November Ends Mild & Wet, but Dry & Cold December Weather Ahead

November 29, 2018

1pm Thursday…

November has been a very cold month across much of the USA, but not west of the Rockies!  We are running a few notches above average in the Pacific Northwest.  

As you can probably guess, it’s been quite dry this month too; Portland has only seen 2.44″ rain so far, less than 1/2 of average.  We’ll see maybe 1/2″ rain at most Friday so November 2018 will go down in the record books as “warmer & drier than normal”

The reason is that we continue to see a weak/splitting jet stream or occasional upper-level ridging overhead.  This has been the case since at least early October.  Let’s go forward through the next week and you’ll see what I mean.  

Here’s the 300mb (around 30,000′ up) chart for today.  A classic “El Nino Winter” split flow.  A “bowling ball low” is sliding by to our south leaving us dry.

Northern/central California is getting a soaking.  All sorts of watches/warnings/advisories down there:

Tomorrow a weak system approaches from the west and moves directly over us Saturday.  This drops snow levels below the passes and brings cold showers to the lowlands of Western OR/WA the next two days.

Note the strongest windspeed is well to our south, so we don’t get a massive soaking or any stormy weather; just cold showers plus maybe 6-10″ snow in the Cascades.  By Monday an upper-level ridge is developing again…while another one of those “bowling balls” is getting ready to slide by to the south.

By Wednesday, California gets another wet (white in mountains) system, possibly aimed more for the southern part of the state.  Over us the upper-level ridging is holding strong, but centered well offshore.

This pattern screams UNUSUALLY DRY START TO DECEMBER IN THE PACIFIC NORTHWEST.  Thus the bold lettering!  The ECMWF has no precipitation over our region from Sunday night through next Friday night

But what happens down near sea level is likely what you’re really going to notice.  Behind that cool system coming in Saturday, a surge of cold/dry Canadian air drops south.  The ECMWF temperature departure (from normal) for next Tuesday through Tuesday the 11th shows very chilly weather across the interior West as surface high pressure traps the cold air in place.  The sun is too weak in December to heat up the cold valley/basin air east of the Cascades.  Inversion season…

The result is this surface map for Monday; cold high pressure has settled in for an extended stay east of the Cascades.  Chilly east wind is already blowing in the Gorge by that time…brrr!

By Wednesday it’s firmly entrenched.  Huge east wind through the Gorge; winter-strength gusts with highs only in the 30s (icy waterfalls?).  Temps peak only around 40 or so east metro and mid 40s for the rest of us.  That dry air will keep us clear so calm areas drop into the upper teens next week  (Battle Ground, Hillsboro, Scappoose).  It isn’t an arctic freeze coming, but you should have your irrigation systems drained by now anyway.

To summarize:

  1. Two days of showers ahead, possibly a few leftover Sunday
  2. 6-10″ snow will fall at ski resorts, but then it’s possible no new snow after that until at least NEXT weekend.
  3. Next week should be all or mainly dry
  4. This is the last of the warm 50+ degree weather, next week will be sharply colder and just plain frigid in the usual east wind areas
  5. That wind begins Sunday night
  6. IF MOISTURE RETURNS FROM THE SOUTH AT SOME POINT LATER NEXT WEEK? Cold air would assure freezing rain or snow in the Gorge and Eastern Oregon.  Of course a “perfect” lineup of moisture plus cold air could bring that threat into at least the eastern metro area.  I’ll be watching that VERY closely.
  7. There is no sign of a typical stormy weather pattern ahead; it’s possible we have begun a “drier & milder than normal” El Nino season.

Chief Meteorologist Mark Nelsen