Northeast Weather Blog

Danny struggling to survive

By: sullivanweather, 12:15 AM GMT on August 29, 2009

Tropical Update



Fig.1 - Current satellite picture of the Northwest Atlantic Ocean. (Credit:NOAA)


The tropical Atlantic remains active this evening with Tropical Storm Danny spinning off the Southeast Coast and disturbance ‘94L’ slowly organizing between Africa and the Lesser Antilles. The threat closest to US shores, Danny, has been struggling to survive as a tropical cyclone over the last 24-36 hours as the storm endures bouts of shear and a dry environment. All of the convection has been well to the east of the center throughout the storm’s life as a tropical cyclone due to the aforementioned hostilities, preventing any organization from occurring. Danny is not expected to come ashore along the East Coast and should only graze the capes running up the coast with some direct effects.




Fig.2 - Current satellite picture of Tropical Storm Danny.
(Credit:NOAA)


The 8pm National Hurricane Center intermediate Advisory places Tropical Storm Danny at 30.9°N, 75.1°W, or about 300 miles south of Cape Hatteras. Maximum sustained winds are 40mph with higher gusts and the central pressure reported by Hurricane Hunters is 1006mb. Danny is moving north-northeast at 8mph.

Danny continues to exhibit very poor organization, with a 75% exposed center of circulation and most of the convection limited to a cluster roughly 40-200km east and southeast of the center. Wind shear continues to be sent Danny’s way as a rather potent upper level low spins over the Southeast US. This shear has been in the vicinity of 20-30kts since Wednesday evening, first from the upper low left in the wake of Bill and now this upper low over the Southeast. Additionally, an upper level shear axis/confluent zone has also hindered development by creating subsidence in the mid/upper atmosphere over the western half of the cyclone. Precipitable water contents continue to run below 2” off the Southeast Coast but have been rising over the last 12 hours.

The intensity forecast for Danny is lowered in light of the hostile conditions and it is now unlikely that Danny will ever become a hurricane. As the storm moves north or north-northeast during the overnight it will find a slightly more favorable environment that could allow for a slight increase in strength. Effective shear over the system will lower by 5-10kts as it accelerates north. High moisture contents can also be found along the storms path north so convection will increase during the overnight and may come over the center. Should this occur Danny may gain 10-15kts in intensity by Saturday evening before stronger upper level winds and the approach of a shortwave trough will induce extra-tropical transition in the waters off Cape Cod. Once caught in the westerlies as an extra-tropical cyclone the storm will continue to produce a larger area of 40-50kt winds over the Canadian Maritimes and the North Atlantic as the wind field expands.

The track forecast for Danny is a bit more straight forward now that the storm has begun moving north (with some hints of a northeast wobble). Danny will move in this 0-30° heading for the next 18-24 hours with an acceleration in forward speed from 8-10mph to 16-20mph. Thereafter, Danny will take more of a turn towards the northeast as Danny approaches a strong belt of westerlies moving off the New England Coast. By this time Danny should be completing its extra-tropical transition.



Fig.2 - Current satellite picture of '94L'.
(Credit:NOAA)



Elsewhere in the tropical Atlantic a strong tropical wave is located along 41°W, south of 17°N with a 1010mb low pressure along the wave axis located near 10°N. This has been an impressive wave since moving off Africa, showing signs of organization as it moves west-southwestward around 18kts. Scattered moderate to strong convection exists within a 100km radius around the analyzed low, with the greatest concentration of convection in the western semicircle. Upper-level winds are conducive for continued development of this system with wind shear of less than 5kts and diffluent flow being located to the south side of strong sub-tropical ridging. The National Hurricane Center gives this disturbance a 30-50% chance of developing into a tropical cyclone over the next 48 hours as the system continues moving west. This system could be a threat to the Lesser Antilles in 72-84 hours. This wave will be closely monitored due to its potential high-impact.


Further east a new wave will be rolling off the west coast of Africa over the next 12-24 hours and another strong wave is lined up right behind it. There is broad model support that either one of these two waves could develop as they move into the Atlantic Basin.




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Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.3 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

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Forecast Discussion


coming later


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Radar: Northeast Region Loop

NE radar
Fig.4 - Radar loop of the Northeast region. Courtesy of Weather Underground.


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Local SST's

Northeast SST's
Fig.5 Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


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Updated: 12:17 AM GMT on August 29, 2009

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Another tropical threat on the horizon

By: sullivanweather, 4:33 AM GMT on August 26, 2009

Tropical Update



Fig.1 - Current satellite picture of the Northwest Atlantic Ocean. (Credit:NOAA)


Almost forty-eight hours have passed since Bill was downgraded to an extra-tropical cyclone early Monday morning, leaving no named systems left in the Atlantic Basin. But this is late August and the tropical Atlantic is brewing a new area of interest, this one beginning its development a bit closer to home. A relatively weak tropical wave interacting with the large upper low left in the wake of Bill has produced sustained convection to the northeast and north of the Lesser Antilles over the last 36-48 hours and has begun building a mid-level low pressure down to the surface along the wave axis. The National Hurricane Center designated the disturbance 92L yesterday and sent a team of Hurricane Hunters to investigate the disturbance earlier this evening. They found no surface circulation but did find winds in excess of 45kts to the northeast of the center. Most regional/global medium-range forecast models develop a tropical cyclone from this disturbance, bringing it close to the East Coast of the US by Friday and Saturday. It should be noted the last landfalling hurricane along the Northeastern shores was Hurricane Bob in 1991.




Fig.2 - Current satellite picture of '92L'.
(Credit:NOAA)



Focusing in on the disturbance reveals a disorganized system at the present time. Starting as a very large system, convection has been slowly migrating towards a common center over the last 12-24 hours as the system slowly gets more organized. Most of the convection associated with 92L is still well away from the developing center, however, in an area of upper divergence and deeper moisture about 50-100km to the northeast. The environment around the low itself, located near 23°N 68°W, isn’t particularly conducive for development with drier air in the mid/upper levels as shown by precipitable water contents of under 2” wrapping into the low from the south and west. Wind shear is also a problem. 15-20kts of southerly wind shear is moving over the center, preventing sustained convection to mix out the dry air in the vicinity of the center, slowing development. However, overall low-level vorticity is increasing as low pressure builds down through the atmosphere to the surface. This process will increase convergence closer to the center as a response and promote the development of convection closer to the center. Sea-surface temperatures are in the 28.5-29°C range in the vicinity of the disturbance and along its projected path ocean heat content rises so there’s plenty of fuel to feed off of.

The expectation is for 92L to be a named tropical cyclone within 24 hours as the process of building low pressure down to the surface should be complete. As mentioned in the synopsis, winds measured by Hurricane Hunters are already above minimal tropical storm force so the prospect that we will see advisories initiated for Tropical Storm Danny as opposed to Tropical Depression #5 is high. Wind shear will relax on Wednesday as the disturbance moves under the upper trough axis, which should allow for convection to develop and become sustained about the center of circulation, although the storm will still be lop-sided towards the northeast. Once named, intensification will occur at a modest pace as varying levels of dry air southwest of the storm will occasionally find itself incorporated into the circulation. Additionally, varying levels of wind shear resulting from the complex circulation pattern surrounding the storm should halt any rapid increases in intensity through the next 48 hours. Despite the negating factors enough development will occur so that by Thursday night a 50-60kt tropical storm or even a minimal hurricane will be located just a few hundred miles off the Southeast Coast. By Friday, with the system approaching the Carolinas, a small window for intensification will open as ‘Danny’ may be in juxtaposition between a weak upper trough over the Mid-South and the deep-layer oceanic ridge near Bermuda, optimizing diffluent flow over the system. Dry air surrounding the system should be all but vanished and the Gulf Stream will be in sight. The combination of these factors should bring a strong Tropical Storm Danny to a hurricane or should the system already be a hurricane, add a category.

