A storm system currently swirling offshore will slowly move inland on Thursday, bringing more rounds of heavy rain showers and thunderstorms that could once again cause some urban flooding issues.
The storm already has a rainy pedigree -- it was what brought some showers and thunderstorms early Tuesday morning that dropped 1 inch of rain in Thurston and Lewis Counties. While we were in a relative lull late Tuesday, a few isolated showers and thunderstorms were possible on Wednesday amid partly sunny skies.
But it's Thursday when the weather is expected to get quite nasty. The low pressure area offshore is expected to slowly drift across the area Thursday into Friday, generating a very wet two days on tap.
First up will be the more intense thunderstorm cells Thursday into Thursday night as the low approaches and the air mass becomes quite unstable, while Friday is expected to be more of a steadier, autumn-esque widespread rainy day as we get on the backside of the storm and the atmosphere stabilizes.
The Thursday thunderstorms have potential to bring as much as 1" of rain per hour -- similar to what happened in Olympia and around Everett/south Snohomish County on Thursday (and double about what fell in the Seattle area). That would be enough to overwhelm storm drains and cause areas of urban flooding.
Once the low passes by into Eastern Washington on Friday, rain will wrap back around the low from the north/northeast. This is a more stable pattern that should be thunderstorm-free but instead be more of a steady nearly-all-day rain.
Forecast models show overall rain totals from Thursday morning to Saturday morning at a solid 1-2" in the Puget Sound area and much of the rest of the Western Washington lowlands, with perhaps 2-3" in the mountain foothills and 4-5" in the Cascades.
Conditions rapidly improve Friday night to where we are still looking at a rather pleasant weekend. Both Saturday and Sunday should be partly to mostly sunny with highs in the upper 70s and sunshine is expected to continue into early next week with highs around 80.