Cahas Mountain in northern Franklin County was outlined by snowfall several days after the December 9, 2018, winter storm that dumped a foot over most of Southwest and Southside Virginia, the last such storm to do so. See the last section of today's Cardinal Weather column for a look back at that event. Photo by Kevin Myatt.
Cahas Mountain in northern Franklin County was outlined by snowfall several days after the December 9, 2018, winter storm that dumped a foot over most of Southwest and Southside Virginia, the last such storm to do so. See the last section of today's Cardinal Weather column for a look back at that event. Photo by Kevin Myatt.

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The 1974 Rankin/Bass animated feature “The Year Without a Santa Claus” had it right – warmth at the North Pole helps it snow in Southtown.

OK, we’re not really talking about the kind of warmth in Mother Nature-enforced dealmaking between Heat Miser and Snow Miser that helps Santa bask in the sun at his North Pole abode to overcome illness and fatigue while the snow-deprived kids in Southtown frolic in their winter wonderland dreams come true.

But when high pressure builds over the North Pole and Greenland, bringing warmer temperatures relative to normal to those regions (still quite cold by our standards), Arctic air typically over those regions is displaced southward.

That is the most common and direct way winter weather comes to Virginia, and it is in the process of developing now, raising the specter of potential cold air intrusions and wintry precipitation flirtations in the weeks ahead.

However – there is often a however where winter weather patterns are concerned — a sled brake is in place slowing down any dashing through the snow in Cardinal News territory, with uncertainty about how long and how strong it lingers. We’ll get back to that.

An example of a negative phase pattern of the Arctic Oscillation that occurred during a cold and snowy period for our region in December 2009. Contrary to some popular conception about the “polar vortex,” it is when the polar vortex is weak, not strong, that colder weather descends southward away from the pole. Courtesy of NOAA.
Examples of how temperature patterns generally line up with NAO-negative and NAO-positive patterns. An NAO-negative pattern features high pressure over or near Greenland that buckles the jet stream southward behind it, bringing colder weather into the eastern United States. Courtesy of NOAA.

High pressure near Greenland, often called the Greenland block, and over the North Pole, causing the polar vortex to loosen and stretch southward beyond a tight circuit around the pole, represent what in meteorological jargon are called the negative phases of the North Atlantic and Arctic oscillations — or NAO-negative and AO-negative — respectively.

And it is during these negative phases that most, though not quite all, of our region’s most memorable Arctic air outbreaks and large snowstorms have occurred.

Upper air wind flow pressed outward by high pressure over the pole generally allows Arctic air to spread farther southward than if pulled in tighter to the pole by strong low-pressure in the Arctic Oscillation positive phase.

Blocking high pressure over Greenland acts like a boulder in the jet stream, the fast-moving river of wind about six miles above us, causing it to flow around it to the south, often digging deeply in a U-shaped polar trough over the eastern United States.

Typically, we would expect prospects to grow for frequent or prolonged below-normal temperatures to occur in Virginia and much of the eastern U.S. as the high pressure blocking in the northern latitudes develops – and this may yet occur toward mid to late month.

Storm systems tracking in the southerly displaced jet stream around the edge of any cold air pressed southward could pose the risk of wintry precipitation – snow, sleet, freezing rain – in Southwest and Southside Virginia.

But snow is never a guarantee. Moisture sources can prove unreliable, storm systems get deflected north, south, east or west by day-to-day atmospheric minutia, the moisture can move in during brief warmups between cold fronts, and sometimes it is cold but just not quite cold enough when the moisture arrives and we get 34-degree rain.

In this case, a third leg of the table being set for a possible wintry pattern is wobbly, at least to start.

The Climate Prediction Center is forecasting heightened chances of colder than normal temperatures over much of the central and eastern U.S., edging into western Virginia, during the Dec. 17 to 30 period covering Christmas. There is much uncertainty about how this will yet transpire, and it is worth noting that snow and ice chances often develop near the rim of the cold air, not the center of it. Courtesy of Climate Prediction Center, NOAA.

Greenland and North Pole blocking high pressure systems work much more efficiently to deliver wintry wonder to our region when another high-pressure system forms over western North America.

This is called the positive phase of the Pacific-North America pattern, or PNA-positive. Clockwise rotation around high pressure in the West can drive cold air forcefully from northern Canada into the central and eastern U.S., where it digs deeply and becomes somewhat locked into place by the high-pressure systems over the North Pole and Greenland.

However, presently the PNA is in its negative phase, with a low-pressure trough over the West. This is delivering more of a west to southwest wind flow that is, at least for now and some days ahead, resisting and delaying any intrusion of Arctic air. Instead, we tend to get mild and showery weather with occasional brief cooldowns.

There are very mixed signals looking toward the middle part of the month, but it seems probable that this PNA-negative pattern will relent in time, or become partly overcome by the cold induced by the NAO-negative/AO-negative combination.

