Warmest Day Of The Year Tomorrow!

After such a cold, snowy winter and the strongest April windstorm in the Portland metro area since April 14, 1957, I’m sure many of you will be relieved to hear that tomorrow is shaping up to be the warmest day of the year! It’s a far cry from the mutant heat wave we saw last April, but the way this winter and spring have gone, I don’t think we can afford to be too greedy.

Tomorrow won’t be particularly sunny, as a low pressure system to our southwest will give us increasing clouds throughout the day. However, a weak thermal trough will be over our region, and this will help us reach reach our warmest temperatures of 2017.

Thermal troughs are local areas of low pressure that are generated by warm air masses. For us, they form when an upper-level ridge of high pressure is just to our east, forcing warm, continental air to flow into our region as it circles clockwise around the high. Thus, we get a tongue of warm air extending to the northwest from the Intermountain West or Central Valley of California, and since warm air is less dense than cool air, this tongue of warm air is associated with a trough of low pressure. With a trough of low pressure over our region and higher pressure to our east, we get offshore flow, which warms us up further due to two reasons.

First off, the Pacific is really cold! I don’t care how sunny you are or how warm the temperatures are below the surface – if you are getting a stiff breeze coming off an ocean that stays in the low-mid 50s the entire year, you are not going to be hot. When you have offshore flow, the moderating effects of the Pacific are much less of a factor. And yes, this means that offshore flow in the winter generally cools us off, particularly for those near the Columbia River Gorge.

Second, we have a large mountain range to our east that offshore flow sinks down when coming into the Pacific Northwest. When air sinks, it warms at the “dry adiabatic lapse rate” of 9.8 degrees C per km. This is due to the compression of an air parcel as the air pressure around it increases with decreasing altitude, and while the total heat content of the parcel stays the same, the parcel becomes smaller, which means the temperature must go up. There is another adiabatic lapse rate known as the “wet adiabatic lapse rate” which applies only when water vapor condenses into water droplets as air rises and cools. The wet adiabatic lapse rate varies with pressure but is generally around 6.5 degrees C per km, and is less because latent heat is released when water vapor condenses into water droplets, warming the atmosphere in the process.

Thermal troughs are responsible for almost all of our record breaking heat in the spring, summer and early autumn, and are most common over our area during the summer and early tend to become more common as the summer transitions into autumn. The aforementioned “mutant” heat wave of April 7, 2016 was was given delivered to us via a thermal trough, as was the even more insane heat wave of April 18, 2016. Not surprisingly, April 2016 was the hottest April in recorded history for many places throughout the the Pacific Northwest.

Tomorrow, temperatures are expected to be warmest near the foothills, where offshore flow is strongest. Expect widspread upper 60s for the Willamette Valley, with some places making it to 70.

Rain is still on track to return for next week, so enjoy your sunny Sunday while it lasts!

Charlie