Northeast Weather Blog

Last Updated: 5:27 PM GMT on Enero 07, 2009 — Last Comment: 3:28 AM GMT on Enero 08, 2009

	Significant winter storm to impact the Northeast

Posted by: sullivanweather, 10:23 AM GMT on Enero 06, 2009

Current watches, warnings and advisories.

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Forecast Discussion

Synopsis - Issued - 1/6/09 @5:30am

A significant winter storm will move into the Northeast on Tuesday and linger into Thursday with lake effect snows left in its wake. Another area of low pressure will approach by Friday and Saturday, bringing more wintry weather to the region. This second system will be followed by an arctic blast that will drop temperatures well below normal for the foreseeable future.

Short-term - Issued - 1/6/09 @5:30am

An extremely complex weather situation is currently unfolding that promises to bring a prolonged winter weather event to the Northeast. The next question is where to begin?? In the southern stream, several shortwave disturbances, along with their attending surface lows, are riding a strong 140-160kt sub-tropical jet stream out of Texas, up the Tennessee Valley, across the Mid-Atlantic and off the coast. Deep sub-tropical moisture (Precipitable water values in excess of an inch) south of the aforementioned line is helping to bring much needed rainfall to the drought-stricken Southern Appalachians; where up to 2-4” of rain is possible. The main pocket of energy in the southern stream is still lingering back over northern Mexico and this will be pulling northeastwards today. Meanwhile, in the northern stream, a tough digging into the Northern Plains will eventually link up with this southern stream energy, partially phasing both jet segments across the eastern half of the country. I mention partially because this entire storm system will remain rather disjointed, a double-barreled storm with no dominant area of low pressure forming until the entire mess reaches the Canadian Maritimes. The result of the northern and southern streams coming together will be building heights off the Southeast Coast that will eventually lift this whole mess towards our region of the country this afternoon. Apparent on satellite imagery is a thickening shield or mid/high level clouds that as already pushed as far north as the New York/Pennsylvania border. These will gradually expand northeastwards as the day progresses with precipitation not far behind.

But first things first. Currently, high pressure rests atop the Northeast, albeit weak, with a central pressure of merely 1018mb. There are still some scattered lake-effect flurries coming off Lake Ontario in light westerly flow on the north side of this high combined with marginally unstable lake-effect parameters but these shouldn’t amount to anything more than an inch, at most. High clouds have already overspread the southern half of the region and these should lower and thicken as the morning progresses. Precipitation is not far behind the advancing cloud shield, with light rainfall being reported just south of the Mason-Dixon line at Maryland ob sites. Increasing isentropic lift will cause precipitation to blossom across Pennsylvania south of the turnpike by noon, eventually overspreading the remainder of the commonwealth, as well as New Jersey and the southern half of New York by evening. The precipitation should start as snow across a majority of the region as the entire column is below freezing. But increasing warm advection will create a wedge of warmer air in the 850-700mb layer that will begin to turn the snow over to a mixture of sleet and freezing rain across southern Pennsylvania but not before an inch or two of snow accumulation. Along the immediate coast the precipitation should also start as snow but an increasing easterly fetch off the relatively warmer ocean waters will make for a changeover to rain as boundary layer temps warm above freezing quickly. The changeover to plain rain should also come rather quickly to southwestern Pennsylvania as there’s little to hold the cold air in place across the west side of the Appalachians as the primary low heads in that direction. To the north there will just be a general increase in clouds today, with precipitation holding off until the evening/overnight hours (upstate New York, southern/central New England) or Wednesday (northern New England). Highs will be in the 20’s across a good portion of the interior east of the Appalachians, with the exception being the higher terrain of northern New York and New England where temperatures may remain in the teens. Along the immediate coast of New Jersey and across southwestern Pennsylvania temperatures should eventually climb above freezing, into the mid to upper 30’s. From New York City northward along the coast temperatures should remain just below freezing, in the upper 20’s to low 30’s. Surface ridge axis across upstate New York across central New England will divide the wind flow across the region. To the north of this axis winds will be from the west to northwest around 5-15 mph while south of this axis (and east of the Appalachians) winds will generally be out of the east-northeast around 5-15mph. West of the Appalachians winds will be more from a southerly to southeasterly direction, gain around 5-15mph.

Things start to get really messy tonight as snow will overspread the remainder of upstate New York and southern/central New England. Increasing warm advection will also start to bring the transition zone northwards, likely reaching the NY/PA border region and eastwards to the Mid-Hudson Valley/Connecticut River Valley just after midnight, and then continuing on north to the I-90 corridor (NYS Thruway/Mass Pike) by daybreak. Along the northern edge of the transition zone a deep enough layer of cold air will exist (for the surface to ~900mb) for mainly sleet. However, south of the NY/PA border, across much of central Pennsylvania and interior southeast New York/southern New England the layer of cold air from the surface on up will be much thinner. Here precipitation will likely transition to mostly freezing rain, which should begin to cause some problems. A coastal low developing along the warm front off Delmarva, along with confluent flow over northern New England, will help to lock in the cold air at the surface, as ageostrophic winds turns to the northeast, despite the primary low heading well west of the region up towards the Great Lakes. After the transition to freezing rain QPF should range from a third of an inch to two thirds of an inch. This may be enough to bring down tree branches, eventually causing power outages over a widespread area in the above mentioned areas. While this event doesn’t look nearly as severe as the December ice storm, there will be significant disruptions. Along the coast precipitation will eventually changeover to plain rain. Strong low-level frontogenesis in the vicinity of the coastal front will lead to heavy precipitation amounts, well in excess of an inch, which may lead to urban flooding, swollen ditches and ponding water in parking lots. To the north where mostly snow falls accumulations overnight will range from 3-6”. Overnight lows are likely to hold steady over much of the region or even rise a couple of degrees, especially along the coastal plain where increasing levels of maritime air will be introduced.

Coastal low begins to get cranking on Wednesday, eventually becoming as strong as the primary by evening. Across the northern interior, precipitation will continue in the form of snow, adding up to another 3-6”. Along the coast precipitation will continue as rain, which may fall heavy at times but in between is where the real trouble will lie as sleet and freezing rain will continue as the low-level cold air will be unyielding. A dry-slot will eventually work into central Pennsylvania and New York, tapering the precipitation off to a showery regime, however, the damage will already have been done with up to an half inch of ice accretion falling across a wide area. Details will be shown in map form following the discussion. By Wednesday night heights crash as the upper trough axis pulls through and cold air advection strengthens, changing any leftover showery precipitation over to snow. Across northern New England snow will be ongoing, falling heavy at times with additional accumulations of up to a half foot.

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Maps.

Ice accretion