Rain Chance Late…Recipe For Severe Weather…Update For Severe Weather in Fox Valley

***Watch WISN 12 News at 5, 6, & 10 p.m. for Milwaukee’s most accurate forecast!***

****UPDATE*****

Severe storms moving through Fox Valley. Possible tornado in Winchester and Kimberly. These storms will stay north of us.

Thank you for stopping by the Weather Watch 12 blog! In today’s entry we will focus on a rain chance for this evening/tonight, and also look at what ingredients are needed for severe weather to occur.

Let’s begin with what to expect with a cold front headed our way this evening. The good news is that we spend the afternoon ahead of the front, and that will translate into our 5th straight day of highs in the 70s or 80s! A strong southwest wind will develop this afternoon, with gusts likely topping 30 mph. That is about the only down side to this afternoon.

By this evening a cold front will begin advancing into southwest Wisconsin. With moisture flowing in on the southwest winds, the moisture will get pushed up by the front. The lifting will then translate into the development of showers and thunderstorms.

Below is the forecast surface map at 7 p.m. Tuesday evening. This shows the likely position of the fronts and low pressure center.

Now the question is will any of the thunderstorms be strong or severe? Before we answer that question, let’s look at the ingredients needed for severe weather to develop. The 3 most essential ingredients are:

  • Moisture (higher dew points)
  • Lift (fronts, low pressure center, jet streak)
  • Instability (strong surface heating, sunshine, inversions)

If one of these ingredients is missing you can still get thunderstorms, but in order for severe weather to occur, you need all three present. In the parenthesis I listed some examples of each. Generally, dew points of 55 or higher are needed for severe thunderstorms.

Now that we know the ingredients, let’s see if severe weather is expected later today or tonight. The Storm Prediction Center issues what are known as ‘Covective Outlooks’ several times each day. The day 1 Convective Outlook is what we are interested in to get a general idea of what to expect for today.

Below is the most recent day 1 outlook. The ‘slight risk area’ means: A SLGT risk implies well-organized severe thunderstorms are expected, but in small numbers and/or low coverage.

Notice that the slight risk area does not include our viewing area. Why you might ask? Remember the ingredient list above? As the cold front advances to the east this evening/tonight it will lose one of the ingredients that is needed…sunshine and surface heating. When the sun sets this evening, the intensity of the thunderstorms will begin to decrease.  Another limiting factor will be the very low dew points.  Dew point is a measure of moisture at the surface, and dew points this evening will only be in the 40s.

We could still see some thunderstorms(best chance in our western and northwestern counties), but they should remain below severe levels. Overall, the rain chance only looks to be about 40% for Milwaukee and the surrounding area this evening/tonight. As thunderstorms develop this afternoon and evening, you can track them on our interactive radar. It is free and you can zoom to any city or state!

www.wisn.com/irad

If you have questions about anything discussed in this blog, please leave your thoughts in the comments section of the blog. For additional forecast information, follow us on Facebook at WeatherWatch 12 and also on Twitter!

Have a great day!

Jeremy Nelson

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2 Responses

  1. Warm sector with surface convergence…sigh, such a waste with waning CAPE. It seems along the warm front, instability, turn, and decent dewponts should exist. It looks to be just a tad north of MKE. Looks like potential of some nice little hail makers north and west of you.

    I often describe the ingredients for severe weather like making a stew. It takes several ingredients for it to turn out right and too much of one or too little of another will not make a good stew.

    I notice you left out shear. I guess you can have severe storms without it, but they just will not last long. Also, might be worthwhile to mention cap/CINH – though in WI, you probably don’t get too much of it.

    I am curious about your 55 degree dewpoint. I guess I have always gone with 58 but ideal in the 60s. Probably my thinking is more surface rooted vs. high based.

    Just some random thoughts…

    • Since this is the first little tutorial…I tried to make it simple. Dew points in WI tend to be a bit less, and yes, elevated convection is usually the rule around here. The stew analogy is good!

      Jeremy

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