The forecast track calls for 92L/’Danny’ to move in a west-northwest direction for the next 12-18 hours with an eventual turn toward the northwest by late afternoon Wednesday, at about 12-14mph. This northwesterly motion should continue for the ensuing 24-36 hours as the system is steered between an upper trough over the Mid-South and a mid-oceanic ridge located near Bermuda. On Friday a much deeper trough will dig into the center of the nation, allowing heights to build over the western Atlantic and increase the southerly flow along the East Coast, of which the storm should be located a couple hundred miles east of. This will cause the storm to turn north-northwesterly and eventually northerly. Where this turn occurs is instrumental for residents living along the East Coast from South Carolina to Canada. A track further west will bring the center onshore near the Carolinas whereas a turn north a bit further east spares the US coast once again but may give Canada its second tropical storm in one week! A turn somewhere between would have the system skirt along the coast, eventually making landfall in Long Island or Southern New England so the possibility that somewhere along the East Coast will be dealing with at least some effects from this system are rather high.

More will follow on this system in the coming days and updates will come more frequently should this storm develop and threaten the local region.


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Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.3 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

--------

Forecast Discussion


coming later


___________________________________________________________


Radar: Northeast Region Loop

NE radar
Fig.4 - Radar loop of the Northeast region. Courtesy of Weather Underground.


___________________________________________________________

Local SST's

Northeast SST's
Fig.5 Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


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Bill challenges Ike's dimensions/Northeast flooding possible from 'pre' event

By: sullivanweather, 4:19 AM GMT on August 21, 2009

Tropical Update



Fig.1 - Current satellite picture of the Northwest Atlantic Ocean. (Credit:NOAA)


The main story in the tropical Atlantic continues to be category three Hurricane Bill, churning nearly halfway between Bermuda and Puerto Rico. Bill remains an impressive looking storm on satellite, with a well-developed outflow pattern covering nearly 20 degrees of longitude across the western/central Atlantic and a clearly-defined 25-mile wide eye located in the western semi-circle of the strong convective ball making up its central dense overcast. A few feeder bands of rainfall extend well to the south of the storm and are bringing a few heavy showers this evening to Puerto Rico and the US/British Virgin Islands. These showers should wane during the overnight as the storm slowly pulls away to the north. Meanwhile, the cirrus canopy to the north of Bill is spreading over the Island of Bermuda which will be the first area of land to see tropical storm conditions from this cyclone starting late Friday night. Bill will pass a couple hundred miles to the west Saturday morning, however, being such a large storm, Bermuda will find itself in the eastern periphery of the system and may experience several hours of squally weather in addition to high battering waves of 10-15’ on top of a 1-3’ rise in tides.



As of the 11pm National Hurricane Center advisory, the center of Hurricane Bill was located at 24.9°N and 64.3°W, a little more than 500 miles south of Bermuda and less than 1,000 miles southeast of the Outer Banks of North Carolina. Maximum sustained winds are unchanged from the pervious advisory at 125mph with higher gusts. The central pressure of Bill has dropped 5mb since this afternoon and now is down to 943mb. Forward movement continues to be northwest at 18mph. The wind field with Bill continues to grow with hurricane force winds extending up to 115 miles from the center with tropical storm force winds extending up to 260 miles away from the center. For comparison, Hurricane Ike of last season had a wind field consisting of hurricane force winds up to 115 miles from the center and tropical storm force winds up to 275 miles from the center. So both storms are very close in terms of size and Ike has been widely recognized as having the largest wind field of any Atlantic tropical cyclone on record.




Fig.2 - Current satellite picture of Hurricane Bill.
(Credit:NOAA)



Bill has maintained a steady intensity since completing an eyewall replacement cycle this morning as the eye continues to have trouble centering itself within the CDO, preventing any prolonged episodes of intensification. Despite the pressure having dropped 5mb since this afternoon, Bill’s extremely large size may we working to his detriment. As seen with Hurricane Ike last season, due to the large size of the hurricane the maximum sustained winds never really caught up to what one would expect from a storm with as low a barometric pressure; the energy seemingly being spread throughout the girth of the storm rather than consolidated in the eyewall. Keeping this in mind only a very modest amount of additional strengthening is expected over the next 12-24 hours as the storm should maintain its current structure and the surrounding environment remains relatively unchanged. As mentioned, the central pressure is down to 943mb and may drop into the upper 930’s before bottoming out. This may add an additional 5kts to the maximum sustained winds yielding a low-end category 4 storm once again. By late Friday night into early Saturday morning, the southerly flow ahead of an advancing trough moving through the Eastern Us will start to increase in the vicinity of Bill. The caveat being that Bill will be now moving in the same direction of the large-scale flow so the effect of these stronger environmental winds will be lessened. Regardless, a slow but steady weakening of the storm will commence as it accelerates north. Though, due to the large size of the storm, it’s likely that the weakening phase will start slowly and Bill is expected to maintain at least category 3 status into Sunday morning when the storm will come under higher shear and colder SST’s which will eventually weaken the storm below major hurricane strength as it approaches Nova Scotia. As Bill reaches the Canadian Maritimes the system will come under increasing vertical wind shear, much cooler SST’s and start to interact with a baroclinic zone. This will bring about extra-tropical transition as Bill races towards Newfoundland, though the storm will still pack 60-75mph winds.

The track forecast for Bill is starting to become clear now and southern New England/Downeast Maine can breathe a little bit easier as Bill is now expected to be caught by the trough moving off the coast in time to spare this region from Bill’s wrath. The northwesterly motion seen now will continue over the next 12-18 hours with an ever so slight bend to the north by 5 degrees of heading. By Friday evening the movement towards to north will be more pronounced as the steering flow over the western Atlantic shifts from the south between the approaching trough and the deep-layer ridge to the northeast of Bermuda. This northerly motion will continue through Saturday. Thereafter Bill will begin to pick up speed with the storm thoroughly caught in the deep-layer southerly flow and start to recurve towards the north-northeast, reaching Nova Scotia by Sunday evening and Newfoundland Monday morning.


The impacts of Bill along much of the East Coast will consist of rough surf, rip-currents, overwash and beach erosion. Mainly a concern of small craft and beach-goers. Along the New England Coast, and specifically Cape Cod, Nantucket, Martha’s Vineyard and along the coast of Downeast Maine, some of the other bands of Bill could move over the region bringing gusty winds, up to tropical storm force, and very heavy rainfall. Coastal tidal rises of a foot or two will occur beginning Saturday and lasting into Tuesday.

The impacts over Canada will be much more severe. Winds of up to hurricane force is expected over portions of Nova Scotia with tropical storm force winds in most other areas of the Maritimes. At the time of landfall a 5-10’ storm surge can be expected to the right(east) of the point of landfall with a 3-6 foot surge to the west. Rainfall of 2-5 inches can also be expected here as well, leading to flooding problems.

Impacts on Bermuda will mainly be felt on Saturday as a few rain bands spiral over the island bringing heavy rain and gusty winds. A few gusts could reach tropical storm force in some of the more intense squalls. Breakers along the shore will be in the 10-15' range so definitely not a day for scuba or small craft boating.


