Examining why Sandy can aspire to make history; gusty winds likely for SW Virginia, snow still possible
The way this whole Weather Journal gig started was with me briefing Roanoke Times editors on weather stories back in the early 2000s. And I still do that today. Here is a photo of me (courtesy of metro editor Brian Kelley) talking to our 4 p.m. news meeting about the situation with the developing powerful East Coast storm, a large satellite image projected on the screen behind me (courtesy of wire editor John Gibbons). Underneath my shadow is the line of clouds marking the Arctic cold front advancing eastward, and of course you can easily see Hurricane Sandy spinning on the screen in front of me. (I didn’t try to blow it out to sea, Phil Connors-style, for those who remember “Groundhog Day.”) Those two features are coming together near the East Coast to create this potentially historic storm. This was my little TV meteorology moment today. I think I’ll stick to blogging and writing columns.
The inset , taken off the initialization of the 12Z European model (in other words, the way the model depicts conditions at 8 a.m. today, based on its data input), and doctored by me scribbling, shows why we’re in this situation. These are atmospheric height anomalies that illustrate vividly where the high and low pressure systems are. The “S” is over Hurricane Sandy (likely soon to be downgraded to a tropical storm — I’ll get to that in a minute). The X’s mark a strong area of high pressure over Greenland (the “Greenland block,” as we like to call it, the key feature of the negative phase of the North Atlantic Oscillation) and also over a big low in the central Atlantic. The curved line with the arrow marks the approximate jet stream flow around the polar trough, or deep displacement of cold air into the central and eventually eastern U.S. Normally, if that big low wasn’t there in the central Atlantic, the trough would just bounce Sandy out to sea. Normally, if that big high wasn’t there over Greenland, Sandy would just scoot on northeastward parallel to the U.S. coast. But they’re both there, and they’re stuck, partly because of each other. So Sandy instead is being drawn northward by the trough over the U.S., and will be caught by the trough as it advances eastward, and at some point will get tugged northwestward into the U.S. coast. Also, the winds in the jet stream trough will blow on top of Sandy’s circulation, allowing air on top of the cyclone to evacuate rapidly. This will rapidly lower the air pressure underneath — you can think of it as less air stacked on top of one’s shoulders — and Sandy will spin faster. The circulation will broaden — already is much broader than it was — as the boundary between cold and warm becomes more of Sandy’s rotation source rather than the latent energy in the evaporation of warm ocean water. This is the process of “extratropical conversion” we’ve been talking about, as a tropical cyclone dependent on warm water for its fuel becomes an extratropical cyclone deriving its energy from larger scale atmospheric boundaries. Most tropical cyclones don’t get this chance, and peter out over cold water or land. But some tropical cyclones get a “second career” as strong low-pressure systems. The “Nor’Ida” storm — Hurricane Ida’s transition into a nor’easter — in November 2009 is a recent example that affected us, with 2-4 inches of rain for Roanoke and east, and near-hurricane winds along the coast. The Galveston Hurricane of 1900, Hurricane Hazel in 1954 and Hurricane Ike in 2008 are among others that lived long and prospered (though many people in their paths didn’t) after coverting from tropical to extratropical. And, perhaps most comparable to this situation, Hurricane
Ginger Grace became absorbed in a polar trough over the North Atlantic in 1991, and became the “Perfect Storm.”
Sandy, as this satellite photo from early Friday evening shows, is looking much less hurricane-like in structure and much more comma-shaped. The National Hurricane Center expects Sandy to be downgraded to a tropical storm
later this evening on Saturday. Eventually, Sandy is expected to restrengthen into a hurricane, partly because it will move over some warm water, but also partly because of the atmospheric processes that will begin its slow conversion into an extratropical cyclone. It will be something of a hybrid — may already be a little bit, judging by its structure on the satellite — but will carry the name “hurricane” if its winds are over 74 mph and it continues to have a “warm core” feeding off the warm ocean energy. It’s meteorological semantics exactly when the flip occurs, and folks on the coast getting pounded by waves and wind won’t really care much what label is on it. The current forecast track still varies, depending on the source, from landfalling on the Delmarva Peninsula all the way to Long Island either late Monday or on Tuesday. Short answer for us: the more south it is, the greater the effects we experience. We get gusty winds (40 mph+) for 24-48 hours just about anywhere it goes, a little stronger and more gusty the farther south it is. We may get some rain circulating around it as passes east and north, but the heavier amounts are expected to the north and east, though the most recent Hydrometeorological Prediction Center map does show some 1-inch plus amounts in some parts of our region, dropping off fast to the south through middle of our area.
And then there’s this whole snow issue. The HPC placed our area from about Roanoke and the Blue Ridge westward under a slight risk of 4-plus inches of snow for the 24 hours ending Monday evening. Greater chances start at the West Virginia border and west of Interstate 77 in Virginia. There will probably be at least a somewhat better chance of measurable snow in much of our area AFTER this map expires, overnight Monday into early Tuesday. But a lot of forecast guidance is showing a somewhat more northern track of the storm, into New Jersey, rather than the D.C.-area track that would be optimal for snow to spread into more of our area. And many models are both bringing less wraparound moisture and shallower cold air into Southwest Virginia. Snow appears likely to be elevation dependent and upslope-enhanced, which favors West Virginia’s mountains and the higher elevations (3,000-feet and higher) in Virginia near the West Virginia line and west of I-77. Winter storm watches are already out for a few counties in the high terrain of eastern West Virginia, near the red-circled “high risk” area at left. This is a situation that will continue to need monitoring, due to the potential high impact even a few inches of heavy, wet snow would have on still-leafy trees and a power grid already being shaken some by wind. At this time, a widespread paralyzing dump of wet snow appears unlikely in most of Southwest Virginia, but there could be pockets of it the farther west and higher up from Roanoke you go, and still some chance of alterations in the track or development of the storm changing that. Just one heavy precipitation band moving through in colder air overnight Monday or early Tuesday could ratchet up the snow chances anywhere in our area.
So, Southwest Virginians: Brace for some pretty strong wind peaking on Tuesday, be prepared for power outages, expect some rain, and we’ll continue to keep an eye on the snow potential. Expect high waves, stronger wind and heavy rain near the coastlines to our east. Travel in any direction, by car and especially in air, will be challenging at some point Monday and/or Tuesday.