May 08, 2020

Stairway to Meteorological Heaven for the Northwest

The next four days will be stunning:  sunny, warm, with many locations enjoying temperatures in the 80s.  But there will be interesting subtleties that the meteorologically informed will enjoy.

Picture courtesy of Ferd Frederick

Let me describe the steps to weather heaven.

The First Step

The surface weather map for Thursday at 5 PM shows step 1 (solid lines are sea level pressure, barbs are surface winds, shading is low-level temperatures).  Higher pressure builds inland over southeastern BC, Idaho, western Montana, and Wyoming.  The resulting offshore pressure difference (pressure higher inland than along the coast) forced easterly (from the east) flow, which warmed as it descended the western slopes of the Cascades, Rockies and northern Sierra Nevada. The warming caused pressure to fall along the Oregon coast, producing a thermal trough that pushed northward out of the Central Valley of

California.   With the core of the trough south of us, a large north-south pressure difference formed, causing the very strong northerly winds of  yesterday.  Ironic.... strong, cool northerly winds are a sign of incipient warming!

But while it was relatively cool and blustery yesterday, something important was happening above our heads here in western Washington: easterly winds aloft from eastern Washington was rapidly warming the air above us!

Below is a plot of the winds and temperatures above SeaTac airport over the last 24h from data provided by aircraft.  Red lines are temperature in degrees C, the wind barbs are blue, time increases to the left, and height is shown on the Y-axis (in pressure.... 700 is about 10,000 ft). 

Yesterday there was shallow northerlies, as noted earlier, with some easterlies aloft.   Note how the easterlies strengthened, deepened and lowered, and are now just above the surface. The air warmed by compression as it sank into eastern Washington.   In fact, the air above Sea Tac Airport warmed rapidly during the day yesterday....from 1 to 8C around 5000 ft (850 on the chart).  A 7 C warming is about 13 F.  Nice.

With so much warming above us, a strong inversion (temperature increasing with height) formed above us last night.

The Next Step:  Today

With clear skies and a strong sun, the surface warmed rapidly this morning, which caused the atmosphere to convect and mix.  This mixing is bringing down the warm air from aloft to the surface.  As a result, temperatures are surging this morning ...and will reach the mid-70s by early afternoon. 

Take a look at the plot of temperatures at SeaTac Airport for the past 48 h. Wow.  At 11 AM it is already 72F at SeaTac, having risen 20F in 4 hours.  Upper seventies are certain in areas away from the water.

The Third Step:  Heaven

By tomorrow (Saturday), the coastal low pressure area...the thermal trough... will move northward to the Washington Coast.  The easterly downslope flow on the western sides of the Cascades will increase.  Temperatures will zoom into the 80s.  Relative humidity will be very low.

The forecast of surface (2-m above the ground) temperatures for Saturday at 5 PM shows a surprising result.....western Washington is warmer than eastern Washington, with 80S over much of the lowlands.   The Willamette Valley will be even warmer.   Why is the west side warmer than the Columbia Basin? 

Because of sinking air on the Cascade and our isolation from the cool, marine air (because the flow is coming from the east).  Plus, eastern WA is getting some of the cooling influence of cold air to east of the Rockies.

Heaven will remain on Sunday--Mother's Day.  Perfect weather for a walk in one of the newly reopened parks. 

And keep in mind that the strong solar radiation with bountiful UV radiation this time of the year is bad for the coronavirus, something highlighted by new research.  And the UV radiation on your skin will aid in the production of vitamin D, which some studies suggest is helpful in fighting the virus.  The UV forecast from EPA shows relatively high values for the Northwest on Saturday (see below).


  1. Love the forecast...but I will not be trudging off to public gathering places like the beach, parks,etc...I fear that this weather will trigger in people the false feeling that corona virus is atrophying...and it is not!..I will be hanging out in the backyard, maybe going for a neighborhood walk, keeping my distance from others. At my age, I feel that I have a big bullseye on my back, for the virus to aim for!

  2. Stairway to Heaven--great analogy for this weekend! I am so looking forward to it and Mother's Day too. Thanks for the good news, Cliff!

  3. Wow. Right now it's 8:44 AM and the temperature in Bellevue is EXCEEDING 70F. I do not know why or how the temperature rose so fast this morning, but it did. This is truly an extraordinary event.

    1. It was explained well in the blog. The thermal trough that was moving north must have reached Whatcom county around that time. So the easterly flow must have really increased on Saturday morning thus preventing any cooler flow off the water from moderating the temperature causing a temperature spike.

  4. It's getting hot I could sure use a haircut

  5. While the PNW has record highs today, here in the great lakes region we had an all time May record low of 23 degrees this morning. Not heaven weather for us.

  6. The KOMO website has today's listed morning low as 63 degrees. If this is correct, it looks like it'll tie the monthly record for maximum lows about three weeks ahead of where the record sits. It's also 11 days earlier than the previous earliest date Sea-Tac failed to dip below below 60 degrees during the night. I'm skeptical of KOMO's reading because it's such an outlier, but if it's true it's kinda neat.

  7. Large diurnal temperature variation in NW Bellingham yesterday and today. On Friday, the temperature rose from a morning minimum of 41F to an afternoon maximum of 79F and on Saturday, from 46F to 83F. These maximum temperatures are the highest at my location since August 2019. Pretty uncommon to be running the furnace and the AC on the same day around here.


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