February 20: Snow On The Way

Weather

A little change of format today, with this winter storm moving into central North Carolina. I’m going to break things down by looking at the most-likely scenario, then a look at the “boom” and “bust” scenarios. There’s always a high degree of uncertainty with winter storms in this part of the country, and this one is no exception.

MOST-LIKELY SCENARIO

As I’m typing this (9:00am-ish), we’re already seeing rain showers moving into our southern counties, mixed with some sleet in spots. Those showers will spread northward, mostly as cold rain showers, into early afternoon. At that point, we’ll start to see a rain/snow mix northwest of the Triangle — that “transition zone” will steadily work its way southward throughout the afternoon. We should see the changeover to snow in the Triangle by late afternoon…earlier north, later to the south.

Even once the changeover to snow occurs, it won’t stick right away — roads will be wet, not icy or snow-covered. The ground will be wet, air temperatures will still be above freezing (barely), and the ground is relatively warm. The greatest accumulations will be on grassy and elevated surfaces — lawns, rooftops, back decks, railings…you get the idea. I anticipate 1″-2″ around the Triangle by the time the snow winds down just after midnight. Areas along the Coastal Plain will see the highest snow totals, in the 2″-4″ range. The rain/snow changeover will happen latest in our southern counties, and that’s where less than 1″ is the most-likely outcome.

Full disclosure: the National Weather Service’s forecast snow totals are similar, but a little higher than what I’m forecasting.

Every county in our viewing area is under an advisory or warning from the National Weather Service. It’s a Winter Weather Advisory for most of us, including the Triangle and Fayetteville. That means up to 3″ of snow is possible, but most locations within the blue-shaded counties will see less than that. A Winter Storm Warning is in effect east of the Triangle, where the highest snow totals will occur and more-widespread travel trouble is likely.

“BOOM” SCENARIO

What I’m wrong? What if the rain changes more quickly over to snow? What if the snow falls heavily enough that instead of melting on the warm ground, we get snowflakes falling onto snowflakes, resulting in faster and higher accumulations? I think we’ve accounted for all of those factors, but winter storms are tricky! If this storm system over-achieves and produces those “boom” numbers on the map above, the radar will look like what the North American Model is simulating from 9:00am today through 9:00am Friday.

That kind of intense snowfall, continuing longer into the pre-sunrise hours of Friday morning, would result in a sizable swath of 6″+ snow reports, shaded in purple on the next map. (Keep in mind, this raw model data doesn’t take melting into account at all, which is why my “boom” numbers aren’t this high.)

That would be a significant headache on the roads, one that would continue through much of Friday, even once the sunshine emerges and temperatures warm up a bit. The National Weather Service’s “high-end forecast” (one they estimate has a 10% chance of verifying) is a snow-lover’s dream and a snow-hater’s nightmare.

“BUST” SCENARIO

What if I’m wrong? What if temperatures warm farther into the 40s before the rain arrives, and the transition to snow is delayed? What if the mix of rain and snow lasts longer before completely changing over? What if the bulk of the moisture misses us to the south, and there just isn’t much available to fall as snow in the first place? Again, I think we’ve accounted for all of those factors. One of the other short-range forecast models, the HRRR, shows a whole lot of rain on its simulation before the transition to snow occurs…and the snow doesn’t last as long. Here’s the HRRR radar loop from 9:00am today through 9:00am tomorrow.

Some snow would still accumulate, but far less than what we’re anticipating. And even these lower amounts don’t account for the melting that will occur with warm ground temperatures.

The National Weather Service’s “low-end forecast” (with the same 10% chance as the high-end forecast) turns the tables for the snow-lovers and snow-haters.

OKAY, SO WHICH MODEL IS RIGHT?

Well, probably neither…and both! They’ll have some of the details right, some of the details wrong. That’s why we look at so many individuals models, so many ensembles of model data, and why we spend so much time obsessing about where we draw those regions on the snow forecast map. Inevitably, something unexpected will happen and we’ll make more adjustments to the forecast. We’ll be on-air with our normal newscasts, and I’ll have updates on social media (particularly on Twitter) throughout the day.

THE REST OF THE FORECAST

It will be cold Friday morning, so slick road conditions will still be a concern. The clouds will clear out early in the day and temperatures will warm up to the upper 30s — not even close to warm, but warm enough for a lot of melting to occur. The melting continues Saturday as we warm up from a frigid start to the weekend.

Back to a chance of plain old rain for the first half of next week, then temperatures will drop off again by Thursday and Friday. No storms expected Monday or Tuesday, just more rain in our already-wet February. The first 19 days of the month rank as the 14th-wettest on record in the Triangle since the late 1880s.

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