This forecast is still likely to require some mostly minor adjustments – but the changes are less likely to be large from this point forward. Very good computer model agreement (for a hurricane 2-3 days in advance) is increasing confidence.
Current: Hurricane Irene continues at category three strength – sustained winds at 115mph. It’s currently centered over the northern Bahamas.
Short-term Forecast: Irene will shift more to the north over the next 24 hours. It is also expected to increase in intensity – likely a category 4 before Friday AM.
Timing/Track: No change to the timing from previous discussions. Begin to see bands of showers/thunderstorms Saturday daytime hours in PA/NJ/Eastern MD/DE (overnight Friday/early AM Saturday south). (There could even be some pop up isolated storms tomorrow PM as the front moving through today interacts with the leading bands from Irene.) The steadier, heavier rain will arrive Saturday night, increasing from south to north. The eye of Hurricane Irene will likely pass along the latitude of southern DE early Sunday morning then along the latitude of Cape May, NJ around or just before noon on Sunday – then approaching Long Island’s latitude Sunday evening (mentioning the word “latitude” as there’s still a potential of a 50-100mi shift east or west for Irene – but that range is narrowing).
Track/Intensity: NC continues to be most likely to experience landfall (first?) in the U.S. (a greater than 50% likelihood for landfall in NC). This will occur during the daytime hours on Saturday. Should Irene track just west of the Outer Banks (over the mainland), that will likely act to weaken Irene, and reduce some of its impact in the Northeast. However, it mostly skirts over the Outer Banks only (or just offshore), intensity loss will be minimal. That’s the ideal path to maintain intensity while maintaining a path that takes it into the high population centers of the Northeast. A track far enough offshore of the Outer Banks could limit the impact in PA/NJ/DE/MD/NY, but pose a greater threat to coastal New England (this is the least likely track). Model agreement is very good – within 40mi of the Jersey Shore on almost every major model. Irene is very unlikely to be a “swing and a miss” storm that shifts harmlessly out to sea. As Irene approaches South Jersey early Sunday, it will likely be no less than category one in strength (74-95mph sustained winds). Should its track across NC be minimal, it could easily be category two (sustained winds 96-110). A track over a larger portion of NC could reduce it to a strong tropical storm – but that’s unlikely. Also unlikely (but not impossible) would be category three hurricane (111-130mph).
Impacts/Precipitation: The best bet for significant impact is the Jersey Shore. It is the midpoint of the computer models (and has been close to the midpoint throughout this week – more so than any one region). Irene will be large enough to affect a very large geographic region – and it’s highly unlikely that anyone in eastern PA/eastern MD/DE/NJ and on up to eastern NY and New England is left out. The concerns over Philadelphia (and even into the western suburbs) have grown during the past 24 hours. This will likely be an area of substantial precipitation – with the possibility that a track farther west brings the heaviest band of rain to this region. Regardless, 4-8” of rain will likely be widespread and common over NJ. Some 8-12” amounts are possible. Philadelphia, Delaware, Coastal MD should see 4-6” on the low end – 8-12” on the high. Western PA suburbs into Lancaster, Berks Counties, 2-4” low, 4-8” high. NYC should see similar to the PHL/NJ totals. (No, these are not snowfall accumulations – though the numbers are more commonly associated with snow!) This is a bit early to post these rainfall projections with high confidence, but it’s a very reasonable expectation given the growing confidence of the track. Amounts may vary by 2-4” depending on the track – but there isn’t a debate if it’s going to be 0-1” or 10-12”.
Impacts/Wind: Again, a lot depends on how much intensity Irene holds going across NC. Also, a lot will depend if the hurricane moves inland over NJ (or west) or stays just offshore. At this point, widespread 50-80mph winds along the Jersey Shore is very reasonable. There is potential for stronger. West/inland, 30-50mph winds will be more common (with higher gusts). Wind projections are more difficult with Irene at this time – and track will dictate the strongest wind speeds (as well as the other factors mentioned above).
Impacts/Storm Surge: Storm surge will be a tremendous concern – especially north and east. Remember, this event will be occurring during high tide. That adds several feet of water to the potential storm surge ahead of Irene (generally north and east of the center). NJ up toward Manhattan, Long Island, Rhode Island all will likely have some serious concerns with potential inundation of high levels of water Sunday. Waves of 10-15ft in some places are possible.
Additional Thoughts: At this point, it appears slightly more likely that Irene will track more west before it would shift back east. In other words, if it’s not along the Jersey Shore, it’s interior NJ or even close to PHL. That’s not a slam dunk at this point – but it is the lean. This would also shift the band of heaviest rain west (and not mentioned previously, but the initial track would bring 2-4” of rain to the Susquehanna Basin – this would suggest 4-6” possible). The wind will be a concern over all of the PHL metro – but it would be less than if it stayed offshore.
