Weather Associated with Rapid-Growth California Wildfires

Weather and Forecasting · 2025-01-14 · 4 citations

Abstract California is vulnerable to large, rapidly growing wildfires that can threaten human lives and damage physical infrastructure. This study analyzes the daily growth of all satellite-observed California fires from 2003 through 2020 and relates that growth to local meteorological and environmental conditions. Fire growth is defined using both absolute and relative metrics (absolute growth—the daily burned area; relative growth—the daily percent increase in burned area). Near-surface environmental conditions are evaluated before, during, and after periods of fire growth for individual fires of varying sizes and spread rates across events with both high and low relative growth. Regional spatial patterns and distributions of near-surface dryness and winds during large absolute-growth events are examined and compared to climatological conditions. Overall, large absolute-growth events generally occur when dead fuels are driest, trailing short periods of increased atmospheric dryness. For large absolute-growth events, high relative growth is usually associated with strong winds, while low relative growth is generally associated with quiescent climatological winds. Winds during high-relative-growth events are greatest in autumn, surpassing the 95th percentile on average. For these autumnal events, atmospheric dryness is less than during other seasons, but atmospheric and fuel dryness is still sufficient for fire. Irrespective of the season, winds during particularly rapid-growth events (high absolute and relative growth) are frequently downslope and influenced by local terrain regardless of the climatological conditions. Significance Statement Wildfires that undergo rapid growth are particularly dangerous to human life and infrastructure. This research finds that, given sufficient dryness, wind is the most important environmental factor in differentiating fire growth of varying sizes and spread rates and for enabling explosive wildfire growth.

CORRIGENDUM

Weather and Forecasting · 2025-05-01

The Meteorology of Large Wildfires over Western Washington and Oregon

Weather and Forecasting · 2025-07-30

Abstract This paper describes the meteorology of large wildfires occurring west of the crest of the Cascade Mountains in Oregon and Washington. The meteorological conditions associated with large west-side wildfires from 1902 to 2020 are simulated at high resolution using the Weather Research and Forecasting (WRF) modeling system driven by reanalysis grids. The northern west-side wildfires are associated with easterly, offshore-directed flow, while wildfires in southwest Oregon are forced by northeasterly flow. Southwest Oregon wildfires are associated with the extension of high pressure into Washington and a thermal trough over northwest California, while wildfires to the north are associated with high pressure over the interior and a thermal trough extending northward up the Willamette Valley. All west-side wildfires are associated with downslope flow, accompanied by strong winds and low relative humidity. Large west-side wildfires occur between midsummer and early fall. This seasonality occurs because summers are dry over the Pacific Northwest, with progressive desiccation of fuels from July through August. Cool, moist onshore flow from the Pacific Ocean dominates early in the summer but weakens by late August and September. Transient synoptic disturbances increase in strength and frequency during late summer, resulting in offshore-directed pressure gradients and strong, dry easterly winds. Large west-side fires are generally associated with warmer- and drier-than-normal conditions from July through August. The identification of the essential meteorological conditions associated with west-side wildfires, coupled with the excellent predictability of easterly flow conditions by modern numerical weather prediction, makes skillful warning of such events possible. Significance Statement This research describes the mesoscale and synoptic conditions associated with large wildfires west of the Cascade crest in Oregon and Washington. Knowledge of such conditions is of great importance for providing warnings for future wildfire events and for preparations to reduce their impacts. Understanding the conditions associated with such fires also facilitates the determination of the potential impacts of climate change on their frequency and severity.

