Northeast Weather Blog

Long duration snow event for New England

By: sullivanweather, 6:30 AM GMT on December 31, 2009

A gathering storm over the Western Atlantic will back towards the Gulf of Maine this weekend spreading heavy snow and strong winds into central/northern New England with high tides and coastal flooding along the shore. Near-blizzard conditions are possible for a time during the height of the storm.


Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.1 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

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Forecast Discussion


Synopsis - Issued - 12/31/09 @1:30am

A very busy week of weather in the Northeast with unsettled conditions each and every day of the forecast period. A series of shortwave disturbances moving through the longwave trough over the Eastern US will coalesce into a rather powerful storm over the Western Atlantic over the next 48-72 hours. As the weaker disturbances feeding into the future oceanic storm move off the coast in the near term they will each bring a round a lighter precipitation centered on the New Year’s holiday - generally falling as snow over the interior and a mix to rain scenario along the coast up to southern New England. The storm begins to form very late on Friday several hundred miles offshore but this storm will then back towards the New England Coast as the persistent strong blocking pattern over the North Atlantic refuses to give any ground spreading heavier snows across New England. Heavier snows should spread back in from the Canadian Maritimes across central/northern New England by the weekend with lake-effect snows gaining in intensity across the snow belts of New York and Pennsylvania as progressively colder and moist air in deep cyclonic flow cranks up the snowguns off the lakes. Elsewhere over the interior lighter snow showers will persist within the upper trough with a few of these even surviving the trip over the mountains down into the coastal plain. Gradual improvement will start to occur to begin next week as the deep low slowly fills and moves away from the Northeast.



Near-term - Issued - 12/31/09 @1:30am


Current regional satellite loops show plenty of clouds spread over the Northeast late this evening moving in out advance of a low pressure trough running from the Great Lakes to the central Gulf Coast. A modest amount of moisture from the Gulf has fed into this feature on a 35-40kt low-level jet, which is currently bringing some pretty heavy rains across the Southeastern portion of the country. While the lion’s share of this moisture will work off the coast before reaching the Northeast, enough will manage to snake far enough north along the trough to provide precipitation upon reaching the region. Thermal profiles show that much of this should fall as snow, except for perhaps extreme southern New Jersey, as precipitation begins. Areas generally south of I-80 should see light snow begin during the pre-dawn hours with as much as an inch coating the ground by sun up, especially across the Laurels. Elsewhere across the region clouds will persist through the overnight with temperatures remaining steady or slowly dropping.


Short-term - Issued - 12/31/09 @1:30am


First round of precipitation continues to move northeastward through the day on Thursday with most of the steadier precipitation falling south of I-90. Embedded shortwave within the overall trough will move from the central Appalachians during the morning to off the northern Jersey Shore by evening, enhancing precipitation across the southern half of the region. QPF should be a tenth to a third of an inch in this region falling during a 4-7 hour ‘burst’ of isentropic lift/PVA with more in the way of showery precipitation to the north. A couple hours of mid-level frontogenesis is also shown moving across northern Pennsylvania and southeastern New York which could bring a heavier burst of snow up to 1”/hr. Rain/snow line should creep inland 20-30 miles during the late morning/early afternoon hours but not before an inch or so of snow falls from central Jersey on north along the immediate coast, including the New York City metro area. A bit further inland it should be all snow with 1-3” of snowfall expected up to about I-90 where areas to the north see under an inch. A few locations from the Poconos to the Catskills and Taconics may even see up to 4” due to the aforementioned frontogenic forcing. Head far enough north to northern New England and it should be just clouds with precipitation remaining south during the day. Temperatures should range from the 20’s across the northern interior with 30’s across the southern interior and coastal plain. A few low 40’s could even be found down to Cape May this afternoon.

First shortwave spawns weak low pressure southeast of Cape Cod during the evening hours with a period of moderate precipitation expected along the immediate southern New England coast before midnight. Most of this precipitation should fall as rain south and east of I-95 with more in the way of snow north and west. QPF across the model solutions range from a tenth to a quarter of an inch, though cold sector QPF is generally a tenth of an inch or less amounting to around an inch of snow. There may even be a ribbon of freezing rain within the transition zone but precip shouldn’t remain as freezing rain for any extended period of time. Most of this precipitation associated with the first shortwave should end by midnight. Meanwhile, the surface trough tied to the northern stream low pressure moves in from the west spreading a fresh round of snow showers across the region. Clouds and precipitation will keep temperatures within a few degrees of their afternoon readings, generally in the 20’s across the interior, 30’s along the coastal plain.


Mid-term - Issued - 12/31/09 @1:30am


Northern stream surface trough moves offshore on Friday (New Year’s Day) and slowly begins to phase with the initial shortwave in the Gulf of Maine as the upper level trough in the northern stream hangs back over the eastern Great Lakes. Meanwhile, an even stronger disturbance at the base of the trough over the Southeast will move offshore and really begin to stir what’s brewing in the cauldron offshore. Strong low pressure forms along the boundary well offshore, nearly halfway between North Carolina and Bermuda, and moves north towards the parent low residing in the Gulf of Maine stuck there due to the blocking pattern over the Davis Strait. Increasing convergence on the backside of the parent low over Maine will cause light snows to break out during the morning hours, then slowly increase in intensity during the afternoon as the Atlantic is tapped and an easterly low-level jet begins to become established. Lighter snow showers will extend back across the North Country in the weakly moist convergent flow, especially across upslope regions of the Adirondacks/Greens/Whites. Elsewhere, mainly cloudy skies are expected with an occasional snow/rain shower depending on location. Yet another cold front will move into western New York and Pennsylvania during the afternoon hours with the lake effect machines beginning to crank after its passage. Status quo on Friday night with the parent low continuing to provide light to occasionally moderate snow over Maine and lighter snow and snow showers extending back across northern New York, Vermont and New Hampshire. By daybreak on Saturday snowfall over Maine should be in the 3-7” range with 2-4” back across the higher terrain of northern New York, Vermont and New Hampshire (1-2” valleys). The cold front pushes to the coast and offshore during the evening hours with temperatures falling back below freezing for just about the entire region aside from extreme southern New Jersey and southeastern southern New England. Lake-effect/enhanced snows will continue across the snow belts with a couple inches of snow by morning.

It’s shaping up to be a very snowy first weekend of 2010 across northern New England as the offshore area(s) of low pressure finally consolidate into a storm center, forecast by some models to get in the 960mb’s range, and slowly back towards the coast. Deeper moisture arriving on a 40-55kt low-level easterly jet will cause heavier snow to develop across Maine on Saturday and back towards the west reaching New Hampshire and Vermont Saturday night. Intense frontogenic forcing/deformation band should cause a several hour long period of 1-2”/hr snowfall rates and QPF over the weekend ranges from three quarters of an inch to as much as 2”+! This could lead to well over a foot of snowfall for many locations across northern New England. Winds will also blow strongly out of the north leading to near-blizzard conditions over the State of Maine at the height of the storm. Additionally, an inverted trough/trowal is quite evident in the models extending back across northern New York and into southern Canada. Significant snowfall may also fall with this feature, 6-12” over the higher terrain; 3-6” in the valleys. Not to be outdone, the Canadian model is also picking up on a second deformation axis across southern New England which may drop a few hours of moderate to heavy snowfall Saturday afternoon/evening. With all the pieces coming together to form this storm just about anything is possible to that effect as inverted troughs and deformation banding will hang all over the backside of this storm. In the cold, moist cyclonic flow around the deep low significant lake effect snowfall will also be possible across the Finger Lakes region of New York and the Laurels/Allegheny Front in Pennsylvania. Temperatures during the weekend should remain in the teens and 20’s across the interior with 20’s and 30’s along the coastal plain.



