Monday, January 16, 2012

Small Storm, Lousy Roads

This morning's storm rates a 1 on a meteorological scale of 1 to 10, but, at least near downtown Salt Lake City, it had a disproportionate impact on the morning commute for those who need to work today.

In particular, although accumulations remain (as of about 8:30 am) less than an inch or two, the roads are slick as snot.  This is because of both meteorological and societal reasons.  From a meteorological standpoint, a key reason is the lag between the frontal passage and the snowband.  This can be inferred from the meteogram from the University of Utah (WBB), which shows a temperature fall into the mid 20s  following frontal passage at around 3 am, with precipitation beginning at around 7 am (inferred from the increase in relative humidity).

Source: MesoWest
The lag between the surface front and snowband can also be seen in the satellite loop below, which includes an overlay of 700-mb temperature contours.  There is a cloud band with relatively cold cloud top temperatures accompanying the leading edge of the cold air, but the main band producer is a cloud  band that lags farther behind in the colder air and has somewhat warmer cloud top temperatures.  This is a good example of why you can't count on precipitation accompanying the surface based cold front.  Sometimes, especially in Utah, the precipitation band lags behind.  It is also a good example of why cloud top temperatures are often not a great indicator of precipitation rates in the wintertime, especially over Utah.

On radar, this is not a strong snowband, and precip rates based on my highly scientific glimpses out the window appear to be near or slightly less than an inch an hour.

So, this is not a big storm, but the commute this morning was difficult.  From a meteorological perspective, the passage of the cold front ahead of the snowband enabled the roads to cool so that snow stuck immediately.  From a societal perspective, the response of road crews, at least in the neighborhoods I travelled through this morning, was somewhat delayed, especially given that this was a well anticipated storm.  Perhaps this was related to the MLK holiday.  On the plus side, the MLK holiday meant that traffic was light.

This is a good example, however, of how even a small amount of snow, when delivered under the right conditions, can make travel difficult.  On the other hand, if you are a late riser, you probably won't experience the full brunt of the white knuckle driving that some encountered this morning.

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