Storm Update: Heavy Rain, Gusty Winds Ahead

Update by 5 PM Friday with latest details & forecast.

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We are less than 24 hours from the start of the first significant storm of the season. Heavy rain, gusty winds, and a chance of thunderstorms are all in the cards.

The rain shield of the incoming system will reach the coast early Friday morning, with rain falling in the Puget Sound area soon after. Expect some rain in the morning, then a potential break in the rain during the afternoon.

Then, on Friday evening, the real action begins. An area of very moist air will move over Western Washington, with a current of air heavy with water vapor directed right at the Puget Sound area.

The UW forecast above is for 8 PM Friday, as heavy rain will be falling in the Puget Sound lowlands. 

Below are 24 hour totals from 5 AM Friday to 5 AM Saturday from the UW model.

From Seattle southward, expect 24-hour totals of 1-2 inches. Up to 2.5 inches near Olympia, and massive amounts of 6-7.5+ inches in the Olympics & Cascades. Notice the prevalent rain shadow northeast of the Olympics. These areas, including North Seattle and Everett, will receive lesser totals of 0.3-1 inches.

This isn’t the end…the rain continues well into Sunday, just becoming more showery. Additional rainfall of 0.5-1.1 inches is expected through Sunday evening.

Make sure that your gutters and storm drains are cleared before the heaviest rain arrives. If you haven’t done this yet, you have until Friday evening!

Now for the wind. Since my last blog post, the forecast winds have increased. Below are 5 forecast model predictions. (How to read this: large numbers=sustained winds, small numbers=gusts, arrows=wind direction)

This forecast (for Federal Way) shows winds gusting 30-40 mph from Friday afternoon/evening through early Saturday morning. Conditions remain breezy (gusts 20-30 mph) through Saturday evening.

Expect slightly stronger winds near Puget Sound, and winds gusting 45-50 mph on the coast and from Admiralty Inlet northward. (Wind Advisory in effect for those areas).

Regarding winds, since this is the first storm of the season, trees are fully loaded and therefore more vulnerable to damage. Add on the fact that many trees have been weakened by this summer’s extreme heat, and the threat for tree damage is higher. Be prepared for scattered power outages and damaged trees.

There is one more threat worth mentioning…thunderstorms.

Behind the cold front that moves through Friday night, the air will be unstable. The best chance for thunderstorms will be midday Saturday to Sunday evening. CAPE instability values of 200-600 will be present over the Lowlands (relatively high for our area). Values of 600+ are possible at times along the WA & OR Coasts.

Wow! A lot of active weather is heading our way! Expect things to calm down by Sunday evening, with dry weather likely by Monday. Below is a recap from NWS Seattle.

Enjoy the storm & stay safe!

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.

What Forecast Should I Use?

One of the most pressing questions regarding weather is what forecast to use. In this age of social media and widespread internet access, there are so many different forecasts to choose from. This blog will attempt to explain which forecast to choose!

My go-to site to begin choosing a forecast is Forecast Advisor. This handy site shows the accuracy of multiple forecasts over the previous month and previous year. Below is Forecast Advisor's accuracy scores for Seattle in 2020.

The Weather Channel and Weather Underground had the highest accuracy score, over 86%. AccuWeather came up just behind at 84%. Notice that the NWS forecast came in at 78.6%, about 8% below the leaders, showing that just because it is run by the government, it isn’t the most accurate forecast available, though it is still a good choice.

Now let’s see how these trends verified last month. Below are the Forecast Advisor scores for Seattle in August.

Weather Underground and The Weather Channel take the cake again, with scores around 84%. Wetter, a German company, came in third. Notice that the NWS came in with a score of 72.5%.

If you were wondering how the accuracy scores are calculated, Forecast Advisor says in their FAQ page that “The overall accuracy percent is computed from the one- to three-day out accuracy percentages for high temperature, low temperature, icon forecast precipitation (both rain and snow), and text forecast precipitation (both rain and snow). Temperature accuracy is the percentage of forecasts within three degrees. Precipitation accuracy is the percentage of correct forecasts. The forecasts are collected in the evening.”

So, what forecast should you use?

The accuracy scores suggest using Weather Underground or The Weather Channel. These companies, both owned by IBM, have a similar forecasting algorithm that has proven to be accurate over the past few years, with consistent ratings of 80+%.

These algorithms take many things into account when making a forecast. Some things include current, future, and historical conditions and weather predicted by multiple forecast models. 

You can download the Weather Underground and The Weather Channel apps on the iOS App Store and the Google Play Store. You can also find these sites on the web, at wunderground.com and weather.com.

Generally, any forecast with 75-80%+ accuracy will give you a good idea of what conditions to expect. However, if you’re able to use the most accurate forecast, do so. It’ll give you the best idea of what to expect, especially regarding temperatures and precipitation.