Wednesday, September 21, 2016

Severe Convective Storms Possible

Forecasts haven't changed much overnight, except that moisture is streaming into northern Utah a bit earlier than expected today, increasing the potential for scattered thunderstorms this afternoon.

Over the years, I've learned to pay careful attention to weather situations that involve the interaction of upper-level troughs in the midlatitude westerlies with monsoon moisture as this often leads to the development of severe convective storms.  The pattern through Thursday night fits this description and demands this careful attention. 

Patterns of this type have the three key ingredients for convective storms: Instability, Moisture, and Lift.  With regards to instability and moisture, I've cherry-picked a couple of soundings from the 0600 UTC NAM forecast for Salt Lake City (below).  In these soundings, plotted on a thermodynamic diagram known as a Skew-T, the forecast environmental temperature and dewpoint are displayed as red lines (with temperature > dewpoint and the right-most line).  The grey line is the temperature of a hypothetical surface parcel if lifted "dry" to the point of saturation or cloud base (grey circle) and then "moist" above that with water vapor condensing and releasing heat.  The hatched area shows how much warmer this parcel would be than the environment and equates to the Convective Available Potential Energy (CAPE) of the parcel, a measure of the amount of energy the parcel would acquire if it were lifted in this fashion.  Although there are problems with this theoretical approach, the magnitude of the CAPE provides a rough guess at the strength of the instability if convective clouds can form.  
The plots above show CAPE values of 709 and 957 Joules.  For our part of the world, those are pretty high forecast values.  In addition, the precipitable water, a measure of how deep the water would be if you condensed all the water vapor out of the atmosphere, is 27.5 and 24.1 mm (roughly an inch).  Again, those are fairly high values for our part of the world, especially in the latter half of September.  

So, we have instability and moisture, but we also have lift.  First, we will be downstream of the upper-level trough axis, an area typically associated with large-scale lifting.  We also have a nearby cold front and in all likelihood a variety of smaller-scale surface boundaries as the upper-level trough moves in to initiate convective storms.

So, three boxes checked.  However, we also have some wind shear, which helps storm organization. 

Based on these ingredients, the Storm Prediction Center has placed much of central and eastern Utah in a Marginal Thunderstorm Risk category from tomorrow morning through Friday morning.   Salt Lake and Provo are on the edge of the Marginal Threat Area.  

A marginal risk means isolated severe thunderstorms (Note: showers and non-severe thunderstorms will be greater in coverage) that are limited in duration and/or coverage and/or intensity.  The main threats from these storms are strong winds and small hail.  

With regards to rainfall, through tomorrow afternoon, the NCAR ensemble is generating the greatest average precipitation, maximum precipitation, and highest probabilities of precipitation greater than 1" or 2" between Salt Lake and Vernal.  
That doesn't mean, however, that other areas won't see some showers or thunderstorms.  Let's see how this comes together and keep an eye on official forecasts, watches, and warnings from the National Weather Service


  1. Jim - that 05Z fcst sounding also appears to have an amazing amount of low-level veering in the wind profile, apparently due to a shallow outflow from the east. Quite something - but of course it's two days out, with much likely to happen at small scales between now and then. Bob

    1. Bob,

      Yes, if you click through the hourly soundings, there's quite a bit of variability in CAPE, shear, etc. The NAM can't really produce convection, but it's got variability in it, so like I said, I "cherry picked" from what was there.

      Was it Chuck Doswell who once said that "storm chasing is about synoptic possibilities, but succeeding or failing based on unresolved mesoscale processes" (or something like that). That comes to mind in situations like this all the time. We're dealing with synoptic possibilities, but we'll see how things shake out.


    2. Jim - Oh, I'm sure he has, perhaps many times. Down here small and mesoscale convective scale events today are so important as to make tomorrow's events almost un-forecastable - course we usually have much weaker synoptic backgrounds. Regardless, looks very interesting up there for next several days. Bob

  2. Today is starting to look real interesting to me actually, not just tomorrow. I am a little surprised the SPC doesn't have us in a risk category for the day 1 outlook. I would not be surprised to see a supercell or two around somewhere later today.

    1. The shear profile is good for supercells today, but the instability is the limiting factor. Storms will be very sheared unless enough of them build up around each other to help counter the shear. Tomorrow definitely looks better.

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