My December 11, 2019 CT Snowfall Bust...It was Very Bad

My snowfall forecast for the December 11, 2019 event across CT…terrible…bad…horrific…there aren’t enough negative terms in the dictionary to describe how awful of a forecast that was. However, I am not mad or angry. The beautiful thing with weather is that it is an imperfect science (something that most people might not really understand) and there are going to be times when you’re wrong. All you can do is learn from any mistakes.

In the case of this storm, it’s not necessarily a mistake I made. It was geared more towards the fact that I placed too much emphasis on a red flag I saw (red flag meaning something that may prevent a certain outcome from occurring) and ignored some of the glaring signals which suggested we would get heavy snow. So, why did I do this and what have I learned from it?

First off, the major red flag to me was the forecast of dry air and subsidence in the lowest ~7,000 feet of the troposphere. Why are these two significant?

• Presence of dry air – Dry air “eats” away precipitation. Basically, what occurs is the process of evaporation. If you have precipitation falling towards the ground through the troposphere and it’s falling through dry air it can evaporate before reaching the ground. Sometimes radar reflectivity will indicate precipitation is falling, however, it is not reaching the ground. This is called virga.

• Subsidence – Subsidence is sinking motion…air is sinking. For clouds and precipitation to form you need rising motion. Warm and moist air rises, as it does so it encounters a cooler environment, the parcel temperatures then reaches the dewpoint temperature and voila…clouds form and precipitation can form (depending on other factors of course). When air is sinking it also warms…not good for the development of clouds and precipitation.

So, what led me to thinking this dry air and subsidence would be a problem? In the essence of time and space I will just pick one of the forecast models. Forecast models differed with just how much dry air would move into place, however, keep in mind my thinking was this dry air would advect in early and quickly. Below, is a cross-section spanning the state of CT from Tuesday’s morning run of the 3K NAM for 3:00 AM EST Wednesday morning. I have highlighted the presence of rather dry air in the lowest 5,000 feet of the troposphere (light shading greens indicating about 75-85% relative humidity). Note the darker green shadings, however, above this along with the purple shaded areas…those are areas of stronger rising motion:

Let’s then compare this with that evening’s 0z run (just hours before the snow) valid for the same time. Notice a SIGNIFICANT DIFFERENCE in the profile:

What stood out as a strong signal for snow to fall (and heavily at times) and accumulate was intense rising in the mid-levels of the troposphere with the most intense lift occurring between where temperatures were -12°C to -18°C. This layer of temperatures is referred to as the dendritic snow growth zone. Snow growth is maximized between this range of temperatures. For snow growth to be maximized you need the following;

•       Very strong upward vertical motion present within the dendritic snow growth zone (referred to as the “cross hair signature).

•       Significant presence of ice crystals (very high relative humidity with respect to ice).

•       Sufficient moisture (relative humidity values > 80%).

If these conditions are present, not only will you develop dendrites (these are the big snowflakes which accumulate very rapidly) but you will also have heavy snowfall rates.

All forecast models were very consistent with this feature. The 12z 3km NAM run from Tuesday morning at Windsor Locks, CT indicated negative values of omega (which indicates rising motion) in between the -12°C and -18°C temperature range. Referring to the relative humidity cross section above, we know it’s plenty moist within that layer. I’ve highlighted in black this area. I’ve also highlighted the yellow-ish shading in the lower-levels of the troposphere. These are positive values of omega (indicating sinking motion). I saw that as a flag and determined this would offset things:

There was also excellent lift provided by a very strong mid and upper level jet stream and a very strong axis of frontogenesis about 10,000 feet above the ground. Below, is the 700mb frontogenesis forecast from the 12z run of the NAM showing intense frontogenesis over CT for 4:00 AM EST Wednesday morning:

After a review, what I’ve learned is DO NOT IGNORE the cross hair signature. When a cross hair signature is present and there is plenty of available moisture…expect snow and heavy snow. While there was dry air involved, the dry air advection was being fought off possibly by evaporative cooling (the process of evaporation leads to cooling). One way to increase relative humidity is to decrease temperature. Also, as snow was being generated and falling, this helped to moisten up the atmosphere as well. The combination of excellent jet dynamics and frontogenesis helped force rising motion.

I incorrectly assessed how everything would come together and how this evolve. This incorrect assessment lead to a horrific weather forecast. You could easily say, “why would you go against everyone else. If everyone else goes a certain way, just go with them”. That seems logical, but that is not the correct way to go. One of the best assets a forecaster can have is knowledge of past experiences. Forecast models are only guidance and if you just forecast based off what a model is spitting out…you’ll find yourself in trouble. After-all, if forecast models were 100% accurate computers would do this job!

So it’s off to the next storm and the next bust and see what can be learned from that 😉