October 31st, 1999 northeast Montana windstorm. A Halloween trick or treat?

Gene Petrescu, WFO Glasgow

Introduction:

Halloween 1999 is a night most residents of northeast Montana will remember for some time to come. Not for the great costumes, but for the weather. During the late afternoon and evening of October 31st, 1999, a powerful, but mainly dry, cold front swept through eastern Montana producing very strong damaging winds. Post-frontal winds were sustained at 45 to 60 mph for several hours with peak gusts to as high as 80 mph. However, the most significant feature of this windstorm was the rapidly moving wildfires that resulted.

The Fires:

Over 30 fire starts were reported during the windstorm. Most were contained in a relatively short period of time. However, a few fires spread out of control at an extreme rate. Two of the major fires were started by sparks produced by passing trains. One such fire, with a flame front moving at over 40 mph, burned quickly through the small community of Outlook, west of Plentywood. Two thirds of the buildings in the town were gutted by flames at a cost of $4 million dollars. This does not include the cost associated with the loss of the train. The train that started the fire could not out run the flames. The locomotive and several dozen cars carrying grain were completely engulfed by flames. The other fire, just east of Wolf Point, Montana, burned 25 miles in 5 hours destroying several buildings and homes, including the local UPS distribution center and an extensive classic car collection.

The fires spread so rapidly fire fighting efforts concentrated strictly on evacuation. People fleeing the fires reported flaming tumbleweeds bombarding their vehicles and shattering into showers of red hot embers. The largest fire burned 107,000 acres in 5 hours east of Sidney, Montana. The burn area was clearly visible on visible satellite imagery the following day. Total acreage burned during the event was approximately 150,000 acres.

Despite the date, mid Fall, conditions for the ignition and spread of the fires on this night were optimal. A very wet spring and summer in northeast Montana allowed the grasses to grow exceptionally tall and thick. The wet summer was followed by an unseasonably warm dry fall. This led to an abundance of dry fine fuels as the grasses cured. That afternoon, temperatures ahead of the cold front soared to a record high of 79 degrees at the Glasgow, Airport. Along with the very warm conditions afternoon relative humidities dropped into the 20 percent range. After ignition of the flames, the strong post-frontal winds resulted in extreme fire conditions. An added threat to fire fighters, was the rapid drop in temperature with the front. As the fires started, temperatures were in the 70s, within 4 to 5 hours temperatures fell into the 30s with scattered light snow showers.

The Forecast:

As early as October 29th, the models were indicating a strong cold front would move through the area on the 31st. The upper level pattern was indicative of a fairly windy cold frontal passage with strong zonal flow across the eastern Pacific along the U.S. Canadian border. A vigorous upper level shortwave and associated surface low were forecast by the models to track eastward north of the U.S. border. The strongest winds with this pattern generally occur in a band to the south of the surface low, just north of the upper jet, in the region of strong subsidence and cold air advection. However, the models moved the surface low ENE through central Saskatchewan with 850 mb flow over northeastern Montana on the order of only 35 kts.

Each successive model run, in particular the AVN, forecasted the track of the low a little further south, resulting in a tighter pressure gradient over the forecast area. By 12Z Sunday, the models were tracking the surface low though southern Saskatchewan, with the upper jet axis over central Montana. 850 Mb winds forecasted by the ETA over the region increased to over 50 kts, The 850 Mb level averages 1000 to 3000 ft above the ground in northeast Montana. Despite the model forecast, the local forecast issued by Glasgow only included winds in the 30 to 45 mph range.

The low wind forecast was partially the result of poor model performance, and partially the result of inexperience with the newly installed AWIPS. The forecasters could now easily display wind forecasts at several levels in AWIPS just off the surface (2m, 60 agl, 925 Mb etc.). However, due to interpolation and friction parameterizations, observed wind speeds, at these levels, can be higher than those indicated by the models. Wind forecasts at 850 Mb and 700 Mb are generally more representative of potential wind gusts at the surface in a well mixed environment. Model soundings after FROPA indicated that the atmosphere would only be moderately mixed, reducing the downward flux of momentum. Prior to FROPA surface temperatures forecasted by the models, as well as MOS, indicated daytime temperature in the 50s.

The Analysis:

A close look at the analysis at 12Z on the 31st indicated an upper trough and closed low over southern British Columbia with southwesterly flow at 850 Mb across Montana into the Dakotas. 850 Mb temperatures revealed a warm pool of air over eastern Montana and the Dakotas resulting from the subsidence in the lee of the Rockies under the southwesterly flow. By 18Z, a noticeable wind shift at 850 Mb was noted as a trough line quickly moved east into the Dakotas. The models forecasted this feature with weak cold air advection near the surface and moderate cold air advection at 700 Mb initiating after the trough passage. This primarily mid-level trough apparently was the remnants of a frontal band that moved into the west coast on the 30th. The lower portions of the front were destroyed by the mountains as the strong westerlies at low levels resulted in subsidence and warming in the lee of the mountains.

With the initiation of cold air advection in the mid levels and the subsidence warming at low levels the atmosphere became increasingly unstable. Surface temperatures over eastern Montana soared to record levels in the mid to upper 70s. Much above the model forecasts, resulting in a much more efficient downward momentum flux. Model trajectories indicated the source of the air at the surface at Glasgow was descending from about 750 Mb Using theta surfaces assuming a dry atmosphere, it was estimated that the actual source of the air at Glasgow was descending from above 650 Mb Pre-frontal wind speeds were already gusting to near 40 mph as the higher momentum air mixed down towards the surface.

As the surface low intensified in the southern Alberta and tracked into southern Saskatchewan after 21Z a surface cold front swept east across eastern Montana. Behind the front very strong winds developed across the area. Strong cold air advection, primarily above 850 Mb as downslope subsidence continued at low levels, maintained steep lapse rates and efficient mixing. Winds at the surface were sustained at 40 to 60 mph with gusts as high as 80 mph. The RUC indicated that wind speeds were fairly uniform to over 700 Mb

Model forecasts and analyses indicated a secondary surface front from the northwest quickly swept over northeast Montana after 03Z. It is very common in eastern Montana to experience these secondary frontal passages. The first front is of Pacific origin with the majority of cold air advection at mid levels. Surface temperatures will often warm with this type of feature initially with downslope subsidence until mid level temperatures cool to the point that the constraint of an adiabatic lapse rate results in cooling at the surface. The second is truly surface based with the strongest cold air advection at low levels. The secondary front moves southeastward entirely on the east side of the Rockies.

Surface winds increased again about 02Z as the second front approached from the northwest, but began to decrease slowly after the second frontal passage as pressure gradients weakened and the atmosphere stabilized with the low level cold air advection. After 06Z, average wind speeds across northeast Montana, diminished to below high wind warning criteria.

A time series loop of a cross section from north of Regina to south of Billings, indicated a moderate stratospheric intrusion north of the jet axis, as well as the descent of high momentum air from mid levels to the surface after FROPA.