Although June has been exceptionally wet in our part of the country – Philadelphia International has recorded 8.31 inches of rain this month, 5.67 inches above normal for the date – it is nonetheless wildfire season across the United States.
Wildfires are making news once again across the West. The Black Forest Fire, near Colorado Springs, Colo., was declared 100% contained late last week, but not after becoming one of the most destructive wildfires in Colorado state history. The picture below, taken by John Wark or Reuters on June 11, shows the smoke plume from the then-out-of-control Black Forest Fire.
A fire devastated parts of the same city last year as well. Colorado Springs lies on the eastern edge of the Rockies; its location and terrain make it a hotbed – both proverbially and literally – for wildfires.
But why is this? In order to grow and move, wildfires needs dry weather, windy conditions, and something to start them. In many cases, upslope convection, caused by moist easterly winds being forced up and condensing into thunderstorms, creates lightning that triggers fires. However, the north-south I-25 corridor through Colorado is also rather heavily populated, and fire officials believe that it was not a lightning strike that ignited the Black Forest Fire, according to CNN. In addition to the three required ingredients, fires also benefit from rapid changes in topography, which is yet another thing Colorado is famous for.
Along with those mountains comes downsloping wind. Yes, Colorado is known for both upslope flow and downslope flow. The difference lies in which way the wind is blowing – up the slope or down it – but you could have probably figured that out yourself.
When fires aren't ignited by lightning from upslope convection, they are often fanned by strong winds from downsloping. As wind travels up the windward side of a mountain, it condenses its moisture out, typically in the form of clouds along the ridgeline. The air then descends the leeward side of the mountain, but is now much drier, which allows faster wind speeds from higher levels of the atmosphere to mix down to the surface more effectively.
These stronger winds are formally known as “Foehn winds,” but are colloquially known as “Chinook” winds in some places, “Santa Ana” winds in other areas, or more generally, downslope winds. Foehn winds leave the lee side of mountain ranges warm, dry, and windy – prime conditions for fires.
Over the inter-mountain west, wildfires are most likely to ignite when an upper-level ridge begins to break down. Before this happens, while the ridge is established, conditions are hot and dry at the surface, which dries out the vegetation. When a new trough moves into the west to break down that ridge, it often brings with it a cold front at the surface. Ahead of the front, winds will turn southerly and increase in speed.
Along the front, high-base thunderstorms will likely develop. Although precipitation from these “dry” thunderstorms hardly, if ever, reaches the ground, the lightning from these storms can still ignite fires. Fires will be ignited in regions that are already dry, due to the ridge that preceded this disturbance, and are now dealing with strong winds as well.
A well-fanned wildfire may travel at a speed of around five to seven m.p.h., excluding any advancing of the fireline or spotfires that may be generated by embers carried by the wind ahead of the initial fire. Regardless of any spotfires, strong wind will still speedily spread wildfires in an elongated oval-shaped pattern in the direction of the wind. Obviously, faster wind speeds translate to faster fire propagation speeds as well.
But wind is just one of the two forces that aid in the spread of wildfires. The other is topography. More specifically, fires tend to travel upslope because as flame lengths increase, they begin to warm and dry out vegetation ahead (upslope) of fires before the actual fire reaches that point. In general, a fire’s placement at the bottom of a steep slope will aid its propagation much more than even a strong wind will.
Contrast all of this with the Philadelphia region, and notice that we do not have the drastic terrain changes that the West has, nor do we normally see dry frontal passages or dry thunderstorms. Of course, wildfires still happen plenty often in the warm season on the East Coast, but they are almost always ignited by a human, and they typically occur when our region is locked in under an upper-level ridge, rather than just as that ridge is breaking down.
When it comes to forecasting fire weather, SPC often looks for an upper-air pattern just like the one described earlier. That’s right, the Storm Prediction Center in Norman, Okla. Issues daily Fire Weather Outlooks just like they do Convective Outlooks. Only they issue Fire Weather Outlooks at 1630z (12:30 p.m. EDT) rather than in the very early morning.
SPC forecasters look for the same basic ingredients discussed here: low humidity, strong winds, the potential for cloud-to-ground lightning strikes, and dry or dead vegetation. Any area that meets most or all of these characteristics will likely receive a “Critical” Fire Weather Area designation from the SPC forecaster on duty. SPC issues these outlooks for Days 1 and 2, and beginning Tuesday, will also experiment with probabilistic forecasts for Days 3 through 8.
Much like when SPC issues a Slight Risk, a “Critical” alerts the weather forecast offices in that region. The WFOs may then choose to issue a Red Flag Warning, which is very much like a Severe Thunderstorm Watch, in that it alerts the people in a specific area that conditions are favorable for wildfire development.
If a Red Flag Warning is issued for your area, avoid camping, or if you must camp, make sure to completely extinguish campfires before leaving a campsite or going to bed, and never leave a campfire unattended. Also avoid setting off fireworks, or doing just about anything that necessitates an open flame. In many states, starting a wildfire, whether intentionally or otherwise, is a felony.