Dangerous Fire Weather Expected on Friday and Saturday

 FastCast—Thursday, Sep. 8 to Saturday, Sep. 10:

Expect a mostly sunny day on Thursday, with highs in the low to mid 70s and lows in the upper 40s to mid 50s. From Thursday afternoon through Saturday, the weather will shift to dominant offshore flow. Offshore winds will dry out Western Washington, dropping humidity values to 20-40%, lower near the mountains. Winds will be strongest in the mountains, especially in the gaps in the terrain throughout Washington and Oregon. Offshore winds will continue on Saturday, and hot temperatures in the upper 80s to low 90s will add to the fire danger. Red Flag Warnings for fire danger are in effect for most of the Pacific Northwest from Friday to late Saturday. Any spark could lead to rapid fire growth, and existing fires will likely grow. Additionally, offshore winds will pull smoke from Eastern Washington into the Lowlands, likely bringing both surface smoke and smoke aloft. Smoke impacts are still uncertain. Important details about this fire weather setup are below, so continue reading to find out about impacts.

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A potentially dangerous fire weather situation is ahead for Western Washington. The combination of strong offshore flow, low relative humidity, dry vegetation, and hot temperatures will create a dangerous environment for fire spread, with the highest fire danger from Friday afternoon to late Saturday night. 

Let’s begin with the main concern…offshore winds. Known as gap winds, Cascadia winds, and easterly winds, these winds dry out the air around them as they descend the Cascades. The European model forecast for peak gusts on Friday afternoon is below.

At this point, winds in the lowlands will primarily be from the north, out of the Fraser River gap. Expect northerly gusts of 25-35 mph on Friday afternoon and into the evening. 

Winds will shift from northerly to easterly (through the gaps) by Friday night and early Saturday morning. This is when there is a higher chance of fire spread, especially in the Cascades. This aspect of the upcoming forecast is more uncertain. The European model forecast for peak gusts on Saturday morning is below.

Notice the yellow colors in the Cascades and the foothills of Washington and Oregon. These are gap winds, gusting 35-50 mph. The NAM model (not shown) is more extreme, suggesting gusts of 45-70 mph in the mountains and 30-45 mph in favored areas of the lowlands. 

The effect of these winds will be exacerbated by the incredibly dry conditions that will accompany them. Below is the HRRR forecast for relative humidity at 5 PM Friday.

The darker the shade, the lower the humidity. Expect very dry values of 15-30% in Western Washington, worst near the foothills. Humidity will be even lower in the Willamette Valley (stronger offshore winds) and Eastern Washington. This combination of strong, dry winds and low humidity led to Red Flag Warnings being issued for the majority of Western Washington, in effect from Friday to late Saturday.

These offshore winds will reverse the recent flow and will pull surface smoke and smoke aloft into Western Washington. Take a look at the HRRR forecast for smoke aloft at 8 AM Friday.

Offshore winds will already be making their presence known, and dense smoke aloft will be present over parts of the lowlands.

By 11 AM, the offshore winds will push smoke toward the surface, likely degrading air quality in parts of the lowlands. The HRRR near-surface smoke forecast is below.

These are moderate concentrations of surface smoke. Be prepared for degraded air quality on Friday. 

Notice that all the smoke flowing into the lowlands is coming from an area near the Canadian border in North-Central WA? That is due to the Chilliwack Complex and NW Pasayten Fires, burning a combined 10,000 acres. A good place to check on wildfires in WA & OR is the Northwest Interagency Coordination Center’s FireMap.

One final note…adding to the misery will be very warm temperatures on Saturday. The European model forecast is below.

Expect highs in the lowlands to reach the mid to upper 80s, potentially near 90. Hot weather will extend to the coast due to offshore flow, with highs reaching the upper 70s to mid 80s on the WA & OR coasts. 

A lot is still uncertain about the upcoming weather, so stay tuned. I will have another update by 10 PM Thursday. Stay tuned to @The_Weatherman2 Twitter (click the Twitter icon on the right side of the blog).

I have also added some more websites to the “Helpful Weather Websites” section, including the WA DNR Burn Portal, WA/OR FireMap, WA Smoke Blog, and the Northwest Interagency Coordination Center wildfire briefing blog.