FastCast—Saturday, Oct. 15 to Sunday, Oct. 16:
A weekend of dangerous fire weather is ahead for Western Washington and Western Oregon. Record high temperatures are expected, with Seattle’s latest 80º+ day in history likely. Highs on Saturday and Sunday will reach the upper 70s to low 80s, with areas near the water in the mid to upper 70s. Due to offshore flow and gap winds, areas downstream of gaps in the Cascades, including the Eastside, Enumclaw, North Bend, and the Portland metro area, will reach the mid to upper 80s, unprecedented at this time of year. Offshore flow will keep the region warm overnight Saturday into Sunday, with lows only reaching the mid 50s to mid 60s. Offshore flow will bring more surface smoke into all of Western Washington, with strong flow dragging surface smoke through the Chehalis Gap and toward the ocean beaches. Air quality across Western Washington will be degraded to the “moderate” to “unhealthy for all” categories all weekend. Additionally, offshore flow will bring gusty winds near the Cascades and terrain gaps, with gusts up to 30-40 mph at peak (Saturday night into Sunday morning). Combined with relative humidity around 15-35%, conditions will be prime for fire ignition and spread, especially near the Columbia River Gorge, where winds could gust over 40 mph. These conditions will enhance fire behavior for existing fires, including Bolt Creek. Keep reading below for important updates on the upcoming fire weather.
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Continue reading the full blog below!
Although it is mid-October, another round of potentially dangerous fire weather is ahead for Western Washington. With high pressure developing over Eastern Washington and thermally-induced low pressure over Western Washington, flow will accelerate through gaps in the Cascades, bringing smoke, wind, very dry conditions, and record setting temperatures.
Let’s begin with the smoke forecast from the HRRR smoke model. Below is 8 AM Saturday.
Notice the moderate to heavy surface smoke concentrations around the lowlands, especially east of I-5 and from Bremerton southward on the Kitsap Peninsula.
By 6 PM Saturday, offshore flow will be more visible, with distinct smoke plumes from area fires stretching westward on the wind.
While AQI will be degraded for the entire region, particularly bad air quality is possible downwind of area wildfires. This is most likely from Seattle northward into SW British Columbia. This will continue overnight into Sunday. Stay tuned for my update Saturday night for more information on what to expect on Sunday.
Due to the pressure difference across the Cascades, gap winds will accelerate from Saturday afternoon to early Sunday morning. We will compare the HRRR and NAM high-resolution forecasts below.
The HRRR forecast shows peak winds around 4 PM Saturday.
The HRRR shows gusts of 25-45 mph in the Cascades and immediately downstream of the gaps, mainly for areas such as the Portland/Vancouver metro and the Cascade foothills. Lesser offshore winds, gusting 20-30 mph, are possible along the WA/OR Coast and Coast Ranges.
The NAM model is more aggressive with the wind forecast, and it shows peak winds later, around 11 PM Saturday.
The NAM shows stronger winds, gusting up to 40-55 mph, in the Cascades, with gusts of 25-40 mph downstream of the gaps. The NAM also shows areas of gusts 25-40 mph along the coast and Coast Range.
Generally, the HRRR forecast has been more accurate in these gap wind events, but gap winds tend to be erratic, so any situation is possible.
Additionally, record-breaking temperatures are expected across Western Washington this weekend. The NAM forecast for Saturday’s highs is below.
On Saturday, expect highs in the upper 70s to low 80s around the lowlands, with some areas (those exposed to gap winds) reaching the mid 80s, with the same expected in the Portland area. Offshore flow will bring rare readings in the low to mid 70s on the coast!
Due to strong offshore flow, temperatures will remain warm overnight Saturday into Sunday. The HRRR forecast for temperatures at 6 AM Sunday is below.
Nighttime temperatures in the upper 50s to low 60s are incredibly rare for October, and these lows are closer to average highs for this time of year.
The potential for extreme warmth is greater on Sunday. The HRRR forecast below for 3 PM Sunday shows what is possible with strong offshore flow and a heat-aiding thermal trough.
If this forecast verifies, all-time October records could be broken. If offshore flow is strong enough, there is a chance that parts of the lowlands could reach the mid to upper 80s, and potentially near 90º. Stay tuned for my Saturday night update for more on this possibility. Notice that cooler (and cleaner) air is moving in from the coast, dropping highs there back to the upper 50s to low 60s.
The potential for fire starts and fire spread will be made much worse due to very dry conditions across the region. This can be shown by the relative humidity (amount of water vapor in the atmosphere). The HRRR humidity forecast for 4 PM Saturday is below.
The HRRR forecast shows that by 4 PM Saturday, humidity across the state will be in the 20-30% range.
However, the HRRR shows this getting even worse by Sunday afternoon. The humidity forecast for 3 PM Sunday is below.
This forecast shows relative humidity across most of Washington State, including the lowlands, dropping to 10-20%. This is rare for a maritime climate like ours, and is very rare for October.
In short, although it is considered autumn, this weekend will feel like summer, with record-breaking temperatures and strong offshore flow bringing gap winds and lots of surface smoke. It is highly recommended to not burn anything during these dry conditions.
Stay tuned for an update by 10 PM Saturday regarding this fire weather and the switch back to normal onshore flow by Monday.
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