A cold air mass returned into the area last night, with urban locations falling into the 10s but locations with strong radiational cooling falling to as low as -7 degrees in Sussex, NJ. Similarly cold temperatures are expected tonight, but with increasing cloud cover on Wednesday as a deepening low pressure tracks up the coast, producing a narrow swath of heavy snow up the East Coast late Wednesday night into Thursday. A brief warm up into the upper 30s is expected on Friday, but with colder than average temperatures otherwise continuing through next week.
Today – Wednesday: Cold Continues
Posted above from left to right are the latest surface analysis and radar composite from WPC, and the initialized GFS 500 millibar heights and vorticity from NCEP MAG; the former is valid at 9z (4am), and the latter is valid at 6z (1am). Over the last day, a cold air mass continued to sink southeastward from the Midwest US, associated with a strong high pressure centered near Iowa, with cold temperatures currently covering the majority of the eastern US. South of this high pressure, a broad area of moisture expands to the north of a stalled frontal boundary near the Gulf coast, with enough cold in place to support snow from northern Mississippi into Georgia. As the high pressure expands eastward into the NE US tonight, the moisture will remain well to the south, extending into North Carolina and South Carolina
With the high pressure overhead, along with light winds and snow cover on the ground, will support strong radiational cooling away from urban and coastal areas; lows are expected to range from the low 10s near NYC and the coast to the single digits further inland and below zero in NW NJ/SE NY and the Pine Barrens in Long Island. The latest high-resolution run of the 4k NAM from PSU e-Wall has been posted to the left, depicting modeled lows for tonight. The cold air mass aloft will depart on Wednesday with southwesterly winds in the mid to upper levels, but with cold temperatures persisting longer near the boundary layer with high temperatures in the mid to upper 20s.
Wednesday Night – Thursday: Heavy Snow, Rain, Wind Expected
The main highlight of the week is on Wednesday night into Thursday, as a strong coastal low pressure tracks up the coast, bringing yet another round of heavy snow across parts of the region. In the 500 millibar map posted above, the two main features to make note of are the southern stream shortwave over New Mexico, and the lead northern stream shortwave just off the coast of the NW US. The southern stream feature will continue to track eastward through the southern US, supporting the formation of a low pressure south of Louisiana on Wednesday. Meanwhile, with a northwesterly flow aloft the western US, the northern stream feature entering the NW US will dive southeastward, reaching the southeast US late on Wednesday where phasing of the north and south streams is expected, leading to a deepening low pressure tracking up the East Coast as the trough becomes neutrally and later negatively tilted, pulling significant amounts of moisture from the Gulf of Mexico and Atlantic Ocean into the system and leading to another round of heavy snow from the Southeast US up the I-95 corridor into Maine.
6z GFS at hour 24, at 6z Wednesday (1am), depicting the three main shortwaves involved in this system; the southern stream feature near eastern Texas, the northern stream near South Dakota, and the kicker shortwave entering the NW US (image credit: PSU e-Wall).
Model Analysis: The main area of uncertainty regarding the storm is the exact positioning and strength of both the north and south stream shortwaves, along with a trailing shortwave expected to enter the NW US early on Wednesday before quickly racing eastward, helping to keep the flow progressive and depending on its proximity to the coastal low pressure affect the dynamics and precipitation amounts in the deformation banding to the west of the low pressure. The model guidance continues to come into better agreement since yesterday morning’s update, with the ECMWF, yesterday’s western outlier, having shifted slightly to the east, and the GFS, yesterday’s eastern outlier, continuing to trend towards a closer proximity between both shortwaves, leading to a stronger phase and a sharper trough axis which becomes negatively tilted further west and closer to the coast. The southern shortwave is already over the US and has been sampled, but the lead northern stream shortwave was still just offshore as of the 0z model runs; the shortwave is expected to land onshore by this morning in time for the 12z runs, with more data sampling for the model guidance to ingest and gain a better handle on the exact positioning of the shortwave feature and its impact downstream on the storm system. With the current trends in the model guidance, confidence is increasing on the scenario described in yesterday’s outlook, although some uncertainties remain as the model guidance continues to narrow down the uncertainty over the next day with some additional changes possible to the forecast; at this time, I am siding towards a blend of the 6z GFS, NAM and CMC, with the ECM considered a western outlier.
