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More from Bob Gardiner at Lake County Stormwater Management.
First I’m going to jump up on my soapbox and talk about what I think I know about weather forecasting…In general, there are a couple kinds of models, global and more local. Global models like the U.S. GFS (Global Forecast System) and ECMWF (“European” model) model the weather for the entire planet and they do a very goo job. There are also more local models like the NAM (North American Model) and a lot of others. The NWS will take a model, like the GFS, and run the model a bunch of times but tweaking the input data just a little to see how the forecast changes. This is called a model ensemble. In the end, the ensemble generates a series of probabilities of various scenarios.
My whole point is that forecasts are not reality. The storms will do what they will. The forecast gives us a range of probabilities of what we might expect. The more complex the storm or the more subtle the changes the lower the certainty for any given scenario. In this case, we have a high pressure system in the southeast rotating clockwise bringing up Gulf moisture. A low pressure system is just beginning to develop over Texas now and is rotating counterclockwise bringing up Gulf moisture. Between the southeast high and the southwest low the Gulf moisture is being pumped north. The NWS stated that the precipitable moisture in our area is about four standard deviations above normal for January and this is a HUGE departure from normal. Now, we have a huge trough of cold air slowly coming down from near Alaska. When that cold air hits all the moisture in the atmosphere it will precipitate out (remember precipitates from high school chemistry?). What the precipitation falls as depends on the temperature. In our case, the cold air is just starting to enter our area. It was near 50° in Lake Zurich at 7:30am today and is now 42° in Libertyville. Still well above 32° so precipitation is still liquid. Next, a couple of things will happen. The cold air will nose into our area late tonight, early Saturday morning. Cold air is heavier than warm, moist air so, under quiescent conditions, the cold air will ooze in under the warm air and once the ground temperature goes below 32° falling rain will freeze on contact. This is the freezing rain that covers sidewalks and roads and tree branches and power lines. The thickness of ice builds up by accretion over time. The current forecast calls for around 0.1” to 0.25” of ice. That’s bad enough but…remember that low pressure system forming over Texas? It is going to ride the jet stream up to here and strengthen along the way. The result will be strong wind and increased turbulence that will more violently combine warm moist air with cold air giving us heavy rain at first, then freezing rain as the cold progresses, going to sleet as things get even colder and the rain drops partially freeze before they hit the ground and, finally, after the entire air column has cooled enough we will start seeing the precipitation fall as snow. All these different types of precipitation depend on temperatures of just a few degrees. So the models tell us that there is a good probability of freezing rain over our area…will we see 0.1” in Lake Zurich? In Libertyville? In Zion? Who knows. However, we do know there is a good chance of seeing heavy rain, freezing rain, sleet and snow in combination in our area and that is enough to make us aware of the risks and maybe even prepare ahead of time. Below are some graphics showing the main players.
So, here’s the latest forecast (i.e. the scenario with the highest probability), heavy rain tonight on the order of 1” to 1.5”, freezing rain starting around 5am Saturday morning, peaking around 8am and continuing most of the day, also sleet from 5am to mid afternoon. Freezing rain and sleet may occur together or more of one or the other. Temperatures in the air column get cold enough to support snow from mid-afternoon Saturday, peaking from about 3pm to midnight and then tailing off to nothing by early Sunday morning. Total snow for Libertyville is about 3”. Strong winds around 40mph for the duration. As the low pressure system strengthens and moves to the northeast the counterclockwise flow will result in strong winds (40+mph) coming out of the east over Lake Michigan and pounding our shore with waves 12’ to 18’ and some as big as 23’! I’m thinking that will cause some coastal damage and flooding. Here is the forecast graph for this storm. Have a great weekend all!
_________________ @audioidkid spaulding wrote: Also if you fuck someone like they are a millionaire they might go try to be one.
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