Thursday, June 16, 2016

Yesterday's Big Blow

Some impressive winds were reported around northern Utah yesterday and I think it's safe to say that they exceeded forecast expectations.  Peak gusts at valley locations reported to MesoWest include the following:

Sherwood Hills: 82 mph (0210 UTC/2010 MDT)
SR201 at I-80: 80 mph (0130 UTC/1930 MDT)
Syracuse: 76 mph (0120 UTC/1930 MDT)
Great Salt Lake Marina: 73 mph (0136 UTC/1936 MDT)

Many mountain sites, especially in the northern Wasatch, also reported strong gusts.

The setup for the event was very similar to many other strong southerly wind events in the spring.  With a developing surface trough extending from the southern Sierra Nevada across central Nevada and northern Utah.
RAP sea level pressure analysis, surface observations, and GOES IR satellite imagery at 0100 UTC 16 June 2016
In such situations, strong surface heating typically has two effects.  One is to contribute to the intensification of the surface trough and the associated low-level pressure gradient.  The other is to grow a deep surface-based mixed layer or what meteorologists sometimes call a convective boundary layer or CBL.  Thermals driven by intense heating penetrate upwards through the CBL, and these updrafts are utilized by gliders for lift.  The updrafts are just one side of the story, however, as the CBL also features areas of descending motion.  These updrafts and downdrafts transport and mix momentum through the CBL and lead to gusty surface winds.

During the spring, meteorologists commonly use the 700-mb flow to get some idea of the potential for strong winds under such conditions, but late yesterday, the 700-mb flow wasn't really all that impressive with modest southerly flow (30-35 knots) east of the trough over Utah.  In the spring, I've seen stronger flow than this produce less impressive winds than we saw yesterday.

RAP 700-mb geopotential height and winds and GOES IR satellite imagery at 0100 UTC 16 June 2016
Yesterday's CBL, however, was remarkably deep and so we were mixing momentum through a much deeper layer, tapping into stronger momentum air above 700 mb.  As shown in the afternoon (0000 UTC/1800 MDT) sounding from the Salt Lake City Airport, the CBL extended to almost 450 mb, or about 20,000 feet above sea level.  Winds between 700 and 500 mb peaked at about 50 knots, suggesting that the 700-mb analysis doesn't capture the strongest winds in the CBL and thus the potential for high gusts at the surface.

Source: SPC
On the other hand, even looking at that sounding in hindsight, I'm not sure if I would have gone for 80 mph valley gusts.  Nevertheless, I'll be making a mental note to pay closer attention to the CBL depth and the strength of flow throughout the CBL, especially for potential events in mid June when the surface heating is so strong.


  1. Is there a good place for me to check wind speeds on various mountains? For instance, we were on the South Ridge of Superior this morning and it was gusting like crazy. I'd love to know what wind speeds were up there.

    1. You'll need to learn to navigate. I think the only near mountain-top location operating yesterday was Alta Collins: Peak gust of 55 there, although strength of the flow at that direction is very sensitive to wind direction.


  2. It looks like the higher gusts (60+ mph) were pretty isolated. I wonder if a sharp leading edge of some cooler mid/upper-level air briefly triggered a large amount of vertical mixing and turbulence. I bet it was a little exciting for local air traffic.