7.2 THE NORTH CENTRAL NEBRASKA PRAIRIE FIRE OF 16 MARCH 1999
John W. Stoppkotte *
National Weather Service Office, North Platte, Nebraska
What came to be the second largest wildfire on record for the state of Nebraska ripped through the sandhills of north central Nebraska during the afternoon and evening hours of Tuesday, 16 March 1999, with flare-ups continuing into the following day. By the evening of Wednesday, 17 March, nearly 72,000 acres of grassland had burned, numerous cattle perished, and one ranch house had burned. The meteorological setup was marked by gusty west winds at the surface, ahead of a cold front due to arrive in central Nebraska during the evening of 16 March. The National Weather Service (NWS) Warning Forecast Office (WFO) in North Platte played a vital role in the ultimate control of the fire by issuing timely spot forecasts.
Forecasters at the WFO in North Platte were called on frequently to provide detailed forecasts primarily of wind direction and speed, which gusted at times, to near 50 mph after the cold frontal passage that occurred around 9 PM CST on 16 March. This paper will look at the meteorological events leading up to the fire, which started near an abandoned house close to the town of Mullen, as well as the use of the Advanced Weather Interactive Processing System (AWIPS) in making those forecasts . This will include a look at the passage of the cold front that changed the direction of fire spread, ultimately culminating in the evacuation of a small town in its path, during the early morning hours of 17 March 1999.
1. Introduction
The wildfire originated near an abandoned farm house located a few miles northwest of the community of Mullen, NE (Fig. 1), caused by a circuit breaker that sparked during a power surge.
Fig. 1 Map of Nebraska with shaded area denoting approximate area burned by the prairie fire.
The intent of this paper is to examine the meteorological events prior to the fire onset, including those in the preceding months which may have been responsible for the ultimate spread of the fire. In addition, a background of basic NWS Fire Weather Forecasting structure will be given, in order to clarify what responsibilities WFO North Platte has with respect to the NWS Fire Weather Program.
2. Background
NWS WFO North Platte has limited Fire Weather responsibility as determined by the NWS Central Region Headquarters (CRH). The only formal Fire Weather product issued by the office is a Fire Weather Trend Forecast for the Nebraska National Forest in Halsey, NE, during the normal growing season. Informal Fire Weather Spot Forecasts are prepared as well, on an as-needed basis. Therefore, the office did not have the capability to issue a Fire Weather Watch or Red Flag Warning as described in NWS Regional Operations Manual Letter (ROML) D-06, drafted by the NWS CRH. However, NWS WFO Valley, NE, is tasked with the issuance of the NWS product known as the Rangeland Fire Danger Index, which is designed to draw attention to how meteorological elements may alter burning conditions across the state of Nebraska. Such a forecast was issued by WFO Valley well in advance of the fire and will be discussed later in this paper.
3.1 Terrain
The wildfire occurred in a region of north central Nebraska known as The Sandhills, which are as the name suggests, made up of gently rolling sandhills that possess a few steep slopes in places. They make up an area about 20,000 sq. mi. across northern Nebraska and southern South Dakota, and range in elevation from around 2,000 ft. above mean sea level (MSL) in its eastern regions to over 4,000 ft. MSL in the west.
The sandhills are covered with a native "bunch grass", of which the species Schizachyrium Scoparium (or Andropogon Scoparium) is a primary member. The common name for this species is "Little Blue Stem", which also happens to be the state grass of Nebraska. This grass grows normally to a height of 2 to 4 feet tall and is hearty even in sandy areas. The common name comes from its stem color, which is bluish-green in nature. It is ideal for range land, which is the primary resource of the sandhills region.
The soils of the sandhills prairie are mainly fine-grained sands and sandy loams, with coarser sands found on the dune tops. Most areas have excellent drainage, except those with underlying clays, with surface sands drying out rapidly after rainfall. The sandhills were formed originally by wind, which continues to alter the shape of the terrain on a smaller scale on a day-to-day basis.
The month of March in Nebraska is characterized by dramatic changes in temperature as well as precipitation. The average high for Valentine, NE (located in the north central part of the state) on the first day of March is 41? Fahrenheit (F), compared to 52? F for the last day of March. Precipitation events have ranged from thunderstorms to blizzards across the state during the month of March, which is a sign of the changing seasons across the central plains. This, coupled with fact that the late winter and early spring months are climatologically favored times for increasing wildfire threat due to the meteorological factors mentioned previously and that the fuels are typically quite dry prior to the onset of the growth cycle. It is this progression of change that can make Fire Weather Forecasting a challenge across the plains states.
