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01/20/2018

An Early Look at Monday's Rain and Storm Potential

There is rain in your forecast Sunday, Monday, and Tuesday with some storms possible.  We are going to show you exactly what to expect. 

SETUP

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The sunshine we enjoyed was thanks to the high pressure to our south.  It's now moving east which is why clouds and mist have now taken over in Louisville and surrounding areas. Coming behind that is a long, draping cold front with an attached low pressure.  That is bringing our rain chances to start the week. 

TIMING

Saturday night a lot of you will see drizzle/mist.  A few light rain showers are possible, but it's not until Sunday morning that the rain gets a little bit more organized.  

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When you leave the house Sunday morning, I suggest taking the rain gear.  While I expect rain will show up on radar in Kentuckiana all day Sunday, it will not rain at your house all day Sunday.  At your house the rain will be scattered (off-and-on) through the day with more dry times than wet ones.  

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That pattern continues all day Sunday.  The cold front moves through Monday with a line of heavy rain and possible thunderstorms.  We see it on AdvanceTrak at 6:30 AM formed to our west. 

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Notice the line looks less strong by the time it gets to our area during the middle of the day.  You still see orange and yellow showing up in the image below, indicating heavier rain.  However, the line at noon does not look as strong or quite as organized as it did to our west (above image). 

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While the line moves through by early afternoon, the rain chance continues Monday night and through the first part of the day Tuesday.  With Tuesday's rain chance, we will need to keep an eye on temperatures.  Behind this cold front, temperatures will fall through the day Tuesday.  

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TOTALS

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Average rain totals through Tuesday afternoon will be 1/4"-1/3", but those of you who see heavier rain will see more than half an inch.  A few forecast models have us closer to one inch of rain, but that seems high to me.  There will be bursts of heavy rain in this line, but it's very difficult to predict exactly where those pockets will hit. 

WIND THREAT

There is a strong low level jet moving through Monday. This 70+ mph wind is not happening in the part of the atmosphere where we live; it's happening above us.  However, if we get some of this wind to mix down lower, we will get some strong wind gusts at the surface.  This low level jet is providing a lot of the energy with this system. 

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That translates to strong winds at the surface, just not as strong. I think it's likely we will see wind gusts over 30 mph as the line moves through.  Remember widespread wind damage happens when we get wind gusts around 60 mph and stronger.

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STORM POTENTIAL

There are four factors I look at for determining storm potential: shear, instability, lift, and moisture.  Shear basically show us how the wind is moving (direction and speed) up through the atmosphere. Instability is a way to look out how much energy there is for the storms to tap in to.  Lift is critical for strong storms to develop, and moisture is always a necessary ingredient. 

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We have limited shear, but it's not strong enough to cause me any concerns.  This factor is most important when looking for tornado potential, which we don't have.  The image above tells me the shear is contributing more to a wind threat than to any strong storm development. 

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Instability is also very low.  For strong/severe storms to develop, I would want this number to be several hundred higher than it is.  The fact that this number is higher than zero is part of the reason we could have a few rolls of thunder in this line, but it's not high enough for strong storms. 

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Lift is important for storms to strengthen.  We do have lift.  A lot of this is driven by the low level jet we looked at above, but this is the other reason I think a few thunderstorms will be possible.  We just don't have enough of all the ingredients we need for storms to become severe. 

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Finally, we also have moisture.  In the summertime dewpoints are much higher than this, but so are temperatures.  With a high temperature in the middle/upper 50s, these dewpoints in the low 50s show we have ample moisture. 

MY THOUGHTS

I do not expect this line will create severe storms.  Thunderstorms are very possible.  Heavy bursts of rain and gusty winds are very likely.  I'll be showing you the forecast on WDRB News at 10, and you can click the links to below to go to my social media accounts if you want to ask a question about the forecast. 

Hannah's Facebook Page

Hannah's Twitter Page

-Hannah Strong

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