Prof. Bosart presented a seminar titled “Impact of Recurving WPAC and EPAC Tropical Cyclones on Extreme Weather and Wildfire Events over the Western CONUS in Late Summer 2020”.
Persistent upper-level ridging over the Rockies and an unusually weak summer monsoon that failed to bring much needed rain to the parched Southwest further favored extensive heat and intensified the overall antecedent dryness in late summer 2020. The amplification of an anomalously strong upper-level ridge over eastern Alaska and western Canada in early September 2020 paved the way for anomalously strong offshore downslope Diablo Canyon, Santa Ana, and Columbia Gorge gap winds along the Pacific coast. These hot and dry offshore winds fueled, concentrated, and exacerbated previously existing spotty drought-related wildfires. These offshore winds occurred with a major heat wave over much of the western CONUS in early September that resulted in all-time record-high maximum surface temperatures being equaled or exceeded in many places. A noteworthy example occurred at Woodland Hills, CA, on 6 September 2020 where the observed maximum temperature of 49.0℃ established an all-time record high for Los Angeles County.
This presentation will document how three recurving WPAC TCs, Bavi, Maysak, and Haishen, that underwent extratropical transition (ET) created favorable dynamical conditions for strong upper-level ridging over western North America via downstream baroclinic development (DBD).This presentation will further document how poleward-directed midlevel moisture surges associated with three decaying EPAC TCs, Elida, Fausto, and Genevieve, enabled midlevel tropical moisture to reach California, Oregon, and Nevada where it fueled high-base dry thunderstorms that sparked the occurrence of numerous wildfires. This moisture surge was made possible by the presence of anomalous weak upper-level southerly flow in the EPAC between a cutoff low situated east-northeast of Hawaii and a subtropical ridge over the Southwest. TCs Bavi, Maysak, and Haishen underwent ET when pre-existing progressive upstream troughs perturbed the NPAC jet stream and triggered DBD that resulted in the formation of a high-amplitude flow pattern across the NPAC. Warm-air advection in advance of a rapidly deepening baroclinic cyclone near the Dateline and south of the Aleutians contributed to the amplification of a strong upper-level ridge over far eastern Alaska and northwestern Canada via the DBD process. This ridge amplification also enabled a strong upper-level downstream baroclinic short-wave trough to drop southward across the CONUS Intermountain region. This trough ushered in the first terrain-channeled autumnal cold surge into the western CONUS and ended a record-breaking heat wave over the Great Basin and Rockies. Alas, cold air was unable to reach the West Coast where a very hot and dry gusty downslope easterly flow that developed across inland parts of the Pacific Northwest contributed to the spread of aforementioned numerous wildfires and resulted in widespread poor air quality up and down the West Coast.
A recording of the presentation is available here.
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