Climatologically, the burned region of the sandhills falls between NWS official observing sites, making analysis of specific annual averages difficult. However, a generalization is possible using the Automated Surface Observing System (ASOS) located in Valentine (VTN), NE, which lies approximately 60 miles north of the burned area. This site was chosen because of the continuous information provided by the ASOS, in addition to the fact that official records for the site date back to the early 1900?s. Therefore, it is concluded that this region receives around 18 inches of rainfall per year, nearly 70% of which falls between the months of May and September. In fact, in the 6-month period between October and March in a typical year, the sandhills region would receive less than 4 inches of rainfall (or liquid equivalent). More specifically, for the period of 1 December 1998 through 16 March 1999, the Valentine ASOS recorded only 1.16 inches of moisture, approximately 25% below the average amount of 1.57 inches on any given year.
Just as important, however, was the fact that average temperatures were much above normal for the period, which yielded many days of low relative humidities across the area. These dry conditions were evident on 16 March, the day of the wildfire onset, as relative humidities remained between 10-12% during the afternoon hours.
5.1 Upper Air Pattern
The 00 hr forecast from the 1200 UTC run of the ETA forecast model indicated a long wave trough moving onto the Pacific coast, putting north central Nebraska in a southwest flow aloft (Fig. 2). However, in time, a more zonal flow was expected, with a short wave trough moving quickly eastward through the northern plains. Since the lower levels were relatively dry (Fig. 3) as forecast by the models (north central Nebraska had 850-mb RH values <30%), precipitation was not expected to be a large part of the forecast problem. The 1200 UTC North Platte, NE (LBF) sounding (Fig. 4) from 16 March indicated the southwesterly flow aloft as noted from Fig. 2, as well as very dry air above the surface with some instability noted aloft. The resulting calculated Haines Index was 6, indicating the potential for large and rapid fire growth (Haines, 1988).
Fig. 2. 00 hr analysis of 500-mb heights from the 1200 UTC 16 March 1999 ETA model run.
Fig. 3. 00 hr ETA analysis of 850-mb relative humidity, valid 1200 UTC 16 March 1999. Shaded areas represent higher humidity. Contours are every 10 %.
Fig. 4. 1200 UTC sounding for North Platte, NE (LBF) from 16 March 1999. Left y-axis is pressure (mb); x-axis is temperature (? C).
5.2 Surface Features
A pacific cold front located by the afternoon of 16 March 1999 in northern Wyoming (Fig. 5) was forecast to move southeast across Nebraska during the evening hours. A tight pressure gradient (Fig. 6) was also forecast to develop at the surface by the ETA by midnight local time, after the passage of this cold front which would switch the winds from their prevailing westerly direction early on in the day, to northwest. This change would likely have a direct impact on the rate and direction of fire spread overnight. Dew point temperatures across northern and western Nebraska ranged from the teens to the low 20s, which, when compared with ambient temperatures in the 75 to 80 ? F range, gave a clear indication of the dry lower levels of the atmosphere.
Fig. 5. Surface analysis from 2300 UTC 16 March 1999. Dashed line represents surface trough, solid lines represent frontal positions (cold front is located across northwest SD into northern WY, with a warm front stretching southeast into southwest MN).
Fig. 6. 18 hr ETA forecast of MSL, valid 0600 UTC 17 March 1999. Contours are every 2 mb.
A number of agencies, local, state and federal, worked together to bring the fire under control. These included : the Nebraska Department of Agriculture, Nebraska Department of Roads, the National Forest Service (NFS), the Nebraska Emergency Management Agency, the NWS, the Bureau of Indian Affairs, and the Nebraska State Fire Marshall?s office to name a few.
The NWS WFO in North Platte played an integral role in the suppression of the fire by issuing numerous Spot Fire Weather Forecasts by phone throughout the two-day event. Forecasters inquired about the fire around 2 PM CST after noticing a reflectivity pattern indicative of a smoke plume on the KLNX WSR-88D radar, originating 25 to 30 miles northwest of the radar tower?s location near the town of Thedford.
The 415 AM CST Zone Forecast product issued prior to the start of the fire by the NWS WFO North Platte for the area, valid for Tuesday, 16 March 1999, correctly identified that sustained winds would become west during the afternoon at 10 to 20 mph, with winds switching in the evening to the northwest and increasing to 15 to 25 mph. NWS WFO Valley, NE also provided beneficial Fire Weather forecast information prior to the wildfire by the issuance of the Rangeland Fire Danger Index (AFOS product OMARFDOMA). The 555 AM CST 16 March 1999 issuance from WFO Valley stated that the fire danger for the area was in the extreme category for the day, and continued to run in the very high to extreme fire danger category during the lifetime of the fire.
The 345 PM CST Zone Forecast issuance from WFO North Platte continued the earlier forecast trend for the evening hours by noting winds would shift to the northwest and increase to 20 to 30 mph with some higher gusts. A wind advisory was subsequently issued for the area at 955 PM CST indicating the potential for wind gusts to exceed 45 mph. Spot forecasts continued through the evening hours of 16 March, as well as into the following day, and were generally provided every 1 to 2 hours or as requested. Updated information was critical in order to keep firefighters abreast of frontal passage timing during the evening hours.