The Meteorology of the August 2023 Maui Wildfire

Weather and Forecasting · 2024-05-24 · 14 citations

Abstract On 8 August 2023, a wind-driven wildfire pushed across the city of Lahaina, located in West Maui, Hawaii, resulting in at least 100 deaths and an estimated economic loss of 4–6 billion dollars. The Lahaina wildfire was associated with strong, dry downslope winds gusting to 31–41 m s −1 (60–80 kt; 1 kt ≈ 0.51 m s −1 ) that initiated the fire by damaging power infrastructure. The fire spread rapidly in invasive grasses growing in abandoned agricultural land upslope from Lahaina. This paper describes the synoptic and mesoscale meteorology associated with this event, as well as its predictability. Stronger-than-normal northeast trade winds, accompanied by a stable layer near the crest level of the West Maui Mountains, resulted in a high-amplitude mountain-wave response and a strong downslope windstorm. Mesoscale model predictions were highly accurate regarding the location, strength, and timing of the strong winds. Hurricane Dora, which passed approximately 1300 km to the south of Maui, does not appear to have had a significant impact on the occurrence and intensity of the winds associated with the wildfire event. The Maui wildfire was preceded by a wetter-than-normal winter and near-normal summer conditions. Significance Statement The 2023 Maui wildfire was one of the most damaging of the past century, with at least 100 fatalities. This paper describes the meteorological conditions associated with the event and demonstrates that excellent model forecasts made the threat foreseeable.

Improving Simulations of Warm Rain in a Bulk Microphysics Scheme

Monthly Weather Review · 2023-11-02 · 1 citations

Abstract Current bulk microphysical parameterization schemes underpredict precipitation intensities and drop size distributions (DSDs) during warm rain periods, particularly upwind of coastal terrain. To help address this deficiency, this study introduces a set of modifications, called RCON, to the liquid-phase (warm rain) parameterization currently used in the Thompson–Eidhammer microphysical parameterization scheme. RCON introduces several model modifications, motivated by evaluating simulations from a bin scheme, which together result in more accurate precipitation simulations during periods of warm rain. Among the most significant changes are 1) the use of a wider cloud water DSD of lognormal shape instead of the gamma DSD used by the Thompson–Eidhammer parameterization and 2) enhancement of the cloud-to-rain autoconversion parameterization. Evaluation of RCON is performed for two warm rain events and an extended period during the Olympic Mountains Experiment (OLYMPEX) field campaign of winter 2015/16. We show that RCON modifications produce more realistic precipitation distributions and rain DSDs than the default Thompson–Eidhammer configuration. For the multimonth OLYMPEX period, we show that rain rates, rainwater mixing ratios, and raindrop number concentrations were increased relative to the Thompson–Eidhammer microphysical parameterization, while concurrently decreasing raindrop diameters in liquid-phase clouds. These changes are consistent with an increase in simulated warm rain. Finally, real-time evaluation of the scheme from August 2021 to August 2022 demonstrated improved precipitation prediction over coastal areas of the Pacific Northwest. Significance Statement Although the accurate simulation of warm rain is critical to forecasting the hydrology of coastal areas and windward slopes, many warm rain parameterizations underpredict precipitation in these locations. This study introduces and evaluates modifications to the Thompson–Eidhammer microphysics parameterization scheme that significantly improve the accuracy of rainfall prediction in those regions.

The Influence of Soil Moisture on the Historic 2021 Pacific Northwest Heatwave

Monthly Weather Review · 2023-02-13 · 19 citations

Abstract During late June 2021, a record-breaking heatwave impacted western North America, with all-time high temperatures reported across Washington, Oregon, British Columbia, and Alberta. The heatwave was forced by a highly anomalous upper-level ridge, strong synoptic-scale subsidence, and downslope flow resulting in lower-tropospheric adiabatic warming. This study examines the impact of antecedent soil moisture on this extreme heat event. During the cool season of 2020/21, precipitation over the Pacific Northwest was above or near normal, followed by a dry spring that desiccated soils to 50%–75% of normal moisture content by early June. Low surface soil moisture affects the surface energy balance by altering the partitioning between sensible and latent heat fluxes, resulting in warmer temperatures. Using numerical model simulations of the heatwave, this study demonstrates that surface air temperatures were warmed by an average of 0.48°C as a result of dry soil moisture conditions, compared to a high-temperature anomaly of 10°–20°C during the event. Air temperatures over eastern Washington and southern British Columbia were most sensitive to soil moisture anomalies, with 0000 UTC temperature anomalies ranging from 1.2° to 2.2°C. Trajectory analysis indicated that rapid subsidence of elevated parcels prevented air parcels from being affected by surface heat fluxes over a prolonged period of time, resulting in a relatively small temperature sensitivity to soil moisture. Changes to soil moisture also altered regional pressure, low-level wind, and geopotential heights, as well as modified the marine air intrusion along the Pacific coast of Washington and Oregon. Significance Statement The record-breaking western North American heatwave of late June 2021 was preceded by below-normal soil moisture over the region. This study evaluates the role of soil moisture on the 2021 heatwave, demonstrating that the anomalous temperatures during this extreme event were not significantly increased by below-normal soil moisture.

