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