In 2021, Nepal underwent its most severe fire season, resulting in a fire rate 10 times greater than the historical average in many areas of the country with record-high air pollution levels. Leading the fire outbreaks in March of 2021, the country experienced an extreme precipitation deficit and drought in the post-monsoon season. Current community forest management practices and resultant forest growth may have exacerbated the conflagration, but an analysis using observational, reanalysis, and climate model ensemble data indicates that climate variability and climate change induced severe drought conditions that resulted in the anomalous fire season. While warning of the likely re-occurrence of extremely active fire seasons in Nepal through the end of the 21st century, this research also proposes a statistical model for sub-seasonal prediction that could help mitigate the projected effects of the drought-fire paradigm.

Drought-busting events in the Colorado River Basin, such as the “Miracle May” of 2015 that greatly alleviated an unprecedented water shortage, have been observed for more than a century. But while such events are much prayed for in times of drought, they have not been well researched or even characterized. In this study, conducted in collaboration with water managers from across the basin, we propose a definition for “miracle events” that reflects real-world, actionable relevance. The resulting characterization offers a framework by which to quantify the frequency and strength of extreme dry-to-wet springtime transitions. While limited by uncertainties in model simulations and the myriad hydrological futures these simulations seek to project, and thus requiring cautious interpretation, this study finds that such transitions may become less frequent and less powerful under climate warming.

This study aims at improving understanding of the environments supporting summer MCS initiation in the U.S. Great Plains. A self-organizing map analysis is conducted to identify four types of summer MCS initiation environments during 2004-2017: Type-1 and Type-2 feature favorable large-scale environments, Type-3 has favorable lower-level and surface conditions but unfavorable upper-level circulation, while Type-4 features the most unfavorable large-scale environments. Despite the unfavorable large-scale environment, convection-centered composites reveal the presence of favorable sub-synoptic scale environments for MCS initiation in Type-3 and Type-4. All four types of MCS initiation environments delineate a clear eastward propagating feature in many meteorological fields, such as potential vorticity, surface pressure and equivalent potential temperature, upstream up to 25 west of and ~36 hours before MCS initiation. While the propagating environments and local, non-propagating low-level moisture are important to MCS initiation at the foothill of the Rocky Mountains, MCS initiation in the Great Plains is supported by the coupled dynamical and moisture anomalies, both associated with eastward propagating waves. Hence, the MCSs initiated at the plains can produce more rainfall than those initiated at the foothill due to more abundant moisture supply. By tracking MCSs and mid-tropospheric perturbations (MPs), a unique type of sub-synoptic disturbances with Rocky Mountains origin, it is shown that ~30% of MPs is associated with MCS initiation, mostly in Type-4. Although MPs are related to a small fraction of MCS initiation, MCSs that are associated with MPs tend to produce more rainfall in a larger area with a stronger convective intensity.

A heatwave and fire outbreak in the western United States in early September of 2020 resulted from an atmospheric wave train that spanned the Pacific Ocean basin. Days before the atmospheric waves developed in the U.S., three western Pacific tropical cyclones underwent an extratropical transition within an unprecedentedly short span of 12 days. Using a climate diagnostic approach and historical forecast data from the Global Ensemble Forecast System (GEFS), it was found that the amplitude of the atmospheric waves accompanying the western U.S. fire weather would have been reduced if not for the influence of these cyclones. Together, the recurving typhoons provided a significant source of Rossby wave activity toward North America-amplifying the ridge over the U.S. west coast while deepening the trough in central Canada. This anomalous circulation was a precursor to the severe frontal system that caused extreme winds in western Oregon-starting and rapidly spreading fire.