April 30, 2019

Wetter than Normal April to be Followed by a Very Dry Start to May

It is not unusual to have a very dry period sometime during the first half of May....and this year won't disappoint.   Particularly, after April ended up wetter than normal over the Northwest.

The precipitation departure from normal over the western U.S for the past 30 days, shows that most of Washington and Oregon enjoyed above-normal precipitation last month (green and blue colors)

Seatttle-Tacoma Airport, for example, ended up with about an inch above normal (purple is observed, cyan is normal in the figure below)

But there will not be any rainfall for many days now due to the development of a major ridge of high pressure over the eastern Pacific.

The UW model shows dry conditions during the next week over most of the Northwest, with the exception of  a few light showers over the north Cascades.  More rain in California.

The origin of this dry bounty?  A very persistent ridge of high pressure over the eastern Pacific.  Here is the forecast upper level map (500 hPa, about 18,000 ft) for Friday evening.  Major ridge over the eastern Pacific, with a low center to the south.   This is called a REX BLOCK and is very, very stable.

The situation Tuesday afternoon still shows a ridge of high pressure/heights along our coast.

Better get use to temperatures in the mid to upper 60s.


  1. I don't think this dry bounty is such a good thing. In your April post (https://cliffmass.blogspot.com/2019/04/no-reason-to-worry-yet-about-summer.html), you indicated we do not need to worry even though the snow pack was lower than normal. You also said to watch the next few months.

    Well, April was not a positive sign. For those who are interested, you should download the Snotel reports today which shows the status on May 1:


    As you can see almost everything in Washington and Oregon has deteriorated since April 1 going down roughly 5% to 10%. Even Oregon which had been above normal, is now mostly below normal. While this does not approach 2015, it is the worst in the last 4 years, which have already seen bad forest fires. And even though the man-made reservoirs may not be in bad shape, it is still a significant effect on nature's reservoir (the snow pack). This will affect stream flows and ground moisture and increase forest fire risk.

    1. Your in Bellingham did you not get the atmospheric river/ heavy rain that hit SW BC and the Fraser Valley on the Easter long weekend? More than 4 inches fell that weekend in some parts of SW BC and I was in the Chilliwack area and the rivers and creeks were flowing at very high volumes. Also id say predicting a fire season at this point is a bit of a crapshoot. Too many variables. The severity of a fire season is mainly dependant on late spring/ early summer rainfall (late May into early July), summer heat waves and also summer lightning storms. It looks like the fire danger will get higher over the next couple of weeks but cool nights, remaining high level snowpack and increased volume in rivers, streams and lakes due to the freshet will help in the short term. The real danger is if we remain warm and dry right through late May and June and into the "normal" warm and dry period of mid July to early September. I also have noticed that the years where we get those warm and dry late spring/early summers we also usually have an earlier start to the fall rains sometimes as early as late August. Like 2015.

  2. Thank you, Cliff.

    Please, save your energy for the intense days coming your way.

    A separate comment I have is that my view of lovely sunsets in Seattle at this time is that it now includes cranes -not the beautiful Sandhill Cranes. It's been quite a loss.

  3. Thanks Cliff!

    April has not been wetter than normal everywhere in the Puget Sound lowlands. Here on south Bainbridge Island, our station (WUnderground Personal Weather Station: "Blakely Avenue"; KWABAINB78) is 23 percent below normal for April, for the 15 years for which we have rainfall data. For the year of 2019 we are 50 percent below normal. Our grass is starting to turn brown on the 1st of May! In 29 years at this location we cannot remember that happening this early.

    Could there be any validity to perception my wife and I have, based on 20 years of watching local doppler radar images, that the Olympic Mountains' rain shadow is forming further to the south over the lowlands and including Bainbridge Island more often than was previously normal?

    Da5id, Bainbridge Island

  4. "Better get use to temperatures in the mid to upper 60s."
    That will be tough for the beginning of May.

  5. Seeyah in November, rain. Been real.

    On the plus side the lawnmower should get maybe one more use before the grass browns and it can get put away for the season.

  6. I think the last frost date for Puget Sound is around the middle or third week in April depending on your location. I live on the Sammamish Plateau (east of Bellevue). I've had freezing temperatures for the last 4 or 5 days! May first was the first morning above freezing. I think my beans have been killed by the cold temperatures and will need to be replanted, and I didn't plant them any earlier than I have for the last 25 years. Any advice on the end of below freezing temperatures or frost would be greatly appreciated!

  7. I'm a very long-time weather observer, and "dry Mays" aren't an oddity. If anything, Whatcom county has historically had dry Mays then wet Junes (with farmers planning for their "first cut of hay in May"). At this low 906 ft elevation we're still having frosty overnight lows, and only three of these days were dry: [freezing=*] 4/25 30.9F*, 4/26 34.5F, 4/27 37.4F, 4/28 28.9F*, 4/29 30.6F*, 4/30 29.3F*, 5/1 34.7F, 5/2 40.1F. "Normal" is clearly relative, where it comes to weather. So far, at least around here, conditions are well within normal variability.

  8. It's not just Bainbridge. It's all of Kitsap which has a bit been dry. Lots of weather has been tracking in from the West instead of the usual Southwest. So the rain shadow has been in control. Welcome to Sequim.

  9. Gee, it sure has not been very wet in the San Juan Islands during the reported period Cliff. If anything we are still 3" down compared to last year out here in the rain shadow. That has a lot of folks worried about the rest of the summer and water issues in a county that is all considered a water recharge area. Lots of wells are going to dry up early this year, not to mention the drop in available forage and grazing pastures for the livestock farms out here.

  10. The National Interagency Fire Center's May 1 outlook should be of concern to everyone living on the West Coast, including Washington. The May wildfire outlook for human-caused fires in Western Washington and Northwestern Oregon is currently among the worst in the US with the rest of the West Coast deteriorating throughout the Summer as non-human caused fires will also become a bigger issue. Most of Washington's risk appears to be driven by moderate drought and abnormally dry conditions in the state.

    A copy of this informative report can be downloaded here:


  11. Scott8 said...

    "Also id say predicting a fire season at this point is a bit of a crapshoot. Too many variables."

    Rather than rely on your personal subjective sense of the risk, perhaps you should look at your government's assessment of the situation, which is similar to the US:



  12. Wells are going dry somewhere now? In May? Weather related? Location and other specifics would be interesting to know. A lot of factors besides weather can and do come into play.


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