An Early Fall “Heat Wave”

September 9, 2013

We’re all set for some hot weather this week.  The stage is set with:

  1. A strong ridge of high pressure in the upper atmosphere developing overhead.
  2. A “thermal trough” of lower pressure develops over the Coast Range/Valleys Tuesday afternoon through Wednesday evening
  3. Offshore (easterly) wind flow shows up due to this Tuesday afternoon and continues through late Wednesday
  4. No clouds to obscure the sun

How warm?  850mb temps peak out around +21 to +23 from Tuesday-Thursday afternoon.  According to my September chart, that puts us in the 91-95 degree range.  However the offshore flow is only for Tuesday and Wednesday.  In early-mid September, as soon as we lose the offshore flow it’ll be hard to get temps to/above that 90 degree mark.   Record high temps for Tuesday-Thursday in Portland are 98-94-92.  As a result I think Wednesday gives us the best chance of tying or breaking a record high.

How about those nights?  Yuck…unusually warm again since we have relatively high dewpoints (for September).  Record warm lows the next 4 days are 63-61-60-64.  We should be quite close to those records.

Check out Brookings today, a high temp of 101 after a low of 80!

MarkWarm_SouthwestOregon_BrookingsEffect

How’s that for offshore flow?  It’s a classic case of the Brookings Effect (also known as Chetco Effect).  I’ll let Wikipedia do the work of explaining it since it appears to be a reasonable explanation.

Let’s move on…quite a change in the 2nd half of the 7 Day forecast the past few days.  Just two days ago it appeared that we would see a rapid cool down Friday and beyond.  Instead it now appears some ridging holds on through at least Saturday, possibly Sunday.  As of this afternoon the ECMWF is bringing a chilly trough through quickly Sunday and Monday.  I totally ignored that while making today’s 7 Day forecast for one reason, that ECMWF operational run was a complete outlier compared to ensemble members for Thursday-Friday.  See how the blue line on the chart below is far below just about all the others?

tseries_850t_000-360_Portland

The GFS is much slower with the cool down…see its chart.  But also notice it’s a little on the warm side for the early part of the weekend.  Due to this, I didn’t ramp temperatues back up Saturday/Sunday (90+) as the 12z/18z GFS would suggest.

tseries_850t_000-384_Portland

Big picture…unusually warm and sunny weather will continue through the first half of September.  There ARE signs of a significant cooldown for next week though, see the below average temps on the charts above, plus here are 4 weeks of ECMWF average 500 mb height maps.  These are from last night’s run:

500za_week1_bg_NA

500za_week2_bg_NA

500za_week3_bg_NA

500za_week4_bg_NA

Chief Meteorologist Mark Nelsen