Active Pattern Continues: Weekend Forecast and a Next-Week-Peek

  Well we had a double-team of cold fronts pass through yesterday and Winter is back. It was never really gone I don't suppose but still, after getting back out to chase a few severe storms in January and experiencing temps that climbed all the way up to about 60F I may not have been completely honest with myself about what time of year it is. Alas, it is still Winter and we have been rewarded with this brief reprieve from mild, comfortable, 50+degree days with temps in the teens. Awesome...

Yesterday's "Snowstorm" (Or Not)

Surface analysis I performed Thursday morning (1 Feb 16z) preceding the onset of precipitation. One of the main things I was interested in was the narrow corridor of moisture (assessed with 700mb Relative Humidity) with a "vort max" (associated with lift) trailing behind it. This moisture was trying to exit the region just ahead of the colder polar-air-mass and strongest ascent. Some overlap occurred resulting in the snow we did see but more widespread moisture that lingered may have aided more snowfall. 

Same analysis chart as the one previously pictured. This analysis was performed Thursday morning (1Feb 16z) to assess the environment before precipitation began. 

What about that snow we saw? If you remember just over a week ago there were whispers about the possibility of major accumulating snowfall yesterday (Thursday 1 Feb). What happened? As I look outside I see about a half-inch of snow on the grass and in eastern KY there are locations measuring just about 1". But the models said 8"! They did in fact, and even more, they were somewhat consistent for a few days. But then it happened. First it was just subtle shifts in location. Then they started trending lower, and lower, and lower. Before you know it we had a whopping 0.5" of snow before the actual system moved out. I'm not going to waste much time on this because there are more interesting things I want to get into but this was a great example of why it's important (to me anyway) to utilize ALL of the tools at your disposal when forecasting. Personally, I was never sold on this being a major snowstorm and I said for days that I was expecting less than an inch of snow with a little more in Eastern Kentucky. Before you think I'm bragging allow me to bring myself down for you real quick: The last system that came through brought 5"-6" of snow to many locations and do you know what I predicted for that event? 1"-3". That wasn't a terrible prediction but I was unable to foresee the storm getting "hung up" and stalling out. The snowfall rates I was forecasting were correct but that detail meant that those rates fell over locations for a longer period of time resulting in more snow than expected. I just want you guys to know that I am not interested in "glossing" things over or anything like that and when I'm wrong I always admit it and figure out why so I can do better. Nature is notorious for bluffing so the more you know about her hand the better chance you have of making the right choice. It's also worth noting that many of the local Meteorologists were on top of yesterday's system and I don't want to sound like I'm trying to bring their skills into question.

"Spaghetti plot" from 25 Jan.
Contours=various model "predictions" for where snow will fall on
a given date (1 Feb).
That's a large spread. That basically said that
snow may fall anywhere in the US from the Rockies to
the Appalachians. Hard to work anything specific into a forecast
based on that. That's why posting one select model
graphic online without context is misleading.

  There were a bunch of folks posting things online as far back as last Wednesday (8 days out!) about major snowfall however. So why? What were they seeing? Well for starters they were seeing aggressive model runs and we have been in a pattern that supports possible systems impacting the region. Actually, the pattern is even supportive of sending one wave after another, and another, and another, much like an assembly line, across the U.S and through the Ohio Valley, each one with potential. Ok, so based on that why did many of us think the models were overdoing it? The short answer is, it was 8 DAYS OUT! I really start to take model guidance regarding large-scale features seriously when it gets to about 96hrs, give or take depending on the scenario. The models were also struggling with a system that was moving through at the time so that really discredits its output. Every time-step of a model run plants the seed for the next time-step and these details amplify exponentially, sometimes into crazy outcomes (check out "chaos theory" as it applies to weather models). The more assumptions you have to make for something to be true, the more likely it is that it's not true. That was the case. IF it's handling the current system correctly, and IF it's handling the initial evolution of the upstream conditions, and IF it's resolving the phasing of upper-level energy correctly, and IF it's handling the moisture trajectories and forcing for ascent correctly, and IF it's handling the precipitation formation processes/physics correctly then we may be able to trust the output. That's a lot of IF's! That doesn't mean you don't look at the models or trust the data just ask yourself the relevant questions and think about the details, particularly how many assumptions have to be made to verify. The trend is your friend when it comes to models. You still have to investigate it further but trends can point you in the right direction. More telling were the actual environmental conditions that were setting up early this week. The biggest thing limiting greater snowfall was the fact that in order to maximize accumulation we would have to see a series of upper-level waves phase flawlessly with an upper-level jet-streak timed perfectly as it entered the base of the parent trough and crossed into the region. Another issue was that we were looking at a deep surface-low with very strong low-level flow, specifically in the warm sector ahead of the cold front/s. Surface gusts were reaching 30kts and this was in effect aiding strong warm air advection in the boundary-layer (Snow can accumulate even when surface temps are above freezing as long as the snowfall rate exceeds the rate of melting). This would have to be overcome by colder air to maintain frozen precip at the surface and the result was a transiton event with rain eventually changing over to snow. This meant less snowfall. The cold air did eventually win out and we even saw some heavy snow for awhile but there can be a big difference in heavy snow falling for a few hours in the afternoon and snow falling all night long.

