Sticking Snow Unlikely Metro Area Tonight

It’s one of those weird nights, in which we are forecasting little to no snow (less than today) in the metro area yet the NWS is forecasting 1-3″ (more than today) for the morning commute tomorrow and has a Winter Weather Advisory up.  We’ll see how it turns out by daybreak tomorrow.

For those just tuning in for the basics, here is our graphical overnight forecast, read the details below:

• Steady rain/snow returns to western Oregon and Washington by midnight.  For most of us, especially north of Salem, it’ll be mainly snow falling, not rain.
• However, it’ll be just 3-5 degrees too warm (35-38) in the Eugene to Vancouver corridor (lowlands) for the snow to stick. 
• The morning commute should just be wet in the majority of the metro area.
• Better chance for sticking snow 1am-4am in Columbia County (Vernonia, St. Helens, Scappoose, & Rainier).  Also northern Clark County (possibly Battle Ground, Amboy, La Center).  We could see a trace to 2″ in all these spots.
• Tomorrow looks very windy and very wet with south wind gusts 30-40 mph and heavy midday-evening rain.

So why am I so “anti-snow” this evening, especially considering the NWS is forecasting a bigger event than last night?

1. Upper levels are plenty cold to start, but even at 1800′ right now (top of TV tower), it’s 30 degrees.  In a well-mixed atmosphere, one would expect 37-38 down at sea level.  That atmosphere does not cool in the next 8 hours, in fact it warms dramatically after 4am.

2. Precipitation rate on models is very light through 4am.  If we’re going to drag the snow level down to the surface, the precipitation had better be at least moderate, preferably heavy.

3. No dry air coming in from the Gorge; actually no real cool air coming out of the Gorge anyway.  Gradients are flat right now and will go to maybe 3 millibars easterly by 4am when it goes calm again.  Either way, the Gorge is not a factor and will not help get us the usual evaporative cooling.

4.  Southerly wind just above the surface will be increasing through the night, with temps holding steady through 3am, then a big jump after that time.  No matter what happens down at the surface, the 1,000-2,000′ temps go well above freezing before sunrise.  Model soundings, cross sections, and meteograms show temps at the surface holding steady or rising the next 6 hours.

Our 00z RPM is pathetic generating very light snow totals as a result of these points:

Wait, there’s more to talk about…

Strong Wind

…in the warm sector tomorrow, quite a strong and gusty south wind up the Valley.  I could easily see gusts 40-50 mph down at Salem during the day, and maybe 30-40 mph here in the Portland area.  That should be fun.

Heavy Rain

We’re really going to get soaked tomorrow.  Models showing a cold front just about stalled over us in the evening and early overnight hours Wednesday.  Our RPM is showing a good 1″ through 8pm, then another 1″ in the following 6 hours.  The WRF-GFS is similar with very heavy rain 7-10pm.

Post Front Heavy Snow Tomorrow Night?

This one has been hinted at by several mesoscale models the past 36 hours.  Immediately behind the stalled cold front tomorrow evening, the atmosphere suddenly cools with the very heavy and steady precipitation.  There is an area of snow levels just about down to sea level as a result.  Last night our RPM was hinting this would occur on the east side of the metro area.  Tonight the WRF-GFS paints heavy snow from 10pm-4am along the east slopes of the Coast Range (Gaston to Banks to Vernonia area).  Note the foot to 18″ forecast by that model over the N. Coast Range?  This will be a very interesting feature to watch.  In fact if you look at the WRF-GFS meteogram for Portland, it drops temps down into the 30s under the heavy precipitation, although it doesn’t generate sticking snow.  Tonight may not be our last brush with snow!

Beyond that…I don’t see a setup for snow as we just have weaker and cool weather systems through Christmas Day.

By the way, Andrew from Silverton pointed out this disparity in forecasts has occurred before.  I had forgotten about the big snowstorm forecast 2 days after Christmas 2007 that never happened.  Some fun old reading on the blog there.  And of course the big event the day the blog started back in 2005.  A winter storm warning was up for a big snowstorm, but I was forecasting no snow, that was a very stressful night.  In both cases it hardly snowed or none at all.

Chief Meteorologist Mark Nelsen