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In this study, our aim is to diagnose how two quasi-linear convective systems (QLCS) are organized so one can determine the possible role of the city of Chicago, IL, USA, in modifying convective precipitation systems. In this Part I of a two-part study, we employ large-scale analyses, radiosonde soundings, surface observations, and Doppler radar data to diagnose the precursor atmospheric circulations that organize the evolution of two mesoscale convective systems and compare those circulations to radar and precipitation. Several multi-scale processes are found that organize and modify convection over the Chicago metroplex. Two sequential quasi-linear convective systems (QLCS #1 and #2) were organized that propagated over Chicago, IL, USA, during an eight-hour period on 5–6 July 2018. The first squall line (QLCS #1) built from the southwest to the northeast while strengthening as it propagated over the city, and the second (QLCS #2) propagated southeastwards and weakened as it passed over the city in association with a polar cold front. The weak upper-level divergence associated with a diffluent flow poleward of an expansive ridge built over and strengthened a low-level trough and confluence zone, triggering QLCS #1. Convective downdrafts from QLCS #1 produced a cold pool that interacted with multiple confluent low-level jets surrounding and focused on the metroplex urban heat island, thus advecting the convection poleward over the metroplex. The heaviest precipitation occurred just south-southeast of Midway Airport, Chicago. Subsequently, a polar cold front propagated into the metroplex, which triggered QLCS #2. However, the descending air above it under the polar jet and residual cold pool from QLCS #1 rapidly dissipated the cold frontal convection. This represents a case study where very active convection built over the metroplex and was likely modified by it, as evidenced in numerical simulations to be described in Part II. 

This study examined atmospheric mechanisms affecting the East Bay Hills Fire (1991) in Oakland, California, using the Advanced Weather Research and Forecasting (WRF) model and North American Regional Reanalysis (NARR) dataset. High-resolution WRF simulations, initially at 16 km, were downscaled to 4 km and 1 km for analyzing primary and secondary circulations at synoptic and meso-α/meso-β scales, respectively, before the fire. Additionally, the interaction between the synoptic-scale and mesoscale environments was examined using backward trajectories derived from NARR data. The findings reveal that a strong pressure gradient created by a ridge over the Great Basin and a trough off the Pacific coast generated favorable meso-α conditions for the hot, dry northeasterly winds, known as “Diablo winds”, which initiated the wildfire in northern California. Mountain waves, enhanced by jet stream dynamics, contributed to sinking air on the Sierra Nevada’s western slopes. The main conclusion is that jet circulation did not directly transport warm, dry air to the fire but established a vertical atmospheric structure conducive to wave amplification and breaking and downward dry air fluxes leading to the necessary warm and dry low-level air for the fire. The hot–dry–windy (HDW) fire weather index further confirmed the highly favorable fire weather conditions. 

On 30 June 2013, 19 Granite Mountain Hotshots firefighters were killed fighting a wildfire near Yarnell in the mountains of Central Arizona. They succumbed when the wildfire, driven by erratic winds, blocked their escape route and overran their location. A previous study is extended to simulate and analyze the downscale organization of convective circulations that redirected the wildfire, which started from the scale of the Rossby Wave Breaking over North America to a convective gust front that redirected the wildfire, trapping the firefighters. Five stages are found: Stage I, the initial deep prolonged gust front; Stage II, a front-to-rear jet and its ascending motions that organized high-based convection; Stage III, high-based dry microburst-induced downdrafts organized initially by ascending flow in Stage II that transported mass and entropy to the surface; Stage IV; multiple meso-γ-scale high centers and confluence zones formed that encompassed the firefighters’ location, which established a favorable environment leading to Stage V, canyon-scale circulations formed surrounding the fire. The atmosphere thus transitioned from supporting a deep and long-lived convective density current to elevated dry microbursts with mass and wind outflow into a canyon, redirecting the ongoing wildfire. 

Abstract Numerical simulations were conducted to investigate the upstream environment’s impacts on the airflow over the lee slope of the Cuyamaca Mountains (CM) near San Diego, California, during the Cedar Fire that occurred from 25 to 29 October 2003. The upstream environment was largely controlled by a southwest–northeast-oriented upper-tropospheric jet streak that rotated around a positively tilted ridge within the polar jet stream. Three sequential dynamical processes were found to be responsible for modifying the mesoscale environment conducive to low-level momentum and dry air that sustained the Cedar Fire. First, the sinking motion associated with the indirect circulation of the jet streak’s exit region strengthened the midtropospheric flow over the southern Rockies and the lee slope of the Sawatch and San Juan Ranges, thus modestly affecting the airflow by enhancing the downslope wind over the CM. Second, consistent with the coupling process between the upper-level sinking motion, downward momentum transfer, and developing lower-layer mountain waves, a wave-induced critical level over the mountain produced wave breaking, which was characterized by a strong turbulent mixed region with a wind reversal on top of it. This critical level helped to produce severe downslope winds leading to the third stage: a hydraulic jump that subsequently enhanced the downstream extent of the strong winds conducive to the favorable lower-tropospheric environment for rapid fire spread. Consistent with these findings was the deep-layer resonance between the mountain surface and tropopause, which had a strong impact on strengthening the severe downslope winds over the lee slope of the CM accompanying the elevated strong easterly jet at low levels.