May 15, 2017

Heavy Rain Followed by Drought

The next week will be a study of contrasts, with a wet weather system saturating the region through Wednesday, followed by a very extended period of sun, warmth, and little or no precipitation.  Get out your hiking boots, bicycles, outdoor gear and will need them.

Tomorrow morning, an unusually strong (for May) upper low will pass through our area (see 500 hPa upper level map).  As Darth Vader would say:  "impressive, most impressive."

And the forecast precipitation for the 48h ending 5 AM Wednesday is also impressive (see below), with the entire northwest from northern CA to BR getting lots of water, including 1-2 inches in most of the mountainous areas.  Even the Sierra Nevada is getting wet, which is really unusual.

But then the long-awaited miracle occurs and the spigot turns off and stays off.   First, a modest ridge of high pressure builds aloft on Wednesday morning.

This low amplitude ridge is distorted a bit over the subsequent few days, with a several weak systems moving to the north.   But the real miracle is later in the weekend when an Olympian ridge develops over the eastern Pacific, and by Monday morning (shown below) extends to Alaska.  Praise to the gods.

Consist with this ridging, the latest NOAA Climate Prediction Center 6-10 day forecast indicates way below normal precipitation for Sunday to Thursday.
Or how about the vaunted European Model forecast?  The 24-h totals ending 6-hr for the next two weeks shows lots of rain the next few days, but dry (or nearly dry) conditions from roughly May 18- 25.

The US/Canadian NAEFS ensemble (many forecast) system is similar to the EC prediction (check out the second panel). So my confidence is quite high in this improving trend.

A fine, dry period sometime in mid-May is not unusual, representing the interregnum between the weakening storms of the winter season and the unpleasant June-gloom of late May and June.  High pressure building north cuts off the storms, but as it strengthens onshore and sinking flow increases the result is  lots of ocean stratus and onshore flow by the end of May.  Only when the ridge builds sufficiently with descending northerly flow does the fine weather of July begin.


  1. I swear every year it's a miracle. After front after front, suddenly that high begins to build.

    I participated in the annual trek northward along the coast in my boat (in my case San Diego to Sequim) in May (many years ago) and like most boats, I'd contracted with a private marine weather service to figure out the best Go time. There were a whole bunch of us wanting to go north.

    Our weather guy earned his fee every Spring by fitting the annual migration into a 7 to 14 day window (slow boats and sailboats) that opened just as the summer ridge started to build but before the land heated enough to get that big thermal low going. Once the thermal low got going with that big ridge parked offshore, then we'd have NW winds funneled down the coast in our teeth all the way north. And that would last all summer, building to a gale-force peak every afternoon.

    Being a newby, I was despairing off finding a big enough window for my very slow boat, but I was told there was always this magic period in May sometime. Just have to be ready at the start of May and then wait for it. People who made that trip every year were all bunched up in marinas across the south waiting for the event you've just descibed. And like magic, it always,always came.

    But I remember one boat, purchased by a couple of just-retired Coast Guard officers from Alaska, who asked for different guidance. They wanted to get pushed north and they wanted to test their brand new boat. So they left two weeks earlier than the rest of us and had a "wonderful time" getting pounded and rolled for two weeks going north. They didn't stop from San Diego to La Conner. They claimed they loved heavy weather. Masochists.

    We were slightly delayed behind the main bunch of boats by a last minute mechanical problem and didn't make it around Flattery before the clear-sky northwesterlies started building each day to gale force just off the coast. Took it on the teeth and got stopped for a couple of days just south of Cape Mendocino in northern CA during the worst of it. But eventually made it.

    Those Alaskan guys, who'd run cutters on the Bering Sea for a living, said they'd sail in any weather if they could avoid taking the winds and seas on the nose. But by the time they got around Flattery, their tune had slightly changed. They claimed they were impressed with how difficult the seas can be off Oregon and WA before that ridge builds.

    So I imagine there are a bunch of boats casting off as we speak in Mexico and CA heading our way for a summer of PNW cruising, sailing north into that annual miracle that only lasts a week or two before the thermal low kicks in.

    For the boats that go back to Mexico for the winter, there is a comparable window in the Fall after the thermal low starts to reduce with weaker sun and before the ridge starts to break down. But if they miss that window, and they can get trapped up here for months or all winter.

    Nobody is as tuned into weather as small, slow boats trying to run offshore up and down the west coast. From Santa Barbara, CA to Cape Flattery, there are few places to get into port when the weather goes bad without having to cross a river bar, and those things are horrific for those who haven't mastered the skill of a heavy weather bar crossing. Just have to stay offshore and slog it out.

  2. I've lived in Western Washington all my life and I'm wondering why some years we don't get a summer. There have been years when I've waited, and waited, and waited for it to get warm and sunny and it never happened. Why is that? Is there any way to tell if we'll get a summer this year?

  3. How long will the warmth last, and do we have to worry about the BLOB being born again?

    1. I'm not sure if we have seen the sun for more than 2 days in a row since last August??.

      A very long 8 month pounding of storms. (Remember the record wet October)!

      A well deserved break from wet is in order.

      Bring on the "drought"...! LOL

  4. Did you mean BC? $10 is such a small investment...

  5. Brenda,

    Two years stand out in my mind as years without a summer: 1993 and 1999. Though 1981 was bad too if I remember correctly. For 1993, you can blame it on the eruption of Mt. Pinatubo. It does seem like such years are less common than they were, however.

    But any NW sun lover such as myself must always keep a trip to a drier place in one's back-up plan. For me that usually means the Rockies in summer, Hawaii in winter.

    Any prediction more than 10 days out is very speculative. But I have learned that the trend can change quite abruptly. Don't despair yet.

  6. Cliff, just curious did we know this winter would be historically bad based on long term weather forecasts?

    Just wondering if those can predict with any accuracy a god awful winter such as the one we just had (which I dare say was not technically over as of Tuesday morning given that it was 40° and pouring rain).

    Also, any way to predict whether this summer will be above or below average in cloud cover and temp?

  7. We had snow showers at Sol Duc hotsprings Tuesday morning, snow all the way down to Lake Crescent.
    Beautiful yet strange at 1700' elevation.

  8. Honestly after the horror BLOB of 2015, to have a mild summer in 2017 would be a relief. A mini-BLOB in 2018 would be ok.

  9. I thought the winter was pretty great, with good skiing more often than not. It all depends on your perspective.


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