Fig.2 - Snowfall through Monday AM.



Long-term - Issued - 12/31/09 @1:30am


Deep stacked low slowly fills and weakens on Monday though the cold moist cyclonic flow around this feature will still be in place providing snow showers across the Great Lakes region and across the upslope regions of northern New York and New England. A few additional inches of snow may accumulate but after a foot or two what’s an inch or two, right? Down along the coastal plain skies should be partly cloudy with breezy conditions throughout the region and seasonable temperatures. This slow winding down process continues into Tuesday and maybe even Wednesday as the pattern remains stagnant across the Eastern US/Western Atlantic. Eyes will be shifting west during the middle of next week as well. Yet another major blizzard is starting to show in the models spreading east from the Front Range of the Rockies being fed by an extremely strong ~1050mb arctic high pressure. The question heading into next weekend becomes does this beast turn the corner or not?

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Radar: Northeast Region Loop

NE radar
Fig.3 - Radar loop of the Northeast region. Courtesy of Weather Underground.


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Local SST's

Northeast SST's
Fig.4 - Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


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Updated: 1:12 AM GMT on January 03, 2010

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Nor'easter on the way.

By: sullivanweather, 8:11 AM GMT on December 18, 2009

Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.1 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

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Forecast Discussion


Synopsis - Issued - 12/18/09 @3:05am


High pressure will crest over the Northeast on Friday before giving way to a major nor’easter slated for this weekend. Low pressure, currently over the Gulf of Mexico, will move to a position near Cape Hatteras by Saturday AM then take an uncertain course northeastward to a position south of Long Island by the wee hours on Sunday before moving east-northeast seaward of Cape Cod Sunday afternoon. This storm has the potential to bring copious amounts of snow and high winds to the coastal plain and across the southern tier of Pennsylvania along with coastal flooding and beach erosion. Cold and blustery conditions follow the passage of the storm along with a couple weak northern stream disturbances which will keep snow showers concentrated around the lakes region to open next week.



Near-term - Issued - 12/18/09 @3:05am


Gathering storm over the northern Gulf of Mexico looks rather impressive on IR satellite loops at this early juncture. The system already displays a classic comma shape to the cloud pattern with a developing baroclinic leaf on the northeastern flanks of the cloud shield while in the southeastern Gulf deep convection is firing as indicated by some -50°C to -60°C cloud tops. Also apparent on Satellite is the small mid-level disturbance diving across the Central Plains that will eventually phase up with the Gulf system as they both reach the East Coast. Meanwhile, here in the Northeast, we’re well into our coldest night thus far this winter with many locations across the North Country already well below zero. Saranac Lake, NY is checking in with a reading of -21°F at the time of this writing! Other areas across the region aren’t that far behind with Saint Johnsbury, VY at -8°F and Rome, NY at -6°F. The reason for such bitter cold is ideal radiational cooling occurring at these locations with clear skies, calm winds, dry atmosphere and a deep snow cover. In areas where the wind is still kicking up across the north temperatures have remained in the single digits, although wind chill values are in that dangerous -15°F and colder range prompting the issuance of wind chill advisories. Further south temperatures are a bit milder, with readings in the teens and 20’s. Some mid/high clouds are streaking out on the jet across this region ahead of the digging trough over the Mississippi Valley. Temperatures during the remainder of the overnight shouldn’t move far from their current readings. Some areas will drop a few degrees while other may rise a few degrees if clouds happen to pass by overhead. But otherwise most locales aside from northern Maine have already seen their coldest temperatures this season thus far tonight.

High pressure will crest over the Northeast Friday morning before retreating back into Canada leaving behind a weakening surface ridge axis. Despite the surface high pressure there will be considerable mid/high level cloudiness across the southern half of the region, which will tend to become thicker during the afternoon as the storm system to the south spread its cirrus shield northeastward. Under the thickening cloud cover temperatures will rise into the upper 20’s to mid 30’s as the airmass over the region moderates some. Across the northern half of the region skies should remain mostly sunny with a few passing mid/high level clouds, though across northern Maine there may be a bit more in the way of clouds due to cyclonic flow thanks to their proximity to the deep stacked low over Newfoundland. Temperatures here will be several degrees warmer than Thursday’s but should still struggle to climb into the low 20’s with the higher terrain remaining in the teens. The wind will also be quite blustery across northern New England adding an extra bite to the air as wind chills are likely to remain near or below zero for much of the day.


Short-term - Issued - 12/18/09 @11:15am


The storm will be knocking on the front door Friday night as clouds lower and thicken across the southern half of the region. Temperatures will be quite chilly under the canopy of clouds, generally in the mid to upper 20’s although locales along the immediate coast of southern New Jersey should remain above 30°F. A raw north to northeasterly wind will begin to pick up with quite blustery conditions developing along the coast after midnight. Light snow will begin to fall in areas south of the PA Turnpike in the pre-dawn hours and may accumulate up to an inch by daybreak down along the Mason-Dixon line with a few inches possible across southern New Jersey. Further north clouds will increase but the cover should remain thin enough to allow for another night of decent radiational cooling. Temperatures should easily drop into the teens and single digits with some of the colder favored locales dropping below zero. Winds will be light and variable but the evening hours across northern Maine could see a few higher gusts over 15-20mph.

The real complications arise on Saturday as the storm clears Cape Hatteras during the morning and heads for the waters off the coast of the Northeast. The track of this storm will be ever so important due to the highly confluent flow to the north of the system. Due to this confluence there will be a very sharp gradient on the northern fringes of this storm between a major snow event and a non-event. This zone is likely to be anywhere from 50-120 miles from the coast then arcing west across Pennsylvania, 30 miles either side of the I-80 corridor. There’s arguments one could make for either solution – coast hugger or seaward bound. As for the coastal solution, the overall trend in the models the last 2 days has been west and the models almost always seems to even underestimate how far west the storm tracks in reality, even at very short timescales before the event in question. There’s also tremendous convection firing over the eastern Gulf of Mexico and Florida, likely building downstream heights and forcing a more northwest track. Additionally, the Gulf Stream is still very warm this time of year and climatologically low pressure tends to follow this naturally occurring baroclinic zone. Furthermore, SST’s off the East Coast are running about 1°C above normal except upon coming across the Northeast where SST anomalies are as much as 3°C above normal. I’m sure this added heat will also trend the system closer to the coast. However, upper-level support isn’t really where one would like to see it in order to produce a blockbuster and the mid-level low pressure features doesn’t close off early enough in latest model guidance to capture the low and hug it along the coast. There’s also lots of dry air to the north of the low for it to tap that could erode the northern edge of the precipitation shield, lessening amounts.

