Monday, September 9, 2013

Monsoon Weakened, Not Defeated

With the passage of a weak trough and the penetration of our first (weak) gasp of flow from the northwest into northern Utah in many weeks, there's been a modest decline in humidity and clouds over the Salt Lake Valley today.  For example, the integrated precipitable water, a measure of the total water vapor within the atmosphere, declined late yesterday from just over 3 cm to something closer to 2.5 cm.

Dewpoints at the Salt Lake City airport also declined somewhat from near 60 to something closer to the mid 50s.

Nevertheless, the monsoon is nearly weakened, not defeated, and we are still seeing some deep convection firing up over the high topography to the south and southwest of the Salt Lake Valley, especially the Oquirrh, Stansbury, and Sheep Rock Ranges.

Source: College of DuPage
Forecasting where and how strong convection of this type initiates remains a major challenge for meteorology.  For instance, why is is that the convection is deeper and more developed over the Oquirrh, Stansbury, and Sheep Rock Ranges than over the Wasatch and western Uinta Mountains just to the east?  It's impossible to anticipate such subtleties with any sort of reliability given todays forecast tools.   

Now that convection is going, monitoring it's movement and intensity will be especially challenging today due to an outage of the National Weather Service Radar on Promontory Point (KMTX).  All that is currently available is this black screen of death.  

Source: NWS
It appears that the radar has been up and down several times the past couple of days.  Hopefully the problem will be solved soon.


  1. While this radar has been out I've been using the other one weather underground shows for the area. It says this is a Terminal Doppler Weather Radar. I wonder what the differences are between the two.

    1. Ah, good sleuthing. The TDWR is located along the southwest shore of the GSL near Centerville and is designed and operated to detect low-level wind shear near the airport. It is, however, useful for detecting precipitation and in some cases severe weather. Because of terrain blockage and a scanning strategy concentrating on the airport, it has a fairly narrow range of coverage.

  2. In mountainous topography, it seems that there are always convective "hot spots" where deep convection occurs much more readily than in other areas. It has always fascinated me to try to figure out exactly what triggers deep convection in these areas (some are more obvious, such as high mountain barriers, and others are harder to figure out). For example, one of the closer ones to SLC is over the south end of the Oquirrhs, where you can often observe deep convection developing in a fairly specific location around midday or soon after. It is also interesting that most of these locations seem to be favored regardless of wind direction. These patterns do seem most pronounced during the daytime hours (suggesting that sun angle is a huge part of it), with the effects of the topography seeming to be much weaker at night. This is definitely an interesting topic to me.

  3. One of the keys here in Utah is to get a wide updraft because instability is pretty limited and any kind of appreciable entrainment to the core of the updraft will kill it. As opposed to most parts of the Wasatch, the southern Oquirrhs can get upslope flow from practically all directions, which probably helps to organize wide updrafts.

  4. I agree. Also most of the east-west oriented ranges (such as the Uintahs and the Raft River range) seem to be very convective during the afternoon, possibly due to broad areas of concentrated solar heating on the south slopes. Other areas, such as most of the greater Salt Lake Basin and GSL area, are often dead spots in terms of afternoon convection (although can be favored areas at night). In Utah in the summer, it seems like almost no matter how bad the sounding looks, some areas manage to generate deep convection. And almost no matter how good it looks, there are areas that will typically not get deep convection at least during the initial round of development.

  5. The additional benefit of the Uintas is that they have a large area of very high elevation above 9-10,000 feet. Even without the peaks, one might expect this area to mix to the LFC before lower areas (neglecting surface differences, which probably play an important in heat fluxes).

    At night, you should expect mountain ranges to no longer support convective initiation due to the downsloping flow. The advantage of open lower elevation basins is that cold pools can better organize to maintain mesoscale systems at night. These often result when enough cold air has built up from daytime convection and it is able to be properly balanced by the vertical wind shear. When those mesoscale systems get large enough, their heating can generate additional mesoscale lift as well.

    You also occasionally get non-surface based convection when there is enough instability and strong mid-upper tropospheric lifting results from a wave coming through. In that case like the mesoscale case, solar heating doesn't matter.

    1. This pretty much seems to agree with a lot of what I have noticed. My observations have been that when nocturnal convection occurs it seems to favor areas which have not had storm activity during the afternoon/evening, i.e. areas where there is a relatively warm land surface or warm boundary layer remaining. There are obviously some topographical influences on nocturnal convection as well, but it is hard to overestimate the importance of the topography in forecasting daytime convective initiation. If anyone knows how to produce (or already has) warm season lightning climatology maps of our region divided up by hour of the day, for example, this could be really useful in terms of understanding these patterns.