How Hot?

July 23, 2009
KPTV's 7 Day Forecast as of 7pm

KPTV's 7 Day Forecast as of 7pm

Of course that’s the big question in the 7 Day forecast isn’t it?

So while I drank my morning coffee today, I looked at some old weather maps.  There is no finer combination at 9am is there?

The maps and data I looked at were from several of our past heat waves.   Now I know several of you regulars do this sort of thing quite a bit and maybe you have numbers written down, so please correct me if you have better data.  The high temps are from PDX, the 850mb A.M. temp from SLE.

1. 107July 30th, 1965   590dm 500mb height, +23 deg. @ 850mb. (Nice surface high to the east, but not too chilly in Montana.  Closed upper level high just to our north, maybe that helped)

2. 102Aug. 9th, 1967   585dm 500mb height, +21 deg. @ 850mb (big push of cool air+high pressure moving into Montana with this one helped offshore flow)

3. 107Aug. 8th/10th, 1981   594dm 500mb height, +23 to +26 deg @ 850mb(Cooler air moving down to the east into Montana helped with offshore flow again)

4. 103July 20th, 1994   594dm 500mb height, +24.3 deg @ 850mb. (Sharp thermal trough)

So what did I notice most? 

No big surprises but some common themes:  One is that 850mb temps sure don’t have to be insanely hot, but the surface flow definitely needs to be good offshore with a sharp thermal trough to push us up around 105.  Seems like it’s all about the surface flow.  Once again, not brain surgery there.  I will admit that I expected 850mb temps of +28 to +30, but apparently that has not happened in Portland’s observational history.  I also noticed 500mb heights are not insanely high either.  I think the shape and placement of the upper level ridge is far more important.  Looks like a closed high is most “efficient” at producing heat when it’s slightly to the north of us (probably helps give more easterly flow in the mid-levels.  That was pretty obvious in the two 107 degree incidents.

Now you may notice (as of this evening) that our forecast 500mb heights and 850mb temps fall into line with all these record setting heat waves, but to get above 101 we’ll need a sharp thermal trough.  The 12z ECMWF as well as the 12z GFS showed this for at least Monday and Tuesday.  That’s why I decided to bump the 7 Day forecast up a bit.  I may end of regretting that, but in the last two big heat waves we’ve dinked around with the high temps in the days leading up to the heat.  Remember the time someone in town went with 107 or 106 and I had a 105?  I think that was 2006.  That was turning more into a “who’s got the biggest number” sort of game I think…maybe too competitive.  Those were the good old days…

Still, the big picture is an extended heat wave from Sunday through sometime later next week.  The latest ECMWF is somewhat historic if it proves correct; the 12z/18z GFS more of a “normal” 100-102 degree heat wave.  We’ll see!

Chief Meteorologist – Mark Nelsen


Hey all, look at the new tab (at the top of page) I just added with some cool cloud pictures from this evening. Here is the direct link too.

-Rob Martin