Localized Chance of Rain on Friday, Changes Ahead

FastCast—Friday, Sept. 16 to Wednesday, Sept. 21:

There is a localized chance of rain in the Central Puget Sound area on Friday, especially in the afternoon and evening due to a potential Convergence Zone. Rain will likely be on the light side, totaling 0.1-0.25 inches. Some areas, particularly the Cascades, will get higher totals, up to 0.5” in isolated areas. Friday and Saturday will be on the cool side, with highs in the low to mid 60s for the lowlands. Temperatures will increase a bit as clouds decrease to begin next week. Expect highs from Sunday to Wednesday in the low to mid 70s, with lows in the upper 40s to low 50s. 

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A weak weather system moving by Western Washington on Friday will bring a chance of a Convergence Zone in Central Puget Sound on Friday, particularly from mid morning to evening. We will take a look at the high-resolution rain forecasts below.

Starting with the NAM model, we see total rain through 12 PM Saturday.

The NAM model shows as much as 0.25” in the Seattle area, with up to 0.5-0.75” in the Cascades and Olympics (potentially higher). However, notice that this rain is confined to a narrow area between approximately Tukwila and Everett, with 0.05-0.1 inches possible from Everett north to Bellingham.

Let’s see what the HRRR forecast says. This is also for rain through 12 PM Saturday.

The HRRR shows much less rain, with totals in the Convergence Zone and associated rain bands only 0.05-0.15 inches, with some areas in the mountains getting closer to 0.2 inches.

This weak system will bring overcast days on Friday and Saturday, with cooler temperatures expected. The NAM high temperature forecast for Friday is below.

It will be one of the coolest days in months for Western Washington, with highs in the low to mid 60s, with some areas (mainly Tacoma southward) potentially reaching the upper 60s. Eastern Washington will only reach the 70s, with a slight chance of the Columbia Basin reaching the low 80s. Statewide temperatures will be below average through Saturday, but that trend will change soon.

The NWS Climate Prediction Center outlook for September 23rd-29th is below.

This outlook shows a 60-70% chance of above average temperatures for the majority of Washington and Oregon. Some extended forecasts have been suggesting temperatures in the upper 70s and potentially the low 80s in late September. Stay tuned for more information!