Unusually Warm Late October Weather; November Likely Starts Cool

October 24, 2017

7 pm Tuesday

I love it when I’m wrong…at least in this case.  A few weeks ago I figured we had seen our last 70 of the season since a week or two of chilly weather was on the way.  But today we just squeaked out a 70 degree temperature at PDX.  That was due to a very warm atmosphere overhead plus a mild offshore wind.

To show you how unusual today was…note we haven’t had a 70+ day in Portland this late in the season (October 24th or later) in 14 years!

We also saw the strongest Gorge wind so far this season…with gusts 50-60 mph in the Corbett area and Crown Point due to over 7 millibars easterly pressure gradient across the Cascades.  I think that Vista House sensor may still be damaged from last winter since it should have read a little higher (compared to Corbett).  That pressure gradient is dropping rapidly this evening and Gorge wind will turn breezy WESTERLY by midday Wednesday.  That’s due to an approaching cold front and lowering pressures east of the mountains.

This front will arrive somewhat dramatically tomorrow afternoon with a cloud deck turning our skies gray quickly the 2nd half of the day (after AM fog breaks out to sunshine).  I think we could get a shower or two but that should be about it with this system; it’s dying as it moves down over the Pacific Northwest.

Upper-level high pressure will rebuild three times over the next 10 days near the west coast of North America or in the NE Pacific.  Models are in generally good agreement on this, or at least their ensembles are.   First ridge is Thursday and Friday; right over the top of us.  Note the ECMWF ensemble 500 mb height anomaly.  This is a dry pattern for us with no wet Pacific weather systems able to make it to the West Coast

Then after a brief wave of energy gives us lots of cloud cover and onshore flow Sunday (but no significant rain), another ridge develops slightly farther west Monday:

This time it is slightly farther west, so we’ll turn cooler for Monday and Tuesday…but likely dry for Halloween.  By late next week, about 10 days out, ridging seems to want to develop even farther west.  That’s over/near Alaska.  The GEFS & Canadian ensembles have it closer to us.  Note the lower than normal heights in both of these charts out over the Easter Pacific west of California.  That implies some “undercutting” of the ridge by moisture and weak systems.

That’s a cool weather pattern for us in November, but still drier than normal.  This is also somewhat similar to what we saw last December/January (far warmer of course this time of year).  When wet systems would sometimes move into California while cool air came down from the north over much of the Pacific Northwest.  The combination of those two brought us numerous snow/ice events.  You can sure see the drop in temperatures after Halloween on the ECMWF ensemble meteogram:

and the GEFS anomaly chart.

“0” means average temperatures.  Note we go below average in early November as the ECMWF shows.

To summarize.  We have 3 more beautiful days Thursday through Saturday, although with another round of gusty east wind.  This will likely be the LAST of the mild/warm east wind patterns.  Next time that wind comes back it should be chilly…no warm east wind for you until at least the Ides of March!  Rainfall will likely be sparse for the next 7-10 days with little or nothing in the next 7.

Chief Meteorologist Mark Nelsen