1:00 PM | ****Quick thumping of snow at the onset in Philly-to-NYC corridor…ice/heavy rain tomorrow night…brutal cold later Sunday into Monday…power outages are a threat****
Paul Dorian
Bitter cold grips the Mid-Atlantic region by early Monday morning (12Z NAM)
Overview
A significant winter storm will impact the Philly to NYC corridor from Saturday afternoon into Sunday with accumulating snow, ice and heavy rain. Snow should arrive during the mid-to-late afternoon hours on Saturday and continue into the early evening before transitioning to rain later tomorrow night. The rain can fall heavily at times tomorrow night and it can freeze on some surfaces for awhile in the far northern and western suburbs where temperatures will struggle to climb above freezing. On the backside of the storm, much colder air will pour into the region on Sunday and rain or a wintry mix early Sunday may end briefly as snow with only minor additional accumulations likely. As the Arctic air floods the region later Sunday, N-NW winds will intensify, temperatures will plunge and any slushy or wet surfaces may quickly ice-up. The plummet in temperatures will continue on Sunday night and overnight lows will likely be in the lower single digits come early Monday – easily the coldest day of the year so far - and afternoon highs will do no better than the teens.
Discussion
Low-level cold air during the upcoming weekend will be slow to retreat and this could lead to accumulations of snow and/or ice in at least parts of the region at the onset of the storm. By early Saturday, low pressure will be intensifying over the Mississippi Valley and it’ll feature lots of snow to its north/west and rain to its south/east. At the same time, an important factor will be strong high pressure that will be building over southeastern Canada and this will push a reinforcing cold, dense air mass into the Northeast US. It is likely the surface low pressure system will advance as far north and east as West Virginia this weekend before jumping towards the Mid-Atlantic coastline and likely located not far from Long Island, NY by early Sunday morning.
Progression of the storm on Saturday as depicted by the 12Z NAM model; courtesy NOAA/ tropicaltidbits.com
Precipitation should arrive in the Philly-to-NYC corridor by the mid-to-late afternoon hours on Saturday and, with the reinforcing cold air mass in place, it is quite likely to be in the form of snow. The snow will likely continue into the early evening hours and it can come down hard for a brief time before a transition. There will be tremendous “frontogenesis” early tomorrow night leading to upward motion across southern PA, New Jersey, southern New York (see 12Z NAM forecast map) and this is likely to contribute to the burst of snow in the early stages of the weekend storm. Accumulations on the order of 2 inches are possible in the Philly metro region with up to as much as 5 inches in the far northern and western suburbs later tomorrow and early tomorrow night with 3-6 inches possible in the NYC metro region (lowest southeast to highest far northwest).
Intense precipitation bands may accompany strong “frontogenesis” later tomorrow evening; courtesy NOAA, tropicaltidbits.com
On Saturday night, precipitation may transition to sleet and then to rain, heavy at times, but with temperatures near freezing in many locations, it is possible that the rain can continue to freeze on some surfaces across the far northern and western suburbs for a brief time. On Sunday, cold air will be advancing southward as the storm pulls off the Mid-Atlantic coastline and the precipitation may change back to snow for a brief period of time before it winds down.
12Z NAM forecast map for late Saturday evening (left) and late Sunday night (right); courtesy NOAA, tropicaltidbits.com
An influx of Arctic air will cause temperatures to drop sharply later Sunday and Sunday night and winds will pick up in intensity out of the N-NW. Any slush or residual wet surfaces will freeze rather quickly late in the day on Sunday. The plunge in temperatures will continue on Sunday night and overnight lows could end up in the lower single digits by early Monday – easily the coldest day so far – and highs on Monday will do no better than the teens. In fact, those areas that receive a fresh, deep snow pack could wake up Monday morning close to the zero degree mark. One of the reasons this Arctic air mass will be so cold around here is the fact that it’ll be coming from the continental part of southeastern Canada and not able to get modified by the more typical crossing of the still-not-completely-frozen Great Lakes.
One final note of concern, the intensification of the winds to 40 mph or so late Sunday and Monday as Arctic air pours into the region may not work too well with potentially ice-covered or snow-covered branches and limbs (i.e., possible power outages) – this is a terrible prospect given the bitter cold.
12Z Euro surface forecast map for January 27th…wow; courtesy WSI, Inc., ECMWF
Looking ahead
This will certainly not be the end of the storminess and the bitter cold air masses for the eastern US. Another storm may impact the Mid-Atlantic by the middle or latter part of next week - perhaps again resulting in mixed precipitation. Beyond that, there are signs for an even colder air mass to invade the central/eastern US in about ten days or (1/27-2/28 or so) and that could be accompanied by a snowstorm in parts of the Northeast US…stay tuned.
12Z Euro forecast map of 100-500 mb thickness for January 27th…wow; courtesy WSI, Inc., ECMWF