Rain this morning has tapered into the afternoon though into this evening we will still have periods of mainly light rain around the area. This activity will continue into Friday morning but be of no significant impact. Milder air is arriving so morning lows will be in the 40s.
Friday afternoon, we’ll have some scattered showers mainly near the coast, near the mountains, and south of Puget Sound. The corridor from about Skagit County south to Tacoma could be much drier during the day in an Olympic Mountain rain shadow, where the Olympic Mountains take out the incoming moisture leaving some partly sunny skies. Highs will be In the mid 50s.
A Winter Weather Advisory has been issued for the Stevens Pass area Friday evening through Saturday. As moisture streams ashore, it could be concentrated in a bit of a convergence zone across the Stevens Pass vicinity, enhancing snowfall totals with 5-10” of snow in that area. Elsewhere, other slopes will pick up some snow, though lesser amounts. We should see mainly rain or a rain/snow mix at Snoqualmie Pass on Friday night into Saturday.
For the lowlands this weekend, Saturday will be the better of the two days with some spotty showers but mainly dry times with some periods of sun. On Sunday, expect a bit more rain moving through with highs both days in the 50s.
Some mainly minor coastal flooding along the coast, the northern waters and possibly around parts of Puget Sound is possible over the weekend around the time of high tide. This is associated with our usual December “king tide” pattern.
HEAVY RAIN NEXT WEEK: Attention turns to next week with the likelihood of several atmospheric river events that could have impact for our area rivers. New forecast data is trending toward a later onset of heavier rain on Monday afternoon, lasting into early Tuesday. There then could be a break before more heavy rain Tuesday night into Wednesday. However, the eventual trajectory of the stream of richest Pacific moisture will determine where the heaviest rain occurs, and we are still too far out in time for finer details, and the timing will likely shift some as well. This will come into better focus later on this weekend.
So at this time, the forecast will call for heavy rain at times in the lowlands and in the mountains below about 5,000-6,000 feet from Monday through Wednesday though we could get a break for a time during the period. Looking even longer range, there could be more rain late next week, though the uncertainty grows significantly with each additional forecast day. Snow levels by midweek could be upwards of 6,000 feet, putting a lot of water into rivers flowing out the Cascades.
While the heavy rain might come in waves, general forecast averages have 2-4 inches of rain through the first half of next week in the lowlands with isolated higher amounts. Though spread out over several days, we could still have some isolated urban flooding during heaviest rainfall times late Monday through Wednesday. Also of note will be the increasing risk of rainfall-induced landslides as well as flooding near burn scars. However, for the majority of people – it will just be several very soggy days.
The most obvious impact from the rainfall late Monday through at least midweek will be rising rivers. It is looking probable that many river systems will get into the minor to moderate flood stage by around Wednesday of next week, as it takes some time after rain begins for rivers to spike up. There is the potential for major flooding on some rivers in a worst case scenario of prolonged heavy rainfall. Rainfall totals of 5-10” are likely in the mountains during that period though higher amounts could occur if these systems last a while. Folks living or having interests in and around the flood plains should start considering preparedness plans for possible flooding next week.
There still remains a prospect of colder weather in the Northwest starting around the end of next week, though the long-range pattern signal for colder weather now isn’t as pronounced as it had been. Snow lovers and those wanting ski areas to open up might have to hang on a bit longer.