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Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.3 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

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Forecast Discussion


Synopsis - Issued - 8/21/09 @1:05am


The Northeast will dodge a bullet this weekend as Hurricane Bill passes a couple hundred miles to the east. The saving grace is an anomalously deep trough digging into the eastern half of the nation, deflecting Bill to the north just in time to spare the Northeast but give the Canadian Maritimes a direct impact from their strongest storm since Hurricane Juan of 2003. The trough, however, will present its own set of problems, mainly in the form of heavy rain and severe thunderstorms on both Friday and Saturday. An extremely moist, tropical airmass will overspread the Northeast in the deep southerly flow ahead of the approaching trough, keeping in mind the remnants of both Claudette and Ana are entrained in this deep moisture plume. Additionally, a predecessor rainfall event in conjunction with the recurvature of Bill over the western Atlantic will only serve to enhance rainfall, possibly leading to flooding. Scattered showers and storms will linger in the upper trough axis on Sunday before high pressure builds in on Monday to dry things out. It remains dry and seasonably warm through Tuesday before another trough moves in for Wednesday and Thursday with a cooler, drier airmass following in behind it.


Near-term - Issued - 8/21/09 @1:05am


A very active day of weather for some sections of the Northeast is beginning to wind down late this overnight. Several tornado warnings were issued this afternoon and evening across southeastern Pennsylvania and western New York. Additionally, several severe thunderstorm warnings had been issued, mainly for high wind knocking down trees and causing power outages. What’s left of today’s activity is across north-central Pennsylvania and central New York State in the form of rain and embedded thunderstorms. A few cells across north-central Pennsylvania are close to severe threshold but, for the most part, the storms have been weakening. Another smaller cluster of showers and thundershowers is entering southwestern Pennsylvania and the Laurel Highlands. Elsewhere skies are partly cloudy with very mild temperatures. 60’s are common across the interior with 70’s in the river valleys and along the coastal plain.

The showers and storms over New York and Pennsylvania will slowly weaken throughout the remainder of the night but hold together through daybreak as they head northeastwards towards the St.Lawrence Valley and the Adirondacks. Additional showers may enter western Pennsylvania from the Ohio Valley but most locations outside of these areas will remain dry through the overnight. Temperatures will only drop a couple more degrees from their current readings in the mid to upper 60’s across the interior to the low to mid 70’s along the coastal plain and low elevation river valleys. Locally dense, patchy fog may present a problem as well across some of the low-lying areas and river valleys. Be prepared for rapid changes in visibility tonight if driving in these areas.


Short-term - Issued - 8/21/09 @2:00am


The morning will break with the line of scattered showers and thundershowers across central and northern New York State with more isolated activity extending southwestward into central and southwestern Pennsylvania. Elsewhere across the region skies will be partly to mostly cloudy and hazy. Dewpoints across the region will be in the mid 60’s (north) to low to mid 70’s (south) so it will feel quite muggy. Across the western half of the region breaks of sun will only serve to destabilize the atmosphere and with the cap virtually gone as heights lowers in advance of the approaching trough convection will fire by late morning. mlCAPE will be in the 500-1,500J/kg range, so not overly unstable but with strong forcing provided by a passing mid-level disturbance and a surface cold front entering the region from the west, showers and thunderstorms will become widespread. Synoptic lift will be provided as well in the form of favorable placement in the right rear entrance region of a 100kt jet. Some of the storms will be severe with a strong low-level jet of 40-50kts whisking through the region and bulk shear in the 30-40kt range organizing storms into bowing line segments. Flow is unidirectional and becoming increasingly parallel to the front so training cells are possible. Precipitable water values range from 1.4-1.75” across the region so heavier downpours will occur capable of flash flooding in localized areas. Further east the airmass is vastly more tropical in nature, with precipitable water values at or above 2” across a wide stretch of the region. Temperatures here will be a few degrees warmer than neighbors to the west as a bit more sun will break through the clouds during the morning hours, hence the airmass will be more unstable, with CAPE values approaching 2,000J/kg! However, there will be a bit of a cap over the region, keeping a lid on convection until the afternoon and keeping it steamy along the coastal plain. The deep southwesterly flow over the mountains will induce the development of a lee-side trough during the afternoon. This trigger will give rise to rapidly building thunderstorm cells during the early afternoon hours from the lower Susquehanna Valley through the Poconos, Catskills and Adirondacks. These cells will evolve into a broken line of storms with bowing segments by later in the afternoon and evening. Winds aloft are similar to the western half of the region but with the deeper moisture in place and elevated 0-1km helicity, an isolated tornado cannot be ruled out in addition to the wind/rain threat. Hail seems less of an issue with very high freezing heights through any stronger updraft will be capable of producing at least small hail. As mentioned above, with precipitable water contents over 2” blinding downpours are possible. Highs today will range from the mid to upper 70’s across the western half of the region where precip will move in during the morning hours. The eastern half of the region will see a few more hours of sun allowing temperatures to rise into the 80’s.

The front begins to slow to a crawl Friday night as it runs into a narrow but strong ridge axis lined up between itself and Hurricane Bill to the east. Showers and storms will continue from eastern Pennsylvania, New Jersey, eastern New York and western New England, aided by a developing pre-event, a combination of mesoscale and synoptic features that give rise to enhanced rainfall ahead of recurving tropical systems. With the tropical airmass in place over the region, rainfall may be heavy at times and could add up to 1-2 inches overnight, with higher localized amounts. This type of rainfall may lead to some flash flooding concerns, especially for flashier creeks and low-lying/poor drainage areas. It will remain quite muggy throughout the overnight with low temperatures east of the front in the upper 60’s to mid 70’s! West of the front precipitation will end and patchy fog will develop in the valleys. Skies will be clouds and lows will range from the mid to upper 60’s.

The front remains stalled along the Eastern Seaboard on Saturday as Hurricane Bill approaches from the southeast. Better moisture convergence along the front will cause precipitation to expand once again and become quite heavy, with an additional 1-2 inches of rain along the frontal axis. Modest insolation through the cloud cover will destabilize the atmosphere with mlCAPE in the 500-1,000J/kg range so convection will fire up causing pockets of intense downpours that could easily drop 1-2” of rain in a short period of time. Localized areas will see more than 3 inches of rain in slow-moving and/or training cells. This, of course, will lead to flash flooding problems and the excessive rainfall over the entirety of the event will cause sharp rises on tributaries and main stem rivers, some of which may go into minor flood. Flows on some area rivers are still running above seasonal averages for this time of year from the heavy rainfall during the first half of the month. Highs will range from the 70’s where precipitation remains throughout the day to the 80’s outside of these areas.



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Radar: Northeast Region Loop

NE radar
Fig.4 - Radar loop of the Northeast region. Courtesy of Weather Underground.


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Local SST's

Northeast SST's
Fig.5 Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


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Updated: 6:02 AM GMT on August 21, 2009

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Bill weakens temporarily

By: sullivanweather, 2:04 PM GMT on August 20, 2009

Tropical Update



Fig.1 - Current satellite picture of the Northwest Atlantic Ocean. (Credit:NOAA)


There’s only one feature worth discussing in the tropical Atlantic Basin this morning and that is Hurricane Bill. During the overnight Bill’s eyewall opened for a time as a modest burst of westerly wind shear was able to cut into the core of the system and disrupt the circulation. In effect, the storm weakened some from a category 4 to a category 3. This is purely cosmetic as Bill remains an extremely dangerous hurricane and the weakening phase was only temporary. Recent satellite imagery shows an expansion of cirrus outflow to the west of the storm indicating a weakening of the shear and a renewed period of intensification should be right around the corner.



As of the 5am advisory issued by the National Hurricane Center, the center of Hurricane Bill was located at 21.6°N and 60.3°W, a safe distance from the Leeward Islands about 350 miles to the south-southwest of the storm. Maximum sustained winds were around 125mph with higher gusts and the center pressure was estimated at 949mb. Bill continues to move at a fairly brisk pace, around 18mph to the northwest.




