Cold-Season Frontogenesis in North American Extratropical Latitudes: the Influence of Subtropical Air Masses on Extreme Precipitation

Moist inversions in the lower troposphere are commonly viewed as indicators of surface fronts. The strength of the front may be indicated by virtual potential temperature contrasts typically horizontally oriented near the surface, and more vertically directed with elevation as the frontal zone slopes to colder air.

Our objective is to identify the role of cold-season frontal lifting of subtropical air in facilitating extreme precipitation. To address this question, we examine instances of extreme values of the maximum value of equivalent potential temperature in the 900-700 hPa layer for rawinsonde-station locations at ~45 degrees latitude in North America during the winter season (December, January, and February). The period of record is 1940 through 2023. We use hourly output from the European Centre for Medium-Range Forecasting global reanalysis (ERA5). This exercise reveals that 1) such extreme values of equivalent potential temperatures often represent the top of a moist saturated frontal inversion with back trajectories traceable to the oceanic regions of the subtropics, 2) frontal ascent of the subtropical airmass often results in high-impact extreme precipitation events, and 3) the saturated nearly moist adiabatic stratification above the frontal inversion facilitates particularly strong ascent and extreme precipitation.

An example of a recent extreme precipitation event in the Montreal, Quebec region will be shown.