Hot Weather Ahead

September 15, 2009

snapshotThe graph tells the story for this water year…it’s been another dry one.  In fact only 2 out of the last 10 “rainy seasons” have been wet.  We definitely seem to be in a drier than average regime so far this decade.  And not much rain in the next 7-10 days either.

Speaking of that, not a whole lot of excitement in the next 4 days with a couple weak systems moving through, so let’s move on to the more “dramatic” weather for next week.  Looks like another one of those episodes where models are in great agreement about an upcoming hot or cold spell.  This time it’s a big ridge.  00z GFS has 500 mb heights over 594dm for Monday-Tuesday next week.  ECMWF is similar, with a slight denting of the ridge around Tuesday by a passing shortwave.  The orientation and position of the ridge appears to be perfect to maximize offshore flow (all the way to the Coast and beyond!).  The new 00z GFS is at the “hot” extreme, with 850mb easterly flow for three days, and a thermal trough west of the Cascades at the surface from Sunday night to Wednesday midday.  850mb temps Monday through Wednesday PM are in the 22-24 degree range. 

For fun, I checked out some of our late season record hot spells…including hourly obs and upper-level heights…more on what I found in a few minutes.  The red numbers below are the highs at PDX.

Sept. 20/21, 1952:  96, 94 500mb heights peaked out around 590dm

Sept. 23/24, 1974: 94, 96 Heights appear to have peaked under 588dm, upper level high was directly overhead with a strong thermal trough (great offshore flow).  850mb temps somewhere between 20-25 deg. C

Sept. 27, 2003: 95 (latest 95 at PDX) Heights around 592 with the upper level high directly overhead.  Peak 850mb temps between 20-25 deg. again.

So nothing in these heat waves was significantly different from what models show for the upcoming week (minus the new 00z Canadian model).  The critical thing is that east wind has to move down into the metro area.  Just like last Saturday, once the wind decides to stay in the Gorge or go westerly temps won’t rise to 90.  The hourly obs. showed the east wind arriving each day at PDX around noon or so, just like it did last week.  Neat how each time the temp jumped from around 80 to 90-93 in just one hour, and then only rose another couple degrees after that.  Once again that’s quite similar to what happened last Friday.  Each evening it was calm again at PDX by 9-10pm, even in the multiple 90 degree day episodes.  Basically you never get a warm night from late September east wind in most of the metro area, except right around the mouth of the Gorge and probably on some of the hills too.  In fact I think it was the 1952 episode in which there were three days in the low-mid 90s, but lows were 52-55 each night! 

Chief Meteorologist – Mark Nelsen