It’s hard to go against a climatologically favored solution, plus a several year long model trend and this forecaster sees no reason to do so but will also give a 20% hedge to the dry air to the north and a short delay of phasing which may prove incomplete. Therefore, expecting low pressure to deepen to ~985mb about 50mi southeast of Cape May by late in the afternoon before starting to feel the effect of the blocking pattern to the north and shearing east-northeast. Development of a coastal front will need to be watched for as this could cause the low pressure to snake up along it a bit further north than anticipated. However, this solution would slowly but steadily spread snow northward during the day on Saturday, reaching to at least the I-80 corridor across Pennsylvania and the I-287 corridor in southeastern New York and southwestern southern New England by mid-afternoon – possibly even as far north as the I-84 corridor. Strong easterly flow should change snow over to rain across southern New Jersey from Vineland to Atlantic City but once north of Seaside Heights much of the precip should fall as snow. Where the snow does fall it will be rather intense as strong frontogenic forcing develops on the northwest side of the low. Snowfall rates across southeastern Pennsylvania and central New Jersey could exceed 2”/hr at time in the heaviest bands late Saturday morning lasting into the evening. Outside of the banded precipitation snowfall should be a general light to moderate, with accumulation rates around ½”/hr. By evening a narrow band of 6-12” of snowfall from southeastern Pennsylvania into central New Jersey is likely, including the Philadelphia metro area with 3-6” of snow falling outside of this band across southern New Jersey and back across south-central Pennsylvania along and to the south of I-70. In addition to the snowfall the wind will really being to howl out of the northeast at 25-35mph along the coast with higher gusts leading to blizzard conditions at times. A bit further inland the wind will be less, around 10-20mph but this combined with the snow and biting cold will make for quite a wintry scene. Temperatures should remain in the 20’s across the interior while sections along the immediate coast could climb into the mid 30’s. Just inland of the coast, where some of the heaviest snow may occur, temperatures should hover in the 30-32°F range making for a thick pasty snow that sticks to everything and should be horrendous to drive or walk in. It would be best to keep it inside tomorrow for those areas mentioned above. Across the northern half of the region it will be just a cold gray winter’s day with high’s remaining in the teens and 20’s under the thickening canopy of clouds. Perhaps northern Maine sees a few hours of morning sun under the strong confluence.

Low pressure winds up Saturday night south of Long Island before starting to pull out to sea. Heavy snow will continue to spread north but not much further north as dry air infiltrates the northern periphery of the storm owning to the confluent flow over northern New England. Northern edge of the precipitation shield should reach a Binghamton-Albany-Concord line as deformation banding should act to generate some precip on the far northwestern fringes of the storm. Closer in a band of very heavy snow should traverse northern New Jersey, southeastern New York - including Long Island and the New York City metro area – and southern New England during the evening and overnight which should also produce some of those same 2”/hr+ snowfall rates. This should continue the 6-12”+ band of snowfall from southeastern Pennsylvania across the urban corridor along I-95 into southeastern Massachusetts. To the north of this heavy band of snowfall amounts should drop sharply to perhaps 3-5” for about 50mi then a general 1-3” across northern Pennsylvania, south-central/east-central New York and along the Mass Pike.

Early Sunday morning the nor’easter will b exiting stage right, east-northeastward away from Cape Cod. Lingering light snow and snow showers should extend over much of southern New England and eastern Long Island through mid-morning while the rest of the coastal plain digs out. Weak high pressure will build in from the west bringing partly cloudy skies. Temperatures should run about 5 degrees below seasonal averages.




Fig.2 - Snowfall through Sunday.

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Radar: Northeast Region Loop

NE radar
Fig.3 - Radar loop of the Northeast region. Courtesy of Weather Underground.


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Local SST's

Northeast SST's
Fig.4 - Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


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Updated: 1:00 PM GMT on December 19, 2009

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Northeast's turn in countrywide winter storm

By: sullivanweather, 3:55 AM GMT on December 09, 2009

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Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.1 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

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Forecast Discussion


Synopsis - Issued - 12/08/09 @10:55pm


A large and powerful winter storm has been churning across the United States over the last 48-60 hours bringing many feet of snow to the Intermountain West, flooding and mudslides across southern California, blizzard conditions to the Upper Midwest and Central Plains, flash flooding, tornados and severe weather in the deep south and high winds from New Mexico and Texas to the Ohio Valley and lower Great Lakes. Even Hawaii wasn’t able to escape the storm as it generated large swells which propagated to the chain of islands producing waves along the northern shores of up to 50 feet in height! Now this overwhelming system has its course set for us here in the Northeast and promises to bring up to a foot of snow for parts of the interior, potentially flooding rains along the coastal plain and high winds across the entire region. The storm will pull northeast into Canada by Thursday with very cold air left in its wake delivered by strong westerly winds with heavy lake effect snows.



Near-term - Issued - 12/08/09 @10:55pm


Precipitation is rapidly spreading northeast late this evening, already through the southwestern two thirds of Pennsylvania. Much of the precipitation east of the Appalachians is in frozen form; mainly as snow. In fact, moderate to heavy snow is falling at rates of over an inch an hour across many locales across central Pennsylvania from the Allegheny Plateau to the Laurel Highlands. Head west of the Apps and the rain is falling hard from Latrobe to Pittsburgh in the southwest corner of the Commonwealth while areas further north are currently seeing a wintry mix of snow, sleet and freezing rain. Meanwhile, areas further north and east the clouds shield advancing out ahead of the storm has spread over much of the remainder of the region, aside from the State of Maine where some high thin cirrus are just now beginning to filter in. Temperatures under the clear skies and calm winds of high pressure in Maine have plummeted this evening well down into the teens and single digits. Clayton Lake, Maine was reporting only 4°F this hour (10pm). Under the cloud canopy, temperatures have generally held in the 20’s in the northern interior although some locales across the North Country have also dropped down into the teens. Across the southern interior and coastal plain temperatures are in the upper 20’s to mid 30’s, coldest across the higher terrain of central Pennsylvania, with the warmest readings right along the southern Jersey Shore as Cape May is coming in this hour at 44°F.


Short-term - Issued - 12/09/09 @1:05am



This is an extremely complicated forecast in terms of timing of changeover and amount of QPF before the changeover. From a meteorological standpoint, this is one incredible storm to encompass. Textbook jet stream structure at upper levels which is bookended by 180kt jet streaks; one feeding into the system coming out of the Four Corners region and another exiting the storm over northern New England. As we march down through the atmosphere we see a sharp, negatively tilting H5 trough over the Mid-Mississippi Valley region of the country pushing strong PVA into the Ohio Valley/Great Lakes region of the country. A bit further east, at the 850mb level a south-southeasterly jet is forecast to develop off the Atlantic in excess of 75kts! This is nearly 5 standard deviations above the norm and at the nose of this developing low level jet feature is an astonishing push of isentropic lift over the northern Mid-Atlantic and Northeast! Then getting down to the surface (where it matters for most of us) low pressure entering central Illinois is forecast to continue to drop from the current 986mb to about 974mb by the time it reaches the shores of Lake Huron.

