Unusually Humid Heat Wave On The Way; Thunder Too

July 11, 2014

Our longest/hottest period of weather in 5 years begins this weekend and continues through at least the middle of next week.  The weather maps and models all scream “get an air conditioner if you don’t have one!” .


  • Temperatures warm today up to around 90 or so in the western valleys
  • Hotter temps are likely (somewhere between 90-100) for Saturday through Wednesday.  We haven’t seen a period like this since late July and early August 2009.
  • At this point it appears unlikely we’ll get above 100 degrees, but we might hit that century mark either Sunday or Monday.  I suppose the point of this statement is that as of now I don’t see one of those heatwaves with several days around 101-105.  But it’ll feel that hot because of the next point…
  • Very high relative humidity/dewpoint levels mean unusually warm nights and sticky weather.  And I wouldn’t be surprised if we have at least one 70+ degree lows in Portland; a first since the 2009 heat wave.
  • Thunderstorms are quite possible west of the Cascades during the period Sunday afternoon through early Monday morning.  They will add moisture to the air at the same time a very hot atmosphere is over us.


I’m still on vacation this week but figured I should chime in on the approaching hot weather.  In one word…yuck for those without air conditioning.  For meteorologists and weather geeks we have a good possibility of thunderstorm action (real summer storms, not the boring spring/fall ones), and some extreme temperatures.  So I’m a bit torn since I feel bad for those that are going to be real cranky, tired, and overheated by midweek.

Why so warm?  An upper-level ridge retrogresses from the Rockies to right over Nevada tomorrow and Sunday, then just offshore by the middle of next week.  500mb maps for Sunday ecm_sun_11am and again Wednesday ecm_wedVery high upper-level heights, but not a sharp ridge.  More of a smothering of warm/hot air.  Without sharp ridging we don’t get easterly flow/thermal trough over us for the extreme heat (103-107) and dry air.  But we also see the cooling effects of the Pacific Ocean reduced to almost nothing with very little onshore flow.  At some point later next week the ridging is centered far enough to the west that the airmass cools some and we get more onshore flow.

Why so humid?  Two reasons, first is that sea surface temperatures are running above average offshore along the north Oregon and Washington coastline, so dewpoints/humidity tends to be a few degrees/percent higher than normal this season anyway.  The 2nd is thunderstorm moisture coming up from the south.  You can already see it on satellite pictures this morning in southern Oregon;  it’ll work its way northward for the next 3 days.  Notice the midday Sunday map above shows an upper-level disturbance stuck on the west side of the ridge.  It’ll move over us later in the day and early Monday.  We’ll see how many thunderstorms it sets off.  Could be a nice outbreak of storms during that period or just a few.  But they will add moisture to the air.  Cloud cover off/on the next 3 days will keep nights toasty too.

Which day will be the hottest?  Tough call due to cloud cover at times.  I’m guessing it’ll be Sunday, Monday, or Tuesday.  ECMWF KPDX_2014071100_dx_240 has all of those days about the same but warmest Tuesday.  WRF-GFS says Monday after a very warm night Sunday night with cloud cover and a few thunderstorms keeping temps up.  Then skies clear out; perfect setup for a scorcher.  Our 00z RPM last night showed low-mid 100s Sunday with very weak offshore flow that day ahead of the approaching upper-level disturbance.  web_RPM_Text_00z_PDX But sometimes it is all out by itself (and wrong), so I don’t put too much faith into it.  Plus the new 12z only says 97 now with a lot more cloud cover.

July is looking to be a scorcher!  Stay cool…

Chief Meteorologist Mark Nelsen