Fig.2 - Current satellite picture of Hurricane Bill.
(Credit:NOAA)



Bill continues to display text-book form and is quite a sight, even on full-disk imagery. A closer view reveals Bill dominates the central Atlantic scene. A very large and ominous storm Bill continues to be with hurricane force winds extending up to 85 miles from the center with tropical storm force winds an astonishing 230 miles away from the center. Upper-level venting, in the form of cirrus clouds, are expanding away from the storm in all directions, with a strong outflow channel extending off to the northeast of the storm then rocketing southward, anti-cyclonically towards South America. There’s little to speak of in the storms wake except for a thin line of convergence that extends back over 1,000 miles to south of the Cape Verde Island where it connects with the next wave emerging off the African Coast. After battling some shear, it now appears that the storm is starting to regroup, with the central dense overcast becoming slightly more symmetric than several hours past and the eye placed closer towards the center. Additionally, cloud tops have chilled some since the overnight as well, now around -65 to -70°C.

The next 12-24 hours should see another round of intensification as the upper environment surrounding Bill becomes more conducive for development with the westerly shear relaxing and highly diffluent flow developing in conjunction with an upper-level ridge located to the northeast of the cyclone. The storm continues to move over waters that are 28-29°C and to a sufficient depth to mitigate upwelling. By afternoon or late evening Bill should once again become a category 4 storm and may see an additional 10-15mb drop in pressure. This favorable upper environment should continue to reside over Bill for the next 36-48 hours so fluctuations in intensity through early Saturday morning should be controlled by inner-core re-structuring and eyewall replacement cycles. Later in the morning on Saturday Bill will be reaching the western periphery of the deep-layer ridge centered at 38°N 60°W, give or take a few ticks, all the while the longwave trough pivoting through the Eastern US will reach the coast. Southerly flow between these two features will increase and Bill will begin to undergo 15-25kts of shear. However, the storm will also be accelerating to the north at this time, which will dampen the effect of the shear on Bill but will still be the harbinger of a weakening trend. Bill should drop to a category 3 storm by Saturday afternoon and a category 2 storm by Sunday afternoon, by which time the storm will only be within a few hundred miles of New England. Late Sunday night into Monday Bill will interact with the baroclinic zone owning to the longwave trough moving off the Eastern Seaboard. This will signal the end of Bill’s life as a tropical cyclone but Bill will continue to remain a very powerful extra-tropical cyclone. The transition to extra-tropical will also expand Bill’s wind field so large portions of the Canadian Maritimes are expected to see at least tropical storm force winds.

The track of Hurricane Bill is well-agreed upon by most global, regional and storm models; a big sweeping parabolic hook, taking the storm between Bermuda and the East Coast of the United States than swiftly moving it through the Canadian Maritimes and off to merge with the semi-permanent Icelandic Low. This track has Bill continuing on its northwesterly course over the next 24-36 hours then gradually swinging towards a more northerly direction as it rounds the western flanks of the deep-layer ridge located just northeast of Bermuda. Thereafter, the storm becomes caught in the deep south-southeasterly flow ahead of the approaching trough which should accelerate the storm northeastwards. There are, however, subtle differences within the model suite, with several models calling for a more rapid advance of the contiguous US trough, hence a faster turn to the north and northeast. Meanwhile, others are forecasting the trough to dig some before swinging eastward, allowing Bill to track a couple extra degrees of longitude west. A completely plausible scenario, and one that would put southern New England and extreme Downeast Maine at risk for catching some of the fringe effects of Bill. In fact, the outer Cape and Nantucket could see conditions approach hurricane force should one of the more western solutions verify, a la Noel. At most risk from Bill is Nova Scotia and Newfoundland. More than half the models take Bill over either of these two areas and does so at hurricane strength, at least for Nova Scotia.

The impacts of Bill along much of the East Coast will consist of rough surf, rip-currents, overwash and beach erosion. Mainly a concern of small craft and beach-goers. Along the New England Coast, and specifically Cape Cod, Nantucket, Martha’s Vineyard and along the coast of Downeast Maine, some of the other bands of Bill could move over the region bringing gusty winds, up to tropical storm force, and very heavy rainfall. Coastal tidal rises of a foot or two will occur beginning Saturday and lasting into Tuesday.

The impacts over Canada will be much more severe. Winds of up to hurricane force is expected over portions of Nova Scotia with tropical storm force winds in most other areas of the Maritimes. At the time of landfall a 5-10’ storm surge can be expected to the right(east) of the point of landfall with a 3-6 foot surge to the west. Rainfall of 2-5 inches can also be expected here as well, leading to flooding problems.

Impacts on Bermuda will mainly be felt on Saturday as a few rain bands spiral over the island bringing heavy rain and gusty winds. A few gusts could reach tropical storm force in some of the more intense squalls. Breakers along the shore will be in the 10-15' range so definitely not a day for scuba or small craft boating.



Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.3 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

--------

Forecast Discussion


Coming later



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Radar: Northeast Region Loop

NE radar
Fig.4 - Radar loop of the Northeast region. Courtesy of Weather Underground.


___________________________________________________________

Local SST's

Northeast SST's
Fig.5 Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


___________________________________________________________



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Updated: 2:15 PM GMT on August 20, 2009

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Hurricane Bill poised to become seasons' first major storm.

By: sullivanweather, 6:47 AM GMT on August 18, 2009

Tropical Update



Fig.1 - Current satellite picture of the tropical North Atlantic Ocean. (Credit:NOAA)


The tropical Atlantic lost two named systems yesterday as Claudette came ashore Florida and degenerated into a remnant low and Ana degenerated into an open wave over the eastern Caribbean as strong trades winds were too much for the small weak circulation to overcome. Bill, conversely, has continued to grow in strength and is now a category two hurricane packing maximum sustained winds of 100mph as it churns over the central Atlantic. Bill is on the cusp of becoming the season’s first major hurricane and hurricane hunter reconnaissance aircraft will begin flying missions into and around the storm tomorrow to get a better picture of Bill’s strength and the surrounding environment which will ultimately dictate where Bill will track.




Fig.2 - Current satellite picture of Hurricane Bill.
(Credit:NOAA)



As of 11pm AST the National Hurricane Center advisory places the center of Hurricane Bill at 15.0°N 48.3°W, about 865 miles east of the Lesser Antilles. Maximum sustained winds are now up to 100 mph and the estimated central pressure has dropped to 967mb. Bill continues to move towards the west-northwest around 17-18mph, little changed over the last 24 hours.

Bill is on the large side for a tropical cyclone, with tropical storm force winds now extending up to 150 miles away from the center. This very large size of Bill has caused a slow organization/intensification process with the storm occasionally ingesting relatively drier, more stable air from the north of the cyclone causing the inner-core of the storm to be disrupted. Sea-surface temperatures along the track of Bill up until this point have been around 27°C, so not overly favorable for rapid intensification in that regard as well. On occasion, a ragged eye has tried to form and microwave imagery from earlier this evening showed concentric ‘eyewalls’. The inner eyewall was open along the southern side of the storm while the outer eyewall wrapped around the western semi-circle of the storm then arcing out into a curved band of convection along the southern flanks of the storm. Meanwhile, the upper environment over Bill couldn’t get much better with diffluent flow over the storm and excellent outflow expanding in all quadrants of the storm.

The last satellite frames before the eclipse showed Bill was finally beginning to draw convection in towards the center of the storm, perhaps the first signs of a strong inner-core of the storm forming. Should this process continue through the remainder of the overnight and into the morning hours on Tuesday a period of rapid intensification may ensue as dry air infiltrating the core of the system gets shunted out and a closed eyewall develops. This would put Bill on track to become a major hurricane by the afternoon hours Tuesday, a status that may be maintained for several days as an upper-level anti-cyclone is expected to persist over top of Bill for the next 48-60 hours. During this time it is expected that Bill will peak in intensity, likely as a category 4 storm somewhere between Bermuda and Puerto Rico. By Thursday Bill will begin to reach the western periphery of the deep-layer ridge to the north as an upper trough approaches from the northwest. Southwesterly shear will begin to impact the system and Bill should begin to weaken some, but retain major hurricane strength into the weekend.

