Short Heatwave Followed by Normal Temperatures and Precipitation

It is now clear that our region will experience a short-period heatwave on Sunday and Monday, with the highs in Seattle reaching near 90F and eastern Washington climbing into the upper 90s.

This is after a relatively cool start to June and the expectation of normal temperatures the rest of the month.

A plot of the temperature at SeaTac, with observations (blue bars), and record highs (red) and lows (light blue) shown, indicates that we have generally been cooler than normal this month.  Record highs on any day are generally around 90F.

The crazy record high in 2021 is evident at the end of the month.  

To put this month in a better long-term perspective, I have plotted the average maximum temperatures at Seattle from 1-10 June for the past 50 years (a trend line is also plotted).

We have started this month, a bit below normal.  But you will notice a small upward trend (brown line) since the 1970s.   That could well be the influence of human-caused warming produced by increasing greenhouse gases.

Now turning to the forecasts, below is the latest National Weather Service forecast for Seattle. Friday and Saturday will be comfortable, but then temperatures increase, reaching 90F on Monday, before declining to normal next week.

Eastern WA will also warm, with the Tri-Cities climbing into the upper 90s before cooling into the mid-90s.   These are not exceptional temperatures in the central Columbia Basin.

 After this warming period, substation precipitation is predicted to return to the region, as shown by the total accumulation through thourgh June 25th (below).    Particularly heavy in British Columbia, which is very good for the Columbia River.  Very favorable for our power generation and irrigation needs.

Recent substantial rain has contributed to Yakima River storage, which is almost exactly at normal levels.   Very good for irrigation in the Yakima Valley this summer.

Meteorological Roller Coaster

June is an interesting weather month in the Northwest.

The sun is very strong, so with the right conditions, temperatures can zoom up to extremes, such as the stunning heatwave of late June 2021 (108°F in Seattle).  On the other hand, with moist onshore flow, our temperatures struggle to rise into the 60s.  The Pacific Ocean off our coast is only around 50°F.

During the next week or so, we will experience a prime case of a meteorological roller coaster.

Consider the latest forecast for Seattle from the European Center model (below).

A high today (Monday) in the upper 50s, with a slow climb into the 70s by Saturday.   Then a sprint to around 90 degrees, followed by a nearly 20F decline next Wednesday.  This is followed by a rise into the upper 80s, with a huge cooling in the subsequent days.

My head is spinning!

The potential to get warm in June is illustrated in the climatological highs (red) and lows (light blue) below for Seattle, with the temperatures this year shown by the dark blue bars (brown indicates the average range).

Once we get past mid-May, high temperature records reach the 90s.  Once in June, low temperatures remain above 40F.

But to get the heat, as will occur in a week, requires offshore-directed flow that pushes the cool, marine influence offshore.  Such easterly (from the east) winds result in downslope flow on the western sides of the Cascades.

Downslope flow warms rapidly by compression as air traverses from low pressure aloft to high pressure near the surface.  

Today (Monday), a very wet weather system is moving in (see satellite image), with clouds, precipitation, and very cool onshore flow.

But in a week, high pressure will build inland, producing offshore-directed winds and profound warming.  

Consider the forecast for Sunday morning for sea level pressure (solid lines), low-level winds (the wind barbs), and low-level temperature (shading), shown below.

A transient high has moved east of us, producing winds from the east to the west. The air warms as it sinks on the western side of the Cascades (pink colors).  Warm air is less dense than cold air, which results in lower pressure over western Oregon and Washington (called a trough in the weather business). 

This situation won't last long, because the high pressure to the east will move away.   As it does so and the winds from the east weaken, the winds from off the ocean will surge in, producing rapid cooling--something called an onshore or marine surge.  

To illustrate, the same map for Wednesday morning is shown below. The reds on the west are replaced by cool greens.  You may need a sweater again.

As discussed in a previous blog, the "locked up" pattern of the past winter has been replaced by one in which pressure systems progressively move through the region.  Thus, we will experience a range of weather conditions, from warm and dry to cold and wet, and meteorologists will have less boredom to contend with.