Yesterday (Friday, April 28) was the first moderately warm day of the “summer” season in the Northwest, with temperatures ranging from nearly 90F along the western slopes of the Cascades to about 70F near the water. (see high temperature map below)
Why are the temperatures warmest along the western slopes of the Cascades?
Turns out the Friday heat was very three-dimensional. Water helps with cooling somewhat, but the situation is much, much more complicated than that.
We need to start with the sea level pressure model, with the model at 11 a.m. Friday shown below. High pressure east of Washington and low pressure over northwestern Oregon and southwest Washington resulted in easterly (from the east) winds moving over the Cascades. But at low levels around Puget Sound, winds are coming in from the north, as pressure is higher in northwest WA and lower in western Oregon. Near the mountains, the wind likes to go from high pressure to low pressure.
A diagram of the situation around Puget Sound is shown below (sorry, I’m not much of an artist). The easterly winds sinking down the Cascades heat up due to compression, and therefore very warm air is found on the lower western slopes of the fence.
But descending air cannot reach the western lowlands because of the cool northerly currents near the surface there. Cold air is denser than warm air and wins the battle near the surface.
Very three-dimensional!
Now, today’s (Saturday) temperature is going to be a complicated story again…but a very different story.
Pressure patterns have changed a lot. The low pressure west of the Cascades has now shifted to eastern Washington. Thus, the Easter celebrations on the Cascades were history. And with higher pressure offshore, cool sea air has begun to move in.
One sign of this shoreline sea air infiltration is the eastward movement of low-seas clouds this morning (see below).
Now, let me show you a fascinating number: the 24-hour temperature change ending at 11 a.m. Saturday.
Mama Mia! It’s a lot colder today along the coast where the sea air comes in. Yesterday, the eastward, offshore current made the coast quite warm
Note that it’s warmer today from Seattle to the north, where the cool northerly flow has weakened due to less north-south pressure difference as the low pressure has moved inland.
East of the Cascades today is generally slightly warmer.
Tomorrow, temperatures will drop in the west as a strong low-pressure system approaches the coast and an associated front makes landfall (see pressure and temperature forecasts for mid-Sunday below).
