As explained in the updates on Hurricane Bill, my time is a bit pressed this season. That's why I dropped coverage of Bill without notice as soon as it was clear he would remain offshore (and I apologize if any of you were awaiting further updates). That's also why I did not post on Danny yesterday. But now that Danny has developed and is a clear threat to the East Coast (though, to be clear, it, too, may remain offshore), I'm trying to make the time to post. So, let's get to it...
From a meteorological perspective, Tropical Storm Danny is a festival of intrigue. The storm's evolution - both track and intensity - is severely complicated by interaction with a system in the Southeastern U.S. Part of this complication is that Danny may not even be a purely tropical system, depending on how this evolution takes place. Also, we're left in a bit of a lurch trying to monitor the current trends. That is, typically, if a system is running north and/or east of the previous expectations, those changes can be applied to the entire forecast going forward and, thus, reasonable track adjustments can be made. Not this time. Part of the complexity with the system in the Southeast is that, if Danny is to make landfall he (sorry, I know some of you don't like the gender pronoun with storms... habit) will need to be ahead of that Southeast system and then merge with it. So, Danny actually needs to get further north for this to happen. Otherwise, that Southeastern system may kick out ahead of Danny, "flattening" the pattern, and allowing Danny to hook more severely to the right and out to sea. Overnight, some model guidance slowed Danny down a bit and, as such, more guidance has shifted out to sea. So, this is a bit of an oddity... a more north Danny track early will allow him to get pulled further in towards the coast due to the interaction with the Southeast low. But if the more north shifts are only subtle, it may simply mean tweaking the forecast more to the east and out to sea. (Of course, southward adjustments early would favor an out to sea track as well - again, counterintuitively - but I'm not really getting into that as the very short term adjustments thus far have been northward.)
The intensity forecast is equally intriguing. The models showing significant intensification of Danny are, for the most part, doing so via an injection of energy from that Southeastern U.S. system. Those not showing much intensification are the further east models, which eject that Southeastern energy out ahead of Danny, so he's not able to tap into it. This is very meteorologically perplexing. The Southeastern U.S. system is a non-tropical upper level low pressure system. Can it effectively inject its energy into a tropical cyclone? And if that happens will Danny be tropical or subtropical (a not-irrelevant question, as that can determine the distribution of the wind field)? Honestly, I'd be lying if I claimed to know the answers to these questions. I do know that some research has suggested that some energy can be imparted this way (a non-tropical low injecting energy into a tropical system); but I'm not sure I've seen it on quite this scale.
With that (long) preface, on to the forecast. Here's what Danny looked like this morning:
The red areas are deep convection (thunderstorms). This looks generally pretty impressive. However, in order to develop these thunderstorms need to be over the center of the storm. They are not. Visible satellite imagery, just becoming available as I write this (sun needs to be up!), shows the surface low off to the west of this convection. So, Danny is likely holding its own, given the strength, coverage and persistence of this convection, but no real strengthening is ongoing. This is not surprising. Some shear is expected to remain over Danny until later today. We'll see what happens later this afternoon.
So, where's Danny headed. Well, here's what the National Hurricane Center (NHC) says:
So, they're calling for a close call to Cape Cod (if you connect the dots it looks to remain just offshore). I rarely differ from NHC, but this time I will... slightly. Here's why: They're sticking with a model consensus - basically an average of all the models. However, their are two "camps". Here's what the statistical models show:
...now check out the dynamic models:
(Note that a new GFDL and HWRF model run have just come in and are slightly east of those plotted on the map above.) The difference in these solutions is not insignificant. The statistical model are almost universally east of the dynamic models. The dynamic models are quite advanced and are almost always superior. There are also a few other dynamic models not on that plot (like the European and Japanese models), which also agree on a more westward - albeit probably just offshore - track. So, I'd lean much more heavily on those dynamic models and lean towards a track slightly west of the hurricane center's. To be clear, my thinking is actually only slightly west of NHC... their forecast really isn't bad at all, and may very well end up correct. But given the focus of the dynamic guidance, I'm expecting landfall on Cape Cod on Saturday... though I must emphasize the uncertainty and the need to check back on this. As good as the agreement seems in those model tracks, this remains an extraordinarily complex setup.
