Amongst the oddities of our most recent event is the duration of the easterlies. Below are 750 mb (near crest-level for the northern Wasatch) wind analyses for 1200 UTC (0600 AM MDT) yesterday and today showing a dramatic drop off in the strength of the easterly crest-level flow.
Although this dropoff can also be seen in mountain-top and pass observations from the Wasatch Range, the corresponding dropoff in wind speed at the base of the Wasatch Mountains is weaker. For example, at the top of the Strawberry Bowl at Snowbasin Saturday night, winds were generally around 50 mph with gusts to 70. Meanwhile, in Farmington (US-89 at Park Lane), winds were just a bit stronger. During the day yesterday, winds at both sites weakened commensurately. However, last night and this morning we clearly entered a new regime. While the flow at the top of Strawberry Bowl weakened, winds of about 30 mph with gusts to 40-50 mph persisted at Farmington.
This potentially suggests two differing regimes for flow dynamics across the Wasatch. In the first, strong flow near the crest plunges into the lee, but experiences only a small acceleration (Saturday night and Sunday morning). In the second, the plunging flow strongly accelerates into the lee (last night and this morning). At least in this event, the latter does not lead to damaging winds, but that's not necessarily the case in all events.
Of course there is always the possibility that the localized nature of the Strawberry Bowl observing site makes it somewhat unrepresentative of the flow moving across the Wasatch Range immediately east of Farmington. It's unfortunate that we don't have a wind observing site in that part of the Wasatch Range and perhaps installing one should be a high priority for the state given the severe impact these events have on the northern Wasatch Front.
It would also be helpful to have soundings taken during future events from along highway 66 south of Morgan. My colleagues at the University of Utah have done this for a couple of events, but it would be great if we could find a way to do it regularly during events. Combined with high-resolution modeling, we could tease out some of these oddities and potentially better predict some of their more fine-scale aspects.
Jim - I'm a new follower and have loved the blog. Interested in your thoughts. In surveying the damage from the windstorm yesterday on the south ogden bench, observed that the higher we got on the bench,the LESS severe the damage. Is this expected downslope behavior or just a terrain based anomaly?
ReplyDeleteTough to say. For idealized, smooth topography, the strongest winds are usually very near the "hydraulic jump" where the flow separates from the surface and there's usually a rapid drop in wind speed as you move away from the mountain. Based on this thinking, I'd probably expect the strongest winds to not be on the bench but a bit further west.
DeleteThe problem with this thinking is that the real world is a complicated place. We're not dealing with smooth topography or a smooth land surface. In addition, wind damage can be produced by brief, intense periods of wind and those are more chaotic than the average wind say over period of several minutes to hours. Wind damage can also be sensitive to other factors, including construction characteristics, tree type, etc.
Bottom line is that this is an interesting question and we would probably learn a lot from careful damage surveys from many events showing where the damage occurs.
I tend to be of the opinion that the Uinta Range has effects on Wasatch Front weather to a greater extent than one would suspect at first glance, and these windstorms are one example. A NE-SW oriented surface pressure gradient like in this recent event results in a huge amount of channeling of easterly flow along the northern flank of the Uintas, particularly if there is much of any low-level stability. It looks like we had only modest low-level cold advection in this event but some of the winter events channel a lot of artic air along the I-80 corridor from east of the continental divide, and the Uinta Range often delineates a sharp separation of air masses. It determines to a large extent where easterly winds interact with the Wasatch Range, although the topography of the Wasatch is very important also.
ReplyDeleteThe role of the Uintas has been examined by colleague John Horel here at the U and John Lawson. See http://journals.ametsoc.org/doi/abs/10.1175/WAF-D-13-00120.1. Full paper access may be paywalled.
DeleteThanks for the link!
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