Thursday, April 17, 2014
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Bolaris: Beware the backlash on this nor'easter

In this satellite image provided by National Oceanic and Atmospheric Administration (NOAA), a winter storm front moves through the Southern states on Wednesday. The nor´easter is expected to dump heavy snow on much of the Philadelphia region. (Photo by NOAA via Getty Images)
In this satellite image provided by National Oceanic and Atmospheric Administration (NOAA), a winter storm front moves through the Southern states on Wednesday. The nor'easter is expected to dump heavy snow on much of the Philadelphia region. (Photo by NOAA via Getty Images)
In this satellite image provided by National Oceanic and Atmospheric Administration (NOAA), a winter storm front moves through the Southern states on Wednesday. The nor´easter is expected to dump heavy snow on much of the Philadelphia region. (Photo by NOAA via Getty Images) Gallery: Nor'easter hits the Philadelphia area

A Nor'easter is locked and loaded with heavy snow, ice and rain - and taking aim at the entire Delaware Valley.

Right now a possibly historical ice storm is making life miserable from Atlanta to Columbia, S.C. and making its way through central and eastern North Carolina, with heavy snow piling up to the west.

Set-up

The storm system is still developing along the Southeast coast and will move northward to a point along coastal North Carolina by early Thursday morning. Dynamics are taking place in the atmosphere that will allow for rapid intensification along the Mid-Atlantic coast as this developing Nor'easter begins to move slowly up the coast.

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  • There will be three distinct parts to this storm:

    1. The snow

    A wall of heavy snow on the front end will result from a strong dome of arctic air in place in conjunction with warm air advection (surging milder Atlantic moisture), thus creating a deformation zone (atmospheric front) that will produce a very heavy band of snow right along this boundary. It appears the I-95 corridor, or just to the west, will see this take place overnight and early Thursday morning.

    2. The changeover

    A lull in the storm will mean the snow will mix and change to rain and sleet during most of the day after 10-11 a.m.

    3. The backlash

    As the storm slowly pulls away, it will pull down colder air which, at times, means more snow. Well, the backlash here is now looking quite formidable with light to moderate and even additional heavy accumulations taking place mainly after 7 p.m. Thursday night into and through midnight, especially across the northern and western suburbs. It's possible that sections of the Lehigh Valley, could pick up another 5-10 inches in the backlash.

    Accumulations

    PHILADELPHIA

    Between 10 p.m.-midnight, light snow arrives. After 1 a.m.: Snow becomes steadier and heavier.

    Between 4-8 a.m.: Heavy bands of snow, with rates possibly reaching 1-2 inches per hour. Greatly reduced visibilities. Rapid accumulatiions on all roadways. Possible isolated thundersnow.

    Up to 6 inches by 9 a.m.

    Between 10 a.m.-noon, a lull in the storm is likely as snow intensities drop and there's a changeover to a mix of sleet and then periods of rain. This will continue through the evening rush.

    After 7 p.m., a band of snow begins to reform and could deposit another 2-3 inches.

    Snow ends between midnight and 2 a.m. Friday.

    Total accumulations: 6-12 inches

    EASTERN MONTGOMERY, EASTERN CHESTER, DELAWARE and LOWER BUCKS COUNTIES

    Timing and snow amounts generally the same as Philly. 6-12 inches

    WESTERN MONTGOMERY, WESTERN CHESTER and UPPER BUCKS COUNTIES

    8-12 inches.

    By noon, up to 6 inches. From 8 p.m. Thursday-1 a.m. Friday, the backlash could mean an additional 5-6 inches.

    BERKS and LANCASTER COUNTIES, plus the LEHIGH VALLEY

    This could be the bullseye for the heaviest amounts of snow as I'm seeing a BIG backlash possible for you - an additional 6-10 inches. But we'll need to see how the backlash develops on Thursday. Total accumulation with backlash: 10-15 inches.

    NEW JERSEY

    From the Delaware River east ending to I-295: looking for up to 6 inches by 9 a.m.

    By 10-11 a.m., a lull in storm as snow intensity decreases and changes to sleet and rain. This will continue throughout the day.

    Backlash after 8 p.m. another 2-3 inches possible, especially in Mercer and northern Burlington counties.

    Total accumulation: 6-12 inches across Mercer and northern Burlington; 4-6 inches across southern Burlington.

    Western Camden, Cherry Hill, Haddonfield, Voorhees: total by midnight Friday, 6-10 inches.

    Western Salem and Gloucester counties: 6-10 inches.

    Eastern Camden, Gloucester and Salem counties: 4-6 inches.

    Shore locations, 2 inches or less.

    DELAWARE

    New Castle County: Same timing as city of Philadelphia. Up to 6 inches by 10 a.m.

    By 10 a.m. snow intensities drop, as precipitation changes to sleet and periods of rain, throughout the day. Between 7 p.m. Thursday and midnight Friday, snow showers add an inch or two.

    Kent County: 6 inches.

    Sussex, 3-5 inches.

    On the good news weather front: significant warming by the middle of next week!!

    John Bolaris

    John Bolaris For Philly.com
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