The Uncoordinated Giant II: Why U.S. Operational Numerical Weather Prediction Is Still Lagging and How to Fix It

Bulletin of the American Meteorological Society · 2023-03-24 · 7 citations

Abstract U.S. numerical weather prediction (NWP) is critical for both economic reasons and the protection of life and property. Unfortunately, the nation’s global model prediction skill substantially lags the performance of leading international NWP centers, with problems extending to regional prediction. This paper reviews the history of U.S. activities in NWP, describes why U.S. weather prediction is not fulfilling its potential, and proposes actions that could restore U.S NWP to world leadership. Key suggestions include the creation of a U.S. Numerical Weather Prediction Center, better coordination of the efforts of the research and operational NWP communities, and increasing NOAA computational resources by at least tenfold.

The Pacific Northwest Heat Wave of 25–30 June 2021: Synoptic/Mesoscale Conditions and Climate Perspective

Weather and Forecasting · 2023-12-18 · 10 citations

Abstract An unprecedented heat wave occurred over the Pacific Northwest and southwest Canada on 25–30 June 2021, resulting in all-time temperature records that greatly exceeded previous record maximum temperatures. The impacts were substantial, including several hundred deaths, thousands of hospitalizations, a major wildfire in Lytton, British Columbia, Canada, and severe damage to regional vegetation. Several factors came together to produce this extreme event: a record-breaking midtropospheric ridge over British Columbia in the optimal location, record-breaking midtropospheric temperatures, strong subsidence in the lower atmosphere, low-level easterly flow that produced downslope warming on regional terrain and the removal of cooler marine air, an approaching low-level trough that enhanced downslope flow, the occurrence at a time of maximum insolation, and drier-than-normal soil moisture. It is shown that all-time-record temperatures have not become more frequent and that annual high temperatures only increased at the rate of baseline global warming. Although anthropogenic warming may have contributed as much as 1°C to the event, there is little evidence of further amplification from increasing greenhouse gases. Weather forecasts were excellent for this event, with highly accurate predictions of the extreme temperatures. Significance Statement This paper describes the atmospheric evolution that produced an extreme heat wave over the Pacific Northwest during June 2021 and puts this event into historical perspective.

The Influence of Regional Meteorology on Carbon Emissions from California Wildfires

Weather and Forecasting · 2022-12-06 · 1 citations

Abstract This paper examines the relationship between daily carbon emissions for California’s savanna and forest wildfires and regional meteorology over the past 18 years. For each fuel type, the associated weather [daily maximum wind, daily vapor pressure deficit (VPD), and 30-day-prior VPD] is determined for all fire days, the first day of each fire, and the day of maximum emissions of each fire at each fire location. Carbon emissions, used as a marker of wildfire existence and growth, for both savanna and forest wildfires are found to vary greatly with regional meteorology, with the relationship between emissions and meteorology varying with the amount of emissions, fire location, and fuel type. Weak emissions are associated with climatologically typical dryness and wind. For moderate emissions, increasing emissions are associated with higher VPD from increased warming and only display a weak relationship with wind speed. High emissions, which encompass ∼85% of the total emissions but only ∼4% of the fire days, are associated with strong winds and large VPDs. Using spatial meteorological composites for California subregions, we find that weak-to-moderate emissions are associated with modestly warmer-than-normal temperatures and light winds across the domain. In contrast, high emissions are associated with strong winds and substantial temperature anomalies, with colder-than-normal temperatures east of the Sierra Nevada and warmer-than-normal conditions over the coastal zone and the interior of California. Significance Statement The purpose of this work is to better understand the influence of spatially and temporally variable meteorology and spatially variable surface fuels on California’s fires. This is important because much research has focused on large climatic scales that may dilute the true influence of weather (here, high winds and dryness) on fire growth. We use a satellite-recorded fire emissions dataset to quantify daily wildfire existence and growth and to determine the relationship between regional meteorology and wildfires across varying emissions in varying fuels. The result is a novel view of the relationship between California wildfires and rapidly variable, regional meteorology.