Cold Nights and Warm Days

September 12, 2016

8pm Monday…

A short post again since I’m doing all 6 shows this evening.  Not much time!

Last night was the coldest we’ve seen so far this early fall in the metro area.  All of us saw lows in the 40s, with some of us making it down into the upper 30s.  Poor Yacolt, all the way down to 36…brrr!

plot_lows_metro

Tonight there is a Freeze Warning for Central Oregon…should be the first killing frost for most of those areas.  It IS time, since in most of Eastern Oregon you can expect your first fall frost in September:

markfrost_falloregondates

Along the Columbia River and along the lower elevations that first frost holds off until October.  Remember these are AVERAGES, based on a 30 year period.  Some years it’ll happen earlier, some later.  West of the Cascades we tend to see our first frost at some point in October in the valleys between the Coast and Cascade Ranges:

markfrost_falloregondates2

Notice that it’s later at the coastline due to the mild influence of the Pacific Ocean.  You don’t get frost very often when you are right beside a 50 degree ocean!  For sure not anything the wind is coming from the northwest, west, or southwest.  One other spot that has a very late frost is right in the middle of the urban areas.  At PDX, although it is surrounded by lots of open land, that station has an average first frost in early November.  It’s not on the map, but downtown Portland is even later…in December.  Basically in the middle of all that concrete freezing conditions don’t show up often.  The KGW downtown COOP site only averages 16 days each winter below freezing!

Looking ahead, we are in what I would consider some of the best weather of the year.  Cool nights and warm & sunny days.  Yet it isn’t hot, but not cold either.  Goldilocks time I think.  Mainly highs around 80 or so through Saturday and then 60s and 70s beyond that.

For the first time in a couple of weeks models are suddenly in huge disagreement over what happens Sunday and beyond.  The ECMWF is pushing zonal flow (moist westerly wet pattern) over or much closer to us starting this weekend.  The GFS is keeping us dry.  Here’s the issue:  normally we can look at the respective ensembles from each model and that will give us a good idea which model is performing better.  For example if most of the ECMWF ensembles support the GFS solution then you go with that instead of the lone ECMWF operational model.  Yet in this case the 12z model ensembles supported their own models…not real helpful!

For example:

  • 28 of 51 ECMWF ensembles (54%) show at least 0.10″ of rain in Portland by Sunday afternoon.  Only 1/21 GFS ensembles show rain by Sunday afternoon
  • By Friday the 23rd, 28% of ECMWF ensemble members say we will have seen 0.50″ in Portland.
  • 84% of the ECMWF ensembles show rain at some point between Sunday and the following Friday.  ECMWF says SOME sort of rain showers are on the way.

But:

  • Only 3 out of 21 GFS ensembles show that 0.50″ or more by Friday the 23rd.  GFS says mild and dry weather will continue.

So I did a 7 Day forecast based mainly on the ECMWF with a little GFS (not quite as wet and cool Sunday) thrown in.  The main point is that showers MAY show up on Sunday.

I used this graphic yesterday at 10pm, showing our string of warm Septembers.

marktempdeviation_septembers

So far this month we are running well below normal for the first September since 2007.  We’ll see how it looks a week from now.  We may end up erasing that cold anomaly.  How dare we let Portland end up with a COLDER THAN NORMAL month!  It seems to take an act of God for that to happen nowadays.  But eventually it will happen again, right?

Enjoy the sunshine!

Chief Meteorologist Mark Nelsen