The Dry Month Begins

junkI have to admit that I stole the idea for this graphic from Cliff Mass’s blog up in Seattle.   This is a graph of the rainfall probability for any one day throughout the entire year at PDX.  Notice the sharp dropoff after the 4th of July.  The month from July 10th to August 10th is the driest of the year here in the Portland area.  Of course each year is different, but the average numbers sure don’t lie do they?  I also see that September is a bit drier than June, but we quickly turn wetter after October 1st.   Basically we turn wetter faster in the fall than we dry out in the Spring and early Summer.

Coming up…looks like possibly some interesting weather for the end of the week.   The upper level low off of Vancouver Island is going to sag to the south and be sitting off the Oregon Coast Friday and Saturday.  It fills a bit as heights rise, but it’s definitely still there through Saturday.  Of course then that sets up the preferred south or southeasterly “thunderstorm” flow that we like to see.  Our RPM model and WRF-GFS imply something could drift off the Cascades Friday and/or Saturday afternoon.  But as we saw in June, we can have this pattern and still get no significant storms west of the Cascades.  It’s something to keep an eye on.

Mark Nelsen

35 Responses to The Dry Month Begins

  1. Karl Bonner says:

    July 5 – Sept. 5 is the hottest two-month period in interior NW Oregon…and just like with rain, the temperature changes more abruptly in fall than in spring. In Eugene, October is basically like April and May on rewind at double speed. Our “winter” – the coldest three-month period – is about Nov. 20 – Feb. 20 while our “summer” lines up perfectly with the official definition. In between are four months of “spring” and two months of “fall.”

  2. Rob "Wrath" S.E. Portland says:

    Depending on the models, where the upper low sets up shop, timing of shortwaves, and surface conditions (Low level heating) I think there is a chance we see a Mesoscale Analysis box for our area Friday/Saturday as well as seeing the SPC take a bit of notice to our region… I don’t know about a ‘slight risk’ zone however like we saw back in early June.

    • Timmy_Supercell (Hillsboro) says:

      Well, CAPE values appear to be just as high as June 4th. and the trough was very similar also. If instability parameters stay around 1500 J/kg and if its moist enough, why not put “slight risk” zone over us?

  3. Timmy_Supercell (Hillsboro) says:

    Has storms been trying to develop off the coast? Or is that chaff?

    • Rob "Wrath" S.E. Portland says:


      I noticed t-storms are actively occurring as far north as the south Washington Cascades. I believe there weren’t supposed to be any today even north of Mt. Jefferson. Hmmm you know the flow may bend around enough later this evening for some good flashes to our east. I like our chances Friday and Saturday as well.

  4. Rob "Wrath" S.E. Portland says:

    They do call them Mid-latitude cyclones…


    CAPE values are 250-1000
    LI 0 to -3
    Flow aloft today isn’t conducive for t-storms here. Friday and Saturday are another story. 12z WRF-GFS Mesoscale model continues the theme of instability developing each afternoon/evening with a slight Foothills eastward Thursday and Friday. For Saturday instability explodes west of the Cascades with CAPE values nearing 1500j/kg. Sure looks possible at this point for some active weather Saturday evening. Surface temp model indicates 88-92F for Saturday. All of this coupled with S-SE flow aloft looks fairly good a reason to be at least optimistic.

    • Timmy_Supercell (Hillsboro) says:

      So Friday and Saturday we (the Valley) should see storms? With the good ‘ol southerly flow and CAPE up to 1500j/kg, “something” should happen. Definitely.

      Its been a while since we had storms.

  5. Jesse-Orchards says:

    Apparently not simple enough.:)

  6. Timmy_Supercell (Hillsboro) says:

    Already some storms in Central Oregon. I’ll keep an eye on that.

  7. …hey all,if you would, please think a good weather thought for the woman who ensured my weather junkiness when, as a child, raised me on the gospel of Bob Lynott and Jack Capell ( Flab5 sidebar…anyone remember the 8am weather bureau report on KPOJ?), helped with my award winning 2nd grade weather project for science fair, and exposed me to any other of our limited
    sources of weather info she could find, and who i talked weather stuff with until just a few days ago….

