July 08, 2009

Dry Sky Parties

One of the inside jokes of the meteorological profession is that summer in the Northwest begins on July 12. And usually that is about right. June often is cloudy and gloomy (but not this year!), and sometimes showers darken the skies around the fourth of July. But in the later portion of July the climatological probability of precipitation plummets to around 10% in the lowlands with many locations receiving less than an inch. Strangely, the lowlands of the Northwest are some of the driest locations in the entire U.S. in July and August. Drier even than the desert southwest, where "monsoonal" rainfall hits during the summer.
Take a look at the plot of the probability at Sea-Tac Airport of at least .01 inch of rainfall (.01 is the minimum required to call precip "measureable"). During July the probability drops from near 30% in the beginning of the month to LESS THAN 10% during the last few days. This is really extraordinary. The driest day of the year? July 29 gets the least on average.... .004 inches. For many meteorologists there is a tradition of having a "dry sky" party during that day or the closest weekend, with suitable libations that I won't discuss in this family-oriented blog. Want to plan a wedding? That is the weekend. Some wise people picked the first weekend of August for Sea Fair.
And you can see why July 12 is often suggested as the start of summer --this is just before the rapid drop off to dry conditions.
Talking about the forecast...a band of showers are now over NW Washington and the coast...and are headed for Puget Sound. But conditions should improve towards the weekend.


  1. This comment has been removed by a blog administrator.

  2. Hi...

    What do you think is going to happen for the upcoming winter season in
    the northeast???



  3. When I was a young Forest Ranger my coworker/mentor always said summer began on July 5th, a cynical reference to the often cloudy/rainy Independence Days. Since then I've never really held my breath for sunny summer days until later in July. Its nice to get the statistical affirmation of my accumulated observations. I always count my blessings when I feel confident in striking out for a mountain hike, unencumbered by raingear which is safely stowed in my backpack! :)

  4. Cloudy skies do not equate to gloomy skies.

    Conditions will deteriorate toward the weekend.

    People who picked things like SeaFair for the last weekend of July are not thinking about people who are not comfortable in those conditions. Looks like early December is the best time to throw a party!

    Not saying you're wrong, but saying that you need to realize people have differing opinions. And will continue to say that until you realize that. Thanks.

  5. Weather is My Life:
    Please send me an email (cliff@atmos.washington.edu)..I want to talk to you offline ...cliff mass

  6. Cliff, very interesting graph, thanks for posting it. What's interesting to me is where this intersects with human experience.

    For example, the jump between Oct 1 and Nov 1 is 20% with a big spike in mid Nov! It's also interesting to note that 20% is the same spread between July 1 and August 1.

    My personal perception is that Nov and Feb are the rainiest and gloomiest months of the year, even though we may have more rain on average in Dec/Jan. It's fascinating to see that big spike in mid-Feb. I took a trip to Florida during that time last year. Maybe that will be an annual trip.

  7. This comment has been removed by the author.

  8. That would be a good graph to show those who think it rains all the time in Seattle, as proof that that statement couldn't be further from the truth. The chance of measurable rainfall is less than 50% for seven months of the year (basically April 1 to November 1), is below 30% from mid-June to mid-September and is below 70% virtually every single day of the year (only very slightly above 70% on one day in mid-November and a day at the beginning of December, but there are more days below 10% than there are above 70%). The average for the entire year appears to be somewhere around 40%, if the chances were equal throughout the entire year.

  9. My birthday is July 11th and my friends have often joked that that's the first day of summer around here. Nice to know that we're not being crazy.

    With my daughter's birthday on January 13th, we've pretty much guaranteed that she will never have an outdoor birthday... we sometimes celebrate her half-birthday (July 13th) for that reason.

    The weather here really is different from North Carolina; over the year, we get about 10" LESS precip overall here than Raleigh gets. It comes down harder and faster when it rains, and they don't enjoy our dry summers. On the whole, I find the weather here MUCH more agreeable, even though we definitely have more grey/drizzly days than they do.

  10. *sigh* I was hoping that perhaps summer had just traded months this year and that we were done with it.

    It's been a wonderful couple of days full of cloudy sixty degree goodness. I shall miss it.

  11. eye-shuh,

    You're not alone. I'm definitely with you, and from the sounds of it, so are many others.


  12. Sweet graph Cliff... it would be even better if you had a state map with average precipitation in July but I don't know if something that detailed exists.

  13. Just read in the paper that El Nino is back. How will that affect our fall and winter weather?

  14. Thunder woke me up in Chehalis at 6:43, storm was directly overhead for about 30 minutes


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