Impacts here in the Northeast are already being felt as discussed above. Snow, due to intense isentropic lift will continue into the overnight hours from central Pennsylvania and points northeast to the southern Adirondacks of New York. Snowfall rates have been observed up to 3”/hr earlier this evening and there’s no reason to believe these types of rates won’t be seen as this area of snow spreads across the landscape. By morning most areas north of I-70 and up to I-90 in central New York should see 2 to as much as 7 inches of snow, depending on location. Areas to the west of the Appalachians should see a changeover to rain, if not already raining. As warmer air pushes up from the south of the heels of the aforementioned low-level jet the snow across Pennsylvania, northern New Jersey, southern New York and southern New England should changeover to a mix of sleet and freezing rain, then eventually plain rain. Snowfall totals before the changeover could reach 10” in areas of the Catskills with a general 3-8” elsewhere. After the changeover to rain in areas across the south, there should be a good 4-6 hour stretch of very heavy rain that could lead to flash flooding. Rainfall could reach 2” from southern New Jersey to southern New England along the coast, especially if a coastal front were to set up, enhancing precip amounts. Further north, over northern New York and central New England the snow will just be getting started around daybreak and will quickly become heavy. Snowfall rates of up to 1-2”/hr will be common during the morning hours as this band of snow moves in. By the afternoon the dry slot advancing from the southwest will have already pushed through much of Pennsylvania, New Jersey, southern/western New York and moving into western New England. But further north, with a lingering surface ridge axis in place, the system will start to shear off to the east. This will halt the northward component of low pressure over Lake Huron and the triple point (secondary) low which will be just off the New England coast, will start to become the dominant feature. Additionally, the transition zone will also start to slow its northward progression, allowing for an extended period of sleet and/or freezing rain across northern New York and central New England, aside valley locations where the boundary layer may warm up enough to support plain rainfall. By the time late afternoon rolls around northern Maine begins to get into the snow and it should stay snow here. Snow should continue through the night and into early on Thursday leaving 6 to 12 inches, basically the only straight forward forecast for the entire region.

Further to the south and west, after the changeover, there will be about 6-10 hours of relative warmth and showery conditions before the cold front comes blasting through. This will turn temperatures sharply colder and quickly freeze any residual moisture on area roadways. Cold, moist cyclonic flow will also activate the lake effect machines, but their top gears will be felt later on in the week. Away from the lakes only a few scattered snow showers will dot the region with most areas seeing just mostly cloudy skies.

Precipitation won’t be the only weather to be dealt with. Out ahead of the system in the strong southerly flow, winds will kick up to 30-40mph along the coastline and higher terrain with 15-25mph winds elsewhere across the interior. Behind the cold front winds will kick up once again, but this time out of the west, at 20-30mph with higher gusts. Good momentum transfer should be realized with strong cold air advection following the front working on a brisk 25-35kt boundary layer flow.






Fig.2 - Snowfall totals through Thursday morning.
(yes, there is a LOT going on in this map but with how dynamic this system is, many colours are needed)

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Radar: Northeast Region Loop

NE radar
Fig.3 - Radar loop of the Northeast region. Courtesy of Weather Underground.


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Local SST's

Northeast SST's
Fig.4 - Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA.


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Updated: 6:07 AM GMT on December 09, 2009

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First snow for many - 2009/10 Winter forecast

By: sullivanweather, 2:38 PM GMT on December 05, 2009

2009-10 Northeast Winter Forecast


It’s been a long wait but once again the winter season is upon us Northeasterners. After a mild and relatively snowless November the snow-lovers are antsy, especially after October brought much below normal temperatures and some of the earliest snowfalls on record for central Pennsylvania. Seemingly a prelude to an early start to winter evaporated into a 15-20 day stretch of above normal temperatures - Indian Summer a month late. Now that we head into the start of meteorological winter (Dec-Feb), signs of a pattern change are beginning to emerge that should bring a taste of what’s to come over the next three months. Beyond the 7-day period to open the season global numerical models agree on a fairly significant blast of arctic air diving south from Canada into the Contiguous United States with a portion of it poised to head to the Northeast. Even before any serious arctic air heads south cold enough air will move in place behind the upcoming midweek storm to set the stage for a possible winter storm this weekend, something hinted at in a few model solutions. But that’s just for the next week or so, what about the rest of the winter? What factors will be involved that should determine what type of weather we’re likely to see this upcoming winter season here in the Northeast?



Fig.1 - Northeast averaged winter temperatures for the period 1895-2009. Credit: National Climatic Data Center (NCDC) of NOAA




Fig.2 - Northeast averaged winter precipitation for the period 1895-2009. Credit: National Climatic Data Center (NCDC) of NOAA


For one, the periodic warming and cooling of the equatorial Pacific Ocean, known as ENSO, tends to have rather large influences on the weather patterns during the winter season in the Northeast. During the previous two winter seasons the pattern of sea surface temperatures across the ENSO region of the Pacific resembled a La Nina pattern. Closer to home La Nina tends to feature anomalous ridging off the East Coast, hence, a more southwesterly flow regime across the Northeast with a storm track that trends towards the eastern Great Lakes. This allowed for many over-running precipitation events from the Mississippi Valley region of the country that was able to tap into milder Gulf of Mexico air, often changing snow over to a mix of sleet, freezing rain and rain. However, the La Nina pattern of last year has fully transitioned into El Nino of moderate strength this season. As expected, with a fully developed El Nino now in place, a much different weather pattern for this winter from the previous two is in order. There’s also another oscillating pattern of sea surface temperatures in the Pacific that can either enhance or diminish El Nino’s effects, and this is the Pacific Decadal Oscillation, or PDO. This second index correlates to sea surface temperatures off the west coast of the US as it’s a measure of the difference in sea surface temperatures in the northeast Pacific compared to the remainder of the Pacific. When the PDO is negative SST’s off the West Coast tend to be below normal, whereas when the index is positive the opposite tends to be true. This PDO is also more slowly evolving than ENSO, trending either negative or positive for decades at a time. A longer-term running mean shows a roughly 30 year periodicity to this oscillation, which has trended back towards negative territory since about 1999. The previous positive phase of the PDO lasted from 1976-1998. The PDO is now trending negative thus eliminating the positive phase PDO years from the analogue. Going back to 1950 there have been 17 individual El Nino events, roughly a third of the time. But many of these are weak, barely registering a 1°C peak 3-month running mean anomaly as determined by the Nino 3.4 index. With this season’s El Nino now far more established and robust, we can eliminate those years for comparison as well (1951, 1969-70, 2004-05). This season’s El Nino, currently peaking in strength as determined by the flattening of the sub-surface thermocline, should barely attain the strong category for El Nino as determined by the Nino 3.4 index for any length of time this winter. Thus we can give less weight to the strongest El Nino events during –PDO for the analogue (1957-58, 1965-66, 1972-73). This leaves 3 years -1963-64, 1968-69 and 2002-03 - as best fit comparisons while adding in the stronger El Nino events for filtering.



