Severe Weather or Not?

The warm weather season is beginning to take shape across North Central Wisconsin and that means the threat for severe weather becomes more likely.  In the winter we worry about where the rain/snow line is, while in the spring & summer we have to account for the location of fronts, the amount of shear going on with the winds from the surface to a few thousand feet off the ground and how unstable the atmosphere is at a given time.  This week I'm going to give you a little insight into what we look for in the potential of severe weather.  In order to do this, check out the graphic below.  It includes a classic storm system with low pressure, a warm front, cold front and occluded front.

As you'll notice one of the key sectors on the map is where we are in proximity to the warm front and the cold front.  Lets start with the warm front, which as Rodney Dangerfield would say, "gets no respect".  A majority of the time, Wausau which is located to the north of the warm front would experience rain which is stratiform in nature, meaning persistent light to moderate rainfall is more likely than widespread thunderstorms.  Yes there could be isolated storms, but many times they'll be garden variety with thunder, lightning, some downpours and perhaps gusty winds.  However, that doesn't necessarily mean that there aren't times when a band of storms develop just ahead of the warm front, known as overrunning precipitation, which can become severe with damaging winds or hail.  If this does happen it will be due to either much warmer air overspreading the cooler air mass in place to the north or a sharp increase in the dew point, from say the 30s to the 50s or 60s.  In other words, a strong clash of two different air masses.

On the other hand, the cold front tends to get more more attention during the spring and summer since a line of storms tends to develop right along it.  Here again is a clash of two different air masses where warm, humid air is typically out ahead of the cold front , along with southerly winds.  Meantime, the cold front most of the time is ushering in cooler, drier air that is flowing in from the west or northwest scouring out that warm air.  Referencing the graphic above, Beloit which is out ahead of the cold front, has the better potential for severe weather to develop.  In this sector, you've not only got more humid air, but also with that switch in wind direction from the south to west, the potential can exist near the cold front for rotating storms to form.  Not only does this translate to storms producing damaging winds or large hail, but also possible tornadoes or supercell thunderstorms which could cause a combination of all of these, along with drenching downpours of rain.  The greatest risk for the most intense storms to develop is near the Triple Point where the warm, cold and occluded fronts meet.  This is the area where the greatest dynamics exist for wind sheer to be the strongest with a higher risk of rotating thunderstorms.  If you need a example where this type of set up impacted our area, look no further than on June 7, 2007, when low pressure which looked a whole lot like that depicted above, tracked northeast through Central Wisconsin.  This storm system produced the 2nd largest hail in state history of 5.25" in diameter near Wisconsin Rapids and an outbreak of tornadoes, one which tracked for nearly 40 miles from eastern Langlade County into NE Wisconsin.

Needless to say, there are a few other parameters that have to be met for the atmosphere to be primed for a severe weather outbreak.  So, not every cold front is going to spark severe storms.  However, these are some of the basic aspects that if they are present, will be the trigger needed to get the stormy weather going.

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