Unsettled Pattern Brings Rain/Snow/Storm Chances Through Next Week

  A pretty stubborn but active pattern aloft sets the stage going forward. Currently we're looking at a single-stream polar-jet with a longwave trough over the eastern CONUS and a Pacific-ridge off of the coast of California. These features have shown no signs of breaking down anytime soon and that means that as mid/upper-level waves develop upstream they have a fairly favorable track across the central and eastern U.S. 

500mb analysis for 12z (6amCST) Fri 2 Feb. Depicting the Longwave Trough Over the Eastern Part of the Country and the Ridge Out West. These Features are Locked in Place for now and Provide the General Track for Systems to Bring Unsettled Weather Throughout Next Week. 

  You can see a shortwave trough along the West coast of British Columbia and that is one of the pieces of energy of our Sunday system. In my opinion the track of this wave is all but decided. There are some questions related to the temperature profiles and moisture quality come Sunday but I'm pretty confident in how the shortwave itself will evolve over the next 48-60hrs. Interestingly enough the upper-level pattern is progressive right now with a high number of longwaves in the Northern Hemisphere but a pair of broad ridges are helping keep our parent-low relatively stationary. We could very likely see our shortwave flatten out and become somewhat disorganized as it crosses the mountains of British Columbia and then the Rockies but the energy should maintain and re-emerge and reorganize over the central CONUS sometime Saturday PM. It's at this point I believe it will deepen as it enters the base of the parent-trough while a strong jet-streak enters its base as well. That's a lot I know, but if you can anticipate these things and time them correctly then you may be able to correctly predict the evolution of these features and in turn you'll have a much better shot at being able to forecast regardless of whether or not the models are performing well (Notice I have purposely avoided mentioning any model data in this part of the thought process). An associated surface-low will be in the Midwest and should deepen a little as it tracks toward the Great Lakes, dragging a cold front through the Ohio Valley. The precip types late Saturday night is where confidence drops off. Right now it's looking like temperatures will be in the low/mid-30's, except for far western Kentucky southward where warm air advection will be a little stronger ahead of the front. Precipitation will likely move in from the SW Saturday evening and temperatures should be near or just above freezing. Initially it should be rain until later Saturday night when the atmosphere has cooled enough and a transition to snow or a rain/snow mix is possible. "Wet-bulbing" (evaporative cooling) could play a big role in precip type as it could serve to cool the low-levels enough to transition to snow fairly quickly. As warmer air continues to work in throughout the afternoon on Sunday a freezing rain/rain/snow mix becomes possible, particularly East of I-65 in Kentucky and Tennessee. Along and North of the Ohio River I expect primarily snow since colder air should be established. A transition to mainly rain could exist along the backside of the precip-shield as it moves eastward. Temps will climb into the 40's Sunday afternoon at which point a transition back to rain will occur. After the front passes Sunday afternoon I think we cold see another quick shot of snow flurries with the NW flow wrapping around the low-pressure system and very cold air interacting with lingering moisture. 

How Much Snow Are We Talking Here?

  I haven't seen anything that suggests a major event but there could certainly be some impacts, especially considering many folks may be having Super Bowl parties or something. If the freezing rain is able to over-achieve then that would be a bigger issue than currently expected so that's worth following. I don't think 1"-3" of snowfall by Sunday night is unrealistic for areas East of I-65, especially in North-Central Kentucky into Indiana, with up to 1" possible West. Temperatures will drop to the teens Sunday night so slick spots will be likely and should be accounted for. 

  So there you have it, rain-to-rain/snow mix-to-snow-to-rain-to-snow. Fun stuff for sure. I didn't post as many graphics as I usually do so I apologize for that. We've put everything together about as well as we can with what we know at this time though so all that's left to do now is monitor the data and observations and we'll be following in real-time tomorrow night.

And Next Week...

  It definitely looks like active weather continues through next week with more chances for rain, storms, and snow across the Midwest. I even think severe weather is at least on the table for the Southern Plains into the Arkansas/Missouri/Mississippi River Valley but there are still some things to work out with all of that. Luckily there's plenty of time and we'll dive into that tomorrow and see if we can put some of the pieces into place but with one disturbance after another set to eject across the U.S within strong flow aloft and a Polar air mass that hasn't entirely modified or cut off the Gulf source I think we have something to work with. Hey, it's a start!

See ya tomorrow, stay warm.

DK