Historic Snowstorm Takes Aim on Mid-Atlantic

It’s already been dubbed Jonas, Snowzilla, Make Winter Great Again, and no doubt some unprintable names, but the winter storm now spreading into the central U.S. East Coast is a formidable entity no matter what you call it. Our most reliable long-range computer models, the GFS and European (ECMWF), have been remarkably consistent all week in targeting the mid-Atlantic for the heaviest amounts. This storm will certainly rival some of the biggest mid-Atlantic storms in recent decades, and some model runs have cranked out snow totals beyond historical precedent. There is every indication that the GFS/ECMWF consensus is on the right track as this sprawling, dynamic system continues to take shape. Blizzard warnings are in effect for the heavily populated urban corridor from Washington, D.C., to New York, as well as from Long Island to Block Island, RI, and Martha’s Vineyard, MA.


Figure 1. Infrared image of Winter Storm Jonas as of 2015Z (3:15 pm EST) Friday, Jan. 22, 2016. Jonas is gradually taking on the comma-shaped configuration of classic East Coast winter storms. Image credit: NOAA/NESDIS.

Washington/Baltimore in the crosshairs
Snow began in the Washington area early Friday afternoon, just a bit sooner than expected, and was intensifying rapidly as rush hour set in. The snow will intensify tonight into Saturday and may continue into early Sunday, leading to some truly phenomenal totals throughout the D.C./Baltimore area. In its experimental probabilistic guidance, the local National Weather Service office lays the best odds on snow amounts in the immediate D.C. area in the vicinity of 24-30”, with the low- and high-end projections published earlier today ranging from 12” to 32”. If this storm produces as expected, it will rank as the city’s heaviest snow since the record 28.0” on Jan. 27-29, 1922 (see Figure 2 below), perhaps even topping that storm. Baltimore could top its all-time record of 26.8” set in the President’s Day 2003 storm (Feb. 16-18), and Dulles has a good shot at #2 behind the 32.4” observed during the Snowmageddon storm of February 5-6, 2010.

If this storm ends up toward the highest-end projections, we may have to go back to colonial days for a historical precedent. In his book “Early American Winters,” eminent weather historian David Ludlum chronicled the Washington-Jefferson Snowstorm of Jan. 27-29, 1772. The District of Columbia hadn’t yet been created, but future president George Washington measured snow at Mt. Vernon, VA, that was “full three feet deep everywhere.” About 100 miles to the southwest, another future president, Thomas Jefferson, called the accumulation “the deepest snow we have ever seen. in Albemarle it was about 3 [feet] deep.” Ludlum also cites the Maryland Gazette (Annapolis): “tis’ supposed the depth where not drifted is upwards of three feet, and it is with utmost difficulty people pass from one house to another.”

It’s worth keeping in mind that official snow measurements are carried out at regular intervals throughout a storm (as often as once each hour), with the snow measuring board cleared off every six hours. This means that at some locations, the total snowfall recorded during a storm will be more than the accumulation on the ground at any one time. Moreover, those snow-board clearings used to occur less frequently during a storm, which means that the total snowfall from some past storms may have been underestimated.


Figure 2. Top-ten snowfalls within a 72-hour period for Washington, DC; Baltimore, MD; and Dulles, VA. Image credit: NWS/Baltimore-Washington.

Further north, nagging uncertainty remains
Friday morning’s computer model guidance offered little help to forecasters in the New York area. Although the NAM model insists that accumulations well over a foot are possible from New York across southern New England, most models are calling for the storm’s heavy snow to have an extraordinarily sharp northern edge located across or very close to New York City. The experimental probabilistic guidance for NYC shows a range of possibilities from less than 1” to around 24”. This range actually isn’t much different from that in D.C., but because it starts at a much lower amount, the range of potential resulting impacts is far greater. Given that strong winds and at least some periods of heavy snow are a good bet, the New York City office opted to go with a blizzard warning. Note that a blizzard is defined not by snow amounts but by at least three hours of high wind (frequent gusts or sustained winds of at least 35 mph) and poor visibility (no more than 1/4 mile). These conditions could be met in or near New York City even if snowfall amounts are relatively low. Southern New Jersey and southeast Pennsylvania, including Philadelphia, are more likely to wind up on the south side of the sharp heavy-snow cutoff, putting them at risk for well more than a foot of snow. Update (5:59 pm EST]: NWS/New York City has raised its expected storm totals to 12-18" and sped up the storm's arrival.

