August 07, 2017

Dry Period Record Tied Today

There has been no measurable rain at Seattle-Tacoma Airport  for fifty-one days.   This ties the record for number of consecutive days without measurable rain, which occurred from July 7 to August 26, 1951.

And considering the current weather situation and the latest model forecasts, we are going to break the record tomorrow.    In fact, we are going to smash this record, with no rain in sight for the lowlands for the remainder of the week.

To illustrate our summer precipitation situation, here is a plot of the normal (blue line) and actual (purple line) precipitation at Sea-Tac for the last 12 weeks.   We had one very wet day in mid-June, but after that precipitation basically ended.
Now it can't be stressed enough that the summer are generally dry around here, and as shown by the figure, we are only about an inch below normal for the 12-week period.   So in terms of real impacts, the lack of summer precipitation doesn't mean that much, particularly after a very, very wet winter and spring.

Meteorologists like records though, and I am no exception.

Our temperatures this summer have been a few degrees above normal, as illustrated by this temperatures plot at Sea-Tac for the same period (average high is purple, average low in light blue)
And the deviation of the average max from normal for the last 60 days indicates that western WA is roughly 0 to 2 F above normal.   The West Coast has clearly been warmer than normal.

Why warmer and drier?    Because of persistent high pressure (ridging) over the West Coast,  which is demonstrated by the anomaly (difference from normal) of the heights of the 500 hPa pressure surface for the past 30 days:

OK, the big question folks are asking are asking is when this warm/smoky situation is going to end? 

 Well, I have good news for them.....the latest forecast ensembles suggest a shift to a cooler, wetter pattern over the weekend.    Here is the latest National Weather Service global ensemble forecasts (GEFS) for 2-m air temperature.  We actually warm up this week (sorry), followed by a distinct cool down on the 13th-15th.  Yes, uncertainty increases (the variability amount the ensemble forecasts), but all go for cooling.


  1. You mean Sea-Tac is breaking a record.

  2. Now that we are getting closer to the Total Solar Eclipse on August 21st, I'm starting to worry about weather and the clear skies forecast for the path through Oregon, Idaho and Wyoming. I had not thought about smoke as one of the variables, but it obviously is one. Also, being extra dry and hot in Oregon means more chance of fires as the tenderfoots head to the forests.

  3. What will it be like for the eclipse on the 21st?

  4. While we have been dry from a precipitation standpoint, it seems to me there have been some days with relative humidity and dew points reminiscent of areas east of the Rockies. I grew up in Seattle and have difficulty remembering a day as humid as it was on Sunday. Does the particulate from the smoke assist in holding moisture in the atmosphere? Do you have historical data to compare the dew points of the past week to periods of yesteryear?

  5. Matter, the highest dew point at sea-tac the last week was 60 degrees that was on Sunday. This would be considered a nice dry air mass east of the Rockies, which commonly are in the 70's for the dew point. You really don't start feeling the humidity it until it gets around 62-65 for the dew point with a temperature with at least 80 degrees.I think the record dew point for sea-tac is 67 degrees.

  6. Is it reasonable to expect weather phenomena such as local temperature, days without measurable rainfall etc. to obey 1/f statistics? Many natural patterns follow this trend. As I understand it, if you are observing something that obeys 1/f statistics, if you wait long enough, you will see the record broken.

  7. I expect that this phenomenon, length of dry periods, obeys 1/f statistics. If you wait long enough, every record will be broken.

  8. Cliff, why do you use Se-Tac as a point of reference for Seattle weather. SEA is rather anomalous because it's in the direct path of outflow from from the east over Snoqualmie Pass. Why not use Boeing Field where the record keeping started 17 years earlier and is closer to Seattle. In addition, BFI is the NOAA data center for Seattle weather, which must count for something. Thanks.

  9. Mike, some people are more sensitive to elevated dewpoints than others. I personally feel sticky when the DP exceeds 55F, so I understand Matter's discomfort. If the dewpoint exceeds 60F, I run the air conditioner regardless of temperature.

    Compared to earlier this summer when the dewpoints ranged between 45F and 55F, this past week of upper 50s dewpoints is comparatively humid.

  10. Consecutive days with 0" rainfall at older stations nearby...
    University of Washington, Seattle: 62 days, ending Sep 1914.
    Kent: 71 days, ending Sep 1914.
    Vashon Island: 78 days, ending Aug 1922.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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