Good morning! Please bear with me since I'm on the road right now but I still wanted to make a quick post to mention the major storm system making its way across the Country right now. This "double-barrel" low-pressure system will bring nasty impacts to the region as we start the weekend. I expect to do another update tonight once I'm set up and stationary and we'll discuss specific local impacts then. The models are tripping up because there will be multiple precip types (rain, freezing rain, sleet, snow) within a relatively short time and they haven't been reliable at all regarding accumulation yet. I expected they would start coming around today anyway so we'll see what they morning runs look like shortly.
What do we know? I'm glad you asked, that is a great question. I actually feel very confident about the large-scale details. That stuff is essentially locked in as I see it. We've still got a split-flow pattern aloft and the southern branch has brought a "bowling ball" upper-low (breaking out the book of weather terms today) into the West/Central CONUS. The northern branch has also brought a shortwave trough across the northern tier which has interacted with a thermal boundary at the surface to develop a surface-low. A second surface-low is developing over the Central Plains as a reflection of the southern upper-low as well. As our shortwave rounds the southern Rockies and ejects eastward it should take a very sharp negative tilt. It will also interact with a noticeable thermal boundary where two distinct air masses are clashing and the "frontogenetical" forcing (where the air masses clash) coupled with a strong upper-level divergence/low-level convergence relationship should result in rapid deepening of our surface-low. (The negative tilt, which means the low-pressure system tilts with height, also results in the surface-low being in a favorable location within the left-front exit-region of the upper-jet where vertical motion may occur, in turn leading to mass being removed from the column, decreasing the pressure).
A surface-low associated with the northern piece of energy will track toward the Great Lakes while the southern low will track toward the Appalachians. At that time, Thursday into Friday, I believe we will see an energy transfer between the northern-southern energy and our system should tighten up again as it passes through the Tennessee region.
I think that is one of the keys to how our transition from rain, to freezing rain/ice, to snow will play out. A quicker energy transfer with a faster pull of deep, cold air into the region would result in a faster transition to snow. We will see a transition with rain/ice/snow regardless but the storm track and duration greatly affect how much of a certain precip type we see. Those are the main questions from my perspective and once we can really answer them we will be able to pinpoint specific impacts more accurately.
Like I said, I really think today's guidance gets a better handle on things with the surface-lows both coming together so that's what's next, watching development and monitoring data trends.
Either way, we'll be under a warm advection regime as southerly flow takes over in response to the system off to our West so for the next 36hrs or so we will experience much warmer temps than we have in awhile. We'll actually get up into the mid 60's Thursday. Winds will occasionally be gusty as the pressure-gradient tightens as well, ~20mph gusts are possible. Rain should begin on Thursday with a transition to a wintry mix of freezing rain & sleet possible some time Friday. Friday night into Saturday we should see a changeover to snow. It is fairly likely we will see accumulating snow with ice being a realistic hazard as well. That's where we stand for now. Now it's back to patiently watching everything as it unfolds. I'll be back later tonight to make an update after I've had the chance to reassess everything.
DKK
Wednesday, January 10, 2018
Sunday, January 07, 2018
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.
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.
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.
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.
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...
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| Leitchfield |
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| Louisville |
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| 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
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