kPa temperature Paci®c Northwest. A maximum likelihood discriminant difference were most useful for discriminating between rule was developed using sounding data, precipitation wet and dry lightning days, the means of these variables records, and thunderstorm occurrence records from Spo- kane, Washington, and lightning-strike data from the were computed at Spokane for ®re days and other days National Lightning Detection Network (Cummins et al. (Table 1). The Student's t test was used to test for dif- 1998). The results of the study indicated the 85±50-kPa ference of means. The t test requires independence of temperature difference and 85-kPa dewpoint depression the data and is not robust against deviations from this were the most useful variables for classifying convective requirement. The daily data used to compute the means days into dry and wet categories. The discriminant rule are not independent; therefore, an alternative to the con- was then used to compute a probability of wet or dry ventional t test was used to compensate for autocorre- lightning in the Paci®c Northwest. The purpose of this lation in the data (von Storch and Zwiers 1999). The t article is to describe how this simple index, developed statistic is computed in the conventional manner and from historical data, identi®ed days in the summer of then is compared with a critical value of t that is ap- 2000 that had a greater risk of dry convection and, propriate for the sample size and the lag-1 autocorre- hence, lightning-caused ®res. lation coef®cient. If t is greater than tcrit, then the dif- ference in means is assumed to be signi®cant. Table 1 includes the computed t values and the critical t values 2. Methods and results for the 95% signi®cance level for each set of means. Dates and locations of large (more than 40.5 ha; The means from ®re days were signi®cantly higher (dri- smaller ®res are not included in the database) wild®re er and more unstable) than those from other days. Tables ignitions in Washington, Oregon, Idaho, and western 2 and 3 include the means for both groups computed Montana were collected for May±September of 2000. from the Boise and Great Falls soundings. The 70-kPa The 0000 UTC upper-air soundings and daily precipi- dewpoint depression and 70±50-kPa temperature dif- tation amounts were compiled from Spokane, Boise, ference are also included, because the 85-kPa level is Idaho, and Great Falls, Montana, and lightning-strike closer to ground level at those sites and may be less data (which include location, date, and time) were ob- indicative of conditions above the boundary layer than tained for the northwestern United States. In addition, is the 70-kPa level. Like Spokane, the differences be- daily precipitation data were collected from 11 other tween all the means at Boise (Table 2) and Great Falls National Weather Service (NWS) sites and 108 Remote (Table 3) are signi®cant at the 95% level or higher, Automated Weather Station (RAWS) sites located indicating there is less moisture and more instability on throughout the four-state area from the Cascade Range ®re days than on other days in southern Idaho and Mon- to the Rocky Mountains. All days were segregated into tana, as well as in eastern Washington. The subsequent two groups: days on which lightning-ignited ®res started analysis will use only the Spokane soundings because (``®re days'') and days on which there were no ®res or the sounding variables from Spokane, Boise, and Great on which ®res were human caused (``other days''). Be- Falls are all signi®cantly correlated (correlation coef- tween 1 May and 20 September there were 57 ®re days ®cients range from 0.61 to 0.74) and because the dis- 788 JOURNAL OF APPLIED METEOROLOGY VOLUME 41

TABLE 2. Mean values of sounding variables (ЊC) at Boise, ID, for ®re and other days, 1 May±20 Sep 2000. 85-kPa DD 85±50-kPa TD 70-kPa DD 70±50-kPa TD Fire days 27.5 36.0 17.1 20.8 Other days 20.4 32.6 12.1 18.1 t* 4.97 5.74 4.35 5.36 tcrit(0.95,140)** 3.97 2.78 3.55 3.17

* t ϭ value of the t statistic.

