In late April of every year for the past 10+ years, scientists and land managers have produced coordinated outlooks for fire season across the western US to better prioritize resources. The primary factors of interest in coming to some agreement on an outlook for a given region involves looking at what has happens and using model guidance to predict what might happen between the forecast date and the end of fire season. These factors include (i) fuel conditions, especially in rangelands where the amount of fine fuels is an important factor in carrying fire and is influenced by antecedent moisture [i.e., a wet year increases fine fuels making them available the subsequent summer], (ii) current drought status, and (iii) seasonal climate outlooks.
Last year was very wet across much of the inland northwest, allowing for ample fuels in rangeland systems headed into summer 2012. Current drought status across much of the inland northwest shows most areas, particularly forested parts of Idaho, wetter than normal per our comeback of water year 2011-12.
However, emerging drought conditions can be seen in the Great Basin in southern Idaho and across southeastern Idaho, northward into southwestern Montana. These drought conditions are mirrored in current snow water equivalent maps, which will likely take a turn for the warmer hues with warm temperatures the next 5-days accelerating snowmelt. The relatively wet conditions circa May 1st across the forests of Idaho and western Montana are not sufficient to ward off a large fire season.
The heart of fire season in the inland northwest typically begins in July. A lot can happen in the next 7 weeks or so. Likewise during the ten-to-twelve weeks from July to mid-September, meteorological patterns can enable, drive, or inhibit large fire potential and the fun that goes with it.
The last piece to the puzzle in making an outlook is seasonal climate predictions that are predicated on long-lived conditions in the climate system that provide memory well beyond that of atmospheric chaos or the limits of numerical weather prediction. These long-lived conditions include things like the state of ocean surface temperatures, sea-ice and land-surface conditions (e.g., soil moisture/snow cover). During the summer months, linkages between the atmosphere and the ocean are rather weak unlike the strong teleconnections that we know about between tropical ocean temperatures and the storm track over the US with El Nino/La Nina thereby reducing the skill of summer climate outlooks. Numerous groups develop seasonal climate forecasts using statistical methods and by running models including NOAA’s Climate Prediction Center.
I’ll focus here on providing interpretation of the Climate Forecast System v2 output which is a great source of ensemble climate forecasting that is updated daily. We’ll look at two maps that show ensemble predictions of temperature and precipitation anomalies for June-August. Regions where there is low skill of the predictions are masked out grey. The dynamical predictions hint at a slightly more amplified upper-level ridge that dominates the western US in summer. In turn, lower tropospheric and surface temperature are expected to be warmer than normal (a whopping 1-2C) and, more important from the perspective of wildfire, drier than normal. The combination of warmer and drier conditions in summer are a natural pairing as a northward shift in the storm track keeps cooler, moister air well to the north and allows strong radiational heating that can dry out soils and fuels and increase fire danger.
We’ll track these conditions throughout the summer and also get a personal view of fire season from Grangeville smokejumpers as the action starts up.
The official seasonal outlook issued May 1st did not feature above normal fire activity this summer in the inland northwest save for parts of the Great Basin, but we’ll keep track of evolving conditions as we march closer to July.