at 6:18 PM by Jacob Wilkins

Where Did The Snow Come From?

So, where did all that heavy snow come from Friday evening? It came as a surprise to most forecasters in the region. A number of factors played into the snowfall event that took place across much of central Kentucky.

Let’s back up a few days. The system really began catching the eye of the meteorological community late Tuesday evening. During that time, the 00z model data suite came out from the GFS, ECMWF, GEM, and NAM models (the main weather models forecasters use for guidance). All the models were in good agreement that accumulating snow seemed likely. The NAM was the most aggressive with the snowfall, placing snow depths of up to 4″ in some spots in western Kentucky.

January 21, 2015 00z NAM Model - Forecast Snow Depth Valid 6 AM January 24, 2015
January 21, 2015 00z NAM Model – Forecast Snow Depth Valid 6 AM January 24, 2015

However, later model runs, some just 12 hours later, began to back off of the idea of that much snow in the region. Many of the models depicted southerly flow at the surface with too much warm air advection for the system to really crank out any major snow totals.

The area of low pressure also seemed to weaken in later model runs, meaning the system would likely be suppressed further south. Remember the perfect track of a Kentucky winter storm? It shifted, and didn’t exist in later model runs. Both warm air advection and the strong suppression of the system on the models seemed to diminish any chance of heavy snowfall for the region. There would be plentiful amounts of moisture available, however, it looked like a nice rainmaker.

A little side note: the High Resolution Rapid Refresh (HRRR) model has been very accurate as of late regarding winter weather events. The downside of the model is that it will only look 15 hours into the future. In the back of my mind, I kept telling myself to wait until the event comes into the time frame of the HRRR and go from there.

We don’t have the luxury of waiting on that model to produce forecasts days in advance. Thus, from one day, it looked like quite the winter storm, to some “plain rain” and perhaps snow flurries the next day. So, we forecast what the models told us. Models are to be used as guidance, and all the guidance pointed toward almost a non-event for much of Kentucky.

Then Friday morning rolled around and we woke up to snow showers throughout most of the morning. These snow showers would occasionally mix with some sleet pellets and plain, cold rain.  This was expected, however, a transition to all rain was also expected, which never took place.

I began looking at the HRRR model and it was depicting a large band of heavy precipitation over much of central Kentucky for the remainder of the day. The problem: temperatures seemed a bit too warm on the model to support snow.

However, the cold precipitation was already set in place across the region; coming down very heavy at times. Thick clouds prevented any warmth from the sun to penetrate into the lower levels of the atmosphere. That southerly flow that everyone was banking on happening… yeah, that didn’t happen.

So, instead of highs being in the low 40s across the region like they were forecast to be, high temperatures were in the mid 30s across much of the region. The temperature didn’t fluctuate much at all after 11 AM. In some cases, northerly winds at the surface caused the temperature to drop steadily into the low 30s by the afternoon.

At this time, a deformation zone formed across parts of the region. This lead to widespread heavy precipitation across the area. You may remember the very large flakes of snow falling in the afternoon hours over Bowling Green. While that was occurring, the deformation zone was developing over the northern part of the region.

By 5:30 PM, moderate to heavy snow was beginning to fall across portions of Butler and northern Edmonson County, where the highest snow totals came from in our region.

Despite surface temperatures being above freezing, the snow began to accumulate on itself, effectively cooling the ground beneath it for significant accumulations to occur. Even through temps were above the freezing threshold, the rate at which it was falling cancelled out the whole “melting on contact” mentality.

After all was said and done, Butler County received over 4″ of snow in some spots. Places further north in Elizabethtown got 6″ of snow.

The majority of Warren County received between 1-2″. Bowling Green specially received over an inch in some locations, which covered most of the roadways as well.  A dusting was reported from those in Simpson and Allen County.

Our forecast for this event was… “A dusting to ~1″ seems plausible. If we’re lucky, we may even squeeze out 2″.”

Even though much of the area saw that, those located along the N tier of our forecast area (Butler & Edmonson County) received more (a few places doubling the forecast amount). When we bust a forecast, we always own it, however, we find it very important to educate the public on why the forecast was missed. Here’s an official look at totals from NWS Louisville:

Snow Totals Map January 23-24, 2015 via NWS Louisville
Snow Totals Map January 23, 2015 via NWS Louisville

Looking back on the event, the main factors at play were:

  • Northerly, not southerly, winds that occurred throughout the day
  • Expected transition period to all rain during the day that NEVER occurred
  • Unexpected heavy banding of snow over a narrow sliver of the region
  • Temperatures not getting as warm as forecast, leading to freezing temps by mid to late afternoon

This event should serve as reminder that meteorology is an ever-evolving, imperfect science. No one’s forecast for this event was correct, no one’s. This event will likely be studied by many meteorology students and professionals for several years to come. Remember, regardless of the forecast, always stay weather aware!