  8. Timmy_Supercell (Hillsboro) says:

    Jesse, I am not being sarcastic. Warmer winters in the Pacific Northwest have lead to more wind storms if you haven’t noticed.

    Look back at last December. Did we have any significant tropical cyclones like the 2007 gale? No. It was too cold, and there wasn’t enough moisture.

    On winters where we have more than one wind storm, I don’t remember even seeing any snow. Like December 1995 for example.

    And I find it very unlikely that any tropical storm originating in the Pacific Northwest would accompany a supercell thunderstorm. A squall line I have heard of.

    • Jesse-Orchards says:

      Timmy, we don’t get tropical cyclones up here.

      The windmakers we get have to do with the interaction of warm and cold air. A completely different animal than warm water induced tropical systems.

      The “Great Gale” of 2007 which you cited occurred during our last big La Nina winter, by the way.

    • Jesse-Orchards says:

      Dude, the winter of 1995-96 had all kinds of snow, and one of our coldest arctic airmasses of the past 20 years in late January/Early February.

    • Timmy_Supercell (Hillsboro) says:

      Maybe I am not too much of a windstorm pro. I thought they called our windstorms cyclones?

    • Jesse-Orchards says:

      They do, but they are not tropical cyclones.

    • Timmy_Supercell (Hillsboro) says:

      It’s easy for me to mix up simple things like that lol

  9. Rob "Wrath" S.E. Portland says:

    Bright sunshine here now.

  10. Valerie (Vancouver) says:

    My favorite time of year!

  11. Debbie says:

    I always say we never get our summer until after the 4th of July.

  12. Rob "Wrath" S.E. Portland says:

    “Here” I think he meant the entire United States.

  13. Andrew Johnson (Silverton) says:

    So how does everyone think the El Nino will affect this winters weather?

  14. Cloudy earlier this morning. Starting to get some partial clearing. Currently 61.6°

  15. Cloudy, cool 63.5F currently here….Looks like a warm start to the weekend followed by cooler weather and maybe some showers late Sunday into Monday…

    If my memory serves me right, Brookings reached their all-time high of 108F on this day a year ago.

  16. Rob "Wrath" S.E. Portland says:

    Ahhhhh it didn’t cloud up here until 4:30 AM… Solid gray now. Not so sure we crack the mid 70’s today.

  17. Tetra says:

    Edit: Dad claimed he saw a few drops of very light rain.

  18. Tetra says:

    I am clouded over from wall to wall. It’s 58.5F but feels a bit chiller.

    It sorta feels like the beach in the morning.

  19. Rob "Wrath" S.E. Portland says:

    Hmmm still crystal clear skies. Currently 49.8 F. I wonder if a tongue of warmer air/higher pressure has moved north up through the Cascades because this doesn’t isn’t like the past several days where we saw clearing only to cloud up quickly during the late evening. This seems more to be like the upper low is losing its marine grasp so to speak and that higher pressure is nosing into our region. I think due to this we may see temps into the 79-82F range today.

  20. Rob "Wrath" S.E. Portland says:

    5$ dolla foot long!

    Skies are crystal clear here no clouds anywhere.

    Mark, thanks for the update.

    00z WRF-GFS mesoscale model looks real interesting for storms Tomorrow through at least Saturday night. Saturday evening especially. Yes mainly over the Cascades, but I would not rule out the chance over the valley if things line up (Temperature, surface flow, flow aloft, moisture content in the mid-levels, instability present, lift.) 00z WRF-GFS also suggests temps in the upper 80s to maybe 91F Saturday with S-SSE flow aloft.. Hmmmm….

  21. Timmy_Supercell (Hillsboro) says:

    It definitely is going to be a dry one, I noticed lately we have been getting below average rainfall. And more of that from here on…

    I say thunderstorms will develop! Five dollars on it.

    • Jesse-Orchards says:

      The question is, where?

      There is a thunderstorm happening somewhere on the globe at all times. That five dollars is already yours!

    • Timmy_Supercell (Hillsboro) says:

      In the Willamette Valley of course. I am not betting if anything will go boom over Portland though, thats a little too risky.

  22. Timmy_Supercell (Hillsboro) says:


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