Atmospheric River: Heavy Rain, Flooding, and High Snow Levels Through Friday

 FastCast—Thursday, Nov. 11 to Friday, Nov. 12:

An atmospheric river will bring heavy rain, urban & river flooding, and very high snow levels on Thursday and Friday. Temperatures will also be on the mild side, due to a stalling warm front. Expect temperatures to rise to the upper 50s by Thursday morning, remaining there through late Friday. Rain will begin early Thursday, gradually picking up in intensity through the day. Heavy rain is expected at times from Thursday evening to Friday morning. Totals of 1-3 inches are expected around the region (except in the Olympic rain shadow). The highest totals in the lowlands (2-3 inches) will likely be between Seattle and Tacoma. On the coast and in the mountains, 2-10 inches are expected. Snow levels will skyrocket due to the stalled warm front, rising to 8,000-9,000 feet. River flooding is expected on most rivers, mainly from late Thursday night to Friday night. Additionally, urban flooding and ponding is expected. Remember to never drive through floodwaters! Expect rain to taper off by Friday afternoon/evening.

————————————————————

Continue reading the full blog below!

For the second time in the past month, an impactful atmospheric river is heading for Western Washington. Heavy rain, urban & river flooding, high snow levels, and mild temperatures are expected. 

Rain will begin early Thursday, continuing through Friday afternoon/evening. The heaviest rain is expected from Thursday evening to Friday morning. Below is the UW forecast for total rain through 4 PM Friday.

Expect 1-3 inches across Western Washington. 2-3 inches (locally higher) are expected between Seattle and Tacoma and in adjacent foothills. Expect less rain (0.3-1.25”) in the Olympic rain shadow and more (2-10”) on the coast and in the foothills/mountains.

This is all due to an atmospheric river, or a narrow current of deep subtropical moisture, aimed right Western Washington. These are also known as the “Pineapple Express” due to their origin in the area around Hawaii. Below is the UW forecast for integrated water vapor transport (IVT), or amount of water vapor in the air, at 1 AM Friday, during the heaviest rain.

The current of high IVT values are aimed directly at Western Washington, which will bring the heaviest rainfall.

As expected, flooding will occur. NWS Seattle has issued a Flood Watch from Thursday evening to Saturday afternoon. The alert is posted below.

Two main concerns when talking about flooding with this atmospheric river.

Urban flooding, ponding, and standing water is expected around the region, especially by Friday morning. Be careful of clogged storm drains and low-lying spots on area roads. 

River flooding is expected on many rivers from late Thursday to Saturday. Below is the Advanced Hydrologic Prediction Service map for Western Washington.

Any river with a yellow (action stage), orange (minor), or red (moderate) dot means it will flood or come very close to flooding from Thursday to Saturday. Nearly every river originating in the Cascades will reach “action stage” or crest above flood stage on Friday or Saturday. Click the link above for frequently updated river observations. 

Remember to never drive through floodwaters, and as the National Weather Service says, “Turn Around, Don’t Drown!”

Expect rain to taper by Friday afternoon. A break is expected through most of Saturday, before rain begins again in the evening. Stay safe & stay tuned here and on Twitter (click the Twitter icon on the right side of the blog)!