If this happens, or even if the PNA-negative pattern hangs on but flings one wet system eastward with the right timing and placement to meet an intrusion of cold air, we could be cueing Bing Crosby for the holiday season. But there is still the possibility the Pacific flow plays Grinch for snow lovers.

Stay tuned.

Rain Hupman of Monterey in Highland County captured this photo of the moon shining through mid-level altocumulus clouds — sometimes referred to as a mackerel or buttermilk sky — on Dec. 1. Mid-level clouds such as these often stream in a day or two before moisture-bearing storm systems. Courtesy of Rain Hupman.

STATS OF THE WEEK

2022 appears to be well on its way to being among the warmest years on record, based on the two major climate stations in the region with most complete historic weather data.

It is one of just 17 years, out of 107 with complete or nearly complete records (fewer than 10 missing days) since 1912, that has averaged 60 degrees or higher from January to November at Roanoke. The 11-month average of 60.5 degrees ties 2022 with 2021 and 1990 for ninth warmest January to November period. Six of the eight ranking higher have happened since 2007, with the 61.3-degree average of January-November 2020 leading the pack.

The first 11 months of the year ranked as the 10th warmest January to November period on record at Lynchburg, among 126 years with complete or nearly complete records dating back to 1893, averaging 60.4 degrees, one of 19 to average 60 or higher. 2020 had the warmest first 11 months at 61.2 degrees and 2021 tied with three other years for 12th warmest at 60.2 degrees. Unlike Roanoke, all of the years with 11-month averages above this year, excluding first-place 2020, were not recent.

We’ll revisit the annual average temperatures, where they rank historically and how they fit into recent trends and broader climate patterns once the year is complete.

Two faces of the same sky from John Holst of the New River community in Pulaski County on Nov. 30, pleasant in one direction and stormy looking in the other. Courtesy of John Holst.

MOUNTAIN LAKE

Earlier this week in Cardinal News, Randy Walker covered the continuing studies and saga of the muddy puddle on Salt Pond Mountain formerly known as Mountain Lake.  

Questions are sometimes asked about the role of weather patterns and long-term climate in Mountain Lake’s dwindling.

I covered the topic of rainfall patterns relative to Mountain Lake in an August 2020 Weather Journal column in The Roanoke Times. The gist is that Mountain Lake has continued to decline despite multiple years in the past decade that are among the wettest on record in our region, especially 2018 and 2020.

The draining and refilling patterns of the lake from the late 1990s to about 2008 seemed to loosely follow wet and dry cycles, but the lake’s level has become decoupled from rainfall cycles since then.

So, as Walker’s article notes, geologic issues are front and center in the Mountain Lake mystery, not drought, which on a scale large enough to dry up lakes has been non-existent in our region for two decades. You can read his article for the details on Mountain Lake.

This map shows snowfall totals across the region after the early December snowstorm in 2018. Courtesy of National Weather Service.

LOOKING BACK

The main subject of today’s column is about looking ahead to weather in the next few weeks, so with the segment that’s usually titled “Looking Ahead,” we’ll look back instead.

Friday will mark four years since the last widespread foot-plus snowstorm across Southwest and Southside Virginia, indeed the most widespread coverage of foot-plus snowfall in what is now Cardinal News territory since 1996 and the earliest large snowfall relative to the calendar on record for many locations.

This snowstorm was caused by a low-pressure system tracking through the South throwing abundant Gulf of Mexico moisture over and into a dome of Arctic air tightly pressed against the Appalachians by high pressure to the northeast, an “overrunning” snow event.

Some snow totals around the region for Dec 9-10, 2018:  Galax, 21 inches; Wytheville, 17.4 inches; Pulaski, 15.8 inches; Danville, 15.2 inches; Roanoke, 15.2 inches; Bluefield, W.Va., 15 inches;  Lexington, 15 inches; Burke’s Garden, 15 inches; Covington, 15 inches; Martinsville, 15 inches; South Boston, 15 inches; New Castle, 14 inches; Stuart, 14 inches; Blacksburg, 13.7 inches; Copper Hill, 13.5 inches; Appomattox, 12.3 inches; Brookneal, 12.1 inches; Abingdon, 12 inches; Richlands, 12 inches; Lynchburg, 11.7 inches; Hot Springs, 11.1 inches; Farmville, 10 inches; Tri-Cities Airport, Tenn., 10 inches; Wise, 8.5 inches.

December snow has tended to be feast and famine in recent years – perhaps a topic to review in the next Cardinal Weather column, if we aren’t pressed by more urgent winter storm potential.

Journalist Kevin Myatt has been writing about weather for 19 years. His weekly column is sponsored by Oakey’s, a family-run, locally-owned funeral home with locations throughout the Roanoke Valley.

Kevin Myatt

Kevin Myatt wrote the Weather Journal in The Roanoke Times for 19 years. He has led students on storm chases and written for “Capital Weather Gang.” Twitter: @KevinMyattWx. Email: weather@cardinalnews.org.