** tcrit(0.95,1.40) ϭ critical t value for the 95% signi®cance level with sample size of 140. criminant rule used in the analysis was developed from the Cascades is typically very different than that in the Spokane sounding data. interior basin), and stations with incomplete periods of The discriminant rule developed in Rorig and Fer- record were discarded. Figure 1 shows the time series guson (1999) from the historical thunderstorm database of the Spokane sounding variables and the daily num- at Spokane was used to assign a probability of dry light- bers of lightning strikes, ®res, and stations with mea- ning for all days from 1 May through 20 September surable precipitation in the study area. Inspection of the based on the values of the discriminating variables (85- data reveals that ®re starts do not correlate well with kPa dewpoint depression and 85±50-kPa temperature large numbers of lightning strikes. There were numerous difference). If the probability was greater than 50%, the lightning strikes but very few ®re starts in both early day was assigned to the dry category, and if it was less to mid-June and early July; in August there were many than 50%, it was assigned to the wet category. The days with ®re starts but relatively fewer lightning likelihood that any given day was correctly classi®ed is strikes. For example, on 1 September there were over greater when the probability is closer to the extreme 20 000 lightning strikes but no ®res. On the other hand, values (0% or 100%). Fifty-one of 57 ®re days were the days with the greatest numbers of ®re starts (10 and grouped into the dry convection category (all 51 days 24 August) had less than 11 000 lightning strikes. Days had probabilities greater than 70%), whereas 58 of 86 with large numbers of ®re starts also, with few excep- other days had probabilities greater than 50% and were tions, have high 85-kPa dewpoint depressions and 85± categorized as dry. Three of the six ``misclassi®ed'' ®re 50-kPa temperature differences at Spokane, whereas days had reported measurable precipitation at more than days having many stations recording measurable rainfall 10 locations in the study area, indicating the fuels at occur on days with low dewpoint depression, relatively potential ®re sites may have been suf®ciently dry for lower instability, and few (if any) ®re starts. ignition despite the high levels of atmospheric moisture. The large number of other days categorized as having On 10 August, 25 large ®res started in eastern Oregon, a high probability of dry convection illustrates the dif- northern Idaho, and western Montana. Figure 2 shows ®culty of using lightning-®re starts to identify dry light- there was extensive lightning activity in Idaho and west- ning days. Whether a ®re will ignite and spread depends ern Montana and less in Oregon and Washington. On on more than the state of the atmosphere. Even though this day, the 85-kPa dewpoint depression at Spokane the atmosphere is dry and unstable, lightning may not (0000 UTC) was 18ЊC, the 85±50-kPa temperature dif- strike where fuels are dry, or, if ®res do ignite, they ference was 33ЊC, and only 4 of 122 sites in the region may either be controlled or never grow large enough to recorded greater than 2.5 mm of precipitation. The mea- be included in the ®re database. surable rainfall occurred in Oregon and western Mon- To understand better the relationship between the tana, where ®res were not observed. Note the ®re in sounding variables at Spokane and rainfall amounts in Oregon, where there is no record of lightning strikes in the study area, daily precipitation data were collected the vicinity on that day. This could have been a ``hold- from 122 sites (14 NWS stations and 108 RAWS sites) over,'' which occurs when lightning strikes but the ®re throughout Washington, Oregon, Idaho, and western smolders for several days until conditions are right for Montana. Locations west of the Cascade Range crest the ®re to grow to a detectable size. were not included in the analysis (the air mass west of On 24 August (Fig. 3), another 25 large ®res started

TABLE 3. Mean values of sounding variables (ЊC) at Great Falls, MT, for ®re and other days, 1 May±20 Sep 2000. 85-kPa DD 85±50-kPa TD 70-kPa DD 70±50-kPa TD Fire days 23.4 35.8 12.7 20.5 Other days 17.3 32.7 8.3 18.1 t* 5.35 5.45 4.78 5.20 tcrit(0.95,140)** 3.17 2.87 2.75 2.38

* t ϭ value of the t statistic.

** tcrit(0.95,1.40) ϭ critical t value for the 95% signi®cance level with sample size of 140. JULY 2002 RORIG AND FERGUSON 789

FIG. 1. Time series of (a) daily 85-kPa dewpoint depression and 85±50-kPa temperature difference at Spokane and (b) daily number of lightning strikes, large ®re starts, and number of stations with precipitation (ppt) Ͼ 2.5 mm within the study area. The vertical lines mark dates used to illustrate lightning and ®re patterns in Figs. 2±4. in Oregon, Idaho, and western Montana. The 0000 UTC but numerous lightning strikes occurred in Idaho and 85-kPa dewpoint depression at Spokane was 20ЊC, and western Montana, with fewer strikes in eastern Oregon the 85±50-kPa temperature difference was 36ЊC. Only and eastern Washington (Fig. 4). Signi®cant rainfall ®ve observation sites in the region recorded signi®cant (substantially greater than 2.5 mm at many locations) rainfall amounts. These sites were located in central was recorded throughout the study area at 66 of the 122 Idaho and western Montana, closer to the ®res, but pre- observation sites. The 0000 UTC sounding variables at cipitation amounts were all less than 5.5 mm. Spokane indicated conditions were marginally unstable On the wet day of 1 September there were no ®res, (85±50 kPa difference of 28ЊC) with an ample supply 790 JOURNAL OF APPLIED METEOROLOGY VOLUME 41

