Potentially historic and unprecedented hurricane potential for the NE US coast.
Over the next 5 days a set of extraordinary meteorological factors will come together to potentially produce a hurricane impact on the NE US coast that has never before been experienced since the founding of this nation and the keeping of written weather records.
In response to this potential threat, all US weather balloon sounding stations are launching upper air balloons every 6 hours to incorporate the latest data into the forecast models in support of NHC and HPC forecasting operations…this has never happened before!
The following discussion will attempt to explain this complex set of parameters for what appears to be an increasing likely extremely high impact but never before seen event!
Hurricanes striking the NE US are not overly uncommon…just last year Irene moved into the same area causing extensive damage and the area which has one of the longest periods of recorded weather history anywhere in the US has been hit several times before by hurricanes. To understand what is forecasted to take place this weekend and early next week, one must look at the fundamental differences between what is called a tropical cyclone…hurricane and what is called post tropical (extra-tropical). To simplify, a tropical cyclone is warm core with no surface temperature gradients (same air mass before and after passage) and they gain intensity from warm waters below. Post tropical (extra-tropical) as usually system higher in latitude than tropical cyclones that do have temperature gradients (cold and warm fronts) and gain intensity not so much from warm water, but the differences in those temperature gradients and jet stream dynamics aloft. Many times as tropical cyclones move out of the tropics and northward into the higher latitude they encounter increasingly colder air masses that wrap into the system and the transition begins from tropical (warm core) to post tropical or cold core (with fronts). An example of a type of extra-tropical system would be a nor’easter.
Upper Air Pattern for this Event:
The upper air pattern over North America and over the North Atlantic is becoming increasingly blocked…weather systems such as big high and low pressure centers are slowing down and will become nearly stationary. Massive high pressure is building over Greenland currently while a large longwave trough is developing over the central and eastern US. Due to the high latitude blocking that is going into place these large scale features will remain nearly stationary in place for the next 5-7 days.
The longwave trough currently developing over the eastern US will become increasingly negative titled (oriented NW to SE instead of SW to NE). Most of the time troughs nearing the east coast are positive tilted (SW to NE) and hurricanes to their SE (where Sandy currently is) are steered N and NE out to sea. In fact all hurricanes that have affected the NE US since 1851 have had tracks from the SW to the NE consistent with a SW to SE steering flow. The difference with this event is the downstream blocking high over Greenland which will prevent Sandy from accelerating NE into the Atlantic and also a strong shortwave trough which moves through the large longwave trough and helps to capture Sandy into the trough.
With the trough anchored over the SE US and mid Atlantic producing an increasingly strong SE steering flow and blocking high pressure over Greenland, Sandy has little choice but to turn back toward the NW or even WNW and strike the US coast. A hurricane has never struck the US mid Atlantic or NE coast moving toward the NW or WNW or at a right angle to the coast as Sandy is forecast to do. Historical storms generally move SW to NE parallel to the coast with the greatest impacts offshore or over extreme east MA and Cape Cod.
As far as the models predicted track go, they are doing a good job with a rare pattern with only modest amounts of spread. Generally the ECMWF continues to be on the west and southern edge of the guidance taking Sandy into the southern DELMARVA area while the GFS and CMC are further north into Long Island (NYC) and southern New England. The current NHC forecast track is splitting the difference with slightly more weight toward the ECMWF which was the first model to predict this “highly anomalous event” and has remained very consistent over the past 2-3 days.
Model Intensification (Tropical vs. Post Tropical):
For reference the powerful 1938 Long Island hurricane holds most of the low pressure records over the NE US. A central pressure of 946mb was recorded as this storm moved inland over southern Long Island producing the highest storm surge on record on Long Island and into southern New England. The 1991 “Perfect Storm” had a central pressure of around 972mb which produced significant coastal damage from North Carolina to Maine. I list these benchmark events as comparisons to what the models are showing for Sandy.
