We have a weather situation evolving over the Lower 48 which is likely to bring an historic blizzard to the Northeast. It’s a textbook scenario with arctic air over southeast Canada drifting into the Northeast US while two branches of the jet stream are active with both northern and southern branches having a storm which will converge. The first, riding the northern branch across the Upper and Lower Midwest will bring as much as 4-8 inches of snow tonight across the Chicago area as well as through central Lower Michigan, 2-4 inches is expected across the Ohio Valley. This system will head towards the New York City area. A second, southern branch system has developed down along the Gulf Coast and this is pulling an abundance of Gulf moisture north bringing big rains across the Mid-South and Southeast part of the country. While this storm heads up the coast in classic Nor’easter fashion, pulling Gulf moisture with it, the system diving southeast across the Great Lakes will be heading towards the New Jersey coast where both meet off the Northeast coast. This is the phasing or convergence between polar and sub-tropical jet streams which are crucial for MAJOR snowstorm over the Northeast. This, combined with arctic air drifting southeast out of eastern Canada, the phasing of the two storms and the abnormally warm SST’s off shore, means major cyclogenesis off Long Island.

Here’s the ECMWF pressure/precip/wind chart for the next 48 hours.

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As you can see, there’s a low over New Orleans which is currently bringing big rains and convective thunderstorms across a large swath of the Southern states while a system drives southeast across the Great Lakes, this is classic when it comes to major Northeast snowstorms.

By 12Z Friday a 1008mb low is diving E,SE across the Ohio Valley while that low which is currently over New Orleans, heads nearly due north, skirting the Carolina coast with pressure steadily falling. This system will continuously draw Gulf moisture up the coast. While this is occuring, arctic high pressure will be sinking out of Ontario and Quebec into the Northeast ahead of both northern and southern branch systems. You can see where this is all going right?

By 48 hours or Saturday afternoon, the northern branch meets southern branch system and they converge, rapidly deepening one main low. As bombogenesis occurs with pressure falling from 1000 to 990mb between the Florida Panhandle and the Virginia Capes, pressure rapidly falls into the 980s and 970s off Long Island. As this occurs, both Atlantic and Gulf moisture gets wrapped into the circulation with the pressure field tightening. Moisture as well as warm and cold air all converge and with the warm-cold as well as moisture rich air meeting, so a maior snowstorm breaks out from New York City up to Maine with southern New England eyeing the worst.

As well as tremendous snowfall rates of 2-5 inches per hour with thundersnow, winds will rage out of the northeast at 40+mph with gusts of 50+mph along the Jersey and Long Island shore including New York City, gusts could reach 65-80 mph along the New England coast. These kinds of winds combined with the tremendous rate of snowfall means an all-out blizzard and a complete shutdown from Providence, RI to Portland, ME.

The most recent runs of several models suggest colder air and more moistur egets in and south of the New York City area, this means a possible 8-14 inches of snow is possible for Manhattan and surrounding boroughs. Hartford, CT looks to see 14_ while Providence up through Boston may pick up an incredible 18-24 inches with locally 30-35 inches possible. The 18-24 inch swath extends from Providence up to Portland, Maine.

The ECMWF and NAM suggest some outstanding snow totals while thw GFS is somewhat more conservative, although the GFS is likely far less reliable.

Here’s the latest 3-hourly NCEP-NAM accumulated snow forecast

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