CW3E AR Update: 16 October 2024 Outlook

October 16, 2024

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AR to Produce Heavy Rain over Pacific Northwest and British Columbia

• An atmospheric river (AR) and low-pressure system will bring heavy precipitation to the Pacific Northwest and Southwest British Columbia from Fri 18 Oct through at least Sun 20 Oct.
• The AR is forecast to make initial landfall over British Columbia Fri 18 Oct and progress down the coast toward the Pacific Northwest through the weekend.
• The ECMWF and GFS models differ regarding the development of a shortwave trough behind the initial AR. The ECMWF is forecasting a more amplified shortwave, leading to a second pulse of IVT that extends AR conditions over the Pacific Northwest.
• The GEFS control member is forecasting AR2 conditions (based on Ralph et al. 2019 AR scale) over the Pacific Northwest and and BC Coast while the EPS control member is forecasting AR4 conditions along the WA coast and AR3 conditions along the BC coast.
• The NWS Weather Prediction Center (WPC), GFS and ECMWF are all forecasting at least 3-6 inches or precipitation over the Olympic Peninsula, Cascade Mountains and Southwest British Columbia.
• The ECMWF is forecasting greater precipitation over the Olympic Peninsula than the GFS due to the longer duration of AR conditions.
• The British Columbia River Forecast Center (BCRFC) CLEVER discharge and return period forecast has three stations in southwestern British Columbia forecast to exceed a 100-year return period and two stations between 50 and 100 year return period
• Despite the forecasted river rise in Northwestern WA, the Northwest River Forecast Center is currently not forecasting any stations to exceed action stage at this time.

Click images to see loops of GFS IVT and IWV forecasts Valid 1200 UTC 16 October 2024 – 1200 UTC 21 October 2024

Summary provided by M. Steen and C. Castellano; 16 October 2024

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*Outlook products are considered experimental

For any unfamiliar terms, please refer to the American Meteorological Society Glossary.