No Wind, But Tons of Rain & Snow On The Way

March 19, 2012

Yes, I realize the title is incorrect because of course there will be SOME wind.  But the highlight of weather this week will be a huge amount of rain (for March) in the lowlands and maybe the heaviest snow of winter in the Cascades, possibly in the foothills and Coast Range too.

Here’s the setup:

A cold front is heading towards us this evening; it’ll spread steady rain inland overnight.  Oh, and maybe 8-12″ snow in the Cascades.   The front, with its steady precipitation, slides to our south tomorrow.  Thus we just get lighter showers most of tomorrow.  Tomorrow will not be a washout, just off/on light rain.

The real action kicks in tomorrow night as the front slides a bit farther north, to around Eugene.  Between tomorrow night and Thursday morning, the front sits right over that area.  It’ll be the focus of the heaviest precipitation as you can see in the 3 day rain forecast graphics from our model, the RPM:

It shows a 6″ rainfall (or melted snow) from Coos Bay up into the Central Oregon Cascades and the WRF-GFS:

Note the red areas indicate forecast rain totals greater than 5″!  I love the Pacific Northwest; the red area is only maybe 20 miles away from little or no rainfall in the shadow of the Three Sisters around Redmond.  Now flooding in March is not unheard of, but very unusual.  If these rain totals actually occur, we may see some flooding to our south, even on larger rivers. 

What about snow?  During the Tuesday night – Thursday morning period that stationary front will be quite a sharp dividing line between about a 6,000′ snow level around Medford to near sea level (at night) north of Eugene.  In the central/northern Willamette Valley, we’ll have calm or very light north wind, heavy and steady precipitation, and a freezing level between 1,500 & 3,000′.  Sound familiar?  It is, like what we saw at the Coast last week.  Check out the Salem cross-section on the 4km WRF-GFS:

It shows near 32 degree temps all the way down to the surface Wednesday morning and close on Thursday morning, and barely above freezing during the day Wednesday.  The soundings show a snow profile as well.  Portland’s forecast sounding is just slightly warmer, but still very close to snow. 

WHAT DOES THIS MEAN?  Some of the lowest elevations COULD see sticking snow Wednesday morning and/or Thursday morning, but as with the Coast last week, we probably won’t have a real good idea until a few hours before.  A bigger, and screaming message is that areas up around 1,000′ and above in the Coast Range, central/northern Willamette Valley, and Cascade Foothills may get hammered with their biggest snowfall of the season.  It’s possible.  IF these models are correct, most or all of the precipitation for about 24 hours in those locations at/above 1,500′ will be snow.  That could be 12-18″!    Here is our 72 hour snowfall forecast from the RPM model, showing light 1-2″ totals in the Valley:

Here is the WRF-GFS snowfall forecast for 5pm tomorrow afternoon through 5pm Wednesday afternoon.  5″ in the south Salem hills?  Interesting eh? 


Now all of this is based on 12z runs…I should point out the NAM has the front farther south with less precipitation over us and slightly warmer air.  So there is quite a bit of uncertainty with the whole plan I’ve just set out above.  If later model runs come in with the front sitting farther north (more rain and less snow over Portland), or farther south (no snow and lighter precipitation), that will make for big forecast changes.

There is some good news…much drier and decent March weather Thursday PM through the weekend as more energy heads south into California.

Chief Meteorologist Mark Nelsen