This is an expansion on a general post on gravity waves that I posted on WeatherNewEngland.com earlier today. The original post is in italics. The new stuff for real weather buffs appears below it. Also, I first mentioned this on Twitter earlier today. Follow me on Twitter by clicking here.
As a major winter storm bears down on New England - the most significant snow of the season for most of Southern New England - there are contemplations and concerns this evening of a potential "gravity wave" traversing New England on Monday morning. What is a gravity wave? These waves can occasionally bring extremely heavy precipitation (snowfall rates of 3"-4"+ per hour), briefly damaging wind gusts that also create blizzard conditions, thundersnow, and some funky readings on the barometer with a fast and pronounced rise and then fall in pressure. As amazing and impactful as these events can be, they are also very tough to accurately predict...
In fact, not too many years ago, prediction of such phenomenon in the atmosphere was mere speculation. The favorable circumstances for gravity wave development include the strong development of a front - a clash of airmasses - developing into an otherwise relatively stable atmosphere. This allows for the wave to continue traveling, and often in the atmosphere, such waves can become quite amplified.
In the last several years, computer guidance in the world of meteorology has improved greatly with regard to the resolution - or grid upon which equations and calculations can be run. This has allowed some of the more sophisticated computer models to acquire the capability of forecasting gravity waves, though the complexity of the wave means these forecasts aren't always accurate. Many times, such waves can develop without any indication ahead of time, and in some instances, gravity waves can be forecasted by never come to fruition.
Monday morning, between 5 AM and 9 AM, we have a fairly strong signal of a potential gravity wave moving from southwest to northeast across Southern New England, then moving into Northeast New
England thereafter. If this wave develops, we should know it well before dawn, as it should start appearing weakly in Central New Jersey just prior to or around midnight. This gravity wave would feature a band of very heavy snow, gusty and perhaps locally damaging winds, and a very fast barometric pressure rise, then fall. This will certainly be a feature we'll be tracking on NECN Monday morning.
For those of you who are able to stomach it, feel free to take a look at the descriptions of gravity waves offered on Wikipedia. First, you may want to read about "gravity waves," a wave that appears in a similar fluid, like the lowest layer of the atmosphere (the troposphere). [8:05 PM Update: I just got a great link from username 'Ginx snewx' on the EasternUSWx.com Forum for a winter storm explanation of gravity waves - thank you!]
Obviously, in the height of storms, gravity waves are indicated by terrible conditions, and cannot be seen in the sky, but can certainly be seen on a trace of barometric pressure, which would show a sharp rise, then fall, then rise again in the atmospheric pressure. This undulation often brings a burst of strong and sometimes damaging wind, and a burst of very heavy precipitation. This would mean a bout with blizzard conditions for a brief time Monday morning from 5 AM to 9 AM from southwest to northeast. First, check out this video of a gravity wave on a quiet day, when they are clearly visible in the cloud structure at the surface - this video was recorded by the Tama, Iowa, KCCI-TV webcam on May 6, 2007. Gravity waves can also be detected on satellite imagery by higher and/or brighter cloud signatures in a banded structure:
Here, I can show you some of the high resolution modeling we use that is forecasting a potential gravity wave in New England for Monday morning. First, this is the NMM computer guidance product solution, which shows a surface low pressure center moving well south of Nantucket at 7 AM on Monday, indicated by an "L." Notice also, however, another L to the southwest of the primary storm center. It's interesting to note that there is no circulation of wind around this supposed storm center, and that's because...it's NOT a storm center. It IS a spot where the pressure is much lower than its surroundings, however, and the solid black lines, indicating barometric pressure, show a well-defined trough (disturbance) extending northwest across Northern New Jersey, with an opposing V-shaped structure just to its northeast, indicating a relatively HIGH barometric pressure. Notice, also, that sustained winds are up to 30-40 knots beneath this disturbance.
Next, the State University of New York at Stony Brook Mesoscale Model Version 5, shown earlier today in one of my posts regarding snowfall amounts. In this image, like the previous NMM product, the solid black lines are barometric pressure, and the same ridge-trough couplet is seen, with localized sustained surface wind to 40 knots just prior to the barometric pressure fall.
Finally, the WRF is another high-resolution computer model that appears here on a slightly bigger scale, but the same feature is evident:
So, in summary, there is sufficient evidence to have concern that a gravity wave will propagate across New England Monday morning between 5 AM and 9 AM, roughly, reaching its maximum intensity directly over Southern New England at about 6 to 8 AM from Hartford, CT, through Worcester and Boston, and into Southeast NH, weakening thereafter as it moves into Maine. Will it verify? Only time will tell, given the difficulty in accurately forecasting such features. We should watch Central and Northern New Jersey starting at 11 PM for signatures, and keep a watchful eye all the way into Southern New England thereafter. These waves sometimes appear gradually, getting stronger all the while, but sometimes can appear rather suddenly, as they penetrate the stable near-surface layer of air.
Matt
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