Saturday, April 1, 2017

A Sneaky Downslope Windstorm

Last night a downslope windstorm impacted the Wasatch Front, including the eastern Salt Lake Valley.  Peak wind gusts reported to MesoWest include:

US-89 at Park Lane: 68 mph
Centerville: 61 and 64 mph (Two sites)
University of Utah: 58 mph
Farmington: 57 mph
Bountiful Bench: 51 mph

As far as downslope windstorms go, this was a modest event.  It was, however, considerably stronger than I anticipated early (yesterday) Friday morning.

There are a couple of reasons why this was a sneaky windstorm.  One is the uncertainty in the amplification of the upper-level trough.  We've discussed this already a couple of times in the context of mountain snow, but it also plays a role in the windstorm forecast.  The second is that the models, while calling for easterly flow, were not giving indications of a significant downslope wind event until the runs initialized yesterday morning.  Easterly flow is not sufficient to give strong winds along the Wasatch Front and often with a "wrap-around" situation we don't get them.  Thus, one looks for key ingredients in larger-scale models and indications of strong easterlies downsloping into the bench areas in the high-resolution models.  For example, if we look at the 3-km NAM initialized at 0600 UTC 31 March, however, one can see a forecast of weak easterlies penetrating across the Wasatch at 0600 UTC (midnight MDT) last night and then a rapid transition to northerly flow immediately to the east.  This is a scenario that one might expect some enhanced easterlies, but nothing as strong as observed last night.

Forecasts began to trend, however, yesterday morning.  For example, the 1200 UTC initialized 3-km NAM, which began to forecast some periods of stronger easterlies penetrating across the Wasatch Crest, especially along the northern Wasatch Front, overnight, such as shown below.

More disconcerning was forecasts by the HRRR.  The HRRR only produces forecasts out to 18 hours, so it's only used in the sort-range portion of the forecast process.  The 1500 UTC 31 March initialized HRRR looked quite a bit more favorable for a downslope wind event.

Forecast discussions from the National Weather Service show the progression of thinking concerning the potential for downslope winds.  Here's the discussion from 5:25 AM Friday morning:
"The other concern is the potential for downslope winds tonight through Saturday morning along the northern Wasatch Front. The models have trended stronger with easterly 700mb flow as well as developed tighter thermal packing along the Wasatch ridgeline this evening. However, the surface pressure gradient is not particularly strong and seems to be oriented in more of a northerly fashion. Additionally the hi-res models which typically hit these events hard seem to be confining any stronger flow to the upper ridgelines and are not developing strong downslope winds downstream into the northern Wasatch. This is causing a fair amount of uncertainty in terms of how this event will actually play out, and as such have held off on the issuance of any wind headlines, although there is certainly some potential for strong easterly winds this evening. Will allow the day shift the luxury of another model run or two, as well as a chance to coordinate with partners."
By 10:24 AM Friday, the models were beginning to line up with stronger easterlies, although there were still some uncertainties.
"Winds will continue to be a concern for tonight but more of the downslope variety for the Wasatch Front. The GFS continues to trend with widespread 45-50kt winds at 700mb across the northern Utah mountains, but cold advection across the terrain is a bit stronger than what it was showing yesterday. The NAM is even more enthusiastic with up to 60kt of 700mb flow noted. The MSLP gradient is still not ideal in the GFS or NAM, but the EC does have a better gradient along with around 40kt of 700mb flow. Given the model trends, will probably have to consider some kind of wind highlight with the afternoon package."
At 1:22 PM, they issued a high wind warning for the northern Wasatch Front.

I think the NWS did an excellent job on this event.  They were alert to the possibility and issued a warning with about as much lead time as was possible given the uncertainties involved.  Warnings are issued for events that have a very high probability of occurring, and that wasn't the case until yesterday.  They have provided longer lead-time warnings for other downslope wind events, but the characteristics of this one precluded that possibility.

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