Enjoy Tuesday’s Sunshine! Rainy Season Arrives Late Week

October 18, 2021

8pm Monday…

October has been a cool but somewhat “dry-ish” month so far in Oregon and SW Washington. Just under an inch of rain in Portland and far less in Eastern Oregon. But we still have 13 days to go this month and a lot can change the 2nd half of October. The PACNW rainy season often arrives during this last half of October and it appears that WILL be the case this year.

First, enjoy your Tuesday because we have a spectacular day on tap. Offshore (easterly) surface flow plus just a few high clouds should allow temperatures to “soar” into the mid-upper 60s. We are in “peak color” in the metro area, so the combo of sunshine and yellows/oranges/reds shouldn’t disappoint. This will be the last guaranteed dry day.

A cool upper-level trough swings through tomorrow night and early Wednesday along with an associated weak front. Expect a bit of rain during this time, but less than 1/4″. Things get more interesting Wednesday/Thursday as a “bomb cyclone” develops out in the Eastern Pacific. A bomb cyclone is a rapidly deepening area of low pressure that deepens more than 24 millibars in 24 hours. This happens several times each cool season in the Pacific, but this one is a bit closer to our region. Who can forget the Thanksgiving Week bomb cyclone that slammed into SW Oregon and N. California two years ago? That one dropped about 40 millibars in 24 hours; similar forecast with this one. Enjoy the memories of closed highways and damaging wind here: https://fox12weather.wordpress.com/2019/11/24/a-bomb-cyclone-southern-oregon-snow-storm-coldest-thanksgiving-weekend-in-years/

Here’s the 11pm Tuesday forecast from the ECWMF (European) model showing a developing low north of Hawaii. About 998 millibar central pressure. Surface pressure is the line contours, colors are upper level heights

Then just 24 hours later…11pm Wednesday. A 954 mb low about 600 miles west of the Oregon coastline. That’s a 42 millibar drop in 24 hours. This is similar to the Columbus Day Storm low pressure center. Except that storm shot south to north quickly up the coastline just 50 miles offshore!

As you may suspect, a storm of this magnitude produces strong wind and a large wind field. This graphic shows the highest wind gust forecast between now and Thursday afternoon. Some gusts around 90 mph over the open ocean, but it’s so far away that gusts 30-50 mph are the most we can expect along the Oregon coast.

Beyond this big storm offshore, a strong westerly jet sets up Saturday through much of next week. It aims first at California, then shifts a bit farther north next week.

This should give round after round of valley rain and mountain snows. This can be a pattern that leads to a windstorm. That’s IF a surface low develops on the north side of the jet and IF it comes up the coastline just right. Here’s a neat tool that allows me to quickly look at all ensemble members from one model. Each colored box and number is a peak gust for the preceding 6 hour period. Each horizontal line is one of the 30 GFS ensemble members. A quick glance shows not too many greens/yellows/reds, but there are a few. As we get closer to a possible event I would expect more to show up in a vertical line, like the gusty southerly wind that shows up midday Wednesday. Of course this is for Portland.

How much rain is ahead? Quite a bit. I see 2-4″ in the next 10 days. Just through next Monday, the ECMWF gives us about 2″ in the western valleys

And fire season will be ending very soon in the northern half of California. 5-10″ in the mountains down there during the same time period

KEY POINTS

  • Tomorrow is your last guaranteed dry day for quite awhile, for at least a week
  • Rain showers both Wednesday and Thursday will not be heavy, although it’ll be breezy Wednesday
  • A very wet weather regime arrives Friday and continues through next week, plan on a wet weekend, but there will be breaks in the rain, not all-day long soakers.
  • At this point I don’t see a windstorm or flooding setup, but will be watching closely since this is the right pattern for something to develop

Chief Meteorologist Mark Nelsen