Arctic Blast to End 2017

  Well, the previously advertised cold blast is upon us and it has not been fun. We've seen temps in the low single-digits with wind chill below zero here in Kentucky and just upstream temps were in the -20's and below. This is the theme and as cold as it has been, it will be getting even colder. How does Alaska, Mexico, and Canadian snow cover come into play you ask? Well, alright! Amplified ridging (anti-cyclonic/clockwise flow) both near Alaska and off the coast of western Mexico aid the development of a deep trough over central Canada and the eastern U.S. This means that the jet stream (Polar Front Jet) dives further South over the eastern U.S. Snow cover in western/central Canada aids the development and strengthening of high-pressure at the surface and these areas of high-pressure are able to tap into extremely cold Arctic air masses and basically bring that air with them as they build into the central and eastern U.S. The troughing over the eastern U.S also means that waves that develop in the atmosphere and can produce different forms of winter weather have a favorable track to impact the central/eastern parts of the U.S as they hitch rides along the jet stream. We will be locked in this pattern until the blocking ridges near Alaska and Mexico begin breaking down. This will happen eventually of course but I think it won't be until we're a week or more into 2018 when we get the chance to thaw out and flatter flow aloft brings more "normal" cold weather. 

Current setup showing a blocking ridge near Alaska, another near Mexico/SoCal, and the associated troughing in the eastern CONUS, helping drive the Polar Front Jet southward. (Map Retrieved from University of Wyoming, Analysis by Myself)

Let's walk through the weekend forecast and see how things look for New Year's. I'll go ahead and say this and odds are you'll see more occasional rants from me about this in the future, I have seen countless people posting a forecast model graphic of their choosing on various sites and saying things like "HUGE SNOWSTORM possible for New Year's!" and "Nor' Easter to slam the East Coast!" or, on the flip side of this, things like "There won't be any snow" and "The snow/ice event in the southeast will NOT happen!". To "couch" what I'm about to say I guess I should point out that I am absolutely not model bashing nor am I bashing people who use them. I'm not even bashing people who post occasional model graphics online because I have done that myself. To the contrary I understand that forecast models are a necessary tool. What I find myself getting frustrated with is when some people post these images without providing context or worse, just taking them at face value with no real "forecasting" at all. What I mean is that it can be beneficial if people seeing these posts also understand that there are countless other models with countless other possibilities or that they understand, at least in conversationally-friendly language, some of the processes a particular image represents. I also get frustrated when it seems that someone completely skips the forecast process and goes straight to model analysis. Let me reiterate that I do NOT hate the computer forecast models, they're absolutely invaluable, I have just learned that if you can only "forecast" based solely on model output then you're destined to live or die by them, so to speak. Personally, the way I like to forecast is by first analyzing and diagnosing the current state of the atmosphere. This is just looking at what is going on and determining why it's happening. I use everything from the ridiculously awesome new GOES East (GOES16) satellite to my own hand-analyzed observation maps. After looking at how things were leading up to a given day and how they are at that time it's possible to develop a general forecast for the near-future, before you've ever looked at the latest HRRR run. Of course there are different things that one would look at for different situations and we will cover those with time but that's how my forecasts start. Simply looking at what is going on now.
After a certain point however it becomes much more difficult to anticipate what will happen and this is where the models can really shine. I like to look at different models/runs and see what they agree on and what they don't and compare this to what I think will happen. If I see that the models disagree with either myself or each other, and they will, then I like to try to figure out where exactly they differ and why they can't come to the same solution. Is there something that it thinks will occur in the atmosphere that alters its output? Is it a result of the model's physics? I find that the more of these questions I can answer the better.
  Last thing and I'll move on. It's also okay to not know something and to say "I don't know what's going to happen...yet". We are constantly playing catch up with mother nature, whether literally chasing a storm, or forecasting, and mother nature doesn't always show her hand truthfully. I would much rather just tell you all that I'm not entirely sure what will happen yet but here's what I know right now and continuously fine-tune it instead of throwing some random data at you for 8 days out and hoping it's right.

  Let's take a look at what's going on right now across the region. There is actually some snow ongoing in eastern Kentucky, mainly along and North of I-64. One snow band in particular from southern Ohio down through Lewis County was producing fairly heavy snowfall. Accumulations are expected to be light as this band moves eastward but hazardous travel conditions are occurring and slick spots will develop tonight. Back across northern and central parts of the State, minimal snow accumulations are possible as light snow showers pass through this evening, 0.5" or so, but the heaviest snow will be North of the Ohio River where the strongest forcing is located. 

Current analysis shows an upper-low just West of the Great Lakes with subtle wave passing through IL/IN right now (1730cst). Another strong jet is loaded upstream across the Northern Plains as it begins to work its way around the parent trough. At the surface, an area of high-pressure is centered over KY with a surface-low over the Great Lakes dragging a cold front through IL/IN. Another strong Arctic High is pushing into the northern CONUS right now and this feature will be a key player in our New Year's weather.

