2 systems of interest
1) Thursday night
2) February 28th
Cold. Pretty much sums up the next 48 hours. Highs will struggle through the 30s...lows in the low 20s with teens much more likely tomorrow night. This cold air mass is crucial in how things play out on Thursday.
This is a classic overrunning type of setup in Kentuckiana. Cold air is heavy...hovers close to the ground. Warm air is lighter, and rises over it until so much warm air flows in---it takes over.
But it is the "takeover" period of the warm air invasion that leads to icing events in our area. Duration and intensity are the key on impact of course.
The warm air initially looks warm enough to keep the raindrop...a drop ...until it gets to power-line level and lower. At that point...the drops will freeze on any object they touch. The only exception may actually be the roads. The cold days/nights in the short-term will certainly lower the pavement temps---but deep soil temps may be warm enough to keep the roads just wet. Sideroads-sidewalks---likely a different story.
Here is a chart showing how the warm air will take over. You look at this with the left side being southern KY...the right----toward north Indiana. You can see how the depth of cold air determines the precip type. It is important to note that this is a snapshot of what it will look like Thursday night---but the warm air will be moving north overnight---so think of this image as a movie as the precip types will keep moving north until all rain is the type.
HPC has outlined a risk for at least .10" ice accumulation..mainly along and north of I 64.
The chance drops off once you get to the risk for .25" of ice accumulation. So .10-.20" looks to be the main risk----but that will have to be watched.
Timing wise---- that will depend on a different factor. The dewpoint.
The radar should start to light up in the afternoon with a rain shield spreading toward us. In the afternoon, we should have a temp in the Louisville region of around 37-39° with dewpoints 20-23°. Remember, the closer those values are---the more saturated you are...aka, precip reaches the ground. The further apart they are---- the more the atmosphere will eat it all up before it reaches the ground---despite what the radar shows.
However, if you can get precip to fall long enough...it will eventually force the two values to come together and meet at the same point. The question is---where will that be. The temp will lower..and the dewpoint will rise. The risk is there for them to meet up in the middle which would be near 31-32°. Once they meet up...they will hold at that value for an hour or so...then the warm air will start to move both numbers upward toward 40° by Friday morning. So the timing of the icing will highly depend on when we start to see saturation occur...and that looks to be near 8pm...and it may take until midnight or after to get us all above freezing. The further north you are (southern IN) the longer this is going to take. So I think areas north/east of Louisville are certainly more at risk at a longer period of danger with the icing.
This looks to be a brief advisory event...if the risk looks to be that we could reach .25" in ice accumulation, then we are talking about warnings. But right now---advisory level looks more likely.
So that is an early stab at the ice thinking. We will see how it looks tomorrow. Lots can still change of course...especially if the cold air mass moving in is COLDER than forecast. Then we have a whole other issue to examine.
An occluded low pressure will will move through the region during this time. This looks to be a rain event by Tuesday...I don't see much wintry precip with it---but it may get pretty chilly/windy behind it.
The occluded low mentioned above is part of the blocking pattern we discussed the other day. This means the next low pressure will dig south. And with the indicies lining up like they are, this low is likely to become intense. So this storm will raise eyebrows in the coming days for many. The EURO is pretty extreme with it now with a deep low over eastern KY.
It then moves into the NE and bombs out more....quite cold here...this would be by March 1/2.
Too early to get caught up on what this would mean for us, but it looks increasingly likely that March is going to roar in like a LION for many of us. Stay tuned.