FastCast—Thursday, Dec. 21 to Sunday, Dec. 24:
Another calm and foggy day is expected across the lowlands on Thursday, with a Dense Fog Advisory in effect for most of the region until 12 PM. Visibility could be as low as 1/4 mile, so be cautious. The next system approaches the area on Friday, bringing a brief round of rain (likely 0.2-0.5” for the lowlands) and winds gusting 25-30 mph. The passes will pick up 4-6” of snow, which could bring brief impacts for Christmas travel. Expect highs in the mid to upper 40s on Thursday, then in the low to mid 40s from Friday to Sunday. Lows will be in the low to mid 40s on Thursday and Friday, then in the upper 20s to low 30s on Saturday and Sunday mornings. The next system will arrive by late Saturday, bringing rainy conditions on Christmas Day. Stay tuned!
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More fog and calm weather is expected across the region on Thursday. NWS Seattle has issued a Dense Fog Advisory through 12 PM Thursday for most of the region. Below is the European model forecast for visibility on Thursday morning.
This forecast shows visibility of 0.1-0.5 miles for most of the lowlands from Seattle southward on Thursday morning. Fog is hard to forecast, so low and potentially hazardous visibility may be more widespread than this forecast shows.
A good marker for where fog is most likely is the extent of the Dense Fog Advisory, seen below on this map.
Basically, the advisory covers the entirety of the lowland regions from Chehalis northward to the San Juan Islands.
Now, let’s take a look at the next system, moving into the area on Friday. Below is the European model forecast for total rain through Saturday.
This forecast shows 0.2-0.5” for the lowlands, with less from Seattle south (as little as 0.1”). Areas from Everett to Bellingham could get up to 0.5-0.75”. The coast will likely pick up 0.6-0.8” with this system.
Let’s compare this to the GFS forecast, also showing rain through Saturday.
This forecast shows less rain overall, with most of the lowlands getting 0.05-0.2” and being in a rain shadow (mainly from Olympia to Seattle), with areas from Seattle north getting 0.2-0.5”. The coast gets 0.4-0.8” in this forecast.
Finally, let’s take a look at the mountain snow forecast. That’s the only part of this system that could have an actual impact. Below is the European model forecast for snow through Saturday.
This forecast shows the passes getting 4-6” of snow, with up to 8” at Stevens Pass. Most will fall on Friday. This could cause brief travel impacts, especially with increased holiday traffic. Areas south of Snoqualmie Pass will only receive 3-4” of snow.
Let’s compare this with the GFS forecast, also showing snow through Saturday.
This forecast shows 4-8” of snow for Snoqualmie Pass and up to 12” for Stevens Pass, with areas south of Snoqualmie Pass receiving up to 4”.
There is potential for brief heavier snow at Stevens Pass on Friday and early Saturday due to Convergence Zone bands in the area. Overall, there is a potential for snow-related impacts, but the brief duration of this system will mitigate a major event.
Stay tuned for updates as somewhat more active weather is possible around Christmas and beyond!
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