The Meteorology Behind the Weather

In this article, I will give a behind-the-scenes view of what goes into my daily forecast for Sault Ste. Marie. A colleague once told me that if someone truly understands a complex topic, then they can explain it in a way that anyone can understand it. I have used thousands of general public forecasts for my region, and now I want to show how I build those forecasts as a meteorologist.

What the Public Sees

Let’s first look at what I issued to the public today on SooToday.com for just today and tomorrow. Notice the terminology and key points about timing and impacts without scientific lingo.

Forecast

Public forecast for Oct.31/17

 

What the Meteorologist Thinks

Now we will look at the thought process of what went into this forecast. A glimpse into what I’m considering at 5am to get a useable forecast to the public by the time they are waking up after 6am. Notice the struggles with various factors and decisions between borderline considerations.

The first thing I notice is the 4:45am ob from the airport of ice pellets. This makes me wonder about the temperature profile aloft. There is no evidence of an elevated warm layer above the surface that would create ice pellets. The forecast soundings suggest that saturation aloft does reach -10C for ice crystal growth and so I can rule out supercooled droplets that could create drizzle or freezing drizzle. I write the ob off and figure that maybe saturation hasn’t reached -10C yet – but will soon and we may see a light snow ob in the next hour or two. (note by 5:21am CYAM started reporting light snow)

I look at water temperatures in Whitefish Bay near +9C and the temperatures aloft at 850mb. Forecast guidance suggests temperatures aloft near -9C. That would give me a delta of 18C, certainly enough instability for lake effect snow. I need an inversion aloft and winds well aligned from the northwest for a squall risk to the Sault with that instability. There is a subtle inversion over 5000′ and winds are generally northwest but veer with height. Certainly, there is a good chance for lake enhancement today, but organized squall-lines just don’t look good today. With a surface trough crossing and weak vorticity aloft I will go on the high end of model guidance due to this likely enhancement.

Upper Air sounding early morning

Guidance shows temperatures a few degrees above freezing which looks likely with the lack of warm air advection, cloud cover and winds off of Superior. The timing of precipitation and risk of snow changing to rain is the big question today.

A look at forecast soundings suggests drying in two parts of the profile this morning and this afternoon. First on and off near the surface with the guidance wants to saturate with each precipitation episode. More alarming in the dry air aloft in the ice growth region of -10 to -15C. If this dry air does develop and cuts off the moisture in this level, it could have a big impact on precipitation types at the surface. What is a snow forecast could quickly turn into a drizzle forecast due to a lack of ice crystal growth.

Web-bulb zero heights remain below 1000′ feet today so even with surface temperatures just above freezing it would be enough to melt any snow growth – but if no ice crystal growth occurs then it will be impossible to have snow, and we end up with drizzle. Very borderline but with the lake enhancement I’m going to hope we can keep enough moisture at height to bring snow that melts at the surface. Another factor would be the presence of organic nuclei in the atmosphere with leaves falling off the trees. This can reduce the temperature needed for the formation of ice crystals below -10C. Snow it is; and enhanced by the lake instability and upper-level QG forcing from weak 500mb troughing will give me 2-5cm – that all should melt on the ground with temps above freezing.

Weak upper-level ridging early this evening and drier air moving into the region should end precipitation by sunset. So for Halloween, I will advise flurries ending, colder than normal temps and cool windchills due to the persistent westerly breeze.

I look at tomorrow morning, and the upper-level guidance suggests saturation at the surface but not up to -10C aloft. The model also shows the risk of a shower or flurry tomorrow morning, but this sounding suggests to me drizzle or freezing drizzle if surface temps dip to freezing overnight. I will role with a chance of morning sun for now with increasing clouds tomorrow with warm air advection. I will have to watch things closely today and likely have to modify the forecast for tomorrow as we get closer. Argh – why can it never be simple!

Now that is just a quick look at the consideration heading into the next 24-36 hours for one location. Winter precipitation definitely adds to the complexity, but in the summer the same debate goes for thunderstorm risks and lake breezes.

If you have any questions about the forecast process for a meteorologist, then please feel free to leave them in the comment section.