The wind profiler located at Merriman, NE provided valuable information during the event as well. The profiler, which produces a vertical profile of the winds every hour from near the Earth?s surface to above the tropopause (van de Kamp, 1988), indicated winds increasing to 50 kts just above the surface through the overnight hours of 16 March (Fig. 7). The profiler wind fields were also helpful in determining the timing of the wind shift associated with the front, as was surface wind fields supplied via the Local Analysis and Prediction System (Albers, 1995), available on the Advanced Weather Interactive Processing System (AWIPS) (not shown).
The wildfire began about 1 PM CST, spreading quickly eastward, fanned by westerly winds of 15 to 20 mph. By early afternoon, wind gusts were around 30 mph from the west, which allowed the flames to quickly spread eastward across the north central Nebraska prairie. By this time, the fire had destroyed one home located a few miles northeast of Mullen.
The winds diminished for a short time to between 5 and 15 mph from the southwest, between 5 PM and 9 PM CST. However, shortly after 9 PM CST, the cold front that was projected to come through the area during the evening hours made its arrival, shifting winds to the northwest as they increased in speed once again. Wind gusts shortly after the frontal passage were as high as 45 to 50 mph, as evidenced by a 46 mph wind gust recorded by the Valentine, NE ASOS at 904 PM CST. Although the front was considered strong, no precipitation was associated with it due to the lack of moisture in the lower and middle troposphere. The fire then began a more southeasterly spread toward the community of Thedford, a town of approximately 250 people. The town would later be evacuated during the early morning hours, although the fire would be extinguished on the northern edge of town with little damage noted in the town itself.
The following day, winds continued strong out of the northwest, gusting to between 30 and 40 mph during the morning and early afternoon. Hot spots quickly evolved into flare-ups at the southern periphery of the previously burned area, which fire fighters spent the better part of the day controlling. Winds began subsiding between 6 and 7 PM CST, dropping to below 10 mph for the remainder of the night as surface high pressure moved into the area.
Fig. 7 Wind profiler from Merriman (MRR), NE, showing winds speeds from 1200 UTC 16 March 1999 to 1200 UTC 17 March 1999 (winds are in knots). Vertical axis indicates height (m) MSL above the profiler site.
The evolution of a prairie fire across north central Nebraska was shown, with an emphasis on the meteorological events leading up to and during the fire. The dry conditions across north central Nebraska in the months preceding the fire likely played a role in the condition of the fuel, whose consumption is usually rapid in wildfire situations (Alexander, 1999). High winds associated with the passage of a pacific cold front helped to speed up the rate of fire spread, allowing the fire to remain out of containment for several hours after its ignition.
Although the wildfire could not have been foreseen from a meteorological stand point, WFO North Platte forecasters did use the wealth of information available to them to make accurate forecasts well in advance of the onset of the event . These tools also helped to provide fire managers with critical meteorological information in a timely manner during the fire. This information was viewed by NWS forecasters utilizing several tools including AWIPS, whose graphical capabilities include viewing weather information on a synoptic and mesoscale domain. (Because the software build of AWIPS at the time of the wildfire (ver. 4.1) did not provide forecasters the ability to archive its information for later use, graphics from the Display 2-Dimensions (D2D) AWIPS graphical interface could not be shown in this text.)
Since WFO North Platte has a limited Fire Weather responsibility from a NWS perspective, little else could have been done to prepare the fire community of the potential fire hazard that existed. An open dialogue existed during the event between the NWS WFO North Platte and the NFS, which ultimately contributed to saving lives and property in the town of Thedford as the fire approached the community during the early morning hours of 17 March 1999. This relationship stems from the increased effort on both sides to develop a working partnership and promote information sharing between the agencies, which has continued to develop since the devastating wildfire.
Acknowledgments. The author would like to thank Liz Page of COMET and Dennis Blondin of NWS WFO North Platte for their help in making the meteorological data available for this case. I also appreciate the efforts of Dave Wert of NWS North Platte and Daniel Nietfeld and Rick Ewald of NWS Hastings for their helpful review of this paper. Thanks are also in order to Doug Alexander from the US Forest Service for his help in understanding the fire management viewpoint.
REFERENCES
Albers, S. C., 1995: The LAPS Wind Analysis. Wea. Forecasting, 10, 342-352.
Alexander, D.G., 1999, personal communication.
Haines, D. A., 1998: A lower atmosphere severity index for wildland fire. Natl. Wea. Dig., 13, 23-27.
van de Kamp, D.W., 1988: Principles of Wind Profiler Operation. Profiler Training Manual #1, NOAA/ERL, 49 pp.
