Thoughts on Snow/Ice Thursday

9:30pm…

The National Weather Service has much of Eastern Oregon under a Winter Storm Watch for Thursday.  Hard to believe it’s November 11th!

That’s because moisture will ride over the top of the chilly airmass in place over us.  Here is a look at my current thinking and forecast:

  

So what has changed in the past 24 hours?  It’s cold!  The cold airmass is sure pouring in quickly this evening.  I see at 4pm over Salem at 850mb the temp was around zero which is pretty close to what models expected.  Low level temperatures were a bit warmer than modeled today, but seem to have lowered down a closer to where we expected them to be at 9pm.  Gov’t Camp is in the upper teens and 850mb temps were forecast around -11…close enough for me.

Assuming models are doing okay on the thermal profile of the atmosphere, what do they show for tomorrow night and Thursday?

1. Precipitation arrives somewhere between midnight and 7am Thursday.  Our RPM is really fast, but the more reliable WRF-GFS has it into the metro area around sunrise or just before.

2. Sticking, or even “in the air”, snow is very unlikely in the metro area.  Our RPM is too warm at the surface and higher up.  The WRF-GFS, which may have had precip placement issues in the past, has generally done very well with the thermal profile of transition events like this.  It shows temperatures over the metro area rising above freezing from 2,000′-5,000′ tomorrow afternoon and never dropping back down again.  Take a look at the sounding for 7am Thursday:

So I have no idea why it is producing light snow totals over us…it can be ignored for now.  The MM5-NAM at 4am Thursday is a little closer to snow,   but it seems to run too cold often so I bet it’ll warm its forecast a bit the next 24 hours.  Not sure why that model is producing snow too.

3.  Isolated spots of freezing rain are possible to start Thursday morning in the metro area, but I sure don’t see a metro-wide ice storm.  Cloud cover doesn’t thicken up until later Wednesday night so temps may drop below freezing under mainly clear skies away from the Gorge wind.  This will be closely watched by forecasters like me.  It’s pretty simple…if the temperature is 34 or under when the precip starts, then freezing rain is likely, at least for a brief period.  That said, for roads to freeze up we’ll need temps down to 30 or lower.

4.  Cold rain the rest of Thursday...high temps in the 30s, a breezy east wind, and rain at times will make for a miserable winter-like day.

5.  A big November snow/ice storm in the Gorge.  East wind isn’t going away in the Gorge through the weekend and a ton of moisture is coming.  I’m thinking 6-10″ is likely in the central/east end with a good 1/2-1″ ice accumulation at the west end.  Maybe as damaging as what we saw back on March 1st.  Could be some big sleet drifts too like we saw at that time around Multnomah Falls.

6.  Cold air is here to stay for awhile.  The 00z models confirm what the ECMWF showed earlier, a 2nd surge of cold air from the north late Friday and Saturday since the northerly flow around the upper high is going to dominate over the undercutting flow from the southwest.  Interesting stuff since that would mean the snow-covered ground east of the Cascades would stay in place.  Winter is arriving early!

One last thought…

If our forecast is about 5 degrees too warm???  The best November snow/ice storm in 30 years will arrive Thursday in the Portland Metro area and the snow/ice will be slow to leave.  Let’s hope that doesn’t happen.

Chief Meteorologist Mark Nelsen