 Weekend Cold Blast 

  Throughout tonight and into Saturday morning a few flurries will be possible across northern KY as a subtle wave passes through but I don't expect much out of this. Models don't show anything from it but I don't feel cool saying there's a zero chance of a flurry so I figured a mention of snow flurries/freezing drizzle would be appropriate. Temps when you wake up Saturday will be in the low-mid 20's and will only be able to max out in the upper 20's, maybe 30, before a weak front passes through and gusty winds out of the Northwest spread over the area. Wind gusts will be in the 10-15mph range, I wouldn't rule out a few gusts over 20mph. Clouds will increase to overcast/mostly cloudy skies as the front passes as well before breaking up somewhat, to partly cloudy, from West to East following its passage.
  The passage of the front will mark the beginning of a true Arctic blast. That surface high that we said will be a key player really gets to shine at this point. It will serve as a streamline for the coldest air of the year to pour right into the region as it tracks Eastward toward the Great Lakes, keeping us under the influence of a cold North wind. Temps Saturday night/Sunday morning will drop into the teens with areas under more dense cloud cover in the upper teens.

New Year's Eve

  Frigid temperatures really arrive on Sunday, just in time for our New Year's Eve festivities (oh yay how fun). I think temps will top out in the upper teens. I'm not saying a few locations in southern Kentucky won't hit 20F, specifically locations that keep clearer skies during the afternoon, but pretty much everyone will be in the teens for HIGH temperatures. We should also see wind gusts in the 10-15mph range which won't help things in any way. Wind chill values are going to be in the single-digits. This is dangerously cold weather and I know many will be going out so just make sure you dress appropriately for temps this low. That might throw off your New Year's outfit plan but you've got to do what you've got to do. Lows Sunday night will quickly drop to single-digits and wind chill values will be below zero. 2017 ends like it started, cold, except it'll be about 20-30 degrees colder than it was on 1 January 2017, depending on what part of the State you live in.

  The good news is that it looks like Sunday will be dry. I have spent the majority of the time on this forecast trying to solve this very detail. Here's where I was tripping up. There is a jet streak/core of energy in Canada right now that is primed to round the base of the parent trough near the Great Lakes on Sunday given its current track. It will likely amplify slightly by then and cross the Ohio Valley. This is a common pattern to see, deep upper-level low sets up over the Great Lakes and a series of waves pivot around it and cross our region. This same exact setup brought respectable amounts of rain to the area just a couple weeks ago. Initially when I saw the jet streak and vorticity max (vorticity is basically a measure of spin in the atmosphere and it's commonly associated with producing lift) way upstream I made a note that it could be the trigger for snow locally. After analyzing this feature I dove into the models and, as expected, they show it deepening somewhat as it pivots around the parent trough and crosses into the region. However, pretty much every forecast model keeps the region mostly dry on Sunday. Now, waves pass through all the time and don't produce much of anything so this isn't crazy but I also didn't like the idea of saying it wouldn't snow because the models said it wouldn't (See what I'm doing here? Remember my rant?). That's where it pays to do your best Lt. Joe Kenda impersonation and get to the bottom of this. After analyzing more data it seems that there is a lack of moisture in the region, reinforced by the high-pressure at the surface (high-pressure is also associated with sinking air, not favorable for cloud/precip development, it doesn't mean you won't get clouds/precip but it requires another source of lift). The models are picking up on this drier air and that could be a reason for the lack of precipitation in their output. They also suggest a lack of, here's another technical term but I like this word, frontogenesis, essentially a tight converging temperature gradient which is often a source of forcing for ascent. Now it all seems to make sense and based on all of the available puzzle pieces I feel a little more confident. I do think a few flurries will be possible, particularly to our North, but overall I think this wave will lead to increased cloud cover for northern and central Kentucky on Sunday. As always I'll be watching this closely and if anything looks a little off then I'll make sure to address it. 

New Year's Day

  When you wake up for the first day of 2018, or are just going to bed depending on how you spent your night, temps will be in the single-digits for most of us. A few locations may make it to the low teens. Clouds will be clearing in the wake of Sunday's shortwave and skies should be mostly sunny. We will still be under the influence of surface high-pressure to our North and breezy northerly winds will keep things super cold. I'm thinking highs in the teens. Clear skies Monday night will allow temps to fall to single-digits again. Wind chill values will range from low singles to a few degrees below zero. 

I know this was lengthy but I wanted to cover a lot. I'm also hoping it's the last forecast I'll do for 2017. I'll have another post this weekend to get into next week's weather but that will be forecasting for the first week of 2018 so I'm hoping the next two days play out as expected and we can start talking next year without having to make adjustments. 

Speaking of 2018, there appears to be a chance for snow mid-week so that is what the focus of tomorrow's post will be. 

I really hope all of you have just an extreme amount of fun for New Year's. Stay safe and don't do anything stupid obviously but have fun. I'll be back tomorrow to discuss next week's weather and I'll also have a recap of my 2017 storm chasing adventures and stuff like that. 

Stay cool and stay warm...

DKK