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Fourteen years ago, just like this Sunday and Monday, a strong low-pressure system moved up the East Coast. But very unlike this same time in December 2009, Virginians aren’t left digging out from piles of snow.
This weekend’s coastal storm did pull some cold air behind it, just enough for a temporary regional detour from an almost nationwide holiday mild period, and causing just enough snow along and west of the Blue Ridge to make me eat some words in a headline for my weather column last week. (More on that in the second section of today’s column.)
But there wasn’t enough cold air ahead of the storm, or positioned where it could get pulled more rapidly and deeply into the storm, for widespread snow with the coastal low itself. Instead, there was a lot of rain, 1.5 inches or more over most of the central and eastern parts of the state, though much of the western side of the state ended up getting less rain than many prior forecasts.
So why couldn’t this coastal low of Dec. 17-18, 2023, have been snowy like the one of Dec. 18-19, 2009?
The first objection raised to this question comes from many people in the southeastern corner of our state, which largely did have a rainstorm in the 2009 event. It started as snow and changed over to rain or a wintry mix in many locations from Southside and Central Virginia eastward toward Hampton Roads, and of course many areas southward. So, the 2009 storm was also a big rainstorm, for some.
But there was much cold air awaiting the 2009 storm, and therefore most of Virginia experienced one of its largest snowstorms in many years 14 years ago this week, with many amounts topping a foot and some over 2 feet in the western and northern halves of the state.
The 2009-10 winter, like this one, was an El Niño winter, with a warm stripe of sea surface temperatures relative to normal across the equatorial Pacific Ocean. But the current El Niño has already exceeded the moderate 2009-10 in intensity. That alone may hold at least a small clue as to why there is less cold air present this December for a storm to interact with than was present in 2009 — stronger El Niños are often correlated with warmer Decembers than weaker ones.
A reflexive answer might be that the climate is warmer now and that is the main reason this weekend’s coastal storm was rain and not snow. The planet is, after all, headed to its warmest year on record. There are some big-picture reasons how a warmer climate is steering patterns differently, but it is important to note that the two storms we are comparing are only 14 years apart, and as recently as five years ago, conditions in our region supported a major winter storm even earlier in December. So, even in recent years with warmer global average temperatures, it can still be cold enough in December, when conditions are supportive, to allow major winter storms in our region.
So why wasn’t it that cold this time?
A big part of the answer lies in what is happening in the northern latitudes.
The North Atlantic and Arctic oscillations are presently in strong positive phases — meaning low-pressure is dominant near the North Pole and Greenland, rather than high pressure. So, the coldest air is held closer to the North Pole rather than spreading southward, as it would with more high pressure pushing it away from the pole and buckling the jet stream southward.
The 500-millibar maps are particularly telling. This is the dynamic structure of the atmosphere roughly 18,000 feet up, or 3 ½ miles, give or take some relative to the thickness of the atmosphere (troposphere is more shallow in a colder regime, a subject for another day).
A quick look at the two maps reveals considerable similarities — the low over the Pacific, and an angling of the isoheight bars (the solid lines marking the height of the 500 millibar level, generally a marker for atmospheric air pressure) from northwest to southeast across the central U.S. toward a dip in the Southeast, which is where the low-pressure systems developed in both cases.
But one critical difference is that row of three low-pressure centers in southeast Canada in 2009, with only one low very far to the north in 2023. This is indicative of a strong polar vortex, with a tight stratospheric low-pressure circulation near the North Pole holding the coldest air in close in 2023, and a looser polar vortex, allowing it to slip to the south substantially in 2009. The terminology may be a bit confusing, because a surge of extremely cold air southward has commonly been called a “polar vortex” in recent years’ media reports, when it is in fact the result of a weakening of the polar vortex that allows this to happen.
Barely off the 2009 map to the north there is high pressure over Greenland, which, like a boulder in the stream, crinkles the jet stream to a lower latitude, forcing cold air southward and keeping that train of Canadian lows trapped beneath it.
You may also notice the gently curved nature of the isobars over the eastern U.S. in the 2009 map compared to more of a sharper almost “V” shaped look in 2023. This reflects a broader trough that allowed much colder air to flow southward over a wider area, and also, allowed a low-pressure system to develop farther westward before making its climb up the East Coast. This latter factor plays a role in why Sunday’s rain diminished in intensity so quickly west of the Blue Ridge, whereas the snow of 2009 spread more westward even into eastern Kentucky and much of West Virginia.