The intensity is similarly convoluted. Here's what the model guidance shows on that:
The models built for tropical cyclones are all the ones on the higher end, showing Danny up in the 80-90kts range (borderline Category 2 hurricane) when he approaches southeastern Massachusetts. Now, since there are questions about whether or not Danny even will be entirely tropical, the NHC has hedged low (the black line on that chart) and puts Danny at 60-65kts when passing Cape Cod. This is a smart "hedge" in my opinion. I think they're about right; I think Danny will be a Category 1 when it crosses (or passes just offshore of, if my track forecast is wrong) Cape Cod.
Needless to say, this means significant impact for far southeastern New England. Here are a couple of images showing the specific wind impacts. Note that I do not want to be alarmist... so keep in mind that the following two images are very high estimates (I'm using them because they are a couple of the best images I could find). They are from the strongest model (HWRF), showing winds several hundred feet above the surface (surface winds would be less), and their is a brand new (further east) run of this model. So, this will almost certainly overstate the impact, but I post it to give you a basic idea:
...and zoomed in for Massachusetts...
The hurricane force winds on the above plot cover all of southeastern Massachusetts. Again, this is probably overdone, but, under my forecast, I do think Cape Cod could experience hurricane force winds, and the rest of eastern Mass (and Rhode Island?) tropical storm force.
In short, this will not be a massive event. This will be no Carol/Hazel/Edna of 1954, nor will it be a 1938-like event. It may even remain offshore (I'm going with a landfall, but barely... clipping SE MA). But, on the other hand, areas receiving infrequent hits (the last hurricane in New England was Bob in 1991) are prone to significant tree damage... and "collateral damage" (power lines, trees on cars and roofs, travel disruptions due to blocked streets, etc). So, while not a major event, I also do not expect it to be trivial.
...but I must continue to emphasize the uncertainty. If one of the more eastern and weaker tracks verify we will, indeed, be looking at nearly a complete non-event and it will be a trivial, insignificant situation, not even worthy of the time you invested reading this! On the other hand, if the stronger more western tracks verify, it is feasible to see a Category 2 hurricane rake across southeastern New England. My expectation is something in between... a Category 1 clipping Cape Cod. Stay tuned... and apologies for any typos (trying to save time, so posting without proof-reading). Though my time is limited, I will stay one and check comments occasionally, in case anyone has any questions.
UPDATE: As of 10:45AM EDT we have our very first model run of the new model cycle (models run every 6-12hrs). It is not a particularly good model with tropical features (it is a model known as the NAM), but it should handle systems like the one over the Southeast well. This new run is noteworthy in that its previous run flipped to a more southward track of Danny, allowing the Southeast low to run ahead of it, flattening the pattern and kicking a very weak Danny out to sea. Now, this run starts Danny weaker, but further north... ending up with a track further northwest, and a stronger storm, clipping Cape Cod on Saturday. To be clear, it would not be an entirely tropical system, and this model is likely not strong enough to deliver anything more than tropical storm force winds. But it is a significant step in the direction of this western solution, and from a model that should handle the interaction with the Southeast low well. So, based on this, I'm obviously not changing my thinking of a landfall - barely - on Cape Cod.
Note that the 11AM EDT National Hurricane Center forecast is also out. It appears to be completely unchanged... both in track (*just* offshore of Cape Cod) and intensity (marginal Cat 1 hurricane or strong TS when it passes by Cape Cod).
On a final note for this update... more visible satellite imagery is making it clear that Danny's low level center is completely exposed from the convection. So, there will be no intensification for the next few hours - at least - until this situation changes.