Early Season “Cold” Snap

October 7, 2009

junkThis is a forecast “time-height” diagram from the WRF-GFS model run by the UW.  It shows the forecast atmosphere over PDX during the next 24 hours.  Time moves from right to left;  the opposite of what you might expect.  The time goes from 5pm tonight ot 5pm Saturday.  Two things to notice here.  One is the green shaded area on Friday morning…possibly another morning cloud day coming up like today.  The second is a bit more significant for our weekend weather.  Strong easterly flow arrives overhead and down here at the surface by Friday afternoon.  Notice the drop in temps and humidity.  This is quite similar to a wintertime arctic front.  In fact all of the atmosphere over Portland above about 2,000′ is at or below 40 degrees!  That’s amazingly cold for this early in the season.  I didn’t put the image here, but 850mb temps east of the Cascades (over The Dalles for example) are forecast down to -6!  That means low-mid 20s at the 5,000′ elevation.  It doesn’t get any colder than that in early-mid October.  Models (especially the GFS) are pushing the colder air much closer to us than 24 hours ago;  that’s why we dropped the 7 Day forecast temps quite a bit.  They may still be slightly high.

This will be about the driest airmass you could get in early October as well.  Looks like dewpoints down into the 20s at least Saturday and Sunday.  I don’t remember a time before Halloween when we’ve had such low dewpoints.  Yes, a few years ago we had a cold blast right at the end of October, but that is 2+ weeks farther into the cool season.  With such dry air, it should be easy to get temps into the mid-upper 20s in the coldest Willamette Valley spots that see the wind go calm at night.

Still looking rainy next week, starting sometime later Monday or Tuesday

Chief Meteorologist Mark Nelsen