Tuesday, June 7, 2016

Surprise East Wind Event

As a meteorologist, there are times when my forecast doesn't verify all that well, but I'm not surprised.  Given the chaotic nature of the atmosphere, one can often envision a range of possibilities depending on how things come together.

So what really gets my attention is when Mother Nature does something outside my envisioned range of possibilities.  Yesterday was such a day.  Yeah, the east winds were moderately strong in the morning, but what really shocked me was the intensity of the east winds yesterday afternoon and evening.  I went back through some of the NWS forecasts and they didn't mention east winds of such intensity in the zone forecasts that they issued yesterday morning either.

Sometimes the Area Forecast Discussion provides a broader perspective, but it also had no mention of the potential for strong easterlies.

We did have some moderate easterlies Sunday night and Monday morning, but downslope winds often weaken in the afternoon, so perhaps that was lowered expectations.  By the afternoon forecast packages the east wind were included, although even then, the forecast was for only 15 to 25 mph.

What ended up happening was that we had a fairly strong east wind event as cooler air pushed across the Wasatch Range late yesterday afternoon.  At the University of Utah things picked up just after 4 PM with a 6ºF drop in temperature, a drop in dewpoint and relative humidity, a shift of the wind from ESE to ENE, and an abrupt increase in wind speed and gustiness.

Source: MesoWest
The peak gust at that time was around 45 mph.  Maximum wind speeds of 36 mph with gusts to 55 occurred a few hours later near 8 PM.  There was some minor wind damage in the area.  Here's a downed tree along Foothill Boulevard reported by @john_strate via Twitter.

Looking at the weather maps yesterday afternoon, it appears that outflow from convective cells east of the Wasatch Range may have reinforced the large-scale easterly and northeasterly flow, resulting in the abrupt wind increase at the University of Utah.  These precipitation cells are evident in the analysis below for 2200 UTC (4 PM MDT) yesterday afternoon.

The MesoWest surface plot for 2200 UTC (4 PM MDT) shows the broad easterly and northeasterly flow over much of the region from Evanston to Randolph and from Kimball Junction to Powder Mountain.

Source: MesoWest
Note also that easterlies were observed along the entire northern Wasatch Front as well.  Locally cool air around Coalville and Wanship supports the view that the passing precipitation cell may have further boosted the event for the U of U.  These sites also exhibited a dramatic temperature drop shortly after 3 PM about an hour prior to the drop in temperature and increase in wind at the University of Utah.

Source: MesoWest
Nevertheless, the strongest winds at the University of Utah actually occurred nearly four hours later, at 8PM when the radar imagery showed only some weak echoes in the area.  Perhaps that's all it took, but clearly more digging is needed to understand this event, including the roles of the broader-scale easterly flow, precipitation outflow, and flow interactions with the terrain.

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