First Storm of the Season to Arrive on Friday

After a long, hot, and very dry summer, we are finally due for a significant storm system, with a major rain event expected! 

The first storm of this storm season will primarily be a rain-producer, but other impacts will include breezy conditions and a chance of thunderstorms. Additionally, this storm system will likely be the end of wildfire season for the Pacific Northwest, as Eastern WA & OR will cash in on rain, cooler temperatures, and much higher humidity.

Let’s start with the forecast. Below is the Weather Underground forecast for Federal Way.

A weak system will move through Western Washington late Tuesday into Wednesday. Below is the UW forecast for rain through 4 PM Wednesday.

The majority of the Puget Sound Area will receive up to 0.03” of rain, except more in the Cascades (up to 1”). A Convergence Zone band is forecast to develop between Kent and Lakewood. These areas may receive up to 0.2 inches of rain.

However, this is a tiny appetizer to the real action that starts on Friday. 

Our most significant rain since late spring is on the way, in the form of an atmospheric river, or a long & narrow current of moisture. Below is the integrated water vapor transport (amount of water vapor in the atmosphere) at 5 PM Friday.

These values, on the order of 1000-1200 over Western WA/OR, are usually seen only in the winter. This means that we will be having a winter-style atmospheric river event to kick off the storm season!

So, how much rain are we expecting? Below is the forecast for Federal Way.

Wow! Well over 1 inch of rain is expected on Friday, with up to 1.5 inches possible. Then, totals near 0.5 inches on Saturday and Sunday.

As you would expect, rain totals will be quite high, as shown in the UW extended forecast below.

Current forecasts show rainfall totals of 1-3 inches for the Lowlands, 2-4 inches on the coast, and up to 7.5 inches in the mountains. Additionally, 1-4 inches of snow are possible above 6,000 feet. 

In Eastern Washington & Oregon, totals of 0.1-1.5 inches are possible, with the most in the mountains. This rain will play a major role in the decline of wildfires.

Another detail of this storm will be breezy winds. As of Tuesday night, forecasts indicate wind gusts of 20-35 mph from midday Friday to late Sunday.

Thunderstorms are also possible behind the main cold front on Saturday. The best chance of thunderstorms is from Saturday morning to late night, with the highest instability on the coast & in the mountains.

Stay tuned for my next update (Thursday night) as details about this storm, especially regarding exact rain amounts & chance of thunderstorms, continue to change.