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.