The track forecast for Bill isn’t as straight forward as quite a few variables will come into play. The current movement to the west-northwest should continue over the next 24-48 hours with slight wobbles to the west as the system is caught in the easterlies to the south of deep-layer ridging. Thereafter, an upper level trough that will move to a position near 65°W by Wednesday evening will begin to erode the western periphery of the ridge. The effect on Bill will be to draw the storm towards the weakness provided by the upper trough, forcing a northwestward movement. This is good news for the northern Leeward Islands which may still be skirted by some of the fringe effects of Bill but certainly won’t see a direct impact. The upper trough is expected to shear out by Thursday and Friday but not before dragging Bill into the Bermuda Triangle on a northwesterly to north-northwesterly course. This will leave the storm in a vulnerable position to be picked up by any ensuing troughs to move off the East Coast in the following days and luckily, there is one in the waiting to move off the coast this weekend. This second, deeper trough will be of sufficient strength to recurve Bill but not before a close call for the Northeastern US and certainly Nova Scotia and Newfoundland may be in the crosshairs. Bermuda cannot be discounted as an eventual destination for Bill as well should the first trough pack more punch than currently indicated. The chances for a direct impact along the East Coast of the US is less than 10% but all residents from North Carolina to Maine should keep tabs on the very latest info concerning Bill.


Elsewhere across the tropical Atlantic Ana’s remnants are crossing the Island of Hispaniola and should generate locally heavy rains there over the next 24-36 hours before moving across Cuba. This system may possibly regenerate upon moving into the southern Gulf of Mexico or the Bay of Campeche in about 3-4 days and will be monitored for development. A tropical wave with a northwest-southeast axis is along 33°W south of 17°N in the deep tropics, west of the Cape Verde Islands. Scattered moderate convection is lined up along the wave axis from 13-15°N and 30-33°W. Another tropical wave is poised to move off the coast of Africa over the next 24 hours with some models hinting at development.





Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.3 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

--------

Forecast Discussion


Coming later



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Radar: Northeast Region Loop

NE radar
Fig.4 - Radar loop of the Northeast region. Courtesy of Weather Underground.


___________________________________________________________

Local SST's

Northeast SST's
Fig.5 Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


___________________________________________________________



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Updated: 6:54 AM GMT on August 18, 2009

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Bill almost a hurricane

By: sullivanweather, 5:03 AM GMT on August 17, 2009

Tropical Update



Fig.1 - Current satellite picture of the tropical North Atlantic Ocean. (Credit:NOAA)


According to climatology, the Atlantic hurricane season sees its third storm named by August 20th, however, a week ago the question was whether we’d see even one by that time. But a lot can change in one week in August in the tropical Atlantic and suddenly, after the emergence of a couple strong African waves, the first and second named storms of the season formed in the Central Atlantic. In addition, a surface trough draped over the Gulf of Mexico interacting with a tropical wave and an upper level disturbance gave rise to the seasons’ third named system this afternoon, the 16th day of August. A very quick ascent from virtually nothing to speak of a few days ago to ahead of the climatology curve a few days ahead of schedule. It is worth noting, however, that since the start of a predominately +AMO multi-decadal signal and increased Atlantic tropical cyclone activity) the average number of named storms by this date is 4.2 over the 14 year period spanning 1995-2008.



Fig.2 - Current satellite picture of Tropical Storm Claudette.
(Credit:NOAA)

Of most concern currently, because of its position just offshore the Panhandle of Florida, is Tropical Storm Claudette. As of the 10pm CDT NHC advisory Claudette is about 25 miles west of Panama City packing winds of 45mph with a central pressure of 1008mb. Forward movement is to the northwest around 12mph, though over the past several hours one could argue for a west-northwesterly motion that has paralleled the coast. This continued motion is expected over the next 3-5 hours, which would place landfall very close to Fort Walton Beach around 12AM CDT.

Claudette formed very quickly as the northern end of a tropical wave moved into the Florida Straits yesterday and interacted with an upper trough over the region and a surface trough draped over the eastern Gulf of Mexico. Models had been hinting at possible tropical development in this region for several days from the above mentioned features and the first signs that something may be forming came yesterday morning. Doppler radar out of Miami and Key West showed an area of vorticity within the overall convective field along the trough axis that moved across the Keys and into the southeastern Gulf of Mexico. Eventually, a surface low formed from this region and was classified a depression this morning. Once reconnaissance investigated the depression winds were found to be sustained around 45mph and Claudette was named.

Most of the deep convection associated with Claudette is contained within the eastern semicircle of the storm with only some isolated showery activity located near the center of circulation as it has become exposed due to shearing winds from an upper trough located over the east-central Appalachians. The strongest winds at the surface (and aloft) are located under the band of convection and this is where tropical storm force winds may impact the coast, from Fort Walton Beach to Mexico Beach, in the heaviest convective elements. As far as highest impact from Claudette, that would have to be rainfall. The convection associated with Claudette is producing rainfall rates of 1-3”/hr, which could easily lead to flash flooding while total rainfall could range from 4-8” across the Florida Panhandle to southern Alabama. As with any landfalling tropical cyclone, the frictional component of the land interacting with the lower-level vorticity provided by the circulation of the cyclone will present the threat of isolated tornados.

Claudette will spin down over the Southeast after making landfall over the next two days, producing scattered showers and thunderstorms that will be most prevalent during the afternoon and evening hours. These storms, in the rich tropical airmass, will continue to produce extremely heavy short-term rainfall rates which could lead to flash flooding and the threat for isolated tornados.



Fig.3 - Current satellite picture of Tropical Depression Ana.
(Credit:NOAA)


Next in line is Tropical Depression Ana. Ana has been barely hanging on as a tropical cyclone and may be dropped soon as the circulation has deteriorated over the last 12-18 hours and satellite presentation now resembles an open wave. Radar animations show what can best be described as the ragged center of circulation passing between Guadeloupe and Montserrat. Convection has flared some this overnight but the system continues to be enveloped by dry air with little moisture seen in water vapor imagery across the eastern Caribbean.

As of the 11pm AST NHC advisory, Ana is located 16.0°N and 61.2°W. Maximum sustained winds are around 35mph and the central pressure is 1008mb. Ana is rocketing off to the west-northwest around 26mph, thoroughly caught in the trades south of a strong deep-layer sub-tropical ridge. Light to moderate southwesterly shear is also impacting Ana from the large upper level low located over the Bahamas. The forecast calls for Ana to maintain strength over the Caribbean before interacting with the island of Hispaniola and degenerating into an open wave. Despite the short-term prognosis for Ana being death there is potential for this system to regenerate (again) once clear of the Greater Antilles and into the Gulf in about 4 days.




Fig.4 - Current satellite picture of Tropical Storm Bill.
(Credit:NOAA)




By far the most impressive storm of the group and also of this young Atlantic hurricane season is Tropical Storm Bill. Spawned from a high-amplitude tropical wave, Bill has been slowly consolidating since becoming a depression on Saturday. The structure of the storm was banded until this afternoon when the first signs of a developing central dense overcast began to appear. Now with the inner core of Bill in tact, steady strengthening has occurred and all signs point towards continued intensification over the next several days. Currently, Bill is very close to hurricane strength with maximum sustained winds of near 70mph and an estimated central pressure of 990mb. Bill is located near 13.4°N and 41.7°W, about halfway between the coast of Africa and the Lesser Antilles, and is moving west-northwest around 20mph, all as of the 11pm AST NHC advisory.