Fig.3 - United States temperature anomalies by climate division for six (6) moderate/strong El Nino events during long term negative PDO signal. Credit: Earth System Research Laboratory (ESRL) of NOAA.




Fig.4 - United States temperature anomalies by climate division for three (3) 'best fit' El Nino events during long term negative PDO signal. Credit: Earth System Research Laboratory (ESRL) of NOAA.


As shown above, the analogue years tend to yield much colder than normal temperatures across the eastern and southern portions of the country, including here in the Northeast. This is partly due to El Nino’s effect of enhancing the sub-tropical jet stream across the southern portions of the country. The greater storm frequency along the sub-tropical jet allows for more cloud cover across the south and a higher number of intrusions of cold air from the north that follow the passing of each low. Since the tracks of these areas of low pressure are across the southern portions of the country, this also curtails the northward flow of milder air ahead of said storms. Closer to our neck of the woods here in the Northeast, El Nino events also tend to displace the polar vortex towards eastern Canada, which allows for the polar jet to dive southeast into our region of the country delivering occasional bouts of arctic air.



Additionally, the interaction/phasing of these two branches of the jet stream (and their subsequent storm systems) can yield rather significant areas of low pressure that bring substantial precipitation events to the region as they track from the Southeastern US and up the East Coast. Such types of low pressure, commonly referred to as Nor'easters, are a much more common occurrence during El Nino years, especially Miller type-A storms which originate along the Gulf Coast and deliver both Gulf and Atlantic moisture to the region in a power-packed system.




Fig.5 - United States precipitation anomalies by climate division for six (6) moderate/strong El Nino events during long term negative PDO signal. Credit: Earth System Research Laboratory (ESRL) of NOAA.




Fig.6 - United States precipitation anomalies by climate division for three (3) 'best fit' El Nino events during long term negative PDO signal. Credit: Earth System Research Laboratory (ESRL) of NOAA.




While El Nino can be used to help formulate tendencies in seasonal patterns, when it comes to individual storm systems looking at oscillating geopotential height indices such as the PNA, EPO, NAO and AO become more useful. Some of these indices, such as the NAO, tend to have a longer periodicity than others, such as the PNA. As a general rule, when the NAO is in negative phase troughiness tends to move over the East Coast and Western Atlantic Ocean along with below normal temperatures whereas a positive stage will tend towards ridging and above normal temperatures. In recent months the NAO 3-month running mean has been trending negative and should continue to do so throughout the winter season as the spatial pattern of sea surface temperatures across the North Atlantic favors the continuation of this pattern. While the NAO is negative, the polar jet tends to dip further south and have more interaction with the active sub-tropical jet stream across the southern portion of the nation. This is a prime set-up for storminess along the East Coast and should be a regular occurrence this winter, judging from past analogue seasons.




Fig.7 - Standardized 3-month running mean of NAO index 1/1950-10/2009. Credit: Climate Prediction Center of NOAA



El Nino winters also tend to give the Northeast quite a bit of variability as faster west/east flow can, at times, push warmth from Chinook winds all the way to the East Coast. These Chinook winds are a regular occurrence during El Nino winters as a split flow jet stream pattern is commonplace across the East Pacific and western North America. The northern branch of this jet, and subsequent storm track, move into a position that’s favorable for warming downsloping winds off the Rocky Mountains. When the polar jet fails to push south across the Eastern US, these warm airmasses are allowed to sweep east, giving a classic January thaw as January is typically the month of fastest zonal winds across the northern hemisphere.


Considering the above, the expectation is for the Winter of 2009-10 to be one of colder than normal temperatures and average to slightly above average precipitation for the region as a whole. However, certain portions of the region should have a better chance at seeing the more anomalous weather than others. For example, areas closer to the Canadian border are more prone to see their temperatures closer to normal averaged over the 3 months of the winter season than folks to the south and along the East Coast where a better chance of seeing below normal temperatures this winter is present. The same applies for precipitation as the closer one heads to the coast the higher one’s chances are for coming in above the seasonal average due to the proximity of the expected dominant storm track, which should be much further south than the previous La Nina winters. Meanwhile, for those to the north, a more average season precipitation-wise is expected.




Fig.8 - Winter of 2009-10 forecast temperature anomalies.



Fig.9 - Winter of 2009-10 forecast precipitation anomalies.



There’s also likely to be local effects that, once averaged into the season as a whole, could tip the scales one way or another. Areas of southwestern Pennsylvania could be one of these areas that this holds true where most of the lake effect remains to the north of the region while Gulf moisture gets shunted south and east leaving the area much drier than their immediate climate divisions. Also, the eastern end of Massachusetts tend to get clipped by strong nor’easters that are just a bit too far offshore for the rest of us, leading to a higher positive precipitation anomaly than other areas of the region.




Fig.10 - Winter of 2009-10 forecast snowfall anomalies.



___________________________________________________________


Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.2 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

--------

Forecast Discussion


Synopsis - Issued - 12/05/09 @9:35am


A vigorous upper disturbance moving out of the Deep South will energize an area of low pressure just offshore Cape Hatteras today as both head northeast. Precipitation moving out ahead of these features is beginning to expand over the southeastern third of the region this morning and will continue to spread northeast throughout the day. For most areas to the north and west of I-95 snow is expected while areas closer to the coast rain should be the dominant precipitation type. As the low continues northeast this evening the storm starts to crank out some heftier snows over Downeast Maine while any lingering rain showers should change over to snow before ending further down the coast. High pressure follows for Sunday ahead of a weakening disturbance glides through on Monday to provide snow showers, mainly across the north. The next major storm arrives later in the day on Tuesday and should continue straight into Thursday and promises to bring another round of wintry weather. Much colder air will pour down from Canada at the end of next week into next weekend that should put the Northeast into the deep freeze.



Near-term - Issued - 12/05/09 @9:35am


Regional radar depicts an expanding area of precipitation along the Eastern Seaboard this morning developing out ahead of a rather potent mid/upper level disturbance pulling out of the Southeast. This upper level feature is responsible for bringing portions of southeast Texas and southern Louisiana some of their earliest snowfalls on record yesterday. This morning it’s bringing the first snowfall of the season to many portions of the Mid-Atlantic and Northeast not affected by the storm in mid-October. Light snow is being reported at several southern Pennsylvania ob sites while rain falls in Philadelphia and other locales across the Mid-Atlantic along and to the southeast of I-95. Further north clouds are quickly moving in with overcast skies for most up to the Canadian border region. Temperatures across the interior are rather chilly this morning, generally in the 20’s with some lower 30’s as one heads down towards the coastal plain. Along the coastal plain temperatures reach higher into the 30’s with some 40’s along the immediate shore. Winds are picking up from the northeast around 5-10mph inland and 15-20mph along the coast.


