Winter Storm System Update #3

  Well...we are in the monitoring/nowcasting phase and honestly, things are still very complicated. I wanted to do a brief post tonight to outline a few of the factors that I think are important in what happens and then it's simply a matter of analyzing things as they unfold. Ultimately I think we will see rain transition to a freezing rain/rain mix across Kentucky over the next hour or two (particularly across North and East Kentucky and locations North of the Ohio River), and we are going to see some problems with ice across the State for our Monday morning. I believe that many locations will see a "glaze" of ice, about 0.01"-0.1" at most, and while this doesn't constitute an "ice storm" by any stretch of the imagination, it would be enough to make travel difficult for some, especially considering the timing coinciding with a Monday morning rush. The good news is that today was our last day of dangerously cold temperatures for a little bit.

A Winter-Weather Advisory is in effect for pretty much all of the Ohio Valley through Monday morning (Until 12:00pmCST). No surprise here, regardless of the outcome there is good potential for negative impacts on the Monday morning commute.

Recent radar was showing a widespread precip shield across much of the Central CONUS. There have been reports of sleet/freezing mixing in across Western Kentucky over the past hour and multiple locations are beginning to report ice on their roadways. This will continue to drift eastward throughout the overnight. 

Now we'll take a look how how everything is set up for the night...

Looking at GOES16 Water Vapor imagery we can see our shortwave trough over the Southern Plains. I've outlined the trough axis/vort max in black. As expected, this trough deepened as it took its southeastward dive and triggered cyclogenesis as it interacted with a baroclinic zone associated with a stalled-front across the Southern Plains. As I mentioned in the previous updates I figured moisture wouldn't be a problem because the Pacific-Jet was tied into an atmospheric river (The "Pineapple Express") and it would be accentuated by moisture from the Gulf. You can see the moisture plume being transported from its Pacific source region and spreading downstream from our shortwave over the Eastern CONUS.

  

 Analysis at 850mb (about 4,700ft or 1400m AGL) indicates a pretty substantial warm layer over Kentucky with a 30-40kt southwesterly low-level jet. This LLJ will continue advecting warm air and moisture across the region while simultaneously enhancing isentropic ascent (overrunning). Futher North over Ohio temps are colder within this layer. I do think evaporative cooling will help cool this layer somewhat but any precip falling will melt as it passes through (temps are about -4 degrees C at 700mb above this layer). Given very dry air within this layer the process of wet-bulbing/evaporative cooling will be key in how our precipitation evolves throughout the night. When precipitation falls it can cool the air it's falling through by evaporating its moisture into it. Naturally this effect is more efficient when you have precipitation falling through drier air. In any case, precip will continue for several more hours as it slowly drifts to the East.

At the surface we can see our surface-low and associated frontal boundaries with high-pressure off the East Coast. We can also see the retreat of our Arctic air mass as the surface flow between our pressure systems funnels in a warmer Gulf air mass. I've also outlined the strongest warm air advection by using the "box technique". By overlaying thickness contours of the 1000-500mb layers and surface-pressure contours and then assessing where the contours overlap you can identify the areas of stronger/weaker advection. Smaller boxes correlate with stronger advection. If you remember, I have said for days that I thought the NAM was underestimating the strength of warm air advection and that's exactly what we've seen. The NAM was consistently spitting out data suggesting temps that didn't breach the freezing mark today but we actually saw temps in Central Kentucky that hit 40 degrees F. Now with the loss of daytime heating we will certainly see those temps fall but starting at 40F as opposed to 31F makes a big difference. The latter would likely result in a snow event. We are not looking at any appreciable snow across Kentucky. Instead, I think we will see the rain currently ongoing transition to a mix of mostly rain with a little freezing rain/sleet. As mentioned before, actual surface temps will still support freezing, regardless of precip type. Our ground is still frozen to a depth of 4"-5" and even though some surface temperature readings made it above freezing today, with the sun down now we should see the frozen sub-surface bring surface temps back down. This effect will obviously be greatest on untreated surfaces.
  Across Indiana and Ohio we see a slightly colder air mass still established and I think this will support more freezing rain/wintry mix than locations southward. Once even colder air moves in I believe we will see a transition to snow in these regions.

 I said from the beginning that the strength of this WAA was going to be critical and that I thought it was going to be stronger than many were anticipating. A lot of opinions that people were sharing were based on output from the NAM model data (because it had the much more aggressive solution) and I wasn't sold on it (I definitely wasn't the only person who wasn't sold on it but some people just ran with it. If you've followed me very long then you know how I feel about that). When I was doing my initial analysis today I immediately knew that the WAA was in fact stronger than a lot of the guidance suggested and that meant that those particular models were probably not going to be too close to what happens tonight. That's why I'm always whining about how important it is to pay attention to detail and consider things like that if weather forecasting is something you want to do. You will still be wrong sometimes, sure, but you'll do so much better overall!

Here's a look at a few station observations across the State from the "Kentucky Roadway Weather Information System" via KY Transportation Cabinet. Notice the "pavement temperature" readings...

Leitchfield

Louisville

Lexington

So there you go, very complicated situation indeed. I've scrutinized the data for a week now, broke it down over and over again, and considered every scenario and variable I can think of. Sometimes you get setups that let you make forecasts with higher confidence and sometimes you get, well, this. That's what makes it worthwhile though. You can't control how things go but you just start at what you have and go from there. Now we watch.

This week will see a little bit of everything so there will be no shortage of things to cover this week. From this winter system, to 50+ degrees, to thunderstorms, to another shot of cold air, to snow chances, we've got it all. I'll break down how the pattern looks after this system moves out tomorrow. 

Stay safe everyone!

DKK