Hi to all. Yesterday I was discussing with colleagues why the temperature reach so extreme values in the coast of Canada and north US. An interesting hypothesis I found is the next:
In the west region of the upper tropospheric ridge (or dome), a cyclonic circulation was present in the low and mid-troposphere (1000-600 hPa), moving northward parallel to the coast as the ridge moved to the northeast. This low pressure system (L in the picture) enhanced strong easterlies in the lower troposphere, bringing warm & dry air from inland to the coast, but also producing an additional warming by subsidence when these winds descend from the mountains to the valleys and shores (process known as lee winds or föhen effect).
Once the upper tropospheric ridge passed, the low pressure system moved far north, now bringing westerly winds to the Seattle area and dropping the temperature from 40ºC to around 25ºC in just a matter of hours (according to this blog, a record dropping temperature too: link). This coastal low was very marked in the 850 hPa and the surrounding levels, and could be the consequence of a mass compensation mechanism, but also, the projection of the upper troposphere trough, which is in fact part of this kind of REX-blocking pattern. It seems like the projection towards the surface was very efficient, leaving the coastal areas without the classical sea-breeze cooling air for several days.
Temperature in Seattle reached 42.2ºC, the highest temperature ever recorded. Other locations also hit all-time records in maximum temperature for several days. Valleys and inland areas almost hit the 50ºC temperature, an extremely weird value for a «»»near»»»-arctic regions. In the case of Seattle, the picture below also shows how the pressure drop to around 999 hPa the day of maximum heat. It is not an effect of the temperature. The low pressure system observed in the lower troposphere had this northward displacement, producing this decrease in surface pressure. Once the low move far norther than Seattle, pressure started to increase again and temperature cooled.
José Vicencio [Meteorologist] 