Crazy Weather Week Ahead

November 9, 2014

Fall 2014 has been the warmest ever so far…but that’s about to change in a big way.  We’re less than 2 days away from wind and temperatures we would typically see in mid-winter.

Models haven’t backed off a bit in the past two days.  The Pacific Northwest will get a chunk of the unusually cold (for November) arctic air mass diving into the USA this week.  We’ll be on the “left” (no surprise?) or west side of a truly massive surface high pressure area sliding down into Montana. MarkColdThisWeek_ThePlan With a huge area of high pressure to our east, the wind will be rushing out to the west, over the Cascades, through the Gorge, and down into the western lowlands of Oregon and Washington.

SUN

Enjoy the next 3 days of bright sunshine!  It’ll be a very dry air mass coming in from the northeast…the driest we’ve seen since last winter.

WIND

The strong wind arrives overnight Monday night and into Tuesday morning.  For most of the metro area, the peak wind will be during the day Tuesday.  In the Gorge the wind will likely be strongest Wednesday night as a weather system approaches and the wind zone gets scrunched down into a smaller area close in to the Gorge.

MarkEastWind_DownslopeToo MarkEastWind_DownslopeToo2

Those speeds will be strong enough to cause scattered power outages and down some trees.  The air mass will be incredibly dry since it came out of the arctic, so leaves will blow around quite easily.  Models have been unusually strong with the low-level wind field, showing 50-60kt wind just a short distance above the surface in the Portland metro area. kpdx.th There is an outside chance we get a mountain wave developing in the lee of the Cascades over Clark County, but generally that has occurred in a slightly different setup in the past…different wind direction higher up in the atmosphere.  I actually found the maps from Dec 4, 2003 and Dec 12, 2004 when 60 mph gusts hit Hockinson and Battle Ground areas, plus some damage around Mt. Tabor too.  Those maps were sitting in a file folder, old style!  A nice dinner break perusing 11 year old cross-sections!

FREEZING WEATHER

– Prepare for mild freezing west of the Cascades (25-30 degrees), but not cold enough to freeze pipes.  Lows in the windy parts of the metro area will hardly get below freezing.  I could see PDX not even dropping to freezing the next 4 nights.  Any spot that goes calm Tuesday night could drop well down into the 20s, but I think those will be very isolated areas.  This is not a major freeze west of the Cascades, but it will be the first frost for many of us…we’re overdue for that this fall.

WHAT ABOUT SNOW OR FREEZING RAIN?

– Just about everyone west of the Cascades (maybe everyone) can FORGET about snow when moisture returns Wednesday night.  The air mass won’t be cold enough through a deep enough layer to support that.  There is a slight chance it starts as snow up against the east slopes of the Coast Range…we’ll take a closer look at that over the next few days.

– Freezing rain and snow is looking very likely east of Troutdale/Washougal in the Gorge beginning Wednesday night.  Where the cold air is thinner (Corbett, east of Washougal etc…) freezing rain is a good bet.  I already told my kids there’s a good chance they have an “ice day” Thursday out there.  Here is the latest WRF-GFS precipitation forecast ending Thursday morning; enough for 2-6″ snow in the Gorge if this is the case.  or_pcp72.84.0000

– It should be slightly too warm for freezing rain in the metro area, but as of tonight it’s still a close call.  Hopefully we can rule that out by Wednesday morning when the air mass is in place.  That “no frz rain” forecast is based on past events and on the assumption that models are bringing too much cold air over the Rockies and through the Gorge.  The latest 00z GFS agrees with this; you can see it’s forecast of each precipitation type.  gfs_ptype_accum_portland_17 Freezing rain or snow is not shown here in the valleys.

Chief Meteorologist Mark Nelsen