Significant August Rain Event Ahead

FastCast--Thursday, Aug. 22 to Saturday, Aug. 24:

Persistent troughing due to an upper level low off the coast of the Pacific Northwest will bring a significant amount of rain (for August) to the region. On Thursday, expect mostly cloudy conditions, with highs in the upper 60s to low 70s. There is a chance of showers in the afternoon & evening, with a chance of thunderstorms Thursday night, mainly from Tacoma southward. As we move into Thursday night, rain will become more widespread, with areas of steady rain likely from Thursday night through Saturday. This will bring an abnormally high amount of rain to Western Washington, with the lowlands likely receiving 0.75-1.5" (isolated higher amounts) through Saturday night. There is also another chance of thunderstorms across the region on Friday, especially in the afternoon. This threat looks to be best from Seattle north and in the Cascades. Friday and Saturday will also be on the cooler side for summer, with highs only reaching the low to mid 60s, over 10° below normal. Overnight lows will drop to the low to mid 50s across the region. Keep reading below for more information about this upcoming interesting period in our weather.

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Continue reading the full blog below!

A significant August rain event is ahead for Western Washington, as a persistent upper level low moves through the Northwest late this week and into the weekend. 

Let's start with the European model's latest rain forecast, showing total rain through Saturday night.

This forecast shows a significant event, with most of Western Washington getting 0.7-1.5" of rain, with areas in the Cascades, Olympics, Cascade foothills, and central coast getting up to 2-2.5". The Willamette Valley gets 0.3-0.7", and Eastern WA gets 0-0.3", except up to 0.5-1" for the eastern slopes of the Cascades (including Lake Chelan).

Next, let's compare this to the high-resolution NWS NBM model, shown below, also showing total rain through Saturday night.

This forecast shows all of Western Washington getting 1-1.6" of rain through Saturday, with the Cascades and Olympics getting 2-2.75". Eastern Washington gets 0.05-0.25", except up to 1" along the eastern crest of the Cascades. The Willamette Valley gets more rain, 0.4-0.9", in this forecast.

What's causing such a big amount of rain in August? Take a look at the European model forecast below to see.

Notice a large upper-level low moving onshore over Oregon on Saturday evening. This is the source of all the rain moving northward into Washington.

This upper-level low will also be the source of potential thunderstorms and much cooler than average temperatures for the region.

First, the thunderstorms...below is the HRRR forecast for potential lightning on Thursday night.

This forecast shows an area of storms moving from SE to NW across the coastal mountains of WA & OR, then moving over the Washington coast. This area of potential storms has also been forecasted from Tacoma to Olympia on previous model runs, and that area remains in play for possible storms on Thursday night.

There is another chance of thunderstorms on Friday, specifically in the afternoon. Below is the HRRR forecast for lightning late Friday afternoon.

This forecast shows scattered areas of storms across Eastern WA, the Cascades, and the metro area from Tacoma northward through the San Juans and BC. This storm chance is less certain, and storms could pop up in different Western WA locations than shown on this map.

Now, for temperatures, let's take a look at the NWS NBM forecast for highs on Saturday.

On Saturday, highs in Western WA will only reach the low to mid 60s, while the Willamette Valley and Eastern WA only reach the upper 60s to low 70s. These temperatures are 10-15° below average for this time of year. That is due to the upper-level low bringing in much colder air (both aloft and at the surface).

That will be reflected in morning lows, as seen on the NWS NBM model below.

On Saturday morning, while Western WA remains mostly seasonable in the low to mid 50s, areas in the mountains and highlands of E. OR will drop to the mid 30s to low 40s, while Eastern Washington will get chilly, down to the mid 40s to low 50s.

Remember to take it slow, watch for standing water/ponding, and be prepared for isolated thunderstorms!