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.
Credit: UW Atmospheric Sciences
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.
Credit: UW Atmospheric Sciences
Rain is still on track to return for next week, so enjoy your sunny Sunday while it lasts!
Charlie
6 Comments
Charlie, I am a newbie to this site, but am enjoying it. I have a question for you or anyone else involved with Charlie’s Weather. My wife and I are thinking about relocating to Eugene. We have heard that in a normal year (so what is normal these days of global weirding) Eugene has a wet season from November to May and a dry season from May to October. Has that been your experience? Sharon2047
Hi Sharon! Thanks for joining, and I’m glad you are enjoying the site! In my experience, the heaviest rainstorms occur from Halloween through Valentine’s Day, with frequent (but decreasing) showers after Valentines day. The ramp up into stormy season is a lot quicker than the ramp down into our quiet season. The hottest months are July and August… it can reach 100 on occasion, and it is very dry. I’ve attached some climate averages for Eugene below.
Thanks, Charlie! A couple of follow-up questions: are there data yet on the impact of global warming on this pattern of 4 months dry (quiet), 5 months wet, and 3 months transitional? Also, do El Nino and La Nina events affect this pattern? Sharon
Hi Sharon. Research done by Mike Warner, Eric Salathe, and Cliff Mass at the UW suggests that the flooding “atmospheric river” events we see in the winter will become more intense and carry more moisture. Dr. Mass and others have also found that global warming will result in an increase in low, marine clouds during the late spring/early summer. Mountain snowpack in the winter will decrease dramatically at lower elevations, particularly after 2050.
El Nino events tend to make us warmer and slightly drier, while La Nina events tend to make us cooler and slightly wetter. However, some years are exceptions – the winter of 2015-2016 was the wettest winter on record for the Pacific Northwest, and it was also one of, if not THE, strongest El Ninos on record. Our biggest storms have tended to come during neutral years, but given our short period of record, it’s still not certain whether this is coincidence or whether there is a strong link. There does appear to be some correlation though.
El Nino and La Nina events primarily affect our weather from January through March. They have a relatively small effect on our weather throughout the late spring, summer, and autumn.
Thanks, Charlie! Now I know more about what to learn about on your site – like atmospheric rivers and marine clouds. I appreciate your time and efforts. Sharon
Of course, my pleasure!