SARANAC LAKE — A mixed bag of wintry precipitation is forecast to coincide with the Thursday morning commute throughout the Tri-Lakes region as a storm system barrels up the St. Lawrence Valley.

While the storm has been on meteorologist’s radars for several days now, the forecast has evolved. Earlier in the week, it appeared that the Tri-Lakes could receive a potentially significant snowfall accumulation. However, computer forecast models trended increasingly warmer throughout Monday and Tuesday, shifting the heaviest axis of snowfall away from the region and introducing the possibility of sleet and freezing rain as temperatures are likely to rise above freezing for a portion of Thursday.

The National Weather Service’s Burlington, Vermont office, which serves the Tri-Lakes region, issued a winter weather advisory for Franklin, Essex, Clinton, Hamilton and southeastern portions of St. Lawrence County in advance of the storm.

The advisory is in effect from 7 p.m. tonight through 7 p.m. Thursday for hazardous travel conditions as a result of a combination of snow, sleet, freezing rain and strong winds. Between 2 and 6 inches of heavy, wet snow and sleet is expected during that time, according to the advisory — which added that a light glazing of ice from freezing rain is also possible.

The snowfall accumulations forecast is more variable than normal, as meteorologists emphasize that snowfall amounts with this storm hinge on how quickly precipitation — which is forecast to begin as snow — transitions to sleet and freezing rain. Small differences in the timing, which are difficult to pinpoint, could produce a large difference in snowfall amounts across the area, according to NWS Burlington Meteorologist Rebecca Duell.

“Some steadier snow in the overnight hours and then there’s going to be a transition to some mixed precipitation probably right around sunrise, but that could change,” she said. “It’s a tricky forecast anytime you have a wintry mix in there. It’s kind of difficult to say exactly what time it will change over, but sometime early Thursday morning.”

Duell said peak wind gusts could get as high as 50 mph, with the strongest wind expected during the early morning before a relative lull in the middle of the day. It is then expected to increase again later in the evening, before beginning to decrease in the early morning hours into Friday.

“It will be quite messy, and that’s going to be timed right around the Thursday morning commute,” she said. “In addition, we’re also looking at some gusty winds.”

The bulk of the precipitation — regardless of what it falls as — is expected to dial down by mid-afternoon Thursday, although scattered patches of sleet and freezing rain look to continue through the afternoon before changing back over to scattered snow showers as cold air moves back into the region behind the storm.

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It’s all in the track

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Earlier in the week, the computer weather models hinted at a different setup that favored more snow and less ice locally.

The models essentially function by taking actual weather observations taken at the ground and atmosphere — through the use of weather balloons — levels from weather stations around the world. For consistency, the observations are taken by meteorologists at the same universal times each day.

The data is then ‘fed’ into a complex set of mathematical equations meant to predict how the weather will act at various points in the future. There are a variety of different weather models, with each having slightly different equations and formulas. These are in part informed by past atmospheric behavior, as that can serve as an indicator of what will happen in the future based on similar real-time data.

Among other things, the weather models estimate a storm’s track, which is often crucial to determining the amount and type of precipitation. The models produce multiple runs each day — some twice per day and others on an hourly basis — that tweak and shift storm tracks as time moves closer to the storms’ occurence, with more uncertainty and variance in track the further out into the future the models are predicting.

Low-pressure systems generally rotate in a counterclockwise direction in the northern hemisphere. The rotation acts like a fan, churning warmer air up from the south on the storm’s right side — areas to the east of the storm’s track — and pulling colder air from the north down on the storm’s left side — areas to the west of the storm’s track.

This is especially important when dealing with winter storms that are moving in a south-to-north or southwest-to-northeast direction. The more southerly origins means that the warm air associated with the storm’s right side is, in fact, often warm enough to change the precipitation type over from snow to sleet, freezing rain or rain.

Conversely, winter storms that originate northwest of the Tri-Lakes region, such as clipper systems that begin over the western Canadian provinces, track through such cold enough air that, even though they have a similar rotation, temperatures on the “warm” side are still below freezing, resulting in an exclusively-snow precipitation type.

Storms that form over the southeastern United States and then move northward and eventually impact the Tri-Lakes area tend to take one of two tracks. ‘Western runners’ take the inland track, moving up the Mississippi and Ohio river valleys before tracking over the Great Lakes and north from there. ‘Nor’easters,’ on the other hand move along the coastline.

The split in the tracks is driven by topography, as storms tend to not track directly along the spine of the Appalachian mountain range. Most of the time, they are pushed to either one side or the other — although under certain conditions western runners can be pulled east and subsumed by a stronger area of low pressure concurrently tracking up the East Coast before the storm impacts the Tri-Lakes.

The ultimate track has implications locally.

Nor’easters position the Tri-Lakes area on the colder, west side of the track — typically resulting in snow — while western runners position the Tri-Lakes on the warmer, east side of the track — typically resulting in the smorgasbord of precipitation like what is expected for Thursday as warm air displaces the cold that is typically over the region in winter.

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Weekend storm and beyond

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A busy weather pattern remains in place after Thursday’s storm. The next storm appears — as of press time Tuesday evening — to take a more eastern track on the models. Like Thursday, however, this is subject to change as models continue to update. As of now, however, Duell said snow is the most likely outcome for the Tri-Lakes region.

“(It’s) looking like mainly snow,” she said. “We don’t have snow totals in there yet, but it looks like we’ll get some plowable snow this weekend.”

Duell noted that the busy weather pattern looks to continue into next week.

“And then there are more systems on the way potentially for next week,” she said. “We stay in a pretty active weather pattern overall.”