Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 2:18 PM GMT on October 23, 2012
And so it begins. Yet again we are kicking off the end of October tracking another potential Nor'easter. Similar to last October, the threat of high winds, heavy rain, and heavy snow are a possibility. The synoptic setup is very different though in this instance and many different outcomes are possible.
Presently tropical storm Sandy is located in the central Caribbean tracking 0 degrees north.
It is encountering very little vertical shear (sub 10-15 knots) and slightly above normal SST's (80F+). Sandy will begin to increase in speed as it tracks north towards Cuba. A bit of drier air and increased shear along with the high mountain interactions with the tropical core, will weaken Sandy and prevent further strengthening. Intensity forecasts through Friday are only expected to peak at a minimal hurricane. As Sandy tracks north of the Bahamas encountering a more hostile environment, she will begin to take on extratropical characteristics. This time frame is around Saturday/Sunday. The track and intensity after this period is a bit more in question and will be detailed further below:
A deep negatively tilted trough will be swinging south through the central part of the nation by the end of the week. This feature is key in predicting the eventual path and impacts of Sandy. Located to the north is a very impressive blocking setup with an amplified ridge over Greenland and a steep west based negative NAO. The North Atlantic Oscillaion will be approaching the lowest value in several months during this time frame.
The blocking setup will allow for a very amplified jet stream pattern that is also slow moving. Three major scenarios are possible for the impacts of Sandy on the east coast.
1) Currently the 10/23/2012 runs of the GGEM and ECMWF are the most amplified and feature a tightly wrapped extratropical Sandy tracking up the eastern seaboard with a slightly northwest motion. They are also supported by this morning's 0z runs which have impressive consistency in allowing the negatively tilted upper level trough to capture and phase Sandy.
A broad precipitation deck forms while the surface low undergoes rapid bombogenesis courtesy of impressive baroclinic forcing. In some recent foreign model runs, the surface low has dropped sub 950hPa. A tight pressure gradient and raging low level jet sends strong winds to the surface in excess of 50mph up and down the Northeast. Meanwhile the cold air behind the front catches up to Sandy in a pocket of sub 0C H85 thermals located west of the center of circulation. Surface temperatures also fall due to dynamic cooling with heavy snow breaking out across western Maryland through all of central Pennsylvania. In fact the 0z 10/23/2012 ECMWF shows snow accumulations in excess of 12-24in from Reading west to the Laurel Highlands south to Hagerstown, MD and north to Elmira, NY.
Knowing a bit about 500mb maps shows that this solution requires many features to time perfectly to be able to capture Sandy and track with a slight northwest component. It is a dynamic setup and the anomalous blocking to the north makes this particularily more interesting than normally. This is also impressive and consistent support from both operation and individual ensemble guidance especially from the GGEM and ECWMF. A few ECWMF ensembles even show the surface low tracking right through southern New England with a landfall coming from the northeast near Cape Cod, Massachusetts. This setup would cause major problems up and down the eastern seaboard with heavy rain, high winds, coastal storm surge, and possibly heavy snow. Looking back at other analogs, Hurricane Wilma is a quick reminder how a system with tropical origins can quickly resemble a nor'easter with winter-like characteristics.
Key players to watch for the scenario require analysis of the 300mb and 500mb trough axis and tilt including time of these features for the upper level trough. Also the evolution will be highly dependent on the upstream position of the upper level heights near Greenland (blocking).
2) Another scenario is supported by the OP GFS and GEFS. In this evolution the overall jet stream is flatter and the trough lags behind a bit allowing Sandy to not phase and instead swing out to sea.
Therefore any precipitation in the Northeast is a direct result of the cold front and a small inverted trough feature in New England. A recent update in the GFS enembles this morning (6z run) show a clear west shift in about half of the models. This is a critical note to make especially when looking at future run times. Presently this evolution has support from the HPC and other NWS offices, although given recent developments for the ensembles and forecasts for blocking, it is starting to lose gravity.
3) For the time being, the following option has the most support looking at the overall orientation of the jet stream and synoptic setup. While this is a middle ground approach, it offers the most logical evolution given past analogs and our recent weather pattern over the last few months. Model support is lacking as most guidance is in the camps of the two extremes listed above. Nevertheless this possiblity allows Sandy to begin to rapidly take on extratropical characteristics and begin to wash out over the Atlantic as power vertical shear takes hold. A negatively titled -2SD trough will move east from the Ohio Valley and the interaction between the two features allows Sandy to track out to sea. But an inverted trough feature forms along the eastern periphery of the trough particularily towards New England. In almost a similar PRE (predecessor rain event) manner, an axis of +2SD PWATs would pool across New England as heavy rain streams through the region. QPF probably would be in excess of 1-2in given the inverted trough has tropical origins from post-Sandy. A screaming low level jet would also allow for gale force winds along the immediate coastline. Farther west and south impacts would rapidly diminish and mainly allow for a small corridor of rain showers along the immediate front. Both the GGEM and GFS show slight hints of this inverted trough feature.
Conclusion) It is a complex setup given both tropical and non-tropical features that need to interact to produce a solution with a surface low undergoing rapid cyclogenesis and pinwheels northwest sub 940hPa. Therefore given that Sandy just developed and is still located in the central to southern Caribbean, forecasts at this point can be taken with a grain of salt. It is important to note at this point the overall orientation of the jet stream and past analogs given the setup. Also the continuous support for dense upstream blocking is beginning to make odds of some sort of impact on the Northeast particularily higher than just a day or two ago. A few forecasts are already rotating about the cyber world concerning a grim storm tracking towards the east coast. These have little to no support and should be thrown out. The time frame for impact is early next week in the 144-174 hour time frame according to 0z 10/23/2012 model runs. Keep an eye the next few days on ensemble shifts and trends particularily in the timing of any possible phase of Sandy with the upper level trough.
As a final note as of 10/23/2012 at 10am, latest upper level air reports out of Sandy indicate the presssure dropping to 996mb with winds aloft up to 60 knots. Impressive strengthening and it is all courtesy of the warm bath water in the Caribbean.
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"Lower Susquehanna Valley Doppler"
(Courtesy of WGAL)
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