From Justin Shaw of Seattleweatherblog.com
SEATTLE — Another Thursday, another deluge.
A potent area of low pressure blasted through the region on a southwest-to-northeast track, triggering strong thunderstorms and heavy rainfall as it marches from the beaches of southwest Oregon to the deserts of Eastern Washington. One to 2 inches of rain is possible across the entire Seattle metro area from noon through late Friday afternoon—an amount that would blow last Thursday’s 0.76-inch soaking out of the water.
The action got underway Thursday morning as the area of low pressure, located in the upper atmosphere, approached the south Oregon coast. As it did, the upper level winds flipped to the southeast, dragging warmer, unstable air up the spine of the Cascades and into Western Washington. This should spark a round of afternoon thunderstorms across all of Puget Sound, complete with driving rain and wind.
As the low barrels northward into Washington Thursday, the thunder and lightning should die down, but the rain won’t be going quietly into the night. Instead, moderate to heavy rain will increase in intensity as darkness sets in, lashing the Sound from late Thursday through early Friday. This, coupled with the anticipated downpours tomorrow afternoon, has led the National Weather Service to issue a flood watch for the entire I-5 corridor from noon tomorrow through 6 p.m. Friday. Minor urban flooding is possible during this time frame.
With the expected drenching, Seattle is almost certain to obliterate the daily rainfall record for Sept.5 —a measly 0.36 inches from 1984. Friday’s mark of 0.93 inches, while substantially higher, is also in danger. Of even greater interest is the September monthly average of 1.50 inches. If current weather models are correct, Seattle stands a decent chance of recording an entire month’s worth of rain in just two days.
The storm will slowly taper off as the low pulls further east on Friday, leaving plenty of clouds behind. Highs will struggle to reach 70 degrees under the drippy, overcast skies.
Fortunately, high pressure builds in quickly over the weekend as we go from partly cloudy and the low 70s on Saturday to mostly sunny and near 80 by Sunday. Sunny, warm weather then looks to hold through much of next week.
Things have to get worse before they get better anyway, right?