A prolonged period of wet weather has begun in the Pacific Northwest to kick off meteorological winter. For most, rain started Thursday and the east wind coming through the Columbia Gorge was cold enough to allow slushy snowflakes to briefly mix in with the rain in parts of Northeast Portland.
Additional rainmakers are forecast to impact the region over the next week or so. Heavy rain is forecast Saturday and Sunday with daily totals likely exceeding 1 inch (25 cm) in Portland both days. Higher rainfall amounts are forecast closer to the mountains.
Saturday night, the moisture plume switches from out of the west to out of the southwest bringing a stark warmup. Snow levels Friday are around 3000 feet (900 meters) but by Sunday morning they are expected to be closer to 6500 feet (2000 meters).
Of top of heavy lowland rainfall, heavy rain falling on the heavy snow currently falling in the mid-elevation Cascades will create a risk of flooding prompting the National Weather Service to issue a flood watch for the Willamette Valley and Portland metro. A flood watch is also in place for the Coast Range but that risk is driven solely by rainfall as snow has not been as robust over there.
Seasonably warm temperatures continue into next week with Portland highs possibly approaching 60°F (15°C) Monday and Tuesday under Pineapple Express conditions as moisture is sourced from near Hawaii. This will promote further melting of the snowpack while additional rain falls.
A widespread major flood event such as what occurred in 1996 is not likely. The rain during that event came on the heels of lowland snowfall. Soils leading up to that Pineapple Express were either frozen or already saturated. We have neither heavy snow in the valley, frozen ground, or saturated soil to worsen the flood risk.
Still, moderate flooding on small rivers draining the Cascades and Coast Range is possible. The Willamette River at Portland is forecast to rise a few feet (about 1 meter) by midweek but will be well below flood stage. Urban flooding is expected in typical low spots.
Cold fronts sweeping across the region will produce periods of breezy conditions in and around Portland during this active period. The risk of fallen trees increases as soils saturate this weekend making it easier for trees to be uprooted by the wind.
Saturated soils also increase the risk of landslides. As usual, this issue will be most notable in the Coast Range and Columbia Gorge where steep and unstable slopes are common and rainfall totals are higher.
Models are indicating a moderation of the wet weather during the second half of next week along with temperatures cooling closer to the average for this time of year. Still, rain showers are favored but snow levels may drop back down to somewhere in the 3000 to 4500 foot (900 to 1400 m) range.
By next Friday (December 8), Portland may see as much as five inches (130 mm) of rain. Flood watches are also in place for most of Western Washington, including Seattle and the Puget Sound Region. The flood watch for the Portland metro ends Tuesday afternoon.
