4pm: Freezing Rain (& Rain) Ending: Icy Spots Possible Westside Tonight

December 12, 2013

That wasn’t real exciting…just some light rain this afternoon for most of us.  But some spots of freezing rain on the west side of the Willamette Valley.  Had a few reports of freezing rain in McMinnville, Lafayette, Dayton, and some other patches in western Washington county.

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As you see on the radar, rain of any sort is about to end as our weak weather system moves on east.

The issue tonight may be the usual case of skies partially clearing behind a cold front; probably only on the west side of the valley again.  Since temps are within a few notches of freezing, any clearing means icy spots could easily develop on roads in those areas…be careful out there later tonight!\

I don’t see a widespread warming southerly wind tonight, but some spots will randomly pop up temp-wise.  I see some mid 40s over on the SE side of the metro area where southerly wind has arrived.  The rest of us stay in the 30s tonight.

Chief Meteorologist Mark Nelsen

 


Noon Update: Going to Be Close Far West Metro!

December 12, 2013

12:15pm…

Rain appears to be on schedule for an arrival by 1pm or so in the farthest western metro locations, and then it’ll spread to the east from that point through 2-3pm eastside.

Here are the latest temps:

PLOT_Temps_MetroYou can see who is out of the “danger zone” for freezing rain.  Temperatures are above freezing pretty much everywhere north of the Columbia River (Clark county) and east of the Willamette River (Multnomah and Clackamas counties).  And dewpoints are rising with the temps, so when rain starts the temperature won’t fall, or at least won’t fall more than a degree or two.  So most of us in the metro area will not see freezing rain this afternoon.

Lots of 29 degree temps across the south metro, but as I mentioned before, at midday/afternoon the pavement needs a temp 30 degrees or below to freeze up the liquid rain that’ll be falling onto it.  So I think those areas are also “in the clear”.  And the inversion is breaking up, thus the freezing fog/low clouds have lifted in these areas.  That helps to slightly warm the ground.

Which leaves one area in the western valleys that could start as freezing rain around 1pm or so.  As I mentioned in previous postings, the spots up against the east slopes of the Coast Range that are coldest and get the rain first:  Forest Grove, Banks, Gaston, and Hillsboro.  Western Washington county could start icy, then warm to freezing within an hour.   Or there may be patches of ice where the pavement has been shaded more the past few days, but not in spots that had sunshine.  Be careful out there if it happens!

Of all the places to warm up, Eugene has finally gone above freezing.  At noon it’s 33 down there, after a low in the mid teens!

Chief Meteorologist Mark Nelsen


Morning Update: Rain Arrival A Bit Later

December 12, 2013

9:30am Thursday…

It’s cold out there!  At 9am temps in the mid-upper 20s across all of the metro area except those spots exposed to the “milder” east wind coming out of the Gorge.  The east wind will gradually die back this evening, but I don’t see any mixing south wind (if any) until after the rain begins.  So no one below 1,000′ (most of us) will see a dramatic warming today.  As mentioned last night, if rain was arriving right now, or earlier, we would have a widespread freezing rain event west of the Cascades…it’s all about timing.  So where is the rain?  Save this link to radar to watch it during the day.  You generally want a “green” area to get rain all the way to the ground.  Those are echoes above 20dbz.  It’s trying to move onto the central Oregon coast at this hour.  All coastal sites have jumped well above freezing, so no freezing rain out there today.

A change this morning in timing though…all our mesoscale models have slowed down rain arrival in the valley yet again; this is very good news.  Here is the 1:30pm RPM rain/snow outlook

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and 1pm WRF-GFS

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Both show no (other than a sprinkle) rain arrival in the valleys until after the noon hour.  They DO still hint that some of those isolated valley locations in the Coast Range could see freezing rain (Vernonia, Manning, etc…) to start.  But with the forecast later arrival of rain, that decreases further the chance that we see anything frozen on roads here in Portland.

I did notice one more thing on morning models/maps.    This system is so weak, there is a possibility we don’t get ANY mild mixing wind today either in the Valley or the Columbia River Gorge.  I think it’ll just be in isolated spots.  So high temps may only be around 35 and we just sit there tonight (assuming solid cloud cover).  And in the Gorge there is plenty of dry air left to allow some evaporational cooling as the precip starts.  Could still be icy out there late this afternoon or evening when the rain moves in.

The layer of cold air over us west of the Cascades is incredibly thin.  32 degree and warmer temps are only around 700-800′ up!  By the time you get up to 2,000′ it was 46 degrees over Salem!  Now that is a good inversion.

More later….

Chief Meteorologist Mark Nelsen