The first widespread snowfall of the season is just days away for parts of the inland Northeast as a fast-paced coastal storm races up the East Coast early this week. The snow will mainly be of the nuisance variety. Meanwhile, more windswept rain is on the way for coastal and low-lying areas of the region early this week, just days apart from the last soaking rainfall.
This is your Sunday Storm.
This upcoming coastal system garnered a lot of attention across social media last week as having the potential to deliver an accumulating snowfall all the way to the coast. Many of those behind the chatter must have forgotten to look at the calendar. The October 29-31, 2011 “Freaktober” and the post-Sandy November 8-9, 2012 snowfall events may still be fresh in coastal snow-lovers’ minds, but accumulating snow so early in November is a climatological extreme. The same is not true for inland parts of the Northeast, which regularly receives autumn snowfall. Model guidance has since toned down on its portrayal of the upcoming coastal storm towards a scenario more typical of November.
Two upper-level troughs will send shots of winter-like air across most of the contiguous US this week. Both of these upper-air systems will contribute to this week’s coastal storm. The first trough will dominate the flow of the southern branch of the jet stream and has already developed. It is digging southeastward across the Rocky Mountains as of Sunday afternoon. Saturday it emerged from Alberta, and has since intensified over the Rocky Mountains and become increasingly more potent with time.
The cold blast contained beneath the southern trough is going to be rapidly spreading southeastward throughout Sunday afternoon and will continue to do so throughout the week, bringing arctic-like cold as far south as the subtropics of central Mexico. This caliber of cold hangs on the fringes of climatological extremes, as a trough digging so far south in November is on the order of three standard deviations below normal as suggested by the GEFS mean below, courtesy of TropicalTidbits. This highly frigid airmass will approach the warm Gulf of Mexico Monday afternoon. As it does so, the Gulf’s warmth will be drawn northward east of the trough. Cyclogenesis, the birth of the coastal storm, will ensue thereafter.
The southern trough will reach the Gulf Coast by Tuesday afternoon. Its arrival will be too late, as by then the storm will have already been responsible for producing modest Nor’Easter conditions over Cape Cod, MA. Instead, the second trough will increase the dynamic support for the coastal storm as well as deliver the cold necessary for snow in the Northeast interior.
The second trough will soon dominate the northern branch of the jet stream. It was crawling over central Manitoba only as a meager shortwave Sunday afternoon. An expanding ridge over the West Coast of the US and Canada will prompt the shortwave to accelerate toward northern New England, intensifying as it does so. Unfortunately, its cold air will arrive too late to allow for snow at the coast. It will, however, arrive just in time to phase with the southern trough to intensify the low pressure system from a baroclinic wave to a closed off coastal storm over the northern Mid-Atlantic and New England coasts.
The storm will initially organize along the cold front associated with the southern trough Monday night over the Deep South. The magnitude of the incoming cold will permit the development of a wide variety of modes of inclement weather. Northwest of the low, a moderate snowfall on the order of 3-6″ will bring a hearty helping of winter to the southern Plains and the Ohio River Valley. Meanwhile, heavy rain showers will organize along the cold front from the Mississippi River Delta northeastward across the southern Appalachian mountains. Some of the downpours near the coast could become quite violent. Strong horizontal temperature gradients along the Gulf and southeast Atlantic coasts could produce horizontal vorticity sufficient to generate a few isolated tornadoes in coastal areas from Louisiana to South Carolina.
The same subtropical airmass responsible for severe weather along the Southeast coast Monday will surge northward up the East Coast. The strong temperature gradient between the arctic and subtropical airmasses will support a vigorous low-level jet streak that will carry tropical Atlantic moisture into New England. The jet streak will help draw moisture and precipitation northward throughout the day, spreading precipitation northward across the Southeast and Mid-Atlantic to the lower Great Lakes before midnight Tuesday. The resulting tropical connection will be so strong that the coastal storm will be drawn from North Carolina to Maine in less than 18 hours between Monday night and Tuesday afternoon. This will ensure that the precipitation clears seemingly as fast it arrived.
One consequence of the storm being drawn north so rapidly is that it will escape the favorable jet dynamics of the southern trough. It will therefore intensify very little until the northern and southern branches phase with the confluence of the northern trough with the southern jet. The steams will begin to phase early Tuesday morning but they will not fully phase until Tuesday night after the storm departs from most of the Northeast. As they begin to phase, the southwesterly flow ahead of them will mutually intensify and expand warm advection from the Gulf of Mexico towards the Northeast, thereby increasing large-scale lift over the region.
The low level jet will intensify over the Mid-Atlantic coast Monday night as the two branches of the jet stream begin to phase. The surface low and the heaviest precipitation will follow the jet as it will produce the strongest forcing. There are modest differences between model guidance as to where this jet sets up. The GFS depicts a slower, weaker cyclone than the NAM, ECMWF, and its ensemble mean. There will likely be deep convection occurring with this system, process that cannot be resolved well by the GFS. Given its slow bias, the NAM and ECMWF solutions are favored.
Recent runs of the NAM and ECMWF have been consistent with the LLJ developing over coastal NJ, Long Island, New York City, the lower Hudson Valley, and southern New England. This region should expect the heaviest precipitation. Given temperatures in the middle 50s to lower 60s, this precipitation will be in the form of rain. The low is expected to pass near New York City. East of here, a quick 1-2″ rainfall is likely, whereas 0.5-1″ of rain can be expected just to the west of the storm track throughout the rest of this region. Rain should clear by late Tuesday afternoon.
Snow will be a concern further north and west. Precipitation will begin as a mix of rain and snow over western Pennsylvania, western New York, the Adirondacks, and northern New England. Evaporation due to dry air near the surface will cool these areas enough for rain to changeover to snow overnight Monday to Tuesday. The Gulf of Mexico will eventually win out, warming all low elevations enough to changeover to rain by Tuesday afternoon. Only the Adirondacks and mountainous terrain of northern New England are expected to evade the changeover.
A coating to 2″ of snow will fall in low elevations across northwestern Pennsylvania, upstate New York north and west of the Hudson Valley, southern Vermont , southern New Hampshire, and central Maine. The Mohawk Valley is expected to fair slightly better, where up to 3″ could fall. The Allegheny Plateau in northwestern Pennsylvania and middle elevations of the Adirondacks and northern New England will hang on to snow even longer, picking up 2-4″. The highest elevations of the Adirondacks, Green Mountains, White Mountains and northern Maine are expected to remain all snow. Most locations here will accumulate 3-6″ of snow, but isolated pockets of up to 8″ are possible here. Snow and rain will clear inland areas of the Northeast by Tuesday evening.
A frigid air mass from the northern trough will usher in behind the coastal storm. Northwesterly wind will provoke lake effect snow for portions of the Lower Great Lakes Wednesday, but otherwise dry winter-like conditions will dominate throughout the rest of the week. Like all coastal storms, the specific details are likely to change as the storm gets closer. Be sure to check back with WeatherOptics as details become more clear.