Wednesday, January 10, 2018

January 12-13 Significant Ohio Valley Winter Storm

A significant winter storm impacting the Ohio Valley is expected to lay down accumulating snowfall, potentially in excess in 12", as well as potentially crippling ice accretion.

This post will cover current model guidance, which appears to be forming a consensus as of the January 10 00z model suite.


Pivotal Weather - GFS
We begin with the GFS model's outlook for snow accumulation. The 12z GFS appears to anticipate a stripe of accumulating snow falling from western Tennessee and Kentucky into southern Indiana, much of Ohio and portions of West Virginia. Particularly heavy accumulations are forecasted for northwestern Pennsylvania and western New York, especially near the Great Lakes. Per this model forecast, accumulations look to be in the 2-4" range in western Tennessee and Kentucky, increasing to a broad 3-6" range for Indiana, especially in the southeast portion of the state. Ohio may receive anywhere from 6-12" of snow, with the higher end of this range most likely in the northeast part of the state.

Pivotal Weather - NAM
Next is the NAM's snowfall forecast. As is typical for this model, snowfall accumulations are enhanced, likely beyond what will actually occur. Western Tennessee and Kentucky are projected to receive 4-8" of snow in this solution, a notable bump from the GFS' guidance. Southeast Indiana and a good swath of Ohio are outlooked for a 12-18" snowstorm as per the NAM model, again a substantial increase from the GFS solution. I do not expect the NAM's solution to work out, partially because of its inherent bias of over-doing snowfall accumulations but also the possibility (if not the likelihood) that at least a portion of this projected snow is actually sleet for some areas. This sleet-forecasted-as-snow problem is present in all models for the winter season (and can vary depending on the website you use to get snowfall accumulation graphics), but is particularly maximized in the NAM model. As such, I would take the NAM snowfall graphic with a grain of salt at this point and refer to the GFS and CMC models, the latter of which is shown below.

Pivotal Weather - CMC
The CMC model seems to take a middle ground between the GFS and NAM in a broader sense. On one hand, the CMC anticipates a similar snowfall accumulation story for Indiana/Ohio/Pennsylvania/New York as the GFS, but also sides with the NAM in the idea that snowfall accumulations may be heavier in Tennessee and Kentucky. Adding to the variance is how the CMC's accumulations for Kentucky and Tennessee appear to be slightly west of other guidance. I personally would side with the GFS for accumulations in Kentucky and Tennessee - there will be a very warm air mass in place as this storm gains strength, and I'm not confident in the storm being able to cool off the temperature profiles enough to lay down over half a foot of snow in those areas. I do believe a broad 8-14" range of snow for southeast Indiana, much of Ohio, northwest Pennsylvania and western New York is preferable for now, with higher amounts likely in places nearer to the Great Lakes for Pennsylvania and New York.

Freezing Rain

Pivotal Weather - NAM

Pivotal Weather - GFS
I don't want to discuss freezing rain too much in large part because freezing rain is often a "nowcast" situation, or can only be accurately forecasted immediately prior to the event. I will, however, show the NAM and GFS forecasts for freezing rain to give an idea of where accumulating ice is most likely from this storm. I want to emphasize that, with the above graphics, you should *not* focus on ice accumulations, but *should* focus on the forecasted placement of ice accretion.

The NAM and GFS agree that an ice storm is possible for western portions of Tennessee and Kentucky into southeastern Indiana, as well as south and east Ohio. Further ice accretion is also a possibility for central Pennsylvania, as well as parts of central and eastern New York. The remarkable similarity between projected ice accretion and projected snowfall accumulation makes me more suspicious of the snowfall forecasts, particularly for Kentucky, Tennessee and southeast Indiana, as model guidance could be falsely interpreting freezing rain and/or sleet as snow. As I said, this will likely become a "nowcast" situation for that reason.

To Summarize:

- A significant winter storm is expected this weekend in Kentucky, Tennessee, Indiana, Ohio, Pennsylvania and New York.
- Snowfall amounts of 2-6" are possible in Kentucky and Tennessee, while a range of 6-12" is possible for southeast Indiana into western and central Ohio. Northeast Ohio and western New York look to receive 8-14" of snow by the time this event is finished.
- An ice storm does look possible from this system. While I am not confident in forecasting accumulations, it looks as if central Tennessee, western Kentucky, southeast Indiana, central Ohio, central Pennsylvania and central/eastern New York will be at risk for non-negligible ice accretion.
- The location and intensity of freezing rain will most likely only be accurately determined immediately prior to the event, if not determined during the event (i.e. "nowcasting").


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