A Soaker on Friday

So far this autumn, most of our precipitation has come in the form of scattered showers as opposed to stratiform rain. That isn’t to say that these showers haven’t been heavy at times… I was biking back home from work on Wednesday, September 12 this past month and got stuck in lock-step with a very annoying hailstorm with some lightning to boot. But we’ve yet to see the steady, heavy rains that herald the beginning of storm season for us.

That will change on Friday. A relatively weak but very wet system will approach the area from the WNW, spreading steady, heavy rain over Oregon and SW Washington. Light rain will begin to fall Friday morning and will become heavier as we transition into the afternoon, and we should switch to showers late Friday night after this system passes through. Expect these showers to persist through the first half of Saturday morning, and we should be dry by noon. At this point, I’m expecting up 0.5-1 inches of rain in the Portland metro area from Friday morning through Saturday noon and around 0.25 inches for the Seattle area. Friday will be pretty darn chilly as well, with highs only in mid 50s for both Portland and Seattle.

Here are some images that show sea-level-pressure and total accumulated precipitation for the previous 6 hours. Note how the low tracks just to the south of Portland… if there was already an arctic airmass in place, this would be a fantastic pattern for heavy snow.

This system will have tapped into some moisture from Hurricane Walaka currently around 800 miles SW of Honolulu, which is being steered northward towards by counterclockwise flow around a cutoff low to its north and clockwise flow around a ridge to its NE.

Things always get exciting when tropical moisture finds its way into the mid-latitudes. Even though this storm won’t be particularly strong and October in general is still relatively calm compared to November/December, the tropics are extremely active at this time of the year, and when deep tropical moisture is injected into the midlatitudes, tremendous storms can result. The Columbus Day Storm of 1962 is the classic example of this and sprung out of Typhoon Freda, and the under-performing “Ides of October” storm/forecast bust was formed from the remnants of Supertyphoon Songda.

I was able to sneak a nap in this afternoon but it’s past my bedtime now, so I’m gonna turn in. To summarize, Wednesday and Thursday should be dry but cool with temperatures in the upper and mid 60s respectively for the Portland metro area, and the heavens will open up on Friday and we’ll have our first widespread heavy-ish rain event of the year for the lowlands.

Thanks for reading!
Chuck