Tropical Storm Danny still looks rather unimpressive on satellite imagery as of Friday afternoon. The storm circulation is well-defined, but consists mostly of bubbling, fair weather cumulus clouds, rather than the towering thunderstorms usually seen with a tropical storm. This certainly means a weaker storm in the short-term, and greatly diminishes the risk of hurricane winds that exceed 74 mph on Saturday, but yesterday’s forecast for damaging wind along the coast with a storm track crossing Cape Cod remains intact…

After a frosty start in the valleys of the far North Country Friday morning, temperatures remained cool in most of New England with the combination of Canadian air holding tough, a relatively light wind meaning that air stayed in place, and cloud cover moving in. Meanwhile, areas of rain and embedded downpours have been blossoming through far Southern New England and gradually moving northward, and will continue expanding northward with time Friday evening and night. Heavy rain is expected to blossom across most of New England from south to north overnight, though Maine may hold out the longest before Saturday morning rain advances. The radar imagery today is quite impressive, with multiple boundaries of airmasses and atmospheric energy centers touching off large areas of thunderstorms from the Midwest to the Northeast, south to Florida! Meanwhile, bands of tropical moisture are already feeding in from the Atlantic Ocean around the west side of Danny’s circulation.

With cool and dry air already in place, temperatures will fall as rain develops, dropping into the 50s Friday night for many spots, except in far Southern and Southeastern New England, where a steady southeast wind will carry increasingly warm and humid tropical air up from the Atlantic Ocean. Even on Saturday, this warm and humid air will have a difficult time penetrating deep into the interior, and that difference in airmasses will likely result in an enhanced band of exceptionally heavy rainfall from Central Massachusetts through Central and Southern New Hampshire and into Southern Maine. In all spots, except for lesser amounts in far Northwestern New England, three to five inches of rain seem likely Friday night and Saturday, but in the area of enhanced rainfall, over six inches of rain seems possible, resulting in flash flooding of streams and creeks that will lead to some road washouts, and by Saturday night and Sunday morning could lead to some river flooding as well, as the rain runs off into the river basins. Of course, for all spots across New England, the periods of heavy rain on Friday night and Saturday will make for treacherous travel conditions at times, resulting in some street and urban flooding, and an increased risk of hydroplaning.

Of course, wind will be another concern with Tropical Storm Danny. As of mid-afternoon on Friday, no watches or warnings have been issued pertaining to wind from the National Weather Service. As best I can tell, this is due to a concern of whether Danny will technically be tropical or non-tropical. Here in the NECN Weather Center, we remain focused on the effects of this storm for the public, and acknowledge that it will likely end up a “hybrid” storm - that is, a storm that exhibits both tropical and non-tropical characteristics. This actually is a worse scenario for New England than if the storm were completely tropical in nature. The reason for this, is because a tropical system passing over Chatham would keep its core of wind completely to the east of the center, meaning there wouldn’t be much wind threat across the remainder of New England. A non-tropical system, however, as we all know from the many nor’easters and other large coastal storms we’ve received, produces strong wind on many or all sides of the storm. In this case, the storm will be a mix of both tropical and non-tropical characteristics, meaning the wind field will likely be expanding as the storm moves over New England, though also is unlikely to be nearly as strong on the southwest side as it will be on the north and east sides of the storm center. So…with an anticipated track across the Cape, that would put the strongest wind field across the Outer Cape and Islands, and then again in Downeast Maine, where gusts greater than 60 mph will be possible on Saturday afternoon, when the storm makes its passage, resulting in wind damage to trees, limbs and powerlines, and tossing lightweight objects like patio furniture. Along the remainder of the eastern New England coastlines, winds still seem likely to gust above 45 mph, which will cause far less damage, but certainly still can bring down widely scattered trees and power lines. I’ve heard from some folks today who’ve wondered exactly why it is we’re forecasting a hybrid storm of this nature here at NECN. I should make it clear that every forecast is a gamble, but we always make our forecasts based on the most sound scientific reasoning we can derive. In this instance, we’re witnessing an amazing combination of the circulation of Tropical Storm Danny, deep tropical air from the Atlantic, warm and humid air from the Mid-Atlantic States, a surge of exceptionally humid air with a strong energetic upper level system from the Gulf of Mexico, another strong energy center out of the Midwest and moving east through the Ohio Valley, and finally, a clash between all of this warmth and humidity, with the cool and dry fall air that we’ve had in place. All of these players will converge along the Eastern Seaboard Friday night through Saturday, and serve as fuel for the developing storm. In fact, hybrid storms such as this one - if it develops as planned - usually do not obey the normal rules of storms. That is, we still expect the storm to strengthen on its way northward to New England, and are curious to see just how much it strengthens. While we are certainly hitting the wind potential hard, there is actually excellent agreement on a eye-type feature developing with this storm. We’d like to see 12 more hours of guidance products before biting off of this potential, but if that solution verifies, the wind core near the center would be even more intense. For more on how this forecast has come to be, you can check out the in-depth weather page of my personal blog at www.mattnoyes.net/new_england_weather/ where I’ve posted technical discussions with more scientific description.

The timing of the strongest wind should be focused on Saturday, from late morning in Southeast MA, through the day along the eastern coastal plain of MA, NH and Southern ME, and the late afternoon to evening along Midcoast and Downeast ME. Although that’s the timing for the core of strongest wind, the wind will actually start freshening from the southeast along our coastlines all night Friday night, reaching over 30 mph gusts by Saturday dawn, and over 40 mph (tropical storm force gusts) by mid-morning the latest. From there, the wind should ramp up to our expected values expressed above.

The storm should keep moving, and both the rain and wind will taper across Southern New England during the late afternoon…early evening at the latest. Farther northeast through Maine, it will take into the first part of Saturday night for things to wind down.

Sunday is still expected to feature an upper level disturbance moving across New England from the west (hybrid storm Danny will be long gone), capitalizing on some lingering moisture in the lowest few thousand feet of the atmosphere to create a few thunderstorms and showers, though the day will be much, much better than Saturday. Drier air is expected for the start of next week.

Stay safe, spread the word on the storm (the lack of official watches and warnings to this point has me concerned about getting the word out), refer folks to this blog post and we’ll keep you posted. Coverage on NECN Saturday morning begins at 5 AM, and from 9 AM to 1 PM we’ll be streaming live right here on WeatherNewEngland.com as the TV side covers the Kennedy Funeral. At some point during the afternoon, I’ll likely head out into the storm with the FlipCam and grab some HD video and keep shipping it back.

Matt

You can find more weather information from Matt, view his live stream with live chat (when actively streaming), or connect with him on Twitter, Facebook and MySpace on his personal website, MattNoyes.net.