Short-term - Issued - 12/05/09 @9:35am


As the day progresses better forcing will move over the Northeast with precipitation becoming more consolidated and falling at a heavier rate. Dynamic processes will slowly cool the column, allowing precipitation to change from rain over to snow for locales along the I-95 corridor. For points north and west much of the precipitation will start as snow and end as snow. There may even be a few bursts of moderate to heavy snow during the late afternoon and early evening hours from eastern Pennsylvania to northern New Jersey and interior southeast New York as frontogenic forcing in the mid-levels increases in the -12°C to -18°C snow growth region. Snowfall should easily accumulate on colder surfaces and will do so on roads in heavier bursts, especially after 3pm when the sun is very low in the sky. Amounts should range from 1-3 inches by dusk. North and west of a Pittsburgh-Ithaca-Montpelier line, then extending east to the coast skies will be cloudy but snow will be held off to the south and east. Temperatures today will only climb into the upper 20’s to low 30’s across the interior. Along the coastal plain temperature are beginning the morning in the upper 30’s to low 40’s but should drop into the lower to middle 30’s after the precipitation commences.

The surface low will intensify this evening as it passes east of the region with plenty of wrap-around precipitation on the backside of the low lingering over the coastal locales. As the wind turns more to the north and colder air is drawn into the system any rain falling along the coast should begin to mix with and change over to snow. The period of snow that follows may be intense for an hour or two across southeastern southern New England and could add up to 3-4 inches before tapering. The big winner from this system may be Downeast Maine, closest to the track of the surface low. Here QPF of over half an inch is possible with all of it falling as snow. This could translate into 4-7” of snow by morning. Meanwhile, areas to the west will begin to see clearing skies as high pressure builds in from the lower Ohio Valley. Temperatures will get quite chilly tonight, especially where skies clear early west of the Appalachians. Lows should fall into the teens and 20’s across the interior with low 30’s down along the coast. Should be new ice on many of the smaller lakes and ponds tomorrow morning across the interior.







High pressure will move overhead on Sunday with clearing skies for most locales across the southern half of the region. To the north a brisk northwest flow will be present to go along with partly to mostly cloudy skies. This should be the coldest day thus far this season for many locations across the Northeast with highs across the interior only in the 20’s to mid 30’s. Higher terrain locales of the Adirondacks and Presidential Range may even hold in the teens. Along the coastal plain temperatures will struggle to climb to 40°F but should get there for an hour or two during the afternoon, especially in the urban centers of New York and Philadelphia. Winds will be brisk, out of the west-northwest around 15-25mph across the northern half of the region. Further south winds will slacken considerably under the dome of high pressure and remain generally 10mph or less.



___________________________________________________________


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: 6:08 PM GMT on December 05, 2009

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Seasonal Forecast - Winter of 2009-10

By: sullivanweather, 8:22 PM GMT on December 01, 2009

2009-10 Northeast Winter Forecast


It’s been a long wait but once again the winter season is upon us Northeasterners. After a mild and relatively snowless November the snow-lovers are antsy, especially after October brought much below normal temperatures and some of the earliest snowfalls on record for central Pennsylvania. Seemingly a prelude to an early start to winter evaporated into a 15-20 day stretch of above normal temperatures - Indian Summer a month late. Now that we head into the start of meteorological winter (Dec-Feb), signs of a pattern change are beginning to emerge that should bring a taste of what’s to come over the next three months. Beyond the 7-day period to open the season global numerical models agree on a fairly significant blast of arctic air diving south from Canada into the Contiguous United States with a portion of it poised to head to the Northeast. Even before any serious arctic air heads south cold enough air will move in place behind the upcoming midweek storm to set the stage for a possible winter storm this weekend, something hinted at in a few model solutions. But that’s just for the next week or so, what about the rest of the winter? What factors will be involved that should determine what type of weather we’re likely to see this upcoming winter season here in the Northeast?



Fig.1 - Northeast averaged winter temperatures for the period 1895-2009. Credit: National Climatic Data Center (NCDC) of NOAA




Fig.2 - Northeast averaged winter precipitation for the period 1895-2009. Credit: National Climatic Data Center (NCDC) of NOAA


For one, the periodic warming and cooling of the equatorial Pacific Ocean, known as ENSO, tends to have rather large influences on the weather patterns during the winter season in the Northeast. During the previous two winter seasons the pattern of sea surface temperatures across the ENSO region of the Pacific resembled a La Nina pattern. Closer to home La Nina tends to feature anomalous ridging off the East Coast, hence, a more southwesterly flow regime across the Northeast with a storm track that trends towards the eastern Great Lakes. This allowed for many over-running precipitation events from the Mississippi Valley region of the country that was able to tap into milder Gulf of Mexico air, often changing snow over to a mix of sleet, freezing rain and rain. However, the La Nina pattern of last year has fully transitioned into El Nino of moderate strength this season. As expected, with a fully developed El Nino now in place, a much different weather pattern for this winter from the previous two is in order. There’s also another oscillating pattern of sea surface temperatures in the Pacific that can either enhance or diminish El Nino’s effects, and this is the Pacific Decadal Oscillation, or PDO. This second index correlates to sea surface temperatures off the west coast of the US as it’s a measure of the difference in sea surface temperatures in the northeast Pacific compared to the remainder of the Pacific. When the PDO is negative SST’s off the West Coast tend to be below normal, whereas when the index is positive the opposite tends to be true. This PDO is also more slowly evolving than ENSO, trending either negative or positive for decades at a time. A longer-term running mean shows a roughly 30 year periodicity to this oscillation, which has trended back towards negative territory since about 1999. The previous positive phase of the PDO lasted from 1976-1998. The PDO is now trending negative thus eliminating the positive phase PDO years from the analogue. Going back to 1950 there have been 17 individual El Nino events, roughly a third of the time. But many of these are weak, barely registering a 1°C peak 3-month running mean anomaly as determined by the Nino 3.4 index. With this season’s El Nino now far more established and robust, we can eliminate those years for comparison as well (1951, 1969-70, 2004-05). This season’s El Nino, currently peaking in strength as determined by the flattening of the sub-surface thermocline, should barely attain the strong category for El Nino as determined by the Nino 3.4 index for any length of time this winter. Thus we can give less weight to the strongest El Nino events during –PDO for the analogue (1957-58, 1965-66, 1972-73). This leaves 3 years -1963-64, 1968-69 and 2002-03 - as best fit comparisons while adding in the stronger El Nino events for filtering.



Fig.3 - United States temperature anomalies by climate division for six (6) moderate/strong El Nino events during long term negative PDO signal. Credit: Earth System Research Laboratory (ESRL) of NOAA.




Fig.4 - United States temperature anomalies by climate division for three (3) 'best fit' El Nino events during long term negative PDO signal. Credit: Earth System Research Laboratory (ESRL) of NOAA.


