Sunny Sunday to Heavy Rain??? How Did THAT happen?

October 8, 2016

5pm Saturday…

No one has asked me yet, but lots of you must wondering…

How in the heck does the forecast of mostly sunny and low-mid 70s on Sunday go to rain, possibly heavy at times, and a high around 60???

TOTALLY REASONABLE QUESTION!

Here’s what happened:

  1. The “plan” for this weekend has been for a few leftover showers this morning to lift north and then an “atmospheric river” of moisture sets up the rest of the weekend over northern Washington and British Columbia.  We’ve known for days that a big wet weekend event was setting up just to our north in the northern half of Washington.
  2. In the past 24 hours it has become increasingly obvious that Sunday is going to be wet, and as of last night’s models…very wet, because…
  3. The whole band of rain is shifting about 200 miles farther south than expected.  That’s due to a stronger northerly jet stream pushing the whole event south.  It’s all getting pushed south right over us and ruining our Sunday.
  4. What’s 200 miles among friends…right?

RAIN HIGHLIGHTS FOR REGULAR FOLKS WITHOUT THE TECHNICAL INFO:

  1. It will likely rain all day Sunday in NW Oregon and SW Washington.  A steady rain much of the day.
  2. Wind is not an issue.  It will be LIGHT after 8am or so from the north
  3. Heavy rain is possible at times from midday through the evening from the metro area south into the Willamette Valley.  Total rain should be in the 1.00-2.00″ range.  Wettest since January!
  4. There COULD be areas of localized flooding if the rain is heavy enough for several hours.  At this point the event looks less intense than the one last October that flooded MAX.
  5. Some models are giving us .20″ to .30″ for a few hours somewhere between Portland and Albany during that time.  With leaves blocking drainage, that could contribute to flooding issues IF the rain is heavy enough…it may not be.
  6. The front moves south and we’ll be all dry Monday.  With dry east wind Tuesday looks spectacular with blue skies and leaves changing color under the bright October sun.

 

For the geeks…

Check out the precipitable water forecast for midday tomorrow:

gfs_pwat_west3_5

Looks juicy with 1.5″ or higher!  Lots of subtropical moisture to work with as a cold front drags south across our area Sunday.  The problem with forecasting tomorrow is that each model is a bit different on where the heaviest rain falls and final totals too.  Of course we all know the heaviest rain will be in the Coast Range and Cascades, but I mean north and south.  Does the Salem to Wilsonville area get a heavy rain band early afternoon as the WRF-GFS shows?  That’s .60″ in 3 hours.  I’ve noticed we start having urban flooding issues with several hours of .30″ so even this is at the lower end.

wrf_2pmsun_3hrprecip

The WRF-GFS has these totals for the entire event…which are pretty reasonable.  That’s maybe an inch to two inches for many of us in the valley, but less for others:

wrf_rain_tuesam

But the RPM (definitely not always a stellar performer) has it right over PDX with several hours of .30″ intensity in the evening hours:

rpm_rainhighlighted

After almost an inch during the day, it then dumps another 1″ in the next 4 hours!  I bet that is overdone.  But the GFS is extremely wet too, showing 2-3″ over the metro area.gfs_precip_mondayam

The ECMWF is more reasonable than the GFS…

ecmwf_precip_monam

Giving us 1 to 2″ in the metro area but lighter totals (more like 1/2″) down around Salem.

I feel confident saying somebody SOMEWHERE in the lowlands is going to really get dumped on tomorrow, but we don’t know exactly where.  Stay Tuned.

By the way, have you hardcore weather folks noticed this is a pattern that would give us a surprise modified arctic blast in winter followed by freezing rain or snow?  Check out the 48 hour old forecast of 500mb for Sunday PM:

ecmwf_500mbthuram

I’ve added the yellow line to show the main flow.  A cool upper-level trough was forecast to stay just to our north (thus the rain forecast up in northern Washington).  Now check out the forecast for tomorrow afternoon but made this morning:

ecmwf_current500mb

Much better!  In January we’d be real excited about a push of arctic air coming in, or at least just scraping by to the north.  In this October case, that means much drier air for clearing Monday afternoon and then a breezy “coolish” east wind Tuesday for a spectacular sunny fall day.  Then moisture returns with brief offshore flow later Wednesday and Thursday.

As for late next week, it looks rainy and windy…our first real stormy pattern of the season and it’s arriving early.  More on that tomorrow when I’m back to work.

Chief Meteorologist Mark Nelsen