Wednesday, September 14, 2011

Something Out of Nothing

The upper-level trough sliding across the southwest United States has organized the monsoonal clouds and precipitation over the past two days, with a nice comma cloud developing over southern Utah.

Beneath the comma cloud is a weak, somewhat organized rain band with a few embedded convective cells.

We often think of monsoon precipitation as airmass convection, but large-scale systems can be important.  This is especially true in September when midlatitude troughs interact with monsoon moisture.


  1. It seems like these might be a major factor related to a precipitation maximum in October in much of eastern Utah. Most of the Colorado Plateau region has a monsoonal precipitation maximum in August, but some portions have an October (or September) maximum which could be largely due to systems like this one. These also seem to be some of the drier areas, so less sub-cloud evaporation in the fall might be a big factor too.

  2. That's an interesting hypothesis for the eastern Utah fall precip maximum, which I've never understood.

    There's a possible variant on it as I think the peak for Tropical Cyclone remnants to move into the southwest is in September. Perhaps a few big events could be contributing. Technically, I suppose it is all monsoon moisture, but there are a few fairly big deluges related to east Pac TCs.

  3. I agree... it is probably a combination of all of these. I also have the general idea that the summer (July/August) monsoon moisture is mostly from the Atlantic basin, e.g. the Gulf of Mexico and Caribbean, while the fall (September/October) monsoon moisture is generally from the Pacific side. This would support your comment as well.