As shown above, the analogue years tend to yield much colder than normal temperatures across the eastern and southern portions of the country, including here in the Northeast. This is partly due to El Nino’s effect of enhancing the sub-tropical jet stream across the southern portions of the country. The greater storm frequency along the sub-tropical jet allows for more cloud cover across the south and a higher number of intrusions of cold air from the north that follow the passing of each low. Since the tracks of these areas of low pressure are across the southern portions of the country, this also curtails the northward flow of milder air ahead of said storms. Closer to our neck of the woods here in the Northeast, El Nino events also tend to displace the polar vortex towards eastern Canada, which allows for the polar jet to dive southeast into our region of the country delivering occasional bouts of arctic air.



Additionally, the interaction/phasing of these two branches of the jet stream (and their subsequent storm systems) can yield rather significant areas of low pressure that bring substantial precipitation events to the region as they track from the Southeastern US and up the East Coast. Such types of low pressure, commonly referred to as Nor'easters, are a much more common occurrence during El Nino years, especially Miller type-A storms which originate along the Gulf Coast and deliver both Gulf and Atlantic moisture to the region in a power-packed system.




Fig.5 - United States precipitation anomalies by climate division for six (6) moderate/strong El Nino events during long term negative PDO signal. Credit: Earth System Research Laboratory (ESRL) of NOAA.




Fig.6 - United States precipitation anomalies by climate division for three (3) 'best fit' El Nino events during long term negative PDO signal. Credit: Earth System Research Laboratory (ESRL) of NOAA.




While El Nino can be used to help formulate tendencies in seasonal patterns, when it comes to individual storm systems looking at oscillating geopotential height indices such as the PNA, EPO, NAO and AO become more useful. Some of these indices, such as the NAO, tend to have a longer periodicity than others, such as the PNA. As a general rule, when the NAO is in negative phase troughiness tends to move over the East Coast and Western Atlantic Ocean along with below normal temperatures whereas a positive stage will tend towards ridging and above normal temperatures. In recent months the NAO 3-month running mean has been trending negative and should continue to do so throughout the winter season as the spatial pattern of sea surface temperatures across the North Atlantic favors the continuation of this pattern. While the NAO is negative, the polar jet tends to dip further south and have more interaction with the active sub-tropical jet stream across the southern portion of the nation. This is a prime set-up for storminess along the East Coast and should be a regular occurrence this winter, judging from past analogue seasons.




Fig.7 - Standardized 3-month running mean of NAO index 1/1950-10/2009. Credit: Climate Prediction Center of NOAA



El Nino winters also tend to give the Northeast quite a bit of variability as faster west/east flow can, at times, push warmth from Chinook winds all the way to the East Coast. These Chinook winds are a regular occurrence during El Nino winters as a split flow jet stream pattern is commonplace across the East Pacific and western North America. The northern branch of this jet, and subsequent storm track, move into a position that’s favorable for warming downsloping winds off the Rocky Mountains. When the polar jet fails to push south across the Eastern US, these warm airmasses are allowed to sweep east, giving a classic January thaw as January is typically the month of fastest zonal winds across the northern hemisphere.


Considering the above, the expectation is for the Winter of 2009-10 to be one of colder than normal temperatures and average to slightly above average precipitation for the region as a whole. However, certain portions of the region should have a better chance at seeing the more anomalous weather than others. For example, areas closer to the Canadian border are more prone to see their temperatures closer to normal averaged over the 3 months of the winter season than folks to the south and along the East Coast where a better chance of seeing below normal temperatures this winter is present. The same applies for precipitation as the closer one heads to the coast the higher one’s chances are for coming in above the seasonal average due to the proximity of the expected dominant storm track, which should be much further south than the previous La Nina winters. Meanwhile, for those to the north, a more average season precipitation-wise is expected.




Fig.8 - Winter of 2009-10 forecast temperature anomalies.



Fig.9 - Winter of 2009-10 forecast precipitation anomalies.



There’s also likely to be local effects that, once averaged into the season as a whole, could tip the scales one way or another. Areas of southwestern Pennsylvania could be one of these areas that this holds true where most of the lake effect remains to the north of the region while Gulf moisture gets shunted south and east leaving the area much drier than their immediate climate divisions. Also, the eastern end of Massachusetts tend to get clipped by strong nor’easters that are just a bit too far offshore for the rest of us, leading to a higher positive precipitation anomaly than other areas of the region.




Fig.10 - Winter of 2009-10 forecast snowfall anomalies.



___________________________________________________________


Current watches, warnings and advisories.


Eastern US current watches/warnings
Fig.2 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA.

--------

Forecast Discussion


Synopsis - Issued - 12/02/09 @6:20pm


Plenty of storminess is in store for much of the nation over the next 7-8 days, including the Northeast. The first system of concern is churning over the Southeast at the time of this writing, producing severe weather across the state of Georgia, including tornadic activity, along with flooding rainfall of 3-5”. As this storm moves northeast into our region it promises to bring at least an inch of rain east of the I-81 corridor along with some high winds along coastal communities. Colder air will slowly spread over the region behind the departing storm late Thursday and Friday possibly setting the stage for a coastal/ocean-bound storm this weekend that could spread a wintry swath of precipitation across the coastal plain and near-coastal interior. While Northeasterners are occupied with the storminess over the next several days, big changes will be occurring off the West Coast of the US that should have big implications for our weather next week and beyond. An anomalous ridge will develop across the Gulf of Alaska which will help to shunt a brutally cold airmass from Siberia across the Pole into North America by the middle of next week. The resulting pattern will turn mid-winter-like – and depending on storm track- could yield a significant storm or two beyond day 7 in the forecast period.



Near-term - Issued - 12/02/09 @6:20pm


Currently satellite imagery depicts a textbook extra-tropical storm system over the Southeast with the cold core upper level low moving out of the Arklatex region with a classic ‘comma head’ shape to the cloud pattern over the Eastern US. Impressive 50-60kt southerly low-level jet is helping to transport copious amounts of Gulf of Mexico and Atlantic moisture northward, which is evident in the precipitable water content fields as the 1” contour has pushed into southern Pennsylvania with values over 1.5” as far north as Newport News, Virginia. At upper levels a remarkable 160kt jet streak resides over northern New England, aiding in the development of a baroclinic leaf noted on satellite. Rain has already spread north into the Northeast this evening as strong isentropic lift is generated by the surge of moisture and warmth at the nose of the accelerating lower tropospheric winds. This rainfall has been steady with rates around a tenth of an inch per hour in a 150-mile wide stripe from northwestern Pennsylvania to southern New Jersey. Across New England and much of upstate New York the precipitation has yet to move in but the clouds have been on the increase since early in the day and skies are now overcast for most areas. This should help to keep temperatures from dropping much this evening as precipitation moves in and most areas will see just plain rainfall. Current temperatures range from the 40’s across much of the interior to the low 50’s along the southern New England coast and points south along the coastal plain, including the urban centers. A few upper 30’s in spots across northern New England and northwest Pennsylvania can be found but nothing below freezing aside from Mt.Washington. These temperatures are rather balmy for early December, but nothing like those of 3 years ago with temperatures pushed 70°F on the 1st!


