11pm: Last Update For Today

If you don’t have ice now, you likely won’t have any…temps have been holding steady or slowly rising the past 6-8 hours:

The only morning commute ice should be up in the hills.  And some improvement during the night too as road crews have a chance to sand and de-ice:

Dangerous West Hills Right Now

7pm

Very icy and treacherous in the West Hills right now.  We’ve had one news crew get stuck in the past hour up on Skyline Road, and reports of multiple cars off-road or in a collision near Barnes and Miller Road.  Avoid hilltop areas the rest of the night!

The rest of this post is just technical talk, for basic highlights read the previous post…

It makes a lot more sense now.  Especially now that the 4pm balloon sounding is done.  Now we know exactly what the temperature profile is overhead, at least what it was at 4pm.  What happened today was quite similar to what we see with a major marine push in summer, except that there was no push from the Coast to the Interior.   Everything else was the same.

1. Top of the moist and cold air mass was around 2200′ this morning, as mentioned in the earlier post.  But look at it on the 00z! 

The inversion is now up around 6,000′.  It’s not that the temperatures dropped in the lower layer, it’s just slightly warmer at 850mb (-3.7), but they warmed dramatically above.  It’s 44 at 7,000′ this evening, it was 16 at the same time yesterday.  That’s from the well exposed Palmer Lift base on Mt. Hood so it’s unrelated to terrain.  So the moist layer west of the Cascades thickened, we had a strong west wind through the Gorge, gusts to 64 mph at Auspurger Mountain there.  Then west of the Cascades waves of drizzle or light showers coming in from the northwest.  Sound familiar?  In today’s case, much of the “marine drizzle” was in the form of snow showers.  All of .01″ measurable at PDX.  All of this occurred in the cold air mass below 6,000′ while it was in the 40s above.  Interesting isn’t it?

Chief Meteorologist Mark Nelsen

Some Re-freezing On Roads Overnight

3:15pm Update…

The Highlights:

• Many streets are wet in the metro area, especially where we’ve seen drizzle (west side right now)
• Temperatures are hovering right around freezing at the lowest elevations
• Temperatures are slightly below freezing on the hills
• Those hilly areas may see roads freeze up soon after sunset 5-7pm.  Watch out if you are driving after dark off main roads and at any elevation!
• Even some lower spots may see roads freeze after dark.   Be careful the next few hours!
• Little or no new freezing drizzle or flurries are expected through tonight
• Tomorrow morning’s commute is a big question mark, it’s possible there are lots of icy spots if all areas drop below 32 degrees again.  Give yourself a little extra time tomorrow morning

It’s been an interesting 8 hours, lots of flurries, although the precipitation has barely been measurable.  Now some spots have turned over to drizzle.  So plenty of wet streets and roads.  Yet just about the entire metro area is at or slightly below freezing. 

So how does it drizzle at 31 or 32 degrees and not freeze on roads and other surfaces?  It’s a dirty little meteorological secret that during the daytime it needs to be around 30 degrees (or even a little lower) for liquid rain/drizzle to freeze to a surface.  I learned this during an ice storm in January 2005 when reporters kept saying the street and sidewalk was melting slightly and it was 29 degrees.  It gets  just barely warm enough during the daylight hours that the weak sun that makes it through the clouds is enough to keep surfaces warmer than freezing.   Here’s a good example using and ODOT road sensor: 

http://www.wrh.noaa.gov/mesowest/getobext.php?sid=ODT14&table=1&banner=off

This is I-205 and Division street showing the air temperature briefly peaked at 33 this afternoon but the road temp was up to 37.  It’ll drop back to the air temperature after sunset, thus my concern about refreezing in the highlights above.

As for precipitation, we shouldn’t get anything other than what we have already seen.  Flurries or spots of freezing drizzle.

Chief Meteorologist Mark Nelsen

Morning Snow Flurries; But a Slow Week Otherwise

9:30am Monday…

If we can’t get “real” snow, this morning’s flakes are the next best thing!

After a very cold weekend, temps have warmed (slightly) here in the metro area a bit this morning; most of us are right around 30 degrees.  The fog layer has lifted up into a low cloud (stratus) layer around 1,500′ at the base of the clouds.  Radar just barely shows a little lifting of the low cloud layer has allowed flurries to develop and drop right over us.  Here’s a link to the radar loop, but by 10am you probably won’t be able to see anything.  In some spots it’s been enough to turn the ground white, not much different from a heavy frost, but it’s sure fun to watch it fall!  Here’s a pic from Rian Muleback in Gresham:

The one thing that has changed this morning can be seen on the visible image,

the low cloud deck has lifted up and the top is now over 2,000′ ; that’s part of the reason temps are a little warmer at the lowest elevations and a little cooler higher up in our little “westside cold pool”.  It’s really cold up on the TV tower, around 22 degrees at 1800, just about the coldest I’ve seen without arctic air or a cold air mass coming through the Gorge.  The past few days have sure been interesting for the weather geeks with regard to the air mass temp!  As a result today should just be gray and a couple of degrees warmer than the last few days.  Maybe a high around 35-36 at PDX.

Check out what happens in the next 36 hours.  Our 850mb temps go from this morning’s -5 to a +11 overhead!  That means at 4,000′, temperatures which are in the mid 20s right now soar to 50 or so by Tuesday evening.  Even a little warmer Wednesday.  And the warming doesn’t really “move in” from anywhere, it is from subsidence.  That’s subsiding warmer and very dry air as a strong upper-level ridge builds right over us.   It’s a little tricky for forecasting, but here’s what I see happening…  The subsidence warm and dry air will “dry up” the 1500-3000′ low cloud layer sometime later Tuesday (maybe not until after dark) and then a new much stronger inversion forms much closer to the surface.  I could see the low clouds dissipating and then a new fog layer developing in the lowest 1,000′ of the atmosphere by Wednesday morning.  If so, the foothills should be dramatically warmer Wednesday but we stay below 40 degrees here in the lowest elevations.  This would be our more typical inversion setup.  I think I’ve decided to call this the STRATUS REPLACEMENT CYCLE; inspired by the eyewall replacement cycle.  There is no help from dry east wind the next 3 days, so if we stay clouded up or fogged in Wednesday, then we’re stuck in the muck.   For several days, the WRF-GFS has shown this whole process quite nicely:

The green is relative humidity, and time goes from right to left.  Look how the dry air works down later tomorrow and by Wednesday morning it’s extremely dry except right at the surface.  SUPPOSEDLY it makes it all the way to the surface, or within a few hundred feet, but I’m assuming that will be wrong.  And look at all the beautiful dry and warm air up above 1,000’…Spring skiing time!

What about the next 7 days and beyond?  A snoozer…  Next chance for real precipitation is still over a week away…

Chief Meteorologist Mark Nelsen