A Warm & Sunny Week Ahead

September 23, 2018

6pm Sunday…

It’s been a pleasant early fall day outside but somewhat chilly.  Portland only hit 68 which ties for the coolest day so far this season.  The average for this date is 74.

We have a lot of sunshine ahead for this work week as very strong high pressure builds just west of the Pacific Northwest.  Here’s the 500 millibar height forecast (in pretty TV colors) for midweek.  Heights get up to around 588dm, that’s summertime warm.
Jet Stream Forecast 2017

850mb temps don’t reach “heatwave” levels though, topping out around +16 to +17 Tuesday-Thursday.  This time of year we CAN hit 90 or even higher, although it’s rare.  In those cases those 850mb temps get up above +20.  We will get some light offshore flow Tuesday-Wednesday, maybe 2-4 millibars worth of easterly wind through the Gorge those days.  Not exactly a big start to the east wind season, that’ll come later in October.  That plus the warm atmosphere overhead should be enough to push temperatures west of the Cascades into the low-mid 80s.  Models seem to be running a little cool for some reason with surface temps.  Notice the ECMWF shows highs only around 80, this is the EPS (ECMWF ensemble system)….

KPDX_2018092312_eps_min_max_15

You can see a pretty clear end to the warm weather next weekend too.  As a cold upper-level trough drops out of Canada next weekend, another wave moves east across the eastern Pacific and merges with it…looking like this by next Sunday

Jet Stream Forecast 2017_b

That’s the GFS, but the ECMWF is similar.  This is the setup we get in winter when we go from cold northerly flow quickly to mild southwest flow.  Of course the jet stream, cold air, and moisture flow is all far less dramatic this time of year.  So we’ll go from very warm weather Friday to cool and somewhat showery by Sunday/Monday.  It appears October may start cool.

The operational run of the ECMWF gives us a ton of rain the first few days of October, but the ensembles say hold on…not quite as wet.  0.60″ instead of 2.50″ as you can see on the ensemble plot (blue is operational run, green is ensemble average)

KPDX_2018092312_eps_precip_240

So enjoy the warm sunshine this week, because it may be the last time we see several (or any) 80 degree days.  Even in September in this “cool-ish” pattern we have gone two weeks without hitting that number.  A cool pattern in early October means even low-mid 70s will be hard to do.

By the way, a quick update on fire season.  Fire danger WILL go up west of the Cascades this week with dry east wind so be careful with any burning.  Of course it’s not going to dry out like it does in summer but a fire CAN spread in the weather Tuesday-Friday.  There are only 3 large fires left burning in Oregon.  All are showing minimal behavior/spread.  The two on the map below west and NE of Grant’s Pass both started during that lightning storm on July 15th…still going 2.5 months later!

Mark Fire All Oregon

Chief Meteorologist Mark Nelsen