Tuesday is “Transition Day”; briefly icy weather ahead!

December 8, 2013

6pm Sunday…

In the Portland metro area we sometimes get a great (or horrendous) transition from a cold arctic airmass back to our usual wet and mild weather.

This won’t be one of those times.  That said, most of us in the western valleys north of Salem are going to get a little snow and/or freezing rain/drizzle Monday night and Tuesday morning.

First, the bitter and historic cold temps this morning for some parts of our region

In the metro area, in general, today saw the coldest weather since December 2009.  We had a low of 12 and high of 28 at PDX.  We haven’t seen temps that cold in 4 years.  For a few spots around town it hasn’t been this cold since 1998.

Down in the southern Willamette Valley and in some areas east of the Cascades this has been a truly historic event:

  • Eugene hit -10, 2nd coldest temperature of all time
  • Redmond hit -27, 2nd coldest temp of all time.
  • Lakeview had an all-time low of -27.
  • Burns also had an all-time low of -30.
  • 850mb temperature at Salem was -14.1 degrees Saturday morning, which makes this the coldest mid-level air mass that has moved over us since 1998.  It just barely squeaked past 2009.  Thanks to Chris Callais for that info.

Here are the highlights for those just wanting the basic forecast info:

  • Most areas will be dry tonight, not as extremely cold, but still cold
  • Mainly or all dry Monday west of the Cascades.  Skies will cloud up and a flurry is possible, but nothing that would affect traffic from Longview to Eugene through sunset.
  • Increasing areas of light snow OR freezing rain/drizzle after sunset Monday night through midday Tuesday
  • Temperatures won’t rise above freezing until midday Tuesday, so whatever falls during that time will be frozen.
  • Not much moisture, so just a Trace to 1″ snow, or light glazing of freezing rain/drizzle.  Many areas from the middle of the metro area south to Salem may see little or nothing, we’re talking real light stuff.
  • Issues on roads, where frozen precip falls, will be sometime after sunset Monday to midday Tuesday.  Before and after that time it looks clear.
  • There is little or no chance this turns into a prolonged or heavy freezing rain or snow event.  We get very little of anything, then we’ll be dry from midday Tuesday through Wednesday night, and temps warm well above freezing.

DETAILS

It’s funny that the perception continues to be that each time we plunge into an arctic air mass  “there’s no way we’re getting out of this without an epic event“.  Or something along those lines.  I think it’s because we remember the “epic” events for good reason!  January 5-8, 2004, December 21, 2008, February 1996, December 25, 1983.  But what about more recent arctic blast events.

  • November 24, 2010:  Just some spots of light freezing drizzle here and there and then a warm up after a high of only 30 degrees on a sunny day.
  • December 12, 2009:  Trace of freezing rain, then 41 the next day
  • December 17, 2008:  Gusty south wind took us from 23 in the morning to 38 in the afternoon with little/no snow accumulation, then mixed showers.   The big snowstorm was after the 2nd surge of arctic air 3 days later.
  • December 24, 1998:  2″ snow in the morning, then wet roads by sunset

I think this will be one of those times too for these two  reasons

  1. This is the big one, no cold east wind pouring out of the Gorge.  All of our significant transition events involve cold and dry air getting reinforced by the freezer door at Troutdale.  Not this time.  Atmosphere can easily (but slowly) warm up with light onshore flow, especially once cloud cover brings nighttime lows up closer to freezing.  That’ll happen Monday night.
  2. Extremely light precipitation to moisten up the air mass, once that happens forget about evaporational cooling helping us beyond Tuesday.

There is no “storm” or even organized weather disturbance moving through our area later Monday through Tuesday, just mid-level flow turning onshore, moistening things up and riding over the cold low-level air mass.   Notice how light the precipitation will be:

precip_wrf

Very little in the middle of the metro area and almost nothing south of us.  Snowfall off that same model is about as pathetic.

snow_wrf

No more than 1″ anywhere except maybe in some foothill locations.  Clark and Cowlitz counties are probably the place to see.  You may notice there is more precipitation than the snowfall coverage shows even though temps are below freezing.   Check out the WRF-GFS sounding, hinting that it may rise slightly above freezing around 2,000-3,000′.  Good setup for freezing rain or drizzle.  Our RPM is hinting at this, showing an area of freezing rain (pink) on the south side of the light precipitation.

rpm_ice

Here’s a great explainer from the Seattle NWS from today’s discussion:

HERE IS A QUICK WEATHER LESSON. IT IS POSSIBLE TO GET LIQUID FALLING FROM THE SKY…EVEN WHEN THE ENTIRE ATMOSPHERIC SOUNDING IS BELOW FREEZING. THE KEY IS THAT PRECIP HAS TO BE GENERATED IN A LAYER THAT IS -5C OR WARMER…IN WHICH CASE SUPERCOOLED WATER DROPLETS ARE FORMED… RATHER THAN SNOWFLAKES. WHEN THESE CONTACT A SURFACE THAT IS BELOW FREEZING…IT WILL FREEZE UPON CONTACT AND BE OBSERVED AS FREEZING DRIZZLE OR FREEZING RAIN. THIS BECOMES A CONCERN IN A SETUP LIKE MON AND TUE…WHERE PRECIP IS BEING GENERATED AT FAIRLY LOW AND RELATIVELY WARM LAYERS OF THE ATMOSPHERE…EVEN IF THOSE LAYERS ARE A LITTLE BELOW FREEZING.

Gotta love those All Caps!

Beyond Tuesday, we’re in the clear here in the metro area as temps warm. There could be a 2nd transition event possibly in the Gorge Wednesday night or Thursday as real moisture arrives with a much stronger front. By that time we’ll be in the 35-45 degree range here in the valleys so ice/snow won’t be an issue.

To wrap it up: We’re going to see a little frozen precipitation at some point tomorrow evening through Tuesday morning that could impact travel during that time, then the weather gets boring for awhile.

Chief Meteorologist Mark Nelsen


Sunday Morning: Historic Cold Temps

December 8, 2013

I just looked at the temperatures across the Pacific Northwest.  Some amazing and historic cold out there!  Here are the preliminary numbers for west of the Cascades, some will change after 10am when the final lows come in:

Image

Eugene appears to have had either their 2nd coldest temp or coldest temperature ever.  We’ll know at 10am when then 6 hour temp comes out.  As of 4am they hit -10!!!  That’s 10 BELOW zero.  The coldest temperature ever recorded in Eugene was a -12 under similar conditions on this date.  Simply amazing that it occurred again, under almost the same conditions, 41 years later on the same day.Image

Here in the metro area, without snow cover for most of us, we generally dropped into the 5-15 degree range.  Portland only dropped to 12 degrees, which was equalled in 2009, so just the coldest in 4 years here.  The numbers (so far) for the metro area:

What about Central Oregon…wow!  Redmond has seen their 2nd coldest night on record…-27 this morning.  The coldest ever recorded there (since 1948) was a -28.  Do your cars still start over there?  Here are the preliminary numbers for eastside:

Image

I’ll do an update later today on the upcoming “transition event” we have on the way Monday night and Tuesday morning.  Not exactly spectacular, but it will likely affect the morning commute Tuesday.

Chief Meteorologist Mark Nelsen