Wet Start This Week, But A Warm & Sunny End

October 7, 2018

7pm Sunday…

For those of you who wondered “Will the hot weather EVER end?”  Here you go…

Live Cam Portland

Cool fall weather along with clouds and rain ALWAYS comes back in our climate…always.  Even in a slowly warming climate the clouds and showers will continue to come back.  Yes, some autumns are warmer than others, but hot weather doesn’t “go on until November” as one viewer worried in a summertime email I received during a hot spell.

Why so gloomy today (and much of yesterday?).  There is a large upper-level high blocking all storminess to our west

Jet Stream Forecast 2017

The big ridge of high pressure is just far enough west that very weak disturbances with clouds & light rain are moving down the backside of the ridge.   Basically “the back door is open” and won’t close until Tuesday PM.  At that point the ridge starts pushing closer and eventually ends up almost right on the coastline by the end of the week.  Here’s Saturday:

Jet Stream Forecast 2017b

Then it appears to linger over us for a few more days.  Models are in excellent agreement on this scenario.  A few key points:

  1. Gray & cool temps continues through Tuesday, although by Tuesday afternoon we should see a few sunbreaks
  2. Rain picks up a bit tonight and Monday, but will be most noticeable east metro and near hills/mountains
  3. Rain ends all areas by Tuesday evening with no rain expected again until at least Tuesday the 16th (at the earliest).  For agricultural folks: you have at least a 5-7 day window of dry and mainly sunny weather beginning Wednesday
  4. Offshore flow later Wednesday through much of the following 5 days = bright blue skies with abundant sunshine and warming afternoon temperatures
  5. Next weekend MAY be spectacular for outdoor activities


Notice the RPM rain forecast for the next 3 days (most of it is done by Tuesday PM) shows a very strong orographic rain event.  That means northwest/westerly flow runs into the north/south oriented mountain ranges, rises, and dumps a load of rain on the windward slopes.  On the lee side of the Coast Range there won’t be much more than sprinkles or a light shower the next two days (Washington & Yamhill counties).  But in the “upslope” areas on the east side of the Willamette Valley and into the west slopes of the Cascades you can expect widespread rainfall much of the next 48 hours.

RPM Precipitation Accumulation

Note the ECMWF ensemble forecast for 24 hour precipitation shows a huge gap (almost no ensemble members expect rain) from Tuesday night through about the 16th as the ridge moves overhead.

KPDX_2018100712_eps24_precip_360 (1)

although many show a wet pattern resuming soon after that time.  The GEFS is similar with most model members showing a return of rain the middle/latter part of NEXT week as the ridge pushes farther east.


So enjoy the gray, and then get ready to enjoy warmer sunshine later this week; quite a change ahead.

Chief Meteorologist Mark Nelsen