FIG. 2. (a) Lightning strikes and (b) large ®res, 10 Aug 2000. FIG. 3. (a) Lightning strikes and (b) large ®res, 24 Aug 2000. of low-level moisture (85-kPa dewpoint depression of 9ЊC).

3. Summary Lightning-caused ®res ignited on 57 days in the re- gion encompassing Washington, Oregon, Idaho, and western Montana from 1 May through 20 September 2000. The 85-kPa dewpoint depression and 85±50-kPa temperature difference were signi®cantly different on ®re days than on the other days at all three upper-air sounding stations in the region (Spokane, Boise, and Great Falls). On ®re days, the atmosphere was drier and more unstable than on other days. Daily precipitation records were obtained for 122 sites in the study area. Very few stations reported signi®cant rainfall amounts (in excess of 2.5 mm) on days when the 85-kPa dew- point depression at Spokane was high. In converse, on days with numerous lightning strikes and ample low- FIG. 4. Lightning strikes, 1 Sep 2000. JULY 2002 RORIG AND FERGUSON 791 level atmospheric moisture, many stations in the study in acquiring meteorological data, and anonymous re- area recorded precipitation, and few (if any) ®res started viewers for their helpful comments on the original man- on those days. uscript. These results show that, if convection is expected, lower atmospheric moisture content is an important fac- REFERENCES tor in estimating the risk of dry lightning strikes that result in ®re ignitions. The 85-kPa dewpoint depression Cummins, K. L., M. J. Murphy, E. A. Bardo, W. L. Hiscox, R. B. and the 85±50-kPa temperature difference are easily de- Pyle, and A. E. Pifer, 1998: A combined TOA/MDF technology rived and appear to be successful in discriminating be- upgrade of the U.S. National Lightning Detection Network. J. Geophys. Res., 103, 9035±9044. tween dry and wet lightning days. Fuquay, D. M., R. G. Baughman, and D. J. Latham, 1979: A model The methodology described here has been developed for predicting lightning ®re ignition in wildland fuels. USDA as a diagnostic tool, but it can easily be applied with Forest Service Res. Paper INT-217, 22 pp. data from prognostic models and con®gured as a pre- Haines, D., 1988: A lower atmosphere severity index for wildland ®re. Natl. Wea. Dig., 13, 23±27. dictive tool. Extreme ®re seasons like the summer of Naden, R., 2001: SPC Fire weather program. Abstracts, Paci®c North- 2000 occur infrequently, yet even in normal years large west Weather Workshop, Seattle, WA, National Weather Service. numbers of ®res burn across the United States. With Rorig, M. L., and S. A. Ferguson, 1999: Characteristics of lightning increasing development in the wildland±urban interface, and wildland ®re ignition in the Paci®c Northwest. J. Appl. Me- it is necessary to improve the ability of forecasters to teor., 38, 1565±1575. Schmidt, K. M., J. P. Menakis, C. C. Hardy, W. J. Hann, and D. L. anticipate weather conditions that are conducive to the Bunnell, 2002: Development of coarse-scale spatial data for ignition and spread of wild®res. wildland ®re and fuel management. U.S. Department of Agri- culture, Forest Service, Rocky Mountain Research Station Gen- Acknowledgments. The lightning data used in this eral Tech. Rep. RMRS-GTR-87, 46 pp. von Storch, H., and F.W. Zwiers, 1999: Statistical Analysis in analysis were obtained from Global Atmospherics, Inc., Research. Cambridge University Press, 484 pp. of Tucson, Arizona. We thank Steve McKay for help Werth, P., and R. Ochoa, 1993: The evaluation of Idaho wild®re with the statistical analysis, Casey Anderson for help growth using the Haines index. Wea. Forecasting, 8, 223±234.