Tuesday, July 9, 2013

Storms Going Out with a Whimper

There's been some fairly exciting weather in the Salt Lake Valley the past several days, beginning with the 4th of July thunderstorms, which continued overnight leading to the closure of Little Cottonwood Canyon, and continuing through yesterday afternoon when we had another set of rumblers drop up to 0.43 inches of rain on portions of the valley.

This period of thunderstorm activity is, however, slowly dwindling.  Observations of integrated precipitable water from the Salt Lake City airport show a gradual decline over the past two days, indicating a drying of the atmosphere that will lead to reduced thunderstorm activity the next couple of days.  

Integrated precipitable water is the depth of water you would have if you condensed out all the water vapor in the atmospheric column above you.  As can be seen in the plot above, we were running near or above 2.5 cm (1 inch) over during the past several days, with some higher peaks (the peaks are likely associated with thunderstorms or their outflows, which typically have higher water vapor contents).  This morning, we are down to about 1.8 cm (0.75 inches), and that loss of moisture makes it more difficult to develop thunderstorms.  Thus, there is a slight chance of thunderstorms today, mainly over the mountains.  


  1. Looking at the forecast models last week, there were some huge differences in the forecast with regard to the convective activity. As a good example, look at the forecasts initialized on July 3. The NAM forecasted essentially only diurnal activity (mostly limited to the higher terrain), while the GFS forecast included a lot of nocturnal storm activity particularly over northwestern Utah. In this case at least, the GFS schooled the NAM. I am wondering why these models have such huge differences with regard to the diurnal variations in convection, there is obviously a lot more going on than just differences in terrain resolution. Any ideas on this?

    1. It could have to do with differences in their convective parameterizations. I think the NAM uses the BMJ scheme, which has a specific set of biases, and the GFS scheme has specific biases as well, but this is not my area of expertise.