On the potential for an east coast tropical hybrid cyclone impact early next week, I can now raise the odds to 60%. A 10-digit damage event (10 billion or greater) carries odds of at least 30% as of this morning (10/25). The models have remained remarkably consistent in their bent-back track solutions as the various upper atmospheric features now come into alignment and appear over our instrument networks. As climatologically improbable as this event is, and as delicate the balances must be for it to take this deadly track to the NW after it passes Hatteras, I still cannot rule out a near miss or total escape, but those odds are dwindling.
Hurricane Sandy – which is now a rapidly intensifying category 2 storm with central pressure of 964 mb – is confidently forecast to head north or NNE across Cuba and the Bahamas, and then turn NNW toward the Carolina coast tomorrow (Friday). It is then forecast to gently turn northeastward and slow WAY down to a near stall as a deep shortwave trough over the Northern Plains drops southeast and carves out a major east coast long wave trough. This larger trough – highly anomalous for this time of year – will work in conjunction with a strengthening area of high pressure south of Greenland to trap Sandy and turn it northward again early Saturday. From this point, little is known about the final trajectory of the storm, but all current operational guidance agrees on a few points:
- The storm will begin undergoing a transition from tropical cyclone to hybrid nor’easter on Sunday – the exact timing of this transition is still open for speculation.
- As its core cools, it will feel the influence of the approaching east coast trough more strongly and begin to turn NW or even WNW on Sunday Night or Monday. The timing of this westward bend in its track is absolutely crucial – too early and it makes landfall (and begins weakening slowly) over south Jersey or the DelMarVa Peninsula, too late and the turn takes it into the Canadian maritimes as a weaker, more sheared system and everyone breathes a sigh of relief.
- At its most intense (and generally, the peak intensity is forecast to be near its time of landfall!), the storm is forecast to have central pressures of between 942 and 965 mb. The October 31, 1991 “Perfect Storm” which appeared on the silver screen had a peak central pressure of 969 mb.
- Its wind field will be MASSIVE – tropical storm force sustained winds potentially extending 500 km from the center of circulation – especially east of the track.
- 25-50 foot swells, 6-10 foot storm surges, and hurricane force sustained winds are entirely possible as it comes ashore, and unlike a fully tropical system (say, Hurricane Irene) these conditions will occur well inland. Tropical systems lose their low level wind field due to friction and increased static stability near the ground. Extratropical cyclones are cool cored, and thus unstable through the air column, meaning high flight-level winds may mix to the ground more effectively over a large part of the NE US.