Prof. Sandra Yuter
Department of Marine, Earth, and Atmospheric Sciences
North Carolina State University
"Characteristics of flooding and non-flooding storms over the U.S. west coast and implications for their prediction"
During the winter months, extra-tropical cyclones develop over the Pacific Ocean and move across the U.S. west coast. The coastal and interior mountain ranges modify these storms leading to enhanced precipitation. This study focuses on mesoscale observations of storms affecting the Oregon Cascades obtained over five cool seasons. The conditional probability of flooding is examined based upon the values of single parameters and the combination of multiple parameters derived from operational radar and upper air sounding data. All flooding storms were associated with land-falling bands of enhanced vertically-integrated water vapor (atmospheric rivers), high 0oC levels, strong cross-barrier flow, and long durations of precipitation. However, only of 56% of storms with these conditions produce floods near Portland, OR. Compared to non-flooding storms, flooding storms had higher precipitation persistence and intensity in Willamette Valley and higher precipitation intensity over the upper portion of the Cascade windward slope. Precipitation enhancement over the valley was related to two-layer flow where the lower, blocked layer acts to increase the effective cross-barrier width. Additionally, low-level flow convergence enhances precipitation within a concave section of the mountain barrier near the Lewis River Valley. Simulation of a flooding storm in northern California indicates that another factor contributing to flooding potential is the relative timing of the short-lived water vapor source from the atmospheric river and the peak strength of the barrier jet. For the storm studied, the timing of the maximum water vapor flux though the gap in the coastal mountains and into the central valley coincident with the period when the barrier jet was well developed yielded northward along-barrier water vapor fluxes that replenished about a quarter of the water vapor lost over the coastal mountains. These results point to the importance of the representation of low-level blocked flows and small scale terrain gaps in predicting flooding storms.
Host: Adam Sobel
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