The rare summertime Flood Watch is for all counties in Western Washington and is in effect from Thursday afternoon through Friday afternoon.
This is a pretty unique storm system for our region in that it is expected to combine a period of strong thunderstorms at first, then more of a steady Seattle November-esque rain second. When all is said and done, rainfall totals are expected to be greater than an inch across the region, with heavy thunderstorms bringing potential for at least that much in an hour.
We are still in a lull before the main storm tonight with just a few isolated showers or thunderstorms amid partly cloudy skies. Lows will be in the upper 50s.
Showers will begin building Thursday morning as a big area of low pressure currently swirling offshore begins its trek inland, pumping a whole lot of moisture into a very unstable atmosphere. The main events are expected to begin around midday Thursday with bands of heavy showers and thunderstorms moving across the region Thursday afternoon through Thursday night. As mentioned, each thunderstorm has potential to bring rainfall at the rate of about an inch per hour -- about on par with the drenching storms we saw last Thursday.
One the low passes by into Eastern Washington late Thursday night into Friday, the atmosphere will stabilize and the thunderstorm threat will end. However, moisture wrapping back around the low will provide another generally rainy day Friday -- this more the November-type steady rains than the convective thunderstorms of Thursday. Rain will slowly decrease through the afternoon and evening as the low pulls further away.
All told by then, rainfall totals are expected to be about 1-2" in the lowlands as a general rule, but any place that gets picked on with roaming thunderstorms might eke out a little more. The Cascade foothills could be more in the 1.5-3.0" range and the mountains themselves could get 2-4".
This is enough rain that urban and small stream flooding is possible -- especially in areas where strong thunderstorms drop a big deluge in a short period. Mud and or rock slides are also possible in steep mountain terrain.
The National Weather Service does say that while mountain rivers will run high, traditional autumn-type river flooding is unlikely.
The weather rapidly improves in the storm's wake with just a few lingering showers early Saturday morning giving way to increasing sunshine and warmth with highs getting into the upper 70s.
By Sunday, we begin a new stretch of more summer-like weather with plenty of sunshine and highs near 80. That sunny and warm pattern is expected to last into early next week.