Freezing Rain?

January 3, 2011

Pretty quiet weatherwise this evening, still cold, but otherwise very similar to the last few nights.  One change…pressure gradient and easterly wind is slowly dropping off.  This is significant for tomorrow night and Wednesday morning’s forecast.

The next 36 hours will be very interesting to see what occurs vs. what models show will occur.  Models say the surface high east of the Cascades weakens dramatically through tomorrow evening, basically killing east wind even in the Western Gorge by tomorrow night.  That allows light southerly wind at the surface, and strong southwest wind a couple thousand feet above, to develop overnight (tomorrow night).  As a result a warm layer of air develops overhead, killing any chance for snow, and probably for freezing rain or sleet too.  Considering how quickly the gradient has dropped off at the same time a nice inversion has formed, I’m inclined to believe models are correct.  That’s the main reason apparently we are the only station leaving freezing rain out of the forecast.  It’s mentioned on the other 7 Day forecasts around town.  I made that choice this afternoon.  Plus, even if we get a spot or two, that’s not worth alarming the entire metro area.  Plus we have one more day to analyze things.

Here’s what I’ll be watching closely:

1.  Gorge gradient…if it keeps dropping off and we have no east wind at Troutdale by sunset tomorrow, then there’s no way we’ll get anything other than an isolated spot or two.

2.  Cloud cover…models imply we get thick cloud cover as soon as sunset tomorrow, that would mean very little radiational cooling tomorrow night…quite a bit different than other times when we’ve had cloud cover arrive right at SUNRISE, keeping the cold surface air in place.

3. Dewpoints…it’ll be interesting to see how they track.  I think the WRF-GFS is way too aggressive showing dewpoints into the 30s after 5pm tomorrow.  That’s very unlikely.  I would think we’ll still see dewpoints in the metro area in the 20s through about midnight tomorrow night.

4.  Temps…due to the cloud cover, I doubt we’ll drop to freezing tomorrow night, but our model and the WRF-GFS shows temps actually rising during the night into the upper 30s.  Once again…something to watch.

Chief Meteorologist Mark Nelsen