A Winter Storm Warning has been posted for the entire viewing area and remains in effect through 7 PM Saturday.
Further west, it's a Winter Weather Advisory across Southwest Indiana and Western Kentucky.
This storm is expected to impact a huge section of real estate across much of the Ohio, Tennessee and Mississippi River Valleys and even down into Dixie but the heart of it might very well end up in our backyard.
So how much snow can we expect?
That is the million dollar question. Even though this storm is only hours away, there is still a tremendous amount of uncertainty in the data. Let's take a look at a couple of the models for instance.
12z AdvanceTrak (RPM)
The most recent run of AT is only showing an inch or two for most of the area with a narrow stripe of heavy snow, up to 6 inches, up along the highway 60 corridor near Seymour or North Vernon.
The 12z NAM
On the other hand, you have the 12z NAM that shows a whopping 9 to 10 inches for areas along and just north of the Interstate 64 corridor.
Meanwhile, only a county to the south, there could only be trace amounts of snow!
What we know
We know that this system is going to bring with it the potential for very heavy precipitation. Our central and southern counties will see a ton of precipitation with most models going better than 2 inches of liquid equivalent.
See GFS below showing 1.5" to nearly 3.0" from near I-64 south into our Kentucky Counties.
What we don't know
The biggest question revolves around what form the precipitation will take. Will it be primarily rain, sleet or snow?
A huge issue with forecasting a storm like this is figuring out the "thermal profile" of our atmosphere.
You see, in addition to accurately forecasting temperatures near the surface, we need to figure out what temps will be doing several thousands of feet above us in order to correctly identify the main precipitation types.
The Forecast Sounding
Take for instance the following series (below) of "forecast soundings" or "forecast profiles" for Louisville of the lowest 20,000' of atmosphere via the 12z NAM.
We start at 2:00 at about the time heavier precip starts to fall. Notice the 0°c (freezing line) that runs diagonally across the right part of the page.
Now anywhere the red (temperature) and green (dew point) lines are to the left of the Freezing Line represents temps that are below freezing. Anywhere they go to the right of the Freezing Line represents where temps are above freezing.
In the case of the 2 AM sounding above there is a pronounced "warm" layer (above freezing) at about 4 to 7,000 feet above us. Below that is a a cold layer that extends from the surface to about 4,000'. This is a classic freezing rain/sleet profile.
By around 4 AM the warm layer shrinks in size and descends down to between 3 - 5,000'. However, the surface temp inches to just above freezing. This would result in a sleet/rain profile.
By 8 AM the warm layer aloft completely disappears as surface temps remain near freezing. This would result in a heavy snow profile.
The profile hugs the freezing line and stays just cold enough to produce a heavy, wet snow profile through the morning hours
At 2 PM the profile all the way up and down the atmosphere remains very close to freezing with temps in the lowest 2,000 feet rising to just above freezing. This would result in a very cold rain.
By 5 PM the profile cools back down enough to switch precip back over to a wet snow.
So what does this all mean?
You see, in the above soundings lies the biggest challenge that local forecasters have to deal with. There will be a very fine line between precipitation that primarily falls as snow and precip that is primarily just rain from this storm.
A couple of degrees really does make all the difference. If temps warm just a little bit anywhere in the lowest 10,000' of our atmosphere then it makes this a rain event. If temps stay as advertised by the NAM above, or if they cool, then this will primarily be a snow event.
The Current Snow Forecast
As of this writing, we are forecasting a stripe of 2 to 5" of snow from the Western Kentucky/Bluegrass Parkways up to near I-64.
Further south, this system looks to be primarily just a rain maker for areas south of the Parkways with totals of up to an inch possible.
For metro Louisville, areas along and north of 64 into Southern Indiana look to have the highest snow potential with 4 to 8" currently forecasted.
What if it is really mostly snow?
First, notice I didn't elaborate on the potential of much ice or sleet. While I certainly think we will see some freezing rain and sleet during the early morning hours on Saturday, this really does look mainly like a RAIN or SNOW event for Louisville and our central counties.
But to answer the above question, if the profile turns out to be cold enough to support mainly just snow, then I think we will be looking at A LOT of it!
Remember, we are talking about a HUGE amount of precipitation. The latest models are all projecting somewhere around 2 inches of liquid equivalent (the amount of precip it would be if it was all rain) for this storm across our central counties and a little more than that for areas to the south.
So if the bulk of that much precip falls in the form of snow, then we are talking about some LARGE NUMBERS.
The 18z NAM
Take for instance the 18z NAM which came in while I was in the process of writing this blog. It has trended a little colder with the forecasted profile and as a result, it is producing A TON of snow for our Central Counties.
Now you see the dilemma. We are dealing with a huge storm that will bring parts of the area a ton of rain and the potential for a stripe of very heavy snow.
I really think that there's a good chance that someone sees up to a foot of snow in our viewing area from this storm.
The question is where?
Right now, it looks like the best chance for this occurring would be somewhere near or north of I-64 where the "cold" thermal profile meets the heaviest precipitation.
With that said, there will likely be a SHARP GRADIENT to the south of this POTENTIALLY VERY HEAVY STRIPE of wet snow.
As this system draws closer, we'll be able to dial in the locations and amounts with much greater accuracy that we can at this point.
Moral of the story... In addition to some freezing rain at the beginning of this event, there is the potential for VERY HEAVY amounts of SNOWFALL and also RAIN from this system and everyone in the viewing area will be impacted by this large and dangerous storm.
Marc will have a full update on timing and totals of this system tonight on WDRB News at 10.
WDRB Meteorologist Jeremy Kappell
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