Let's start with radar. This screenshot was taken at 2:25amCST and shows the rain just along/behind the surface front, freezing rain from southern Indiana southwestward through Owensboro & Evansville, sleet is the most dominant now, and off to the West as we get close to the limits of the particular radar in use you can notice a transition to snow where the frontal-surface aloft has passed. I will add that you should always use various analysis tools together to verify as best you can what you're looking at. The delineation of the transition zone is accurate in this image however. Dual-pol radar data along with environmental analysis and ground reports from the area strongly support what the radar is showing at this time.
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| Color Legend: Green/Yellow= Rain, Red/Pink=Freezing Rain, Orange/Peach=Sleet, Blue=Snow |
Now that we know what's going on locally and upstream how will it evolve over the next several hours? Ultimately the precipitation is going to work across the region over the next several hours. Then it's a matter of assessing the environment it will be moving into. Full-disclosure: I finally saw a doctor today because I've been really sick and unfortunately I have the flu so I won't necessarily share everything I'm looking at right now. We'll look at some of the key features as they relate to local impacts though.
I always start my analysis in the upper-levels of the atmosphere and work my way down but for the purpose of this blog I am just sharing some of the most notable parts of my analysis. The picture above just shows surface station observation plots with a regional radar composite overlay. I have outlined the cold front in blue as well as the freezing line in pink. Also worth noting is the respectable wind gusts behind the front. Wind gusts up to 30mph have been measured following its passage.
One of the main factors in precipitation type during this event is this layer of warm air (relatively speaking of course) at 850mb(about 5kft). The area where this warm layer overlaps the sub-freezing air at the surface is where we are seeing the freezing rain/sleet. The surface-front is located across Western KY with the freezing line just West of the Ohio River. You can see that the freezing line at 850mb is located across Central Illinois through Southeast Missouri and Central Arkansas. Any precip that falls through this layer will melt however it will freeze again when it reaches the surface. This is the reason for the ice concerns. You can also see that the temperature lines (isotherms) are packed very closely, indicating a strong thermal gradient which is associated with our frontal surface aloft (Sometimes it seems people get so caught up with other things that they forget to look at the atmosphere is 3-dimensions, 4-dimensions if you consider time). This correlates with an area of strong "frontogenetical" forcing. We have obviously been chasing model trends for days now and there is tremendous consensus showing our precip continuing but it is still important to look at analysis data and be able to support that. This area of strong forcing with our FROPA (Frontal Passage Aloft) will support continued precip production as it moves Eastward. This particular factor is important as I see it because it means we have strong ascent within our "warm over cold" sector. This would support the continued production of freezing rain/sleet, and thus a continued threat for ice accumulation throughout the night. I wouldn't be surprised to see ice totals of 0.2" to 0.5". That is certainly enough to make travel dangerous and cause occasional power issues. Don't get too locked in on the max of that range and understand that most will see something near the median of that and that is what I'm considering when mentioning possible impacts. Obviously more ice would mean more problems but I don't want to be the "worst case scenario" guy. Same deal with snow accumulations. Many forecast maps have between 2"-4" of snow across the State. I personally think some people will see a little more than that, up to 5"-6", but that doesn't mean everyone will. For some reason it seems like when snow forecasts come out, whether it's a blogger like myself, or an official forecast from the NWS etc, there are people who will see 1"-3" of snow possible and they will say "they're predicting 3" of snow!".
So now that we have an idea of what our environment looks like locally let's look at how this hand might play. Upper-low is to our West over the Central Plains. Upper-level divergence associated with the upper-jet over the region is resulting in large-scale ascent in addition to the focused forcing with the frontal surface. Forcing for ascent is going to continue to move Eastward. Also in place is southerly/southwesterly mid-level flow thanks to the upper-low to our West and this will serve to keep sufficient moisture advecting over the area. This will also help keep a warm layer above the colder surface layer. Based on this I would expect most locations, at least to the I-65 corridor to experience rain transitioning to freezing rain following the surface front's passage followed by a transition to snow following the FROPA (frontal passage aloft). Once the front passes aloft we will have a sub-freezing profile and in turn, snow production will more efficient. Precipitation will continue tracking generally eastward. Rain will transition to freezing rain and approach I-65 over the next couple of hours. It looks like there is some erosion of the warm layer aloft and this would support more of a sleet precip type but I still believe locations, primarily West of I-65, will experience decent showers of freezing rain. Later this morning and into this afternoon we will see wintry precip tracking across East KY, although there could still be some freezing rain it looks like it will be more supportive of sleet/snow.
One more interesting thing to point out. There is some instability across the region which would support convective development and any showers that develop and take advantage of that instability would be capable of higher rates of freezing rain/sleet.
There you have it. If I felt a little better I would have really broke down some of the data I'm using tonight but hopefully this will help with understanding the "why's" of this event as we go through this weekend. The one thing that will initially limit ice accumulation is the pavement temps. I say initially because it's not going to take long to get pavement temps back down below freezing.
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| Observation Data from the Kentucky Roadway Weather Information System via KY Transportation Cabinet. Pavement temps are still well above freezing right now but that will change. Also see if you can determine when the cold front passed each site. |
I will be continuously watching how things play out all day and will make another post should the situation call for it. Hopefully we will be good to go though and we can wait until later this weekend to do a summary of the event instead of making another update to address hazardous weather.
Be safe everybody!
DKK
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