As alluded to above, the structure of Bill improved markedly during the last 12 hours with the development of the CDO. Additionally, Bill is becoming more symmetrical with time, as the center of circulation is tucked right in the middle of the newly formed CDO with strong convection in all quadrants and good outflow in all quadrants except for the western quadrant where it is fair. Strong banding is also evident outside of the main CDO wrapping up the eastern and northern semicircle of the storm with a strong moisture feed/inflow from the ITCZ. All are indicators of a text-book Cape Verde development and points towards a future classic Cape Verde storm.

Bill should continue to move in a general west to west-northwest motion around 18-20mph over the next 24-48 hours as it spins south of a deep-layer ridge axis forecast to remain parked along 25°N over the course of the next 3 days. In fact, this motion may be more westerly in the near-term as the angular momentum of the strong convective banding wrapping into the eastern and northern semicircle of the storm helps to drive it west. On this track, Bill may come close enough to the northeastern Leeward Islands to skirt them with tropical storm conditions, keeping in mind that Bill should be a major ‘cane by then. After 72 hours Bill will begin to approach a weakness in the deep-layer ridging as an upper low is expected to move to a position very near Bermuda; this is the same upper low currently over the east-central Appalachians shearing Claudette. In response to this weakness Bill is expected to take on a more northwesterly course, followed by a turn to the north-northwest in days 4 and 5. How sharp of a turn Bill takes will be depending on several factors, including the strength of the upper low and the track Bill decides to take in the short-term. A trough moving off the East Coast days 6 and 7 should bring about recurvature but until this occurs all coastal areas from Florida to Canada, including the Bahamas and Bermuda cannot be eliminated from the threat of Bill yet.

The intensity forecast for Bill calls for a steady gain in strength over the next 48-72 hours. The cyclone lies in a moisture-rich environment with a very large circulation surrounding the storm, which should fend off any intrusions of drier air located far to the north and northwest. There’s little shear to speak of and the cyclone will be moving over increasingly warmer waters as it heads west. Bill should become the seasons’ first hurricane later this morning and reach category 2 strength by Tuesday. There’s a chance that Bill could peak as a category 4 storm while moving to the northeast of the Lesser Antilles before shearing winds begin to cut into the circulation of Bill as it heads north of Puerto Rico. This shear should weaken Bill and cause the storm to become more ragged in appearance but it should remain a powerful storm at the end of day 5.

Several more waves will move off the African Coast over the next 3-5 days and forecast models are calling for development from at least one of these features. The tropics are certainly heating up.




Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.3 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

--------

Forecast Discussion


Coming later



___________________________________________________________


Radar: Northeast Region Loop

NE radar
Fig.4 - Radar loop of the Northeast region. Courtesy of Weather Underground.


___________________________________________________________

Local SST's

Northeast SST's
Fig.5 Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


___________________________________________________________



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Tropical Depression #2 and the Caboose

By: sullivanweather, 3:42 PM GMT on August 12, 2009

Tropical Update



Fig.1 - Current satellite picture of the tropical North Atlantic Ocean. (Credit:NOAA)


Tropical Depression #2 formed yesterday morning southwest of the Cape Verde Islands and is on the cusp of becoming the Atlantic season’s first named system. A ball of deep convection has roughly maintained intensity since developing last night but over the last several hours stronger shearing winds from the east have begun to show signs of affecting TD#2 by displacing most of the convection to the western semicircle of the circulation. Dually noted is the entrainment of drier, more stable air to the north and northwest of the depression, which has cause a slight weakening of convection in the northwest quadrant. Despite these modest inhibiting factors TD#2 should become Ana by this evening as QuikSCAT has pinged a few tropical storm force wind barbs, though rain contaminated, in its ascending pass early this morning.



Fig.2 - Current satellite picture of Tropical Depression #2.
(Credit:NOAA)

With the expectation of this system becoming Ana by this evening, the following 24-36 hours looks to be a period of treading water as bouts of moderate easterly shear impact the system and some intake of drier, more stable air to the north hinder development beyond a minimal tropical storm. Thereafter conditions become slightly more favorable for slow strengthening, assuming it survives the shear/SAL intrusion. Sea-surface temperatures warm above 27°C and the atmospheric column will moisten, dampening the SAL . However, shear will be ever present; this time from the west thanks to an upper trough forecast to dip into the sub-tropical central Atlantic. Depending on how much latitude is gained over the next 72 hours will determine how much of an effect this upper trough will have on TD#2/(Ana?) as much stronger shear will run north of the 18th Parallel. Current thinking is the easterly shear encountered over the next 24-36 hours will keep the surface circulation moving along with the easterly trades, keeping the system further south and out of the stronger shear. This further south track also puts the northeastern Leeward Islands at risk for direct impact from this system in about 5 days. Residents there should make preliminary preparations and are urged to stay up to date with the very latest information concerning this system.

The two waves highlighted yesterday between 50°W and 60°W have shown no signs of development over the last 24-30 hours and, in fact, did weaken some. A lone convective cell lies along the leading wave axis, now along 65°W, south of 21°N, just north of the South American Coast with just a few isolated showers along the remainder of the wave axis. This leading wave is moving quite slowly, chugging westward around 10mph. The trailing wave is still tilted northwest to southeast, along 58-61°W, south of 22°N. Widely scattered weak to moderate convection exists along the wave axis, with a higher concentration of convection along the northern end of this wave in a more highly sheared environment. This second wave is moving a bit faster than the wave preceding it and may eventually merge with the leading wave in a few days. A few speculative models even develop a system in the Gulf of Mexico this weekend as these waves link up with a pre-existing surface trough forecast to be draped across the Gulf. Waters in the Gulf are very warm, nearly 30°C basin-wide, so should something develop it will have the opportunity to spin up quickly.

If that’s not all in this suddenly active Atlantic Basin, what’s perhaps more ominous than TD#2 is the new wave which emerged off the African Coast over the last 12-24 hours. This is a very high amplitude tropical wave currently along 20°W, south of 22°N and is moving west around 18mph. Moderate to strong convection is running out ahead of the wave axis between 12-15°N and 21-24°W with scattered moderate convective flare ups within the ITCZ along the southern flanks of the wave. Additionally, broad cyclonic turning is noted in the low cloud field, circulating around a presumed area of low pressure along the wave axis located near 12°N. Satellite presentation is of a text-book seedling and this system has good potential to become the next depression and soon after, a named system. Conditions are ripe for development with low shear and high absolute humidity and with this system being of a lower latitude than its predecessor it will benefit from warmer sea-surface temperatures of nearly 28°C along its projected track. Most global models develop this wave into a named system and do so quite vigorously, taking it to hurricane strength in about 5 days and bringing it close to the Leeward Islands in one week. Once again, it is worth mentioning that forecast steering patterns in week 2 will bring whatever develops in the Atlantic very close to the United States, namely from Florida and on up the East Coast. This would be a good time for residents of these areas to review their disaster plans and make sure needed supplies in an emergency situation, such as bottled water and batteries, aren’t in short supply.

Elsewhere in the Atlantic a weak tropical wave is interacting with an upper level trough over the Yucatan and waters of the western Caribbean, producing scattered weak to moderate convection. Otherwise deep-layer ridging dominates much of the scene over the Atlantic with two areas noted. One just off the Southeast Coast and another extending over much of the sub-tropical central Atlantic east of 55°W. Between these two ridges is a surface trough extending southwest to northeast between 25-35°N, 50-70°W marked by scattered convection. There also exists one upper level low located at 23°N, 62°W and an upper trough along 45°W. The latter is what may eventually shear TD#2 in about 2-3 days time as it digs towards the southwest. Lastly of note is the trough currently moving of the East Coast. This trough is forecast to stall and eventually wash out as it runs into that strong ridge located off the Southeast Coast.




Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.3 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

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Forecast Discussion


In the Northeast a rather active day is expected as a surface trough running along the coastal plain and an upper disturbance pulling through ignites scattered convection. Modestly warm/moist airmass is in place this Wednesday with dewpoints up into the 60’s for most locales, save northern New England where dewpoints are in the mid to upper 50’s and temperatures that will rise into the 70’s and 80’s from north to south. Filtered sunshine is making its way through the mid/upper-level cloud deck allowing for enough insolation to adequately destabilize the atmosphere. Isolated to scattered convection is already underway, primarily terrain enhanced across the interior, although a few stronger cells have popped along a theta-e ridge extending north-south across central New Jersey. mlCAPE values will rise to 500-1,500J/kg this afternoon across the Northeast with lifted indices dropping to -2°C to -4°C. Additionally, colder air has moved in aloft, -10°C to -12°C @500mb, helping to maintain higher lapse rates in the 1000-500mb layer during peak heating. Winds aloft are quite weak within the trough axis, generally under 15kts to 500mb, so storms will be slow movers. With precipitable water contents of ~1.5” across the southeastern half of the region the slow movement of these storms may pose a threat for flash flooding. These weaker winds aloft and lowered effective shear will cause storms to be of the pulse variety today with severe weather limited to high winds in collapsing cells and large hail in the strongest cells which are able to take advantage of the cold pool aloft.

The loss of insolation will bring about an end to much of the convection across the interior during the evening hours, however, along the coastal plain convection may continue a bit longer into the evening in the higher theta-e air and given the proximity to the surface trough. During the overnight, just offshore, a ripple of weak low pressure will move along the surface trough supported by a mid-level disturbance moving off Delmarva. This should ignite an MCS that may track close enough to brush Long Island very late in the overnight into early Thursday morning. Otherwise, expect partly to mostly cloudy skies with a stray shower or two not out of the realm from I-90 south given the region’s position within the trough axis. It will continue muggy along the coast but the airmass will start to become more comfortable across the northern interior as high pressure builds down into Maine. Lows will range from the upper 40 to mid 50’s over northern New England with 60’s common across the remainder of the interior. Along the coastal plain thicker clouds and higher humidity will keep temperatures held in the low 70’s.




___________________________________________________________


Radar: Northeast Region Loop

NE radar
Fig.4 - Radar loop of the Northeast region. Courtesy of Weather Underground.


___________________________________________________________

Local SST's

Northeast SST's
Fig.5 Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


___________________________________________________________



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Updated: 4:27 PM GMT on August 12, 2009

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Pop goes the tropics

By: sullivanweather, 6:28 AM GMT on August 11, 2009

Tropical Update






Activity is beginning to pick up in the tropical Atlantic Basin as we ramp up towards the peak of the Atlantic Hurricane Season in about a month. Thus far the tropical Atlantic Basin has yet to produce a named system and has yielded only one depression, a short-lived system that formed off the Chesapeake in late May and dissipated the very next day. Since then there’s been barely anything to speak of with invests only slapped on a few sub-tropical in appearance areas of low pressure and a couple of modest waves. But all that’s changing this second week of August as the strongest African Easterly Wave of the season to date emerged off the cast a couple days ago with another strong wave on its heels. Additionally, there’s a couple areas of interest closer to the Lesser Antilles.

The overall picture of the tropical Atlantic paints a scene that’s becoming more favorable for development, not surprising as we head into the climatologically favored part of the season. Upper level troughiness, which has produced unrelenting shear across the main development region throughout much of the season, has relaxed and been replaced by broad upper ridging and lessened shear. One upper level low is over the western Yucatan and the southern Bay of Campeche and is slowly pulling west. A second upper level trough is positioned over the central Atlantic with a SSW-NNE axis between 45-50°W. This upper trough is of fairly low amplitude and only extends down to roughly 30°N. As mentioned above, broad ridging now extends across much of the sub-tropical Atlantic from just south of the Azores to the eastern Greater Antilles. Closer to home a 590dm+ 500mb ridge is positioned over the Southeast with a sphere of influence that covers the western Atlantic to Bermuda. Between the Southeast ridge and the Yucatan trough is an axis of weak convergence extending from the central Gulf of Mexico to the Bahamas with scattered moderate convection firing along it.

In the deep tropics between the Lesser Antilles and Africa is where things are beginning to heat up with three areas of interest currently churning over the warm Atlantic waters. The first is associated with a tropical wave that’s currently along 60°W and south of 18°N. A very ill-defined and broad 850-700mb low is along this wave axis positioned near St.Vincent and is producing scattered moderate to isolated strong convection in intermittent bursts. This wave is expected to move into the eastern Caribbean without much in the way of development expected over the next 48-72 hours as it interacts with the South American landmass. Conditions become more favorable as this wave heads into the western Caribbean but feeling is that this will cross over into the Pacific without development here in the Atlantic.

The second wave is about 10ticks to the east and is oriented on a northwest to southeast axis with broad cyclonic turning noted around 12.5°N, 48°W. Scattered weak to moderate convection extends in a broken line to the northwest of the presumed area of low pressure with only some isolated showers popping within the cyclonic field. Only slow development of this feature is expected over the next 24-48 hours as it battles a bit of shearing winds and a relatively dry surrounding environment. An increase in coverage of squally weather can be expected as this wave moves through the Antilles in 36-48 hours.

This third, and most impressive feature of the group, is a strong tropical wave that emerged off the African Coast two days ago. This wave axis lies just west of the Cape Verde Islands along 28°W, south of 20°N. Along this wave axis is a well-defined area of low pressure just south of 15°N and registering a roughly 1012-1013mb central pressure. Flare ups of strong convection has occurred each overnight as we approach diurnal max and tonight is no exception, with a round ball of deep convection now firing over the center of circulation. In addition, the wave axis is now starting to slightly out pace the forward movement of the low pressure, moistening the atmosphere along its track and limiting the dry air inflow from the SAL to the north and west. The broad upper level ridge axis over the sub-tropical Atlantic is sending some light northeasterly shear through the disturbance but is having little effect on the continued development of this wave. I fully expect this system to become the second tropical depression of the 2009 Atlantic Hurricane Season sometime over the next 24-48 hours as upper level shear remains on the light side and in the direction of the systems’ forward movement and the atmosphere should contain enough moisture along the projected path to maintain convection. There’s even a slight chance we see our first named system by Wednesday evening.

This system should continue on a general west to west-northwest track over the course of the next 5 days, slowly gaining forward speed from the snail-like 10mph now to roughly 15mph by Thursday afternoon. The waters this wave is traversing are currently running around 26.5-27°C and should remain near those temperatures over the next 2-3 days. Beyond that time the waters increasingly warm west of 50°W to 28°C and climb to 29°C around the islands. It’s too soon to definitively say this system will move north of the islands, though is it starting on a more northerly latitude than one would expect for a threat to the islands. However, strong upper level ridging is forecast by global models to develop along 20°N by the end of the week which could help to suppress the system to the south. Residents across the islands are urged to keep abreast of the very latest concerning this system. Just throwing it out there but the GFS model also has been consistent in giving the East Coast of the US a brush from this system in about 12 days. Just a little something to keep an eye on in the coming week.


Update!!

The National Hurricane Center has designated the disturbance southwest of the Cape Verde Islands as Tropical Depression #2 of the 2009 Atlantic Hurricane Season. Current NHC forecasts take Tropical Depression #2 to tropical storm strength early Wednesday morning and continue to slowly strengthen it thereafter but caution that environmental factors 3-5 days out is not well agreed upon amongst the latest model suite as they take divergent courses in both track and intensity.



Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.10 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

--------

Forecast Discussion

Coming later



___________________________________________________________


Radar: Northeast Region Loop

NE radar
Fig.11 - Radar loop of the Northeast region. Courtesy of Weather Underground.


___________________________________________________________

Local SST's

Northeast SST's
Fig.12 Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


___________________________________________________________



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Updated: 11:17 AM GMT on August 11, 2009

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Cha-cha-cha-changes!

By: sullivanweather, 2:25 PM GMT on August 04, 2009



Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.10 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

--------

Forecast Discussion


Synopsis - Issued - 8/04 @10:20am

The trough that has more or less been a constant feature over the eastern half of the nation since the end of May will see its long stretch of plaguing our weather come to an end by the weekend as building heights push eastward. This will be in response to the first significant trough moving into the Western US since June. Unfortunately, the weekend is still four days away and the ever-present trough still has several more disturbances lined up to round out the work week, providing the opportunity for showers and thunderstorms, mainly during the afternoon and evening.


Short-term - Issued - 8/04 @10:20am


A pleasant start to the day for most locales across the Northeast with mostly sunny skies and temperatures ranging from the mid to upper 60’s along the coastal plain with mid 50’s to low 60’s over the interior. There are a few minor exceptions, however, and mainly concerning locally dense valley fog from central Pennsylvania to the Connecticut River valley in southern New England. Stratus/marine layer is also making a gloomy go of it from the Twin Forks of Long Island and for several locations along the New England coast. Besides the fog some mid/high-level clouds are streaming northwards from northeast Pennsylvania to the Adirondacks, filtering the morning sun. There could even be a few sprinkles around the St.Lawrence Valley under the thicker canopy of this cloud deck.

Deep-layer flow from the southwest will advect a warmer airmass into the region as 850mb temps rise to 15-17°C by day’s end. Combined with copious sunshine, temperatures will have no trouble pushing into the 80’s today across most areas. There may even be a spot 90°F degree reading or two across the urban corridor from Philadelphia to New York City but with the very wet ground that’s currently in place belief is these will be few and far between. Dewpoints will be on the rise as well, reaching the low to mid 60’s by afternoon so it will definitely feel like summer. The increase in heat and humidity will also create a modestly unstable atmosphere, with mlCAPE of roughly 750-1,200J/kg during peak heating. The impetus for convection will come from several areas today but neither one being a particularly efficient producer. Weak convergence in the low-levels thanks to a lee-side trough east of the Appalachians should provide a trigger for convection in this region while areas further north and west see better forcing due to their closer proximity to the mid-level disturbance. Any storms that do develop should be of the garden variety and not reach severe limits. Winds aloft are rather weak and wet-bulb zero heights are rather high, generally above 12,000’asl. Shear will be under 20kts so storms should be pulse-like and not able to maintain themselves over longer distances. Precipitable water contents will rise to nearly 1.5” so heavy downpours will be the main threat from these storms, especially should one pop over one of the more saturated parts of the region, of which there are many.

The loss of insolation will cause most convection to weaken during the evening hours, especially across the coastal plain. Further north and west the approach of a more defined trough and increasing winds aloft will keep the threat for showers and thunderstorms going throughout the overnight. Skies will be partly cloudy for most with lows dropping into the mid 60’s to low 70’s along the coastal plain. The interior will get slightly cooler, mainly in the upper 50’s to low 60’s but the increase in clouds should keep temperatures several degrees warmer than those of the night previous.

Chances for showers and thunderstorms increase on Wednesday as a weak cold front dips into the Northeast from Canada. Broad southwesterly flow will increase ahead of this boundary, continuing to pump warm, humid air into the region. Dewpoints rise into the upper 60’s to near 70°F as moisture pools ahead of the front. This will lead to a more buoyant atmosphere with mlCAPE values approaching 2,000J/kg during peak heating and lifted indices reducing to -2 to -4. Winds increase aloft to nearly 40-50kts in the 700-500mb layer leading to bulk shear of over 30kts. The Northeast will also come under better dynamics in the right rear entrance region of a modest 80kt jet over New England. That said, the potential for severe weather is there, mainly from high winds in bowing line segments that develop from stronger cells that manage to capture those stronger winds aloft. One deterring factor is that the trough will be lined up to the deep-layer flow from the west, limiting convergence along its flanks and out ahead of it. Temperatures will be similar to those of today across the southeastern half of the region in the warm sector with highs in the mid to upper 80’s with some low 90’s in the urban corridor. To the north and west clouds will have more of a presence and cooler air will be filtering in behind the front so temperatures will remain in the 70’s to low 80’s.

The front pushes off the coast Wednesday night with clearing skies and noticeably drier air dropping south. Across the interior it will be another great night to open the window and let in some crisp late summer air. Meanwhile, along the coastal plain, it will take until the overnight to get the good sleeping weather to press in, so the a/c may be needed at bedtime. Lows here along the coast will only drop to the mid to upper 60’s. As one heads inland there will be a fast drop off in temperatures with 50’s for most and 40’s over the higher terrain and northern New England.


Mid-term - Issued - 8/04 @10:20am


The longwave pattern over North America begins to show signs of breaking down and reorganizing during the mid-term period. Out West, a deep cut-off low off the California Coast moves inland, forcing the western ridge to build east into the Plains and eventually the Mississippi Valley region. Here in the Northeast the trough that has been a semi-permanent feature for most of the summer also progresses east, becoming squeezed between the building ridge to the west and the Bermuda high to the east, which will be giving little ground. The end result is the trough axis moving to a position over the eastern Great Lakes by Thursday and along the East Coast by Friday. Unfortunately, this puts the Northeast in position for more scattered afternoon and evening convection. This convection will be focused over the western half of the region Thursday and the eastern half on Friday. Temperatures will run below seasonal averages during the period and could be especially chilly over northern New York and New England Thursday night, with some locations in the higher terrain even dropping below 40°F!


Long-term - Issued - 8/04 @10:20am


The big changes come this weekend as the deep-layer ridge builds over the Eastern US. There may be one more dreary day on Saturday to open the weekend as models are indicating MCS activity moving down from the Great Lakes in the strong northwest flow developing along the northeastern periphery of the building ridge. Current indications are for this complex of showers and storms to cut from the Niagara Frontier to the southern New England Coast with mainly fair conditions on either side of the track of this system. By Saturday night a developing warm front will usher in a much warmer airmass as 850mb temps rise to +20°C across the western half of the region overnight! The strong warm advection will likely trigger MCS activity along the flanks of the warm front draped across northern New York and New England and will need to be monitored in the days ahead.

By Sunday the blast furnace is switched to the on position as strong ridging finally comes over the region. 500mb geopotential heights rise to 590dm+ across the southern half of the region with 1000-500mb thicknesses over 580dm and 20-22°C 850mb temps! Deep-layer ridging will provide plenty of subsidence and hinder the development of clouds allowing full sun to bake the ground. Temperatures will rise into the 90’s for the first time since April for many locales and could approach 100°F in the normally warmer locales such as Newark Airport, especially given downslope flow east of the mountains. Only northern New England will escape the heat, as the warm front will linger in the region providing cloud cover with scattered showers and storms. A trough slicing into the oppressive airmass to open next week will ignite showers and thunderstorms. It’s likely a few of these storms will be severe given the high heat and humidity in place over the region as this trough moves in. With the more progressive pattern in place this trough will not stick around long and ridging is forecast to build back east during the middle and later parts of next week.



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Radar: Northeast Region Loop

NE radar
Fig.11 - Radar loop of the Northeast region. Courtesy of Weather Underground.


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Local SST's

Northeast SST's
Fig.12 Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


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About sullivanweather

Thomas is an avid weather enthusiast, landscaper and organic gardener. This blog is dedicated to Northeast and tropical weather forecasting. Enjoy!

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