The Last Heatwave of the Year

After examining a wide variety of forecast models and considering the region's climatology, it appears we will have to endure only one more warm spell for the remainder of the year.

And by a warm spell,  I mean an event where western Washington rises into the mid-80s or above and eastern Washington nears 100F.

 I will start by showing you the forecasts.  

At Seattle, the skillful NOAA/NWS National Blend of Models (NBM) prediction system forecasts warming to the mid-80s on Thursday and Friday before cooling over the weekend.  Temperatures cool to below normal (upper 60s) next week.  The average high-temperature today in Seattle is 75F for reference.

As a check, consider the highly skillful European Center model ensemble of many forecasts (the mean of these forecasts are shown below).  Quite similar to the NWS prediction.... and this forecast extends through mid-month.

 

Next moving to the Tri-Cities, the prediction for Pasco shows a torrid high above 100F from Thursday to Sunday, with cooling to the lower 80s next week

The temperature anomaly (from normal) forecast (by the European Center) for the mid-month (Sept 13-20) indicates cooler than normal temperatures over western Oregon to near-normal temperatures for the remainder of the region.

Solar radiation and the length of the day rapidly decline in September making it increasingly difficult to get into the 80s by the end of the month.  Check out the monthly solar radiation at Seattle for last year to see what I mean.  September values are WAY down from July.

So why is the heat wave followed by cooling?    

Easy to explain by looking at the upper-level height/pressure pattern at 500 hPa pressure (around 18,000 ft above sea level).  The shading shows the difference from normal, with red indicating ridging or higher pressure aloft.  Blue is roughing or lower than normal pressures.

On Thursday, a strong ridge will develop over British Columbia, with the center in an ideal location to warm Washington State. If this happened in July, we would be talking about mid to upper 90s.

But by Monday,  the ridge had moved inland and a cooling trough of lower pressure will approach. Maybe even a few showers.

The situation later in the week is similar (Thursday).  So still cool.

So enjoy the heat, wear some shorts, and get that swim in.   The heat will soon be over and we will be moving towards fall-like weather starting next week.

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Reminder UW students:  I will be teaching Atmospheric Sciences 101 this fall:

And if you are over 60, you can take the class for $5 through the UW Access Program...either online or in person.

An Impressionist's Sky

 The sky this morning was beautiful (see below).   An impressionist's dream.  

What you are seeing is mid-level convection:  shallow cumulus clouds resulting from instability in the middle troposphere (I will explain this later).

French impressionists loved such beautiful skies, as illustrated by this lovely painting by Monet (The Port at Argenteuil)

Here is another one by Monet that was even more similar to what I viewed this morning.  Monet should have been a meteorologist.....

The Seine at Vetheuil

In 2024, we could do something Monet could not have dreamed of....to observe such clouds from space.  And for your viewing pleasure, below is what NOAA's GOES-West satellite observed at almost the same time as my picture above.  

Do you see the small cloud elements over western Washington and northwest Oregon?  These are the features of interest.

We can tell the elevations of these cloud elements by using another high-tech sensor system that Monet could not have dreamed of: laser ceilometers maintained by the Puget Sound Clean Air Agency.   These surface-based units use vertically pointed laser beams that scatter off clouds and smoke (see below).  The time it takes for a scattered beam to reach and return from the cloud provides the cloud height.

The clouds in question formed 5 and 6 kilometers above the surface (roughly 16,000 to 20,000 ft above sea level ) around 1:30 AM this morning.

But our high-tech bag of tricks does not end there! We have balloon-launched weather stations called radiosondes around the region, including Quillaytue on the Washington coast and Salem, Oregon.

Here is the temperature (red line), dewpoint (blue dashed line), and winds from thelayer radiosonde launched this morning at Salem, which is positioned in the middle of the cloud field.   Temperature (°C) is on the x-axis and height in pressure (hPa) is on the y-axis.  500 hPa is about 18,000 ft.

There is a layer (indicated by the red arrow) in which the dewpoint and temperatures are the same.  Such a layer is saturated (100% relative humidity) and is associated with clouds.  Matches the ceilometer height quite well.  The red temperature line also shows that the temperature was declining rapidly with height in that area, consistent with an unstable, well-mixed layer.

So why do we have these beautiful cumulus clouds in that shallow layer?

It was associated with an approaching low-pressure center aloft (see upper level, 500 hPa, about 18,000 ft)--as shown by the forecast map for 8 AM this morning.  In front of the low (to its NE) there is both upward motion and southerly winds.   Upward motion causes air to cool (air cools by expansion when as it moves to lower pressure), which causes the relative humidity to increase (because cool air can hold less water vapor than warm air).   The southerly (from the south) winds also helped move some moist air up from the Arizona "Monsoon" region.

Another high-tech capability is to view water vapor from space! Such imagery shows the moisture plume this morning

The motion of the low into the region today is going to initiate a significant onshore push of cool marine air tonight.  Tomorrow you should expect MUCH cooler temperatures and a lots of low clouds.

Enjoy the weekend.