Rain Showers Then Clearing = Icy Roads!

January 5, 2016

10pm Tuesday…

The ice all melted today, except in spots at the top of the hills and in the Gorge.  That was great…life has resumed.

But something we see from time to time in the winter means we may have more school delays and icy roads for a different reason.

A band of showers passed over the metro area around 8-9pm.  Now skies are clearing behind it.  Temperatures are dropping quickly:

PLOT_Temps_Metro

Check out the dropping road surface temps as of 9:45pm:

MarkODOT_RoadTemps

Not good.  This is a classic situation of rain showers and then roads that freeze during the night.

It’s quite possible we get school delays in the morning due to the icy roads.  Click for a larger image.

Stay safe if you are out driving between now and morning!


The Slow Thaw Continues

January 5, 2016

Whew!  Close call, but temperatures rose after midnight sparing us from an ugly mess this morning.  Clouds returned and as expected most of the metro area saw only regular rain this morning.

  • There was generally no refreezing of wet roads for the morning commute.
  • Roads will only get better today as temperatures rise into the upper 30s, a bit warmer than yesterday.

I see lots of schools with wet roads are being extra conservative with delays or closures; that may be due to leftover ice on sidewalks?

Of course in the West Hills/Mt. Scott areas and near the Gorge where temperatures were slightly cooler and ice is already on the ground, it didn’t matter…still icy roads for a 2nd morning.

The world’s slowest thaw continues today.  But we are running 3 degrees warmer this morning at PDX compared to 9am yesterday.  Two things are going on.

 1. The cold airmass about 3,000′ thick in the Eastern Gorge and Columbia Basin continues to slowly moderate.  It sure isn’t going to suddenly warm up, but it’s moderating.  The Dalles is 5 degrees warmer than 9am yesterday, Troutdale is 2 warmer.  Baby steps… You can see it on the Troutdale profiler too:

Capture

Time runs from RIGHT to LEFT.  Note the slowly warming colors as you move left on the graph and also note that at Troutdale the cold airmass pouring out of the Gorge is very shallow, less than 1,500′ thick.  It’s in the 40s above that elevation.  This low level cold airmass coming in from Eastern Oregon isn’t going to disappear, but it’s becoming a “little less cold”.

2. The leftover cold air in the valleys west of the Cascades that is “detached” from the cold Gorge wind will continue to slowly warm up as well.  No or little/weak sun this time of year sure doesn’t do us any good for warming, that’s why it’s so slow.  As a result we’ll see temperatures only rise into the upper 30s in the valley.  If somehow the warmer air overhead could mix down to the surface where we all live (it wont), the valley would jump up to around 50 degrees today.

Moving On…

Models are having a tough time with timing/amount with one last surge of showers and/or light rain moving north from California tomorrow.  It might arrive during the AM commute OR later in the day.  Or it may hardly make it here and just be a few sprinkles.  My one concern would be that we could see clearing tonight and colder temps than forecast (like the first half of last night).  It’s something to keep an eye on.  As of this moment I don’t see another round of freezing rain for the Wednesday AM commute except possibly in the Gorge.

It is pretty clear that with cold air still coming out of the Gorge the next 5-7 days we’ll be close to a frozen precipitation event again IF moisture decides to show up.  At this point it looks mainly or all dry though through early next week.

Chief Meteorologist Mark Nelsen