For many places in western Washington, temperature stayed fairly excessive in a single day.
Normally, when there are clear or almost clear skies aloft, temperatures cool quickly within the night because the earth radiates warmth to house.
But a lot much less so final night time.
Consider the temperatures at Sea Tac Airport over the past week (beneath). Red strains are the noticed temperatures and the purple/cyan strains are the typical highs and lows, respectively. The highs on the final two days had been manner above regular, however take a look at the lows! The low temperatures had been roughly the identical as the standard excessive temperatures.
But there’s extra attention-grabbing elements of the low temperatures. Below are the minimal temperatures final night time (click on on picture to broaden). In locations like Cle Elum temperatures dropped into the 30s, and in a lot of jap Washington 40s had been prevalent. But western Washington was manner hotter, starting from the LOW SEVENTIES to the mid-50s. It was notably alongside the Cascade foothills in locations akin to Enumclaw. Why? And SeaTacwas hotter that Boeing Field and North Seattle. Why?
The principal clarification: sturdy easterly winds above us.
Easterly winds (wind from the east) end in air sinking down the western slopes of the Cascades, warming by compression because it declines from roughly 5000 ft to sea degree. Such compression causes adiabatic warming, by roughly 10C per kilometer (roughly 27F in 5000 ft).
During the final two nights the easterly winds had been fairly sturdy aloft, one thing proven by the winds above Sea Tac over the previous 24 h (see beneath). Time is alongside the X axis, top within the Y axis (700 is round 10,000 ft, purple is temperature, winds in blue). Easterly and southeasterly winds of 15-25 knots had been proper above us).
During the day, floor heating causes mixing that brings down the nice and cozy air to the floor…that’s the reason we had file breaking heat on Saturday (and later in the present day).
But at night time, there’s a battle happening. Without photo voltaic heating, there’s cooling because the floor radiates warmth to house. Without any mixing of heat air down from aloft, our temperatures would cool considerably.
But the sturdy winds aloft trigger sufficient turbulence on their very own to combine heat air down, maintaining the temperatures up at night time. Places with the strongest easterlies (from Sea Tac Airport to Encumclaw) downstream of the Stampede Gap within the Cascades had the best temperatures. And the best warming tended to be within the foothills of the Cascades–places uncovered to the uber-warm downslope air. The temperature prediction by the UW high-resolution mannequin at eight AM this morning exhibits this impact. Warm temperatures had been additionally discovered downstream of the Olympic Mountains.
And in locations just like the University of Washington, you can see the “battle” going down (see beneath). The plot beneath exhibits situations on the high of the UW Atmospheric Sciences constructing in the course of the previous day, with time growing to the proper. The high panel is wind velocity, the second is wind course, the third exhibits temperature (black) and dew level (purple) and the fourth is relative humidity. Temperatures solely dropped into mid-60s.
But look fastidiously. The temperatures, dew level and relative humidities had been leaping up and down in a single day (indicated by purple arrow). Now take a look at the wind course….leaping between northwesterly circulation (cool, low degree air) and northeasterly air (heat downslope air). Turbulence as sometimes mixing the nice and cozy air down….and when it did the relative humidities would drop.
As quickly because the solar got here up the battle was over….the nice and cozy, downslope air was being blended to the floor. And we’re going to get very heat due to it.