Every forecast model except the NAM is forecasting a sub 960mb storm which would be significantly lower than the 1991 event. However most of the model guidance is producing a sub 940mb storm which has never occurred over the NE US before (946mb is the lowest recorded pressure). To understand what this means, the lower the pressure the stronger the storm system…in the tropics such pressures as this would equal to a category 4-5 hurricane. However, it will not be that simple! Sandy is already starting to undergo changes in her internal structure this morning with the inner core winds weakening and the wind field expanding outward a feature closer to a post tropical system (large weaker wind field) than a tropical system (smaller more concentrated wind field). Even if these super low pressure are to verify, the system would not produce category 4-5 winds as the pressure gradient would be spread out of hundreds of miles with a massive area of 70-90mph winds.
The question is why are the forecast models predicting such “historic” low pressures with this event and are they correct?
1) One would expect a true hurricane (tropical cyclone) to weaken as it moves northward into a colder and drier air mass over the NE US and adjacent Atlantic waters. Additionally water temperatures along the track are generally below the threshold needed for tropical cyclone intensification.
2) If the real world data does not support tropical cyclone intensification, then why are the models all showing it? The system is expected to gain more and more extra-tropical features as it moves northward which means it starts to draw its energy sources from temperature gradients and jet stream dynamics instead of the warmer waters. What is likely happening in the models is that the warm and moist influx of Caribbean air with Sandy will greatly intensify the temperature gradient with the SE moving polar air mass from Canada and help intensity the overall storm. Additionally, Sandy’s surface low will be in a favorable region of the trough to provide strong upper level divergence aloft which will help promote intensification.
3) However…what is interesting is that many of the models keep Sandy warm core or tropical toward landfall while intensifying in conditions that would be marginal at best for a tropical cyclone to hold its intensity. It is likely that Sandy will be something of an “in between system” drawing energy initially from the warm Gulf stream below and then more and more from the mid latitude jet and temperature influences.
4) The models could be completely wrong on the pressures (it would not be the first time). However, it must be noted that nearly every model is showing the same very low pressure and that does add credibility to their solutions.
As with any rare weather event the impacts are somewhat of an unknown. Additionally the uncertainty of how Sandy will evolve and how is curves back toward the US coast will drive where and how severe the impacts will be.
It can be easily stated that should the forecast track and models “spectacular” low pressure verify a storm of rare intensity and tremendous impacts will be felt along the NE and mid-Atlantic coast. Given the forecasted perpendicular strike on the coast, onshore winds will push the Atlantic Ocean inland along the New Jersey and New York coast including New York City. Due to the high blocking over Greenland, the fetch of wind will extend nearly across the entire Atlantic Ocean and this will result in massive wave action aimed at the NE US coast. Lunar tides are also near peak with the full moon on Monday and this combined with the wave run-up and long duration of onshore winds (20-30 hours at 60-80mph) will result in potentially record breaking storm surge values. The potential is there for coastal inundation of sea water never before experienced in the NE US including New York City, but this depends heavily on the exact track of the center of Sandy.
Strong winds will batter much of the mid Atlantic and NE for not hours but days as Sandy moves NW to WNW and slows. These winds will last anywhere from 20-30 hours at 60-80mph with higher gust resulting in widespread power outages and downed trees. Strong winds will spread well inland from the coast into Canada and the OH valley.
Rainfall will be extensive as tropical moisture is brought northward with Sandy and pushed against a stalled front nearly along the higher terrain of the Appalachian mountains. Flooding rainfall due to the high rainfall rates and slow storm motion is likely and it is possible some rivers will reach record crests.
Impacts over the open Atlantic will be severe with a massive area of sustained winds of 60-70mph over hundreds of miles. Wave heights will build into the 20-30 foot range and I would not be surprised to see heights build toward 40-45 feet. Visibilities will be reduced to near zero in blowing sea spray and heavy rainfall.
Potential for widespread travel and commerce disruption as air, surface, sea, and rail travel will likely be significantly impacted along with widespread long term power outages which could last well into November.
michmich wrote:I'm surprised there's not more discussion about this. Sandy looks like one crazy beast.
Karen wrote:Couple of questions.
1. what changed the original track had the storm going to sea a couple of days ago
2. is there anything that will change the direction and possibly push this back to sea
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