14:20 Thu Apr 05, 2018
One Windy Day
Although not quite the storm I was hoping for, the storm that has affected the summit over the past 24 hours has still been very impressive due to the duration of our high winds. For 10 hours straight we observed winds equivalent to a category 3 hurricane (111 mph or greater), with 16 consecutive hours of 100 mph or greater winds. Despite the long duration of winds of this magnitude, our peak gust from the storm was “only” 120 mph, meaning these were extremely steady, but very strong winds. We’ll take a look below at the overall set up for this storm, why we were so excited for high wind potential, and one theory as to why the summit did not see higher winds.
It was anticipated that the highest winds with this storm would occur on the tail end of the system, as high pressure situated over the Southern Appalachians began building into New England from the southwest.
GFS Lamp model numbers for Mount Washington roughly 18 hours ahead of the highest expected winds. Highlighted are gusts that were expected to reach or exceed 112 mph for 7 consecutive hours!
GFS Model showing the expected surface features across the U.S. Note strong low pressure (974 mb) over eastern Canada and a 1024 mb high pressure system. Image courtesy of Tropical Tidbits.
A very tight pressure gradient between the strong storm system that tracked through the Saint Lawrence Valley and this high pressure set the stage for widespread high winds across New England. This type of set up has resulted in some of the highest wind events ever recorded on the summit (gusts of 154 mph in 1996 and 175 mph in 1942 come to mind, to name a few), but for a system to produce winds of that caliber, a few other factors need to align. Firstly, the location of the high pressure is extremely critical to the strength of the winds (by setting up the steep pressure gradient between the passing Low and the building High pressure systems). Additionally, the atmospheric profile must show a few key elements as well.
Surface weather features for Dec 3rd, 1942. Note the similarities in the location of high and low pressure to our most recent storm. Low pressure on this date was significantly stronger than the more recent storm, however, and resulted in a peak gust of 175 mph!
Forecast sounding around the time of highest winds on the summit. The red line (temperature) shows a sharp increase starting near 800 mb. Mount Washington was at about 785 mb during this time, or within this temperature inversion, likely contributing to a more steady, less gusty wind.
These factors are where our current passing system met its downfall, in terms of generating impressive wind gust values. The summit was situated in a very strong and relatively deep temperature inversion, which likely dampened our gust potential. This inversion, or increase in temperature with height seen in the forecast sounding above diminished the air turbulence within that layer and resulted in a very steady wind. This is at least my leading theory on why we did not see higher gusts as one would typically expect with winds of this strength. Still, this was one of the more impressive storms of the winter season, and one I won’t soon forget!
Tom Padham, Weather Observer/Education Specialist