Many other locations in the eastern U.S. are feeling the effects of Winter Storm Jonas, including:

Center, KY: 15” already on the ground late Friday
Nashville, TN: 5” already on the ground late Friday
Asheville, NC: 7-12” possible by late Saturday
Charlotte, NC: 0.1-0.2” of ice, topped on Saturday by 2-4” of snow and sleet (the storm is expected to clear in time for Sunday’s NFC playoff game)
Roanoke, VA: 11-21” possible by Saturday night

Top amounts recorded by late Friday afternoon included 15” at Center, KY, and 18” near the appropriately named Jonas Ridge, NC. Thanks to Nick Wiltgen and Dr. Greg Forbes (The Weather Channel) for these statistics.

What’s making this storm so powerful?
At Capital Weather Gang, Jeff Halverson has an excellent analysis of the factors leading into this remarkable storm, with the main player being a pronounced upper-level low over the Southeast that will trigger a surface low developing off the East Coast. The upper- and lower-level centers will align just east of the Delmarva in an occlusion, allowing the entire “stacked” system to slow down and prolonging the heavy snow. Also in the mix is a rich feed of moisture streaming up from the northern Caribbean, with pockets of as much as two inches of precipitable water (the amount that could be squeezed out above a given point). Temperatures are considerably warmer than average over the Northwest Atlantic, ranging from about 1°C above average near the Bahamas and northern Caribbean to 2-3°C above average off the Virginia coast northeast to the Canadian Maritimes. (Warmer-than-usual oceans are commonplace across the globe right now, as December was the warmest month in more than a century of global recordkeeping.)


Figure 3. Departures from average sea-surface temperature for this time of year, calculated for the week ending on Jan. 16, 2016. Image credit: National Hurricane Center.

Which of the factors driving Jonas is most unusual? Joe Romm (Climate Progress) and Andrew Freedman (Mashable) point out the strong, important links between rising global temperatures associated with human-produced climate change and the observed tendency for high-end precipitation events to become even heavier. Many of the record U.S. rainfall events over the past year have benefited from vast amounts of precipitable water, often at record amounts for the time of year. Winter storms rely on ample moisture as well as strong temperature contrasts and large-scale dynamics. In the case of Jonas, a crucial factor is the extreme intensity of the low-level jet stream pulling moisture from the Caribbean and slamming it around the north side of the surface low into the snow-generating machine over the mid-Atlantic. These powerful winds will replenish the atmospheric moisture being used up rapidly in the snow-making process.

I asked Richard Grumm, science and operations officer at the National Weather Service office in State College, PA, what he saw as the most noteworthy aspects of Winter Storm Jonas. He replied in terms of sigma levels, which express how often you’d expect a particular situation to recur based on previous weather patterns. A 2-sigma event (two standard deviations) occurs about 5% of the time at any given point, while a 6-sigma event is far more rare--in theory, a one-in-500-million likelihood, although the exact recurrence period for such extreme events is impossible to nail down. Grumm calculated sigma levels for the imminent storm using output from the GFS ensemble system (GEFS). Grumm noted that the precipitable water flowing into the storm is in the 1- to 2-sigma range: high but not extraordinary. However, the winds flowing from the Atlantic into the Delmarva peninsula about a mile above the surface could exceed 80 mph (see Figure 4). The east-to-west component of this wind qualifies as a 6-sigma event. Such anomalously strong winds, said Grumm, are “about as good as they get.” He added that 2012’s Hurricane Sandy was the last East Coast event to produce such a high-sigma east-to-west wind anomaly. The intensity of the upper-level front is also apparent in the very sharp south-to-north drop-off in heavy snow depicted by most models. The contrast between cold air over the Northeast and the unseasonably warm waters just offshore may help to enhance the front even further.


Figure 4. Winds at the 850-mb level (about a mile above sea level) projected by the 12Z Friday run of the GFS ensemble system (GEFS) for 7:00 am EST Saturday, Jan. 23, 2016. The lines at each point are directed into the wind; barbs along each line show the wind speed in knots (see this NWS guide). The colored areas denote estimated sigma levels (see legend at left) corresponding to the rarity of the east-west wind component. Higher sigma levels correspond to especially strong easterly or westerly wind components. Image credit: Richard Grumm, NWS.

Live-blogging the storm
As we move into the active phase of this blockbuster storm, it’ll be time to watch short-term mesoscale models more closely. These will help tell us where the most intense bands of snowfall will set up and how far that all-important northern edge of the storm will extend. Mesoscale snow bands can goose totals across southwest-to-northeast strips that may only be a few tens of miles across.

We’ll be documenting the progress of the storm through this weekend at our WU live blog. I’ll have another post by Saturday evening at the latest.

Bob Henson