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What happened with Sunday's snowstorm? Shifting details left room for big snow.

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So, what happened?

Sunday’s snowstorm certainty didn’t spring up out of thin air — the potential for flakes had fueled discussion for days.

By the end of the last workweek, it seemed that the worst of it would land in the mountains and foothills far southwest of Richmond.

And it did. The epic snow predicted for the southern Appalachians and the Piedmont of North Carolina was a successful forecast.

If only we could say the same here.

Instead of a manageable 1-inch or 3-inch coating, Richmond dealt with its second-biggest December snowstorm and the 12th-biggest one overall in 120 years of records.

The Richmond International Airport snow total for Sunday ended up at 11.5 inches, according to the National Weather Service.

The track of the low-pressure center itself wasn’t dramatically off, the way you might picture a hurricane veering off course. The forecasts had a reasonably good handle on the “big picture” surface weather map from Tuesday onward.

At issue all along was the northward extent of the system’s moisture from the Carolina coastline toward Virginia.

A lot of that is dictated by the shape and strength of the upper-level trough swooping over the Southeast behind the surface low.

It appeared late last week that the trough would emerge from the Southern Plains in a weaker configuration. The system had already drastically underperformed winter weather forecasts in Oklahoma, for instance.

There’d be a coastal low, regardless, but similar setups in the past had blasted our neighbors in North Carolina with huge snow totals while leaving us on the fringes, or high and dry.

Then, there were the other question marks: Would storms in the Gulf of Mexico and Atlantic hamper the flow of moisture up the East Coast, or would we be dominated by the bone-dry high pressure over the Northeast?

And even if we were to have abundant moisture here, there was the issue of how much of that potential snow would turn to sleet or rain instead.

A lot could have played out differently.

The resulting 11.5 inches of snow and sleet had a liquid equivalent of 1.04 inches — a perfectly typical snow-to-liquid ratio for this area.

That amount of moisture is already significant when you’re talking about rain. As we saw, it’s a ton when you’re talking about turning that into snow.

The computer models didn’t really lock in the right amount of moisture for us until Saturday morning.

There had been some outlier models indicating a moisture amount that high — or even higher — all during the workweek preceding that storm.

The 1-inch-plus scenarios didn’t appear with enough consistency to where you’d want to bank on a blockbuster snowstorm so far out, or assume that it would all come as snow.

Then, as the trough looked weaker, computer model consensus sank to about 0.3 inch of moisture between Thursday and Friday and portended something like 3 inches of snow.

Extrapolating that trend led to the concern that we’d end up with little to nothing by Sunday.

Forecasts usually get more accurate the closer you get to an event, after all.

But by Sunday, every feature on the weather map and every layer of the atmosphere overhead was in the perfect configuration for snow.

Or for the humbled forecaster, the makings of an imperfect storm.

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Check Richmond.com/weather for John Boyer’s videos and forecast updates. Contact him at JBoyer@timesdispatch.com.

Meteorologist

John Boyer is the first staff meteorologist for the Richmond Times-Dispatch. He joined the RTD newsroom in November 2016. Boyer earned his degree in meteorology from North Carolina State University in Raleigh.

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John Boyer

John Boyer, the RTD's staff meteorologist

John Boyer is the first staff meteorologist for the Richmond Times-Dispatch. He joined the RTD newsroom in November 2016 after covering severe weather on television in Tulsa, Okla.

As a native of the Roanoke area, the region’s heavy snowstorms started his fascination with Virginia’s changing weather.

Boyer earned his degree in meteorology from North Carolina State University in Raleigh. He is a member of the American Meteorological Society and earned their Certified Broadcast Meteorologist seal in 2012.

Look for his stories in the RTD and on Richmond.com, along with videos and forecast updates for major weather events in our area.

Email him your story ideas and weather tips.

Tuesday Weatherline

Central Va. may see first snowflakes today

Morning rain could briefly switch to afternoon snowflakes in central Virginia as colder air sweeps in today. If its rate is heavy enough, grass and elevated surfaces could see a light coating. Temperatures will fall from 50s at daybreak to 30s by sundown.

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