Combine the stronger polar vortex keeping the cold air wrapped up near the pole (though there has also been a significant lobe of deep cold on the other side of the pole, over China and Russia) with a fast-moving jet stream over the northern Pacific, spraying milder air across North America, and the tendency in a strong El Niño for warmer air to spread over much of our continent in December, and there not only wasn’t any deep cold air awaiting the coastal low, but none located where it was easy to be tapped and pulled into the precipitation shield of the coastal low.
So that is why we got rain, and not snow, from this particular coastal storm.
There may be several different reactions to having had an all-day rain from a coastal low in mid-December rather than a widespread snowfall.
· Man, we missed the big one. Only time will tell if this is the only winter setup that features a strong coastal low, but yes, with even a reasonable amount of cold air available ahead of the storm or where it could be pulled into the storm, this would in fact have been a large snow dump for much of our region along and east of the Blue Ridge, and probably farther west than that with the somewhat different dynamics of a colder setup.
· Good. Those who don’t like snow, those who like snow fine but would rather not get 2 feet of it at once, and those who might like a big snowstorm but maybe not right now just before the holiday should be thrilled.
· A sign of things to come. Patterns tend to repeat themselves so it could well be that we see another similar setup — perhaps with colder air — later this winter. It does look likely we’ll see more wet storms that will sometimes interact with some colder air as we move toward January.
· Lots of help for the drought. Indeed — for areas east of the Blue Ridge.
‘Snow may be Virginia’s last for weeks’
There’s no point dodging the words that headed my column a week ago — “Probably no white Christmas – this week’s streaky snow may be Virginia’s last for weeks.”
The first part of that headline, about not having a white Christmas, is still very much in effect. And for most everybody east of the Blue Ridge, the second part of about having the last snow for weeks, as of last week, or perhaps even from right now, still looks intact.
But from about the Blue Ridge westward, there was some pretty intense — though still quite streaky — snowfall late Monday and early Tuesday that defied the “last for weeks” prediction of snow a week ago.
While last week’s column did offer the caveat that upslope snow showers in the mountains would be possible behind cold fronts during the coming mild period, what happened late Monday into early Tuesday exceeded the typical snow shower episode deriving primarily from the air lifting over the Appalachians. Many locations, even into parts of the Roanoke Valley that usually only get flurries in upslope snow shower events, got a coating of snowfall, and some spots in the higher elevations along the Blue Ridge and in patches westward got an inch or two.
This was definitely not expected as of the middle of last week, when the eventual evolution, intensity and track of Sunday’s coastal low was not generally known. But by the weekend, it became clearer that the coastal low would help pull down some colder air behind it, though only for a couple days, accompanied by some energetic lift aloft that would enhance snow squalls. Even at that, Monday night’s episode ended up a little more widespread and intense than expected, and caught many by surprise who may have not closely followed more recent forecasts.
Moving forward, it’s important to note that weather is like a game of chess in which moving the pieces can change the shape of the board. The coastal low was such a board-shifting piece.
We are still expecting a mild period of several days ahead, with warmer than normal temperatures almost nationwide and expanding into our region through Christmas and a few days beyond. There is a possibility something could disrupt that locally or regionally for a day or two, but the trend is toward mild 50s and maybe some 60s for highs approaching Christmas, when there may be some rain.
At some point next week or perhaps in early 2024, a colder pattern is expected to develop, but for now it looks to be near-normal to slightly below normal in temperature, with the deep Arctic air bottled up near the North Pole and on the other side of the globe in parts of Eurasia. Some of that may eventually make a move over the North Pole our way later in the winter.
Chances of wintry precipitation are likely to increase in the new year, but a full-on deep wintry pattern might take longer to develop, if it does at all, given the historically low snow cover over North America. Snow cover helps hold in colder air once it overspreads a region.
So that is where we are at in the 2023-24 winter, and it may yet take weeks for a truly widespread snowfall across Southwest and Southside Virginia to occur. But let’s watch the chessboard for disruptions to expectations.
Journalist Kevin Myatt has been writing about weather for 20 years. His weekly column, appearing on Wednesdays, is sponsored by Oakey’s, a family-run, locally owned funeral home with locations throughout the Roanoke Valley.
