Freezing Rain Update: Looks A Bit Less Likely

December 11, 2013

9pm Wednesday…

I think it’s very unlikely we see anything other than a spot or two of ice in a valley out near the Coast Range late tomorrow morning.  The chance for a metro-wide glazing of ice?  About 10%

Look at the high temps for today; warmest in 8 days here in Portland:

PLOT_Highs_Metro

You can see where the mild east wind cut through the middle of the metro area (at least above the surface) and helped to clear the fog out.  Anywhere north or south of us stayed near or below freezing.

This doesn’t affect tomorrow’s weather at all because now with the clear sky temps have dropped below freezing again; back to the usual freezing fog programming overnight and tomorrow morning.

I don’t have any new thoughts on tomorrow’s chance for freezing rain west of the Cascades, EXCEPT that this evening’s models are slightly slower with the rain tomorrow.  That’s very good news if you don’t want ice on roads.  Our RPM precip forecast at 11am shows the edge of rain just coming down off the Coast Range into the farthest western portions of the metro area:

rpm_11am

and by 1pm it has spread to about Camas/Gresham.

rpm_1pm

The 00z WRF-GFS is even slower, holding off precip for most of us by 1pm.  I’ve never seen freezing rain start DURING THE DAY (after 9am) unless the temperature is real cold…like 25 or so.  Not happening this time around.

One more thing…

Air temperature doesn’t equal road surface temperature.  Even on a cloudy winter day the road surface is going to be warmer than the air temp, often significantly warmer.   Take a look at the Rocky Pt. ODOT sensor along Highway 30 in the fog and clouds today.  http://www.wrh.noaa.gov/mesowest/getobext.php?sid=ODT95&table=1&banner=off

At 5am, air temp=27, road temp=28.

9am, air temp=28, road=33

11am, air temp=30, road=39

In this case cold rain might lower the road temp a notch or two, but today you wouldn’t have seen freezing rain glazing over the road after 8am or so.  At the Enchanted Way sensor south of Salem along I-5 it was 28 at 11am and the road temp was 37.   Same thing.

As a result, I don’t believe we’ll have anything other than a spot or two of ice in a valley out near the Coast Range tomorrow morning.

Chief Meteorologist Mark Nelsen

 


Will We Get Freezing Rain Thursday? It’ll Be Close!

December 11, 2013

12:30 pm Wednesday…

junk

Here are the highlights…

WE ARE CONFIDENT WITH THIS:

  • Most of us will see regular rain arriving between 11am and 1pm tomorrow.
  • Anyone above 1,000′ will definitely see rain only.  It’s warmer higher up.
  • It only takes 5 minutes of freezing rain to turn a dry road into an ice rink.
  • The morning commute should be dry.  But spots of freezing fog again where skies clear this evening and fog redevelops.
  • The evening commute should be fine, with rain showers and temps 35-40.
  • The central/eastern Gorge won’t thaw until evening, that includes anywhere east of Multnomah Falls.

TIMING IS EVERYTHING TOMORROW:

  • To get freezing rain to “freeze” on pavement, the air temperature needs to be about 28-30 degrees or lower during the daytime.  Even at night, to get roads iced over, it needs to be down around 30-31.  Most spots will be at that temp or warmer at 11am-1pm, thus the reason I think most of us west of the Cascades will not see traffic issues.
  • IF THE RAIN ARRIVES BEFORE 10 AM, MORE WIDESPREAD FREEZING RAIN IS POSSIBLE!  Tomorrow is definitely a “watch the radar” day.
  • If the rain waits until noon-1pm, none of us westside should get freezing rain.

So what has changed since I posted about this event yesterday? 

Not much, but I’m being a bit more cautious.  It IS warming up dramatically above 1,500′ today.   I see some 50 degree temps in the foothills of the Cascades and the KPTV Tower temp keeps rising.  Thanks to Jim Little for this graph:

kptv_tower_temps

In the lowest elevations we had a round of freezing fog last night with the cold air trapped below an inversion; a common occurrence in wintertime here.  I was really surprised a few districts in the metro area cancelled classes based on freezing fog!  I’ve never seen that.  Both Portland and Vancouver did so, although none did in Washington County or Clackamas County.  And Portland was the only district to close in Multnomah County.

Skies are clearing in some spots, mainly where some east wind from the Gorge is drying things out.  Just a little mixing going on, I’m seeing temps getting close to 40 in parts of Washington County and near the Gorge.  Other than those “warm” temps, no changes through this evening.  It’ll be another mainly clear and cold night with spots of freezing fog developing.  Areas that don’t clear out today will just sit under low clouds/fog tonight.

So obviously that sets us up for possible freezing rain Thursday morning if the precipitation arrives early enough.  Models are still insistent on nothing before 10-11am.  They have been very consistent for quite a few runs.  That includes the new 18z NAM and 18z RPM.  There won’t be ANY mild mixing wind before the precipitation arrives, so that won’t help at all.

As a result it comes down to an epic race between the rising temperature and the incoming rain tomorrow morning/midday.  The best chance for freezing rain may be the calm pockets in Washington, Columbia, and Clark counties.  Areas near the Columbia River Gorge in the metro area might have the LEAST chance because of the mixing east wind going tonight keeping temps up.  Just a guess.  Our RPM even shows a few spots in western Washington County as the precip shows up around 10am:

rpm_ice

The east wind in the Gorge will be dying quickly tomorrow, so I think it’s most likely we’ll only see freezing rain east of Multnomah Falls, assuming the rain waits until 11am or later to show up.  It’s possible the freeway will be mainly clear if temps rise above 30.  But I do not expect a widespread and dramatic warming during the afternoon, thus some ice accumulation is likely at Stevenson, Hood River, and maybe The Dalles.

Chief Meteorologist Mark Nelsen