Branches always fly during the first storm of the winter and even a few trees were lost. Forecasts were decent.
But the most interesting aspect of the storm was not the general power outages and wind gusts, but the strong downslope winds and AMAZING and sudden jump of approximately 20F around Port Angeles, on the northern Olympic Peninsula.
Lots of power outages occurred in Seattle yesterday with the strong winds.
Here is the observed temperature plot at Port Angeles during the past few days (time in UTC/GMT and increases to the right). Yesterday, the temperatures surged by 20F over roughly 2 hours, from around 46F to 65F, stayed there a few hours and then dropped to around 40 F over a few-hour period. Talk about extreme temperature changes!
Mama MIA! What was happening there?
The answer: a strong mountain wave circulation forced by the interaction of increasing southerly (from the south) flow and the Olympic Mountain range. This resulted in strong downslope flow descending into the Port Angeles area. The air warms rapidly as it sinks due to something known as "adiabatic compression" resulting in the brief heat wave.
Let's take a closer look at the conditions at Port Angeles during the crazy heat wave. The plot below
shows the sustained winds (blue lines), gusts (red lines) and sea level pressure (green lines) over the same period. There were two events (low centers) that occurred during the period shown, with the second (Friday's) being the stronger one. Winds surged to around 40-45 kts, just as the temperatures surge. A separate plot (now shown) indicates that the strong winds were from the south, thus descending down the steep northern slopes of the Olympic mountains.
And look at pressure (green line)--a period of very low pressure occurred when the temperatures surged....keep this in mind, it will be important.
A plot of the temperature, wind gusts (red numbers), wind barbs, and dew point (blue numbers) show the conditions around 2 PM Friday. Strong southerly winds reached Port Angeles (gusts to 45 knots) and you can see the localized warm temperatures (click on plot to expand).
The winds were strong enough to down trees and cause extensive power outages on the north side of the Olympic Peninsula.
So why was there strong northerly winds descending down the Olympics into the Port Angeles area?
A large-scale low pressure trough was approaching the Washington coast, increasingly southerly winds reaching the Olympic mountains. The air pushed against mountains and a mountain wave developed over and downstream of the mountain crest. As the air rose, it cooled as it expanded, with the cooling causing the air to become more dense, resulting high pressure on the windward (southern) side of the Olympics
Subsequently, as the air sank on the northern side of the Olympics, it was compressed and warmed. Since warm air is less dense, an area of low pressure (or a pressure trough) developed over the northern slopes of the Olympics. To illustrate this situation , here is a simulation of the sea level pressure (solid lines) and wind speed (shading) for 10 AM on Friday. You can see the high pressure/low pressure couplet and the large pressure difference between them. That pressure difference accelerated winds to the north and down into Port Angeles.
The flow configuration is called a mountain wave, because of the wave-like up and down of the airflow, something illustrated by a SW-NE vertical cross section across the Olympics at 10 AM Friday. The air flow is roughly parallel to the solid lines (potential temperature) ....you see the wave-like structure? The shading show clouds-- plenty of them on the SW side of the mountain, but none in the descending air to the NE.
And as noted above, the descending air rapidly warms, creating the warmth on the north side of the Olympics. The high temperatures recorded in the region yesterday show the extent of the warmth, with the highest temps around Port Angeles (click to enlarge).
Clearly, the incoming flow was just right for the development of strong northerly flow and warmth on the north side of Olympics. But unfortunately, our friends around Port Angeles will have to accept the fact that they won''t always have Los Angeles weather in December.
from Cliff Mass Weather and Climate Blog https://ift.tt/2EkZybw
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