Temperatures will soar to record levels through midweek in the Western US as warmth expands with an intensifying ridge of high pressure. It will feel more like June than March in low-lying coastal areas and valleys from San Francisco to Vancouver as the mercury climbs above 70 degrees. The warm temperatures are the result of easterly winds compressing as it flows down mountains. But as winter-weary coastal residents bask worry-free in the spring sun, concern grows across inland communities that the inevitable snow melt will lead to severe flooding.
The Western US underwent a brutal winter featuring a parade of powerful storms that contributed to record snowfall for the month of February in Seattle and over 50 feet of seasonal snowfall in the Sierra-Nevada. Snowfall was not quite as extreme in the Cascades but snow was nonetheless overly abundant. The snow pack depth currently ranges from roughly 3 feet in the Columbia Valley to 20 feet on the peaks of the Cascades.
Represented by the snow pack is a copious quantity of liquid water–up to 6 feet in mountain peaks. High temperatures this week will climb into the 50s in the Cascades. Matched with continuous sunshine, these late-spring conditions will prompt the snow pack to surrender significant quantities of liquid water much earlier than usual.
As the sun angle increases and temperatures rise, liquid water from melting snow will flow down mountainsides into valleys, streams, and rivers. Hydrological systems have a limited capacity to carry snow-melt. This week’s sudden temperature upswing could result in more runoff than the region’s rivers can carry.
Further complicating matters, rivers and soils are still frozen across most of the interior Northwest. Water will be unable to soak into the ground so most of it will runoff towards rivers and low-lying areas. But with streams and rivers still frozen, there are few existing paths for water to flow out of the region, causing it to pool in low elevations. Water that does reach rivers will be thwarted by ice jams, a buildup of broken ice on river surfaces that impedes the flow of water. They act like dams that force water to pool up stream. Ice jams on the Missouri River alongside snow melt from warming temperatures and heavy rainfall recently contributed to the ongoing catastrophic flooding in eastern Nebraska, southwestern Iowa, and northwestern Missouri. The flooding in the Midwest has resulted in at least two deaths and hundreds of inundated homes. Fortunately, fears of flooding of this caliber can be quelled, as rain is not expected with this week’s warm-up.
Though rain is not expected, The National Weather Service Offices of Spokane, WA and Billings, MT warn that the snow runoff due to the earlier described conditions could lead to significant flooding in coming days. Since river sources have been frozen until now, rivers have extra capacity to carry snow-melt compared to later in the spring, when gradual snow melt and rainfall will have already raised river levels. The scale of the flooding in the Northwest is therefore expected to fall short of that of the Midwest, but dangerous conditions are still poised to ensue.
Farmland and low-lying communities in eastern Washington, northern Idaho and western Montana is highly susceptible to field flooding over the week. Water will accumulate on the poorly-drained soil, especially near streams and rivers. Without tree roots to uptake some of this water, cleared farmland is especially susceptible to this flooding.
Farmers aren’t the only residents who ought to worry. Basements across the region are expected to take on water as it pools on the thawing ground searching for a path downward through the soil. Drivers should to heed caution too. Mud washed down from mountain sides and from water flowing over top-soil is expected to pile on roads. The mud can strand vehicles and isolate rural communities from main interstate arteries like I-90 and I-15.
The ongoing warm-up will continue through the week, but the June-like temperatures will only persist through Wednesday. Temperatures will tumble to April-like temperatures behind a Pacific-based storm expected to bring heavy rain to the southern California, Oregon and the Great Basin and snow in the Sierra and southern Cascades. A second storm expected to bring light rain and snow to the Northwest this weekend will bring temperatures back to seasonable levels by Sunday. Despite the return to seasonable temperatures, flooding is expected to continue into next week.