Tonight lots of viewers have asked us to explain why Hurricane Sandy has become a Post Tropical System and does this mean Sandy is dissipating? Regardless of the title, this is a very large storm and it continues to produce hurricane force wind gusts. Pressure differences in our atmosphere drive wind, as air flows from regions of higher pressure to regions of lower pressure. So a lower pressure and compact pressure gradient results in stronger winds; resulting from air rushing towards the low pressure center.
I want to first discuss tropical systems in general, these are what meteorologists refer to as “warm core” low pressure systems. They gather their energy from warm ocean waters and release that energy near the storm center. These tropical lows typically weaken quickly as they encounter land. We’ve witnessed this countless times as named tropical systems impact Alabama and the southeast.
What makes tonight’s setup so unique is that the parent low pressure center associated with Sandy is transitioning into a low pressure type often associated with frontal systems. The low will continue to gather energy from large temperature discontinuities across the eastern United States. While lots of focus has been on the hurricane force wind gusts impacting the populated tri-state region, 50 + mph winds will continue well away from the storm center. The center of this new low will become larger and the wind field will become much broader. I observed one wind gusts of 55 mph on the opposite side of New York in Rochester.
Check out the pressure chart that I included from NOAA. This shows the center of this transitioning low over the Philadelphia area. All of the lines on this analysis chart represent lines of equal pressure and you can see the rapid drop off in pressure from the midwest to the northeast. This tight gradient is expected to lessen as the system slowly weakens. Keep in mind we’ve had gusty winds in Alabama because of a much lesser gradient over the past few days. Given this much larger gradient, as you can probably imagine, high winds will continue to plague this region for the next 24-48 hours.
It is interesting to note that a small tornado threat did exist for the upper portion of Maine earlier tonight, with Blizzard conditions over West Virginia. I think its safe to say Sandy has lived up to the name ”Superstorm of 2012.”
Now that the system has become post-tropical the updates will no longer be issued by the National Hurricane Center (NHC). The new products are the responsibility of the Hydrometeorological Prediction Center (HPC). You can find the latest updates here: http://www.hpc.ncep.noaa.gov/
Fox6 Meteorologist Wes Wyatt
wwyatt@wbrc.com
Twitter @weswyattweather
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