From The Cliff Mass Weather Blog
Cliff Mass
A tragedy occur yesterday (Sunday, September 4) with the crash of a float plane over Mutiny Bay of southern Whidbey Island just after 3 PM (see map below, the star indicates the accident site).
The aircraft was en route from Friday Harbor to Renton. Like all aircraft accidents, it will be examined comprehensively by the National Traffic Safety Board (NTSB), which is very valuable. Flying is extraordinarily safe because the causes of past accidents are painstakingly investigated and the system is improved as a result.
Several folks have asked me whether there was a meteorological contributor to this terrible occurrence, and after examining the data, I did find something significant.
Let me show you now.
During the exact time of the accident a front was moving through the region, and this passage, interacting with local terrain, produced a very sharp low-level wind shift in the lee of the Olympics.
Here are the surface observations at 3 PM. To the south of the accident site the winds were from the south (the barbs face the direction from which the wind is coming) while to the north the winds were from the northwest.
Temperatures were much warmer in the southerly flow compared to the northerly flow.
Such convergence of low-level airflow often accompanies frontal passage over Puget Sound and often produces a band of clouds and precipitation: the infamous Puget Sound Convergence Zone.
This transition can be very sharp, as a cool wedge of cold, northerly air undercuts and pushes aside warmer southerly flow (see schematic vertical cross section below).
To see how sharp the transition was, let me show you the observations from a surface observing location (Bush Point), just north of Mutiny Bay (courtesy of the WeatherUnderground).
The bottom panel is wind direction. Wow… a VERY sharp, almost instantaneous wind shift from southerly to northerly at 3 PM. The top panel (red line) shows temperature, which plummeted after the windshift. Other nearby observations showed the same thing.
The visible satellite image 3 PM shows a band of clouds associated with the windshift line.
Really not that impressive and the weather radar showed no active precipitation with the windshift line (see below).
3 PM weather radar
The vertical structure of the leading edge of the wind shift line (illustrated by the figure above) would be a narrow wedge of northerly winds at low levels, with the southerly winds right above. High-resolution UW WRF model simulations indicated such a structure (see simulated sounding with height at 2 PM for the point in question). The Camano Island radar noted the same thing.
With all this meteorology in mind, let’s get back to the float plane accident. According to flightradar24 (below), the plane was flying around 600 ft AGL, climbed to around 1000 ft, and then rapidly lost altitude at 3:09 PM (2209 UTC). Aircraft speed was around 125 knots earlier and then lost speed just after 3:08 PM.
Flying south they had the northerly winds behind them. Then they hit the convergence zone line. Moving from northerly to southerly winds could explain their rapid loss of speed. Although their ground speed would decline, their airspeed could increase, causing more lift. Did that contribute to their increasing elevation at the end?
Not only did the wind reverse suddenly, but there would have been considerable turbulence due to the strong vertical wind shear associated with the convergence/windshift line.
Did this have anything to do with this accident?
I do not know. All I can say is that the accident occurred in an area of an active wind shift and potential low-level turbulence. Is it a coincidence that the accident occurred simultaneously with their traversal of the big wind shift?
