If I had to pick a favorite weather phenomena or occurrence to watch unfold, it would most definitely be the development, maturity, and strengthening of a cold conveyor belt in association with a nor'easter or deepening low pressure systems. Strengthening nor'easters can feature some of the most impressive and majestic storm structures on the planet. We have had the fortune of experiencing several over the past decade plus years on the east coast, including several that have directly affected the New York City metro area. That said, there are a few very important ingredients that go into the melting pot that eventually produces these complex and powerful systems.  We've included an image to the left, depicting the classic rapidly deepening low pressure area from the 'Superstorm' of 1993. This system already had a well developed CCB (cold conveyor belt) and the surface low was barely out of the Gulf of Mexico. You can view distinct features of the system just by glancing at the satellite image. We've highlighted the warm front surging northeast of the surface low, the cold front (blue line), the dry slot, and the CCB. The dry slot on this image happens to be very impressive, a result of the H7/H5 lows closing off and the dynamics rapidly shifting northeast.  To see the re-analysis around the time this satellite image was taken, click here. You can see a few distinct features including the rapidly deepening H5 trough, the intense PVA underway (and strengthening), and the favorably closed H7 and H85 lows. Notice the impressive thermal packing near the H85 low as well. This storm ended up becoming one of the deepest, more famous systems of all time. Click here for a later image showing the obscene deepening of all features. You can still see, when analyzing the 700mb RH field (the bottom left of the four images), the classic CCB deformation band and dryslot.

That said there are more toned down, yet still classic, examples of CCB development that we can draw upon as we head towards meteorological winter. February 11-12, 2006 featured a classic deformation band that developed directly over the New York City metropolitan area, and eventually resulted in Central Parks greatest single storm snowfall total on record. The environment surrounding the storm, pattern wise, was not very favorable. The Pacific Jet was in full force for most of the winter, and the NAO remained positive. Still, we were able to properly time a +PNA ridge and some semblance of a -NAO blocking feature or 50/50 area block to our north. This helped to slow down the pattern just enough to allow for a H5 longwave trough to form over the Central US and a shortwave to progress eastward through the Mid Atlantic/Northeast. The surface low rapidly deepened off the Mid-Atlantic coast and progressed northeast. Click here for a surface re-analysis depicting the situation. The result was a very strong cold conveyor belt/deformation band which formed over much of Northeast New Jersey, New York City, Southeast New York and Connecticut. A rather classic radar reflection of deformation band snows.

As we head towards winter, it's important to note and learn that there are several different types of cold conveyor belts and associated deformation bands, some associated with cyclone occlusion. The "Classic Norwegian Occlusion Model" details this in the well-known polar front theory as a development from a wave stage. The basic idea is that the cold front moves faster than the warm front, therefore the warm sector continuously becomes more narrow until finally the cold front overtakes the warm front completely, thereby lifting the warm air. This is the typical situation for the occlusion band which turns cyclonically around the core of the cyclone.  Image 1, Image 2, Image 3.

The more common type of deformation bands/conveyor belts in our region stem from the conveyor belt theory. All of these features were highlighted on the satellite image in the first part of our post. The warm conveyor belt is a rising relative stream from south, south-eastern directions turning to north, north-eastern directions; it transports warm and moist air. The dry intrusion/dry slot is a sinking relative stream from north-west to south-east, splitting into two branches: a further sinking one to the south-west and a rising one to the north-east. The cold conveyor belt is a rising relative stream from east, south-east which is initially below the warm conveyor belt, but then emerges from below and extends to northern directions. See the image to the left for a visual depiction of the cold conveyor belt.

Now that we know what to look for, we conveniently have a forecast to get to this evening which involves this very process. As a longwave trough progresses eastward through the MS/OH Valley's this Thursday and Friday, a piece of northern stream energy is forecast to phase into the system. A rapidly deepening surface low, owing to this H5 phase, is likely to form somewhere near or east of Cape Cod and then into the Gulf of Maine. The orientation of the features aloft will dictate if portions of New England will in fact experience an early season CCB. This evenings 21z ETA offers an interesting and dynamic solution. As it phases the shortwaves aloft, a vorticity maxima translates northeast from the Mid Atlantic coast to just northeast of long island. Once it strengthens, the translation to the surface is robust with a deepening surface low east of Boston. The H7 and H85 lows rapidly close off and the CCB responds over interior New England. Although somewhat obscene, this guidance ends up with QPF totals nearing 3 inches over portions of New England. A great example of how powerful these features can be. We've uploaded the entire 21z ETA run from 11/25 onto our server. We encourage you to loop through the images, it's really a great example of a strengthening nor'easter and draws upon everything we touched on in this post. Click here for the images.

The sensible weather across the New York Metro Area likely won't feature any dynamic/enhanced precipitation in the near future. The surface low doesn't get captured by the upper level features until it's well to our northeast. We will have to keep an eye on the pattern over the next several weeks as we head into winter. Hopefully this post will be a good reference to draw back upon when and if we finally do have a threat for one of these powerful systems. All of that said, we can still expect some rain on the front end of the system as well as some possible flakes/flurries on the back end over north and northwestern zones. The bulk of the rain should hold off until Thanksgiving Night and into Friday. Speaking of Thanksgiving, as we wrap up the post, all of us at New York MetroWeather would like to wish you and your family a happy and safe Thanskgiving this year. We are especially thankful for your views and comments, every one of you help to keep this blog running and you are our motivation for continuing to forecast. Have a safe and happy holiday.