Short-term - Issued - 12/02/09 @6:20pm


The surface low, still over the upper Tennessee Valley, will begin to rocket northeastwards later this evening as the upper low opens and becomes caught in the fast southwest to northeast flow over the eastern half of the country. Rain will overspread the entire region by daybreak with locally heavy rainfall rates at times. A couple sheltered valleys across far northern Maine may see the precipitation begin as a period of freezing rain and/or sleet but this should quickly changeover to rain as warmer air surges northward on the heels of strong southerly winds. This southerly flow will push a warm front into the region during the overnight before the occlusion blasts trough towards daybreak. The warm front currently rests just south of the region, behind which dewpoints surge into the 50’s. As this warm front pushes north temperatures will rise and the atmosphere will become ever so slightly unstable with mlCAPE’s producing up to 100J/kg and total totals approaching 50. This may be enough to initiate a rumble or two of thunder ahead of the cold front in the warm sector from eastern Pennsylvania across southeastern New York and southern New England. Should convection develop some stronger winds from aloft could mix down to the surface, possibly causing some tree damage and sporadic power outages but no major wind event is expected from convection. Along the coastal plain winds will also be strong from the south due to the strong environmental flow ahead of the cold front. Wind advisories are posted with high wind warnings for the outer Cape and Islands where winds could gust as high as 55mph. Higher winds will also effect the higher terrain of the Catskills, Berkshires, Adirondacks, Green and White mountains where southerly wind gusts of 45-50mph are possible. Behind the cold/occluded front, which should make quick progress north and east to central New York State by daybreak, steady precipitation will abruptly end with just a few scattered showers left in its wake. Rainfall amounts during the overnight should range from a third to three quarters of an inch across the western half of the region while across the east rainfall amounts should come in over an inch, with possibly up to two inches in some locales. Of course, this excludes northern New England where the bulk of the precipitation will fall during the morning hours on Thursday.


In fact, by Thursday morning low pressure will have rocketed to the eastern Great Lakes with precipitation a done deal for the southern two thirds of the region as the dry slot rides north. This low will begin to feel a bit of blocking over northern Quebec and will shear east during the day on Thursday, helping to keep northern Maine stuck in a band of frontogenic precipitation for much of the day. Across the remainder of New England, the cold front will sweep eastward during the morning/early afternoon hours with moderate to heavy windswept rain out ahead of it. Once again, some weak instability is present so a rumble of thunder may be heard in some New England communities. Rainfall amounts should range from three quarters of an inch to an inch and a third across much of New England on Thursday. Behind the front brisk winds from the west will pick up in the cold advection pattern. Cyclonic flow will lead to mostly cloudy skies for most areas across the interior though breaks of sun are likely by late morning down along the coastal plain. Lake response should be weak with 850mb temps ranging from -2°C to -4°C by late afternoon leading to marginal instability. Only a few scattered snow showers along the spine of the Appalachians are expected with boundary layer temperatures along the lake shores warm enough for rain to mix in. As far as temperatures go, ahead of the front strong southerly winds will help to pump some rather mild air across New England where highs will reach into the 50’s and maybe even low 60’s towards southern New England. These temperatures will flirt with record highs for the date. Behind the front temperatures should fall through the 50’s and into the 40’s by afternoon with areas further west that see the front pass before daybreak should drop back even further by afternoon, into the 30’s.


Cold air advection will continue Thursday night with 850mb temps dropping another 3-4°C. With cyclonic flow will in place over the region lake effect should start to have more of an impact with some light accumulating snowfalls downwind of Erie and Ontario. Elsewhere skies should be partly to mostly cloudy with lows dropping into the 20’s across the interior, aside from the immediate lake shores, and 30’s down along the coastal plain.


Mid-term - Issued - 12/03/09 @9:00am


It will be a tranquil Friday outside of the Great Lakes region as most will enjoy partly sunny skies, especially down along the coastal plain. Temperatures will remain above normal, though colder air aloft will continue to bleed into the region. A brisk wind will gust at times up to 15-20mph out of the west but with temperatures climbing into the upper 40’s to mid 50’s along the coastal plain many folks will take that and run with it for early December. Further inland more clouds will be present but there will be some breaks of sun and here too temperatures will climb to slightly above normal readings from the upper 30’s to the mid 40’s. However, as mentioned above, in and around the Great Lakes region that colder air aloft flowing over the relatively warmer lake waters will initiate some lake effect precipitation which will be aided by a passing surface trough. Lake effect precip will fall as snow away from the lakes where up to an inch may accumulate across the higher terrain of northwest Pennsylvania/southwest New York and the Tug Hill Plateau region. Temperatures around the lakes will hold in the 30’s with the clouds and showers/snow showers.

Weak high pressure will build into the Northeast on Friday night which will provide enough subsidence for skies to clear for several hours during the late afternoon into early evening. This will allow for several hours of radiational cooling across the region and temperatures will plummet during the first half of the night. Across the interior temperatures will fall back into the 20’s by midnight with teens across the higher terrain of northern New York and New England and 30’s along the coast. After midnight high clouds will begin to stream in from the southwest carried on a 160-180kt jet streak riding up the East Coast as a storm system organizes in the Gulf of Mexico. This storm could potentially bring the first snowfall to a wide region of the Mid-Atlantic and near-coastal locales of the Northeast over the weekend.

The low moving of out of the Gulf will be a quick mover and will move several hundred miles east of the region. However, dynamic lift and frontogenesis in the mid-levels of the atmosphere might just squeeze out several tenths of an inch of precipitation on the backside of this low which could fall as snow as one heads away from the immediate coast with cold air in place on Saturday and Saturday night. North and west of the I-95 corridor 850mb temps will be in the -4°C to -6°C range but boundary layer temperatures will be in question, especially south of the Philadelphia metro area for those residents living in the Mid-Atlantic. Believe air will be just cold enough for a wet snowfall across these areas with accumulations possible on colder surfaces with main roads and highways remaining just wet. Once north of Philadelphia the airmass will be colder throughout the column and snow should have a better time sticking to roadways. Head far enough north and west (I-81/I-90 corridors) snow will taper with the storm too far east to affect these areas. Along the immediate coast boundary layer should be warm enough to support mainly rain. The precipitation should clear out of the region by Saturday evening with clearing skies for the remainder of the overnight. Some lake effect snow showers are possible as the wind will continue from the west but no organized activity is expected. Highs on Saturday will be in the 30’s along the coastal plain and across the interior with 20’s across the higher terrain. Lows at night will dip into the teens and 20’s with temperatures close to the freezing mark along the immediate coast and urban centers.

Much uncertainty remains regarding the eventual evolution of this storm. Should it track closer to the coast precipitation amounts may be considerably higher and the transition zone could be pushed further inland. On the flip side of the things the low could move a bit too far offshore to directly affect the sensible weather across the Northeast except for maybe the eastern shore points. Will have an update on this as the event draws near.


___________________________________________________________


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: 2:06 PM GMT on December 03, 2009

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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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