To coin the cliché "A year and a day"
Current watches, warnings and advisories.
Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.
Barring the unlikely rapid development of 95L in the western Gulf of Mexico, Tropical Depression Nine in the central Atlantic will be upgraded to Tropical Storm Isaac later today (and will be referred as such throughout this blog). Every so often, a tropical wave moves off Africa and perks the interest of every and any observer of weather near or far. It screams out, "You better watch me closely," as if we didn't already notice its presence; a large, well-defined circulation accompanied with strong convection moving off the coast of Africa. Model guidance need not apply to aid in the forecast of the development of the particular wave into a tropical cyclone because there's that certainty that's there - this one is going to develop. And it isn't just that. Our weird sixth sense kicks in and infers to us this isn't going to be the typical storm but something much worse.
Tropical Depression Nine, in its entirety, is currently displaying a stunning satellite presentation for being just a tropical depression. The center of circulation is displaced slightly to the west of the massive overall circulation envelope almost 800 miles in diameter, covering a huge region of the central Atlantic Ocean basin. Several banding features are becoming well-defined as the storm breaks away from the ITCZ. In addition to the impressive presentation of the depression itself, a large arc of Saharan dust extends for some 2,500 miles from the northwest of the storm and trailing off into the eastern Atlantic, distinctly showing the extremely large amplitude of the wave spawning Isaac.
Tropical Depression Nine visible satellite image 12:15UTC. Courtesy NOAA
The forecast for Isaac is quite foreboding, as it should begin to make its first presence felt over the Leeward Islands in about 24 hours as a strengthening tropical storm. The official NHC forecast from then takes the storm over the northern Caribbean Sea where it will strengthen to a hurricane, skirting to the south of Puerto Rico and Hispaniola, then taking a turn toward the northwest in the general direction of the Florida Straits by early next week. On this track Isaac would affect all of the Greater Antilles with tropical storm conditions at the very least and be on track to affect the US mainland next week. However, for reasons I will explain, my reasoning is Isaac will move north of the HNC projected path and set a course for a potential historical impact along the East Coast of the United States.
Official NHC track for Tropical Depression Nine as of 8/21 8AM EDT.
The big picture is always important to take note of when forecasting tropical cyclones. A trough ten-thousand miles away over eastern Russia might become what ultimately determines the fate of a tropical cyclone in the Atlantic so it's very important to identify all potential features which may interact with the storm. As it currently stands, the strong +590dm 500mb ridge anchored over the Sargasso Sea driving Isaac west will begin to build toward the northeast in response to the deep trough slowly progressing eastward over the eastern United States. Several shortwave disturbances are clearly defined within the overall trough, aiding in the maintenance of such a deep August trough. Also of note is a broad southwest-northeast oriented tropical upper-tropospheric trough (TUTT), the base of which is centered around 75°W over Cuba. The erosion/displacement of the center of the 500mb ridge to the north and east combined with southwesterly upper flow emanating from the TUTT should allow Isaac to add a more northerly component to its track starting later today as it approaches the Leeward Islands.
As Isaac moves through the northeastern Caribbean Sea Wednesday evening into the day on Thursday toward the general direction of Hispaniola the TUTT to the west of Isaac will yield little ground and reorient itself on more of a north-south axis. This feature greatly factors in my belief of this storm gaining latitude as the expected more-vertical storm will feel greater effects from the developing mid/upper southerly flow over the central Caribbean.
0600UTC GFS depicting lingering TUTT over the western Caribbean, inducing southerly upper flow over the central Caribbean Wednesday evening.
One of the first effects Isaac will feel of the stagnant upper pattern ahead of the storm will be a slowing of the forward speed. This will allow for Isaac to be more easily influenced to the upper-level steering patterns, which should pull the storm northward. Additionally, due to the aforementioned large circulation of the storm, should the storm move into Hispaniola most of the storms angular momentum will lie on the Atlantic side of the island by that time, possibly allowing for the center to 'jump' north of Hispaniola and into the Atlantic basin. With this expected interaction with the terrain of Hispaniola Isaac may emerge as a large, disorganized minimal tropical storm but with plenty of warm water and a favorable upper environment Isaac should quickly take advantage and reorganize.
Friday afternoon through the weekend will be the time period in which Isaac will begin garnering the attention of the national media. It will be during this time frame in which Isaac will strengthen into a large formidable hurricane as it tracks west-northwest to northwest through The Bahamas around 10-12kts. The deep trough, doing its part of lift Isaac north, will be in the process of lifting out to be replaced by a transient ridge riding rather far to the north over southeastern Canada and northern New England to open next week. This should keep Isaac on a path that will bring it very close to the Carolinas by Tuesday. One saving grace might be due to the transient nature of the ridge quickly moving through westerlies to be replaced by a broad through over the eastern US by the middle of next week. This may act to recurve Isaac out to sea quickly, unlike Irene from last year which churned slowly up the entire stretch of the East Coast, dumping copious amounts of rain. The irony of this potential East Coast storm to last year's Irene is freakish. Both will be the "I" storm moving up the East Coast almost exactly a year apart (a year and a day to be precise). Just like the huge circulation Irene sported last year, Isaac is expected to be equally as large. This carries with it all the same issued Irene presented last year - a greater potential for coastal storm surge, a greater potential for inland flooding and the potential Isaac may not pack the expected punch in the wind department. Although the potential wind impact will ultimately be determined by the inner structure of the storm which is practically impossible to forecast just a few hours hence, let alone a week in the future.
This is just my preliminary forecast reasoning based on what I'm "seeing" might be potential important factors later on down the line at this very early stage. I will be here to track this storm and the one following it, currently developing southwest of the Cape Verde Islands. This, too, has the potential to become another storm to watch next weekend into the following week but should pose more of the threat to Bermuda and Atlantic Canada.
Radar: Northeast Region Loop
Radar loop of the Northeast region. Courtesy of Weather Underground.
Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.