Rocky Mountain
Similar to other GACCs, the Rocky Mountain Area has one general critical fire weather pattern, the breakdown of the upper level ridge. However, three patterns were identified within the broad definition of the breakdown of the upper level ridge. These patterns are: 1) thermal trough; 2) southwest flow; and 3) downslope flow. Within these three patterns, there are different varieties of critical fire weather patterns. The four corner’s high or pattern is not discussed here. While it does provide lightning, it is usually not dry and the effects come days later when one of the other critical fire weather patterns develop. There is one major caveat with all of these critical fire weather patterns, which have also been discussed for other GACCs. Fuel conditions must be dry enough to support ignition and large fire growth. Without this antecedent condition, critical fire weather patterns effects are dramatically reduced.
Thermal Trough
The thermal trough critical fire weather pattern affects the Plains and the Western Slope of the Rocky Mountain Area (RMA). It occurs with a strong upper level ridge centered over the Plains during the summer months. Deep mixing, instability, and hot, dry air are prevalent near the ridge axis. Additionally, a reverse wind profile develops, which is when strong low-level winds develop in the absence of strong upper level winds. This occurs due to the strong heating at the surface inducing a strong surface pressure gradient, which increases winds. These winds help drive the fire and also create surface convergence. The strong winds along with deep mixing, instability, and hot, dry air produce an environment conducive for plume-dominated fires, while vertical wind shear allows for continuous development and redevelopment of the plume. This occurs for similar reasoning that a thunderstorm grows with strong surface convergence, instability, and vertical wind shear. A positive feedback loop develops where fire spread increases all of the previous ingredients, which then fuels more fire growth.
The case study chosen for the thermal trough example is the Region 24 Complex that ignited in north-central Nebraska 20 July 2012. The lightning ignited wildfire burned more than 76,000 acres during 20-30 July 2012. Unstable, moist west-northwest flow to the north of the ridge in the preceding days generated thunderstorms, which produced lightning across much of the northern Plains (Map 1; Map 2; Map 3). As the ridge shifted and expanded eastward, hot, dry, and unstable conditions pushed further east on the Plains (Map 4; Map 5; Map 6). A surface thermal trough developed under the upper-level ridge as the surface heating helped produce a pressure gradient. This gradient induced a surface convergence zone and stronger, opposing winds on either side of the boundary combined with the hot and dry conditions (Map 7). The hot, dry, unstable, and windy conditions extended through the 850 hPa level. The reverse wind profile can be inferred from looking at the wind change that occurs between the 1000-850 hPa and 850-500 hPa layers (Map 8; Map 9; Map 10).
Southwest Flow
A common pattern occurs during fire season especially in May, June, and into July across the Rocky Mountain Area (RMA). This pattern consists of a persistent strong upper level trough to the west, which brings strong southwest flow, warm, and dry conditions to the RMA. This pattern can be stationary due to blocking over the eastern United States or Atlantic Ocean. The pattern can be progressive with shortwaves passing over the RMA, but the long wave pattern stays relatively stationary with a trough to the west and a ridge to the east. Additionally, dry cold fronts can accompany these progressive shortwaves, which creates very dry, windy, and unstable conditions. This pattern can also bring lightning to the RMA if access to moisture is available which occurs more often later in the summer. It has a longer residence time than a breakdown of the upper level ridge, which allows critical fire weather conditions to persist for days to weeks at a time. Examples include Alpine and High Park fires (2012), Hayman (2002), and South Canyon (1994).
We will examine the Storm King/South Canyon fire of July 1994, which burned just west of Glenwood Springs, CO. Thunderstorms produced lightning across the RMA early in July, which ignited the South Canyon Fire on 2 July 1994. The southwest flow accompanied a slower moving upper level trough just north of the Canadian border (Map 1). Initially, the southwest flow brought moisture and instability into the RMA, which generated the lightning that ignited the South Canyon Fire (Map 2). As that upper level trough continued to move slowly across southern Canada, shortwave moved in behind it moving to the east-southeast through the Pacific Northwest and eventually through Wyoming (Map 3; Map 4). Strong southwest winds advected warm, dry air across Colorado ahead of the trough. During the afternoon of 6 July 1994 the winds increased as the pressure gradient increased due to the approaching front and upper level winds mixed down (Map 5; Map 6). The upper level jet was directly overhead at this time, which helped transport higher momentum to the surface (Map 7). The atmosphere was unstable ahead and just ahead of the front with the Haines Index plots illustrating the front (Map 8; Map 9). The increase of winds coupled with the dry and unstable conditions created extreme fire behavior conditions over the fire, which led to the deaths of fourteen wildland firefighters and burned approximately 2,000 acres.
Downslope Flow
Downslope windstorms affect the lee slopes of the RMA especially the Front Range in Colorado. When flow is perpendicular to the mountains, downslope winds develop and are accelerated due to the cross-mountain flow generating a standing mountain wave. This mountain wave helps focus momentum down the slopes increasing winds and decreasing RH. An upper level trough to the north will keep westerly flow over the Front Range, which induces a mountain wave and strong downslope winds. This can occur ahead or behind a cold front that is associated with the upper level trough. The front represents a strong pressure gradient, which will increase the cross-mountain flow and further accelerate the downslope winds. The air mass change is negligible with the front therefore dry, windy conditions persist through the change. This pattern tends to occur in spring and late summer or early fall. Examples include the 4-Mile fire (2010), Bobcat-Gulch (2000), and Lower North Fork (2012).
The Lower North Fork Fire was a prescribed burn that was lit the week prior to 26 March 2012 to the southeast of Conifer, CO. The Lower North Fork Fire burned over 4,000 acres and killed three people while destroying twenty-three structures. An upper level trough was approaching the RMA the morning of 26 March 2012. Mid to upper-level clouds developed ahead of the approaching upper-level trough and surface front (Map 1). Poor overnight RH recovery existed along the Front Range due to overnight and early morning cloud cover. The air mass was already warm and dry and drier air continued to advect over the Front Range due to the southwest flow from the upper-level trough (Map 2). Strong southwest winds accompanied the upper-level trough as the center of it moved into Wyoming through 26 March 2012 (Map 3). Strong winds, low RH, and high Haines values contributed to the extreme fire behavior and rapid fire growth during the afternoon and evening of 26 March 2012. The winds moved from south-southwest to west-southwest during the afternoon as the upper-level trough continued east into Wyoming. The change in wind direction helped create perpendicular, cross-mountain flow, which accelerated the already gusty winds down the lee slopes (Map 2; Map 3; Map 4; Map 5). Additionally, synoptic scale downward motion due to being in the convergent region of the upper level jet also helped force momentum downward (Map 6). Wind gusts exceeding 50-60 mph, RH of 5-7%, and Haines of 6 were near the fire (Map 7; Map 8). The smoke plume was detected via visible satellite imagery before 2300 UTC and by OOOO UTC the smoke plume was prominent in the visible satellite imagery (Map 9).