Impact Analysis: This storm system will produce widespread impacts along the East Coast, ranging from more snow and freezing rain in the southeast US, including locations such as Atlanta which were significantly impacted by the winter storm late last month, to an axis of heavy wet snow along the I-95 corridor into Maine and strong north to northeasterly winds near coastal locations. As the system spreads into the region late on Wednesday night, the strong cold air mass aloft will have departed the region, but with the air mass remaining in place cold enough to support snow, along with surface temperatures still in the 20s, holding steady or slightly falling from Wednesday’s daytime high temperatures. A band of high 700 millibar upward vertical velocity and strong frontogenesis is expected to expand northeast into central Virginia and near Washington DC into Maryland and southern NJ, supporting strong lifting in the mid levels which will likely lead to heavy snow rates near or over 1-2 inches per hour; with the most favorable dynamics in this region, the highest snow accumulations from this storm are favored to set up over central-western Virginia towards near or west of Washington DC.
The strongest lifting is then expected to continue along the I-95 corridor extending into the NYC area into New England until the low pressure rapidly deepens into the 960-970mb range near Maine, with heavy precipitation expected to reach the area early on Wednesday morning, supporting a period of 1 or more inch per hour snow rates for most locations, including coastal areas which may eventually change over to rain. By the late morning, the initial front end banding will depart the area, but with support for a deformation band setting up northwest of the low pressure. The exact location of this band remains to be determined, although at this time based on the anticipated setup, a likely positioning of the deformation band after the front end round of heavy snow is between about central-northern NJ into central New England and central Maine, pivoting east through the area and eastern New England later in the evening and overnight hours as the low departs the region while rapidly deepening near Maine. Southeast of this deformation band, warm air advection is expected to peak by the mid afternoon hours before east-SE winds turn northerly as the mid level and surface lows pass to the east of NYC. While the snow banding temporarily weakens while placed over northern NJ into central New England with moderate snow likely, a changeover to rain is expected over Long Island but with the inland extent the main uncertainty; at this time, coastal southeast CT and Long Island are most likely to change to rain and/or sleet in this time period, with the possibility of at least brief mixing with rain up to the rest of coastal CT and NYC. As the deformation band likely wraps around through the area again in the evening, some intensification of snow rates are possible with a changeover to snow likely near and east of NYC, but with snow rates not expected to be as heavy as the morning.
As this is still at least 2 days away with some short range differences with the model guidance, uncertainty continues regarding the exact setup of snow bands which is subject to additional changes. The ECMWF remains the westernmost model at this time, and depicts the heavy snow banding progressing NNE through New England with any pivoting of snow bands towards the area in the evening less evident than the NAM, CMC and GFS models. Confidence is highest at this time regarding the front-end heavy snow band, although how much precipitation is left over behind the initial banding, as well as the inland extent of the mid level warm air advection and positioning of the deformation band, which will determine how much snow falls after the morning hours. These details will continue to be narrowed down over the next 1-2 days as the storm approaches.
Forecast for NYC Area: Snow is expected to develop in the area towards 2-4am on Thursday, tracking northeast while becoming heavy towards the early morning hours as temperatures slowly rise into the mid to upper 20s. Heavy snow is expected to continue through at least the late morning to possibly the early afternoon hours before lifting northeast of the area, with northeasterly winds turning to the north at 10-25 mph. Through the mid-late afternoon hours, light to moderate snow is likely to continue north and west of NYC, while locations near and east of NYC observe lighter precipitation rates as precipitation type mixes with and changes over to rain. Slightly cooler temperatures are possible in the evening with additional light to moderate snow, likely to end towards 9pm-12am on Thursday night.
There remains some uncertainty regarding the exact snow totals, although preliminary thinking at this time is for widespread moderate to significant wet snow accumulations, with potential accumulations ranging from 4-8 inches in Long Island, including coastal SE CT, and 7-12 inches for the rest of the area, possibly up to 14 inches. As the model guidance has not reached a consensus regarding the exact details yet, additional changes in the outlook are still possible; one potential scenario would be a slightly further east storm track with Long Island and coastal locations seeing higher snow totals up to 7-12″ and a lower probability of a changeover to rain, or a slightly weaker low pressure which also can’t be ruled out. Another forecast update will be posted late this evening after 7-8pm along with a preliminary snow map, and the next full forecast discussion will be posted early on Wednesday morning.