After a winter that featured frigid temperatures and above-average snowpack, and a spring that was marked by high streamflows and a lengthy runoff, western Montana is now roiling under one its hottest and driest summers on record.
The aptly termed “flash drought” has devastated locations throughout the state, impacting agricultural productions and fueling massive wildfires that continue to burn and degrade air quality. Just 3 months ago the US National Drought Mitigation Center (NDMC) classified only 1.5% of the state as experiencing drought conditions. The latest release from the NDMC shows that approximately 90% of Montana is now identified as having drought conditions present, with 12% of the state experiencing “exceptional drought,” the most severe classification (Figure 1).
Figure 1: Left: Montana’s drought status worsened dramatically from May to August of 2017. Right: Table showing percentage of state experiencing various intensities of drought over the last year. Three months ago (green oval), only 1.55% of the state was experiencing drought, compared to 88.51% now (red oval). Source: National Drought Mitigation Center: https://droughtmonitor.unl.edu.
Although Montana experienced an above-average snowpack last winter, extremely warm spring and summer temperatures (and a lack of any significant precipitaiton the last 6 weeks) has all but negated the hydrologic benefit from last winter’s storms. Recent research shows that spring and early summer temperatures are now one of the major drivers of late season streamflow in the Northerrn Rockies and other snow-dominated regions.
How abnormal has this summer really been?
Although dry spells and heat waves are common occureneces in Montana during the summer, this summer’s drought has been extremely unusual. Statewide, July 2017 ranked as the second-driest and third-warmest July in Montana over the past 123 years of record-keeping (Figure 2). Several cities in the western part of the state have experinced their longest dry spells in hisory, going more than 40 days without measurable precipitation.
Figure 2: Rankings for statewide precipitation (left, top) and average temperature (right, bottom) for July 2017. Montana reaches both ends of the scale with “Much Below Average” precipitation, and “Much Above Average” temperature for the month. No other state overlaps on both extremes. Source: NOAA, National Centers for Environmental Information: https://www.ncdc.noaa.gov.
Is global warming causing this summer’s drought?
The direct cause of Montana’s hot and dry summer this year may be traced to a persistent ridge of high pressure that has remained entrenched over the Northern Rockies since mid-June, blocking any significant moisture from making its way into western Montana. While global warming is not the primary factor that hastened this summer’s extreme conditions, it is very likely that it has influenced the current drought’s intensity and duration.
A report released this spring by the National Academy of Sciences found that four-out-of-five record-hot days globally are now amplified by the trend in global warming. Record-breaking temperatures are a classic signal of climate change, and are anticipated to become even more common in the coming decades.
In a stable climate, the ratio of days that are record-hot to those that are record-cold is roughly even. However, in Montana’s warming climate, record-high temperatures have begun to outpace record lows – an imbalance that has been growing since the 1980’s (Figure 3).
Figure 3: As the climate warms; Montana can expect to experience more extreme hot weather events and fewer cold weather events. Source: US EPA.
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