April 17, 2020

A Very Dry April with Extraordinary High Pollen Levels: But Change is Coming

April in our region has not only included "dry storms" of desiccated air and lots of sun.  Not surprisingly this month has been extraordinarily dry in terms of precipitation, with Seattle only receiving .01 inch this month.

Some folks are already asking me about drought---but as we will see below, such worries are premature. 

Let's start with the percentage of normal precipitation for the past two weeks, which is a study in contrasts.  Portions of the Northwest have received less than 25% of normal, while southern California have been crazy wet, with a large area being hit by 800% of normal.  You read that right.
Los Angeles and San Diego have been like Seattle in January.  Obvious this is good for delaying the wildfire season in a large portion of the Golden State.

Although April has been dry in the Northwest (because the weather systems have been going into California), our long-term precipitation is generally fine.  For example, below is plot of the accumulated precipitation at Seattle over the water year starting October 1 (purple is observed, cyan is normal).  Almost exactly normal right now.  That is why our plants look green and happy--the subsurface moisture is ok.
This dry spell was timed perfectly with the release of tree pollen in our region to provide a tree-pollen storm during the past two weeks (precipitation reduces pollen concentrations in the air).  Take a look at the values from pollen.com for Seattle, shown below.  Yikes.  It has been very high for the past week or two.  Miserable for those afflicted with allergies.

But relief is on the way....eventually.

The latest European Center (EC) model runs are insistent that rain will return in the second half of April. 

 The high resolution EC forecast (see below, from the wonderful weatherbell site) shows substantial rains during the last week of the month  (green bars are rain)--about 1.1 inch in total.  Temperatures will fall as well, mainly back into the upper 50s, which is normal.

A more regional view, looking at the average of the EC ensemble (running the forecasts many times, each a little different) for the total accumulated precipitation through May 2, shows a return to the normal pattern, with several inches (2-4) over the western side of the Northwest and drier conditions over California.   So enjoy the beautiful weather today, by the end of the month cloudier, cooler, and wetter conditions will return.  And thus suffering from tree pollen allergies can expect relief.


  1. I would normally like warmer, sunny weather to continue, however I'm not gonna complain about this. I actually kind of miss the rain, believe it or not. So, bring it on!

  2. Here in Yakima, it feels much drier than normal for spring. We've received less than 1/2 inch of precip since Feb 1. Our ridges are usually lush green this time of year (before they burn out in June and July) but this year, they're still dried out and barely any new veg is popping up. Cold mornings too - the orchard frost fans have been getting used.

  3. Its been very dry in the Methow valley. Per the Winthrop weather station the last significant precipitation we had was on March 13 (0.12-in as snow). The precipitation in March was only 0.21 inches vs a normal of 1.03 inches. I hope the models are right but I will believe it when the rain is in my rain gauge. Pardon my fire paranoia but I've seen this movie in recent years and it often doesn't end well.

  4. "The American West May Be Entering a ‘Megadrought’ Worse Than Any in Historical Record"


  5. Trace of precip today in NW Bellingham.

  6. Though my sinuses and allergies have been going nuts... I will deeply miss the sun and warmth.

  7. Intense rain in parts of Puget Sound Apr 18 945 PDT, with a counterclockwise spin to it. Where did that come from?

  8. I logged onto pollen.com to check and see what trees were shedding pollen down here in Olympia. It said mulberry, sycamore and ash. While there are some ash trees south and east of Olympia, neither sycamore nor mulberry are native anywhere near here. There may be a few planted landscape sycamores and mulberries, but certainly not enough to be shedding large quantities of pollen. Probing a little further, I looked up sycamore and they got the genus right (Plantanus), but their photograph of leaves and seed pods were of sweetgum (Liquidambar), not sycamore. What goes with pollen.com?


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

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