Wednesday, February 16, 2011

A Surprise Outflow Boundary with a Remarkable Wind Gust

Some remarkable wind observations were recorded as the front penetrated across northern Utah.  In particular check out this meteogram from the UDOT Lake Point I-80 site (UT9).

There is a wind gust of 80 mph at 0040 UTC.  This is a prefrontal wind gust (SSW flow before, during and after) that is accompanied by a brief 4F temperature fall.  The front then comes through at 0110 UTC.

I was ready to dismiss this as a bad ob, and then I looked at the traces for Lake Point (LAK) and KSLC.  They both show the same behavior, although the peak gusts are not as strong.

How do we explain this unique southerly wind burst?  The 0000 UTC sounding at KSLC shows a classic inverted-V sounding with large values of downward convective available potential energy (DCAPE).

Looking at the radar, one can clearly see a pre-frontal precipitation band with bow-echo-like structure moving over the observing sites during the period of peak gusts.  It appears the strong winds were produced by moist downdrafts and related outflow.

With an incredible front approaching from the west, it's easy to overlook such an innocuous feature, yet it produced the strongest lowland gust observed during this event.  You might also check out the cell that develops where the apparent outflow boundary collides with the approaching front.

1 comment:

  1. One thing that I think some of these lightning-generating fronts have in common is an overhanging area where the mid/upper-level portion temporarily outpaces the lower (near-surface) boundary. This could be due to terrain resistance that slows down the surface portion. This might also explain the rapid development of convective cells slightly ahead of the surface wind shift, and the extreme gusts in this zone. I have noticed that surface fronts typically slow down a bit along the Wasatch Front which I think can contribute to this situation. It would be interesting to look at the geographic distribution of lightning with these types of winter cold fronts.