Only half-jokingly I call the period from middle spring into early summer, the roller coaster temperature season here in the Pacific Northwest.
During the last month or so we have experienced a number of roller-coaster temperature changes (see the temperatures at Seattle-Tacoma Airport below). An earlier major event around March 13 had a rise to about 73F, followed by a drop to 45F. Or the recent one around April 25th, with a rise to about 80F, followed by a drop to around 50F. Plus, several more minor declines.
The biggest one-day temperature changes in our area are NOT in the middle of winter when fronts and storms are strong, but during spring.
Don't believe me? Here is the proof from a paper I did a number of years ago. The figure shows the average number of days per month with one-day temperature drops of certain magnitudes.
For the biggest one-day declines (10°C or more), May is the biggest month by far. For somewhat lesser drops (7.2-9.4°C), June takes the lead. For moderate temperatures drops, the summer is tops, with August taking first place.
Temperature drops in winter are small in comparison.
How does one explain this bizarre state of affairs? Why do temperatures drop more when it gets warm around here?
As we will see, the big issue is the vast Pacific Ocean and the seasonally changing temperatures differences between land and water.
During winter, we are dominated by onshore flow off of a cool (roughly 50F) and vast Pacific Ocean. Air temperatures over land (west of the Cascades) are just a minor tweak of the ocean temperatures.
Weather fronts coming across the vast Pacific Ocean are heavily modified by their long traverse across water, with temperatures changes at low levels greatly weakened. A strong front coming off of Asia, with large temperatures differences at low levels, barely produces a few degree change when it hits are shores.
Yes, the Pacific Northwest suffers from wimpy fronts at low levels. Something I try not to admit to outsiders.
But in spring as the sun warms up and clouds abate, something changes. The land starts warming up, particularly east of the Cascade crest--and yes, even on our side to a lesser degree. But the eastern Pacific Ocean sea surface temperatures hardly change. Thus, a large contrast between ocean and land temperatures can develop over the Pacific Northwest. And there lies the answer.
When we have days with onshore (westerly) flow off the ocean, our temperatures are controlled by the ocean temperatures, and thus remain cool. By when we get periods of offshore (easterly) flow, our temperatures zoom up.
Cool, onshore flow is the default. But in spring, there are still upper level weather systems moving through, which can result in high pressure building west of us, resulting in offshore flow...and thus we can get periods of warming.
Here is the plot of wind direction at Hoquiam on the Washington coast for the past four weeks. Some major swings of direction as weather systems move by. But look closely around April 24th when we got warm...the winds were easterly (or offshore), pushing the cool, ocean influence out to sea.
The warming tends to take time as offshore flow brings warm air down into western Washington, but the influx of cool air (often associated with an approaching trough) comes in fast...and we have a name for it: the onshore or marine push.
Why are the largest temperature changes in May, rather than in early August when the temperature contrasts between ocean and land are greatest? Because weather systems that cause the offshore flow and incite rapid onshore flow tend to weaken during the summer over the midlatitudes.
Why? Because north-south temperature differences--the drivers of midlatitude disturbances--are less during midsummer.
Anyway, being on a roller coaster can be fun, particularly if you understand how it works.
from Cliff Mass Weather and Climate Blog https://ift.tt/2FWdQfF
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