Now the Freezing Rain…11pm

December 18, 2005

A quick changeover this evening to freezing rain.  I noticed a sensor at 2,000′ east of Silverton went from 24 to 43 in just a couple hours this evening.  As I mentioned on the 10-11pm shows, temperatures are a few degrees warmer than the ice storm last January, so now I’m thinking the surge due in at daybreak will be it.  Although models say it could contain up to .50" rain…that’s a lot of ice!  After that we should gradually warm, but of course the Gorge (east of Troutdale) should remain near or below freezing through later Tuesday.  There is no good westerly, warming wind through the Gorge itself until a system passes by to the north Thursday.

By the way, I made brief reference to "evaporative cooling" on the show, although I didn’t use those words.  Here’s a better explanation:  As rain or snow falls into a dry airmass, some of that moisture evaporates.   But to change a liquid back into a gas (water vapor), it takes heat, which comes out of the air.  So basically the temperature drops until it meets the dewpoint and the dewpoint rises too.  If the temperature is 35 and the dewpoint is 20…when precipitation falls through it, the temp may drop to 31 and the dewpoint comes up to 30.  That’s called evaporative cooling.  It happened when the snow fell today and should happen again when the heavier rain arrives near daybreak (or else the forecast busts and we get mainly plain rain).

It Has Begun…5pm

December 18, 2005

Well, now we know when the storm will arrive…about 2 hours earlier than expected.   Looks like lots of you cut it a bit to close today!  Drew mentioned around 4-5pm for a start, and it seemed to start around 2-3pm instead.   I couldn’t believe how many folks were at the local stores as it was beginning.

As of 5pm we have made it through one wave.  The next may not arrive until close to sunrise.  That one should be quite a bit warmer above and very wet.  I expect a lot of solid freezing rain with that one.  Forget the morning commute if you have a choice.

Wind should taper off (at least to regular windy instead of crazy windy) overnight after this first wave finishes going by.