A Hydrologic Outlook has been issued by the National Weather Service for heavy rain and snowmelt heightening flood potential across Southwest Washington and Northwest Oregon Tuesday and Wednesday. A wet and mild series of storms will move into the region next week. The signals have been consistent for a heavy rain event somewhere in the region in the Tuesday to Wednesday timeframe. Wherever the heaviest rain axis sets up, well over 5 inches of rain can be expected on the Coast and Coastal Mountains and 2-4 inches in the interior lowlands. Rainfall amounts in the Cascades are expected between 3 and 5 inches with over 5 inches possible in the South Washington Cascades. Since the arrival of the first storm is still 4 days away, the exact track of these storms, and the heaviest rain, is still uncertain.
In addition, snow levels will be rising Monday night, reaching near 7,000 feet Tuesday morning and 8,000 feet by Tuesday evening. This warmer air, combined with the rain, will lead to snowmelt. Expect significant amounts of snow in the Coast Range to melt Tuesday and Wednesday which will add runoff in addition to the heavy rain. Current snow water equivalent estimates place between 0.25″ and 1.0″ below 1,000 feet, 1.0 to 2.5″ between 1,000 and 2,000 feet, and over 3.0″ in the limited areas above 2,000 feet in the Coast Range. The snowmelt component to the runoff is more complicated in the Cascades. Snow water equivalent is generally 120% to 200% of normal, but with much above normal snow depths in some areas and the lack of a thawing cycle yet this season, snowpack will likely
be able to absorb a lot of rainfall, especially above 3,500 feet. This uncertainty about the runoff contribution from the snowpack leads to highly variable flooding potential for rivers draining the Cascades.
While snowmelt is a contributing factor to the flooding potential, the amount of heavy rain will remain most significant to flooding magnitude. The details of rainfall totals and expected flooding will be refined as these storms approach the Pacific Northwest, however confidence is increasing for flooding on area rivers, possibly approaching major flooding in some locations.
Be sure to follow Weather or Not’s Twitter feed to keep current on these developing flood conditions. You’ll get updated travel info and notification of any advisories, watches or warnings. Follow @chrisburnswx.