Arctic Blast ’09 Almost Here!

December 4, 2009

No changes in the forecast this evening.  Cooler air is filtering down across the Pacific Northwest now, but the really cold modified arctic air moves in Sunday.  NO model shows precipitation in the western valleys anytime in the next 4-5 days.  So this will be like the beginning of the December 1998 cold spell when we saw just a scattered flurry or two as the arctic front passed by.

So the big arctic blast is on, although I still can’t figure out why some are holding back so much…very strange.  00z NAM was slightly warmer, the 00z GFS actually a bit COLDER.  Minor differences that we shouldn’t worry about. 

We are locked in cold northerly flow through at least the middle of next week with the coldest air stuck in the lower elevations east of the Cascades.  Check out this WRF-GFS map of surface pressure for next Wednesday night:

Next Wednesday night. Notice about 10 millibars easterly flow and the classic "dome" of high pressure trapped in the Columbia Basin.

Notice the difference between that pressure pattern and what we see for Sunday and Monday.  This is the setup where the wind really rages in the Gorge and east metro area but backs off all other areas.  Most important, very cold air continues to move through the Gorge, keeping us from warming up much.  We’re forecasting 35 & 39 Wednesday and Thursday.  That may not happen, it could be colder…we’ll see. 

There actually isn’t much more to talk about.  Late next week some moisture COULD try to move in on us, but model solutions seem a bit chaotic beyond Wednesday.  So we’ve thrown an obligatory snowflake and freezing rain symbol on Friday’s forecast.  Don’t read too much into that.

On the Home Front: I cut down the top half of 12 foot noble fir tree on my property for a Christmas tree today.  I know it’s a bit trashy, but I’ll cut the rest down in the spring.    Got the Christmas lights up too since I figured it would be way too cold on the fingers to do it early next week.  Tomorrow it’s time to wrap the mimosa tree so it’ll survive the blast and blow the water out of the outside drip irrigation stuff.  I had to replace a bunch of pipes this last spring after I didn’t do that last year.  By the time I’m 70 I’ll figure all these things out.  Stay warm and I’ll post again either Sunday night or Monday.

Chief Meteorologist Mark Nelsen

Models Keep Improving…

December 4, 2009

12z RPM Forecast for 5pm Today

Here is the 5pm forecast for today from WSI’s 4km RPM model (the same thing we use on the air).  Looks like it did pretty well.  When we saw this yesterday we put flurries in the forecast for the Cascades late this afternoon and evening.  That’s exactly what is happening.  Great forecast by a mesoscale model.  A few showers have even appeared over the lowlands, although very light.  So what does it show for Sunday morning?  Here:

18z RPM Forecast for 8am Sunday AM.

Nothing too significant.  On later images the snow over the Cascades stays put…it’s upslope flow into the east side.  But cloud cover coming over the Cascades moving west out over the Valleys could produce some snow showers.  That’s it, then it’s on to dry and COLD!  More later…