The cold and snowy pattern which has been in place since late January continues into the beginning of this week as another storm affects the region on Tuesday, producing 2-6 inches of snow away from coastal locations mainly during the morning hours. A quick change in conditions will occur by the late week with rain, strong wind gusts and even thunderstorms possible on Friday, followed by a gradual return to colder weather by the end of the month into early March (image credit: PSU e-Wall).
Tonight – Tomorrow: Snow Returns
Posted above from left to right are the latest surface analysis and radar composite from WPC, and the initialized NAM 500 millibar heights and vorticity from NCEP MAG; both are valid at 18z (1pm). A strong high pressure is currently positioned over the area, leading to fair skies and continued cool temperatures in the upper 20s to low 30s. Since the high pressure was not positioned overhead until this afternoon, with a northwest wind having continued last night before subsiding this morning, conditions were not ideal for strong radiational cooling, and temperatures in NW NJ and interior SE NY only fell into the low 10s. The main highlight over the US is an area of low pressure near Illinois tracking ESE, with widespread heavy precipitation associated with strong upward vertical motion ahead of a strong shortwave trough over Iowa, under regions of positive vorticity advection. Most of this precipitation is falling in the form of snow, with reports of heavy thundersnow in parts of Illinois earlier this afternoon. With a progressive, zonal flow aloft behind this shortwave, indicated by the flat 500mb geopotential height contours outlined in black to the west of this feature, the shortwave will continue to progress eastward while remaining slightly negatively tilted, as indicated by the SSE-NNW orientation of the trough axis. The surface low pressure will track northeast into western NY state early on Tuesday morning before a coastal low pressure develops just south of New York City, continuing NNE up the coast. While the low pressure will be fairly weak and compact, strong lifting ahead of the shortwave will be capable of producing a short period of moderate to heavy precipitation across the region.
18z run of the 4k-NAM model at hour 21, at 15z Friday (10am), depicting 700mb geopotential heights, relative humidity (green shaded areas) and vertical velocity, also referred to as omega (brown shaded areas). Negative omega values can be found near NYC and Long Island, indicating strong upward motion supporting the development of moderate to heavy precipitation (image credit: NCEP MAG).
The main uncertainty regarding the impact in the area is the development of the coastal low pressure and the anticipated warm air advection, which will influence the total snowfall amounts and how far inland rain extends from the coast. The high resolution model guidance, such as the NMM/ARW and the earlier runs of the high-resolution 4k-NAM, indicate over 0.50 inch of precipitation over most of the area, which would support widespread moderate to heavy snow totals across the area, while the global models, such as the GFS and ECMWF, are drier but have been trending slightly wetter with their recent runs, with at least 0.25 to 0.35 inch of liquid-equivalent precipitation over NYC. Some of these high-resolution models have a tendency to depict too much precipitation, and another area of uncertainty is regarding the exact positioning of the strongest upward vertical velocity – as shown to the left, the latest 18z run of the 4k-NAM depicts this near and southeast of NYC, which is ultimately where heavy precipitation develops and expands as it tracks northeast along with the surface low pressure. At this time, I am siding closer to this scenario, with the heavier precipitation from about NYC into Long Island and further NNE into Connecticut and most of New England.
The next area of uncertainty is the rate of warming and how it will affect precipitation types; surface temperatures will be in the 20s at the onset of precipitation, supporting accumulating snow, but due to the high pressure departing to the east, an increasing SSE flow will set up in the boundary layer from the surface to about 850mb, leading to warm air advection. Coastal southern locations in Long Island are most likely to quickly change to rain with little accumulations, although the moderate to heavy rate of precipitation may at least initially offset the marginal boundary layer temperatures. As the morning hours progress, warmer temperatures should be able to more easily spread inland, with Long Island and NYC expected to change over to rain, possibly including NE NJ and coastal CT, before precipitation ends in the early afternoon hours. This mixing is expected to limit snow totals in Long Island to mostly under 2-3 inches, with the highest totals in the area likely over CT away from the immediate coast.
Forecast for NYC Area: Light snow is expected to develop tonight between at least 3-5am, remaining fairly light and scattered until heavier snow bands develop around 7-9am; these are primarily expected to be focused over NE NJ, NYC, Long Island and Connecticut, while light to moderate snow continues further inland. A quick changeover to rain is expected near southern and eastern Long Island, transitioning to rain elsewhere in Long Island and NYC by at least 10-11am as surface temperatures warm into the low to mid 30s. Locations further inland are expected to remain with plain snow, with a risk of some light rain at the end near coastal CT and northeast NJ, with precipitation ending by 12-1pm for most locations.
Snow totals of at least 1 to 4 inches are expected across Long Island, ranging from near to below 1 inch near coastal southern and eastern locations and 4 inches near the north and west locations; 2 to 4 inches in NYC; 2 to 5 inches over SE NY, northeast NJ and southern CT, perhaps below 2 inches in western NJ, and around 4-6 inches in interior Connecticut. Snow totals are subject to slight changes, with the biggest areas of uncertainty regarding Long Island, with the possibility for more snow than forecast if warm air advection is not as significant as currently anticipated, and snow totals from NE NJ into NYC and Connecticut which depend on the western extent of the heavy precipitation bands. Occasional storm updates will be posted on Tuesday morning.