Mar 22, 2011: Late Season Snowstorm Tomorrow

After a storm brought accumulating snowfall to the interior parts of the area yesterday, today brought a brief return to drier conditions with high temperatures peaking in the mid to upper 40s inland and in the upper 40s to lower 50s for the rest of the area with a maximum temperature of 53 degrees in Newark.
This return to dry conditions, however, is already ending as a storm is producing clouds in the area with temperatures and dew points dropping, resulting in snow tomorrow morning, and wintry precipitation will continue to affect the area until Thursday morning. This is not the last of winter, however, as a cold pattern continues through at least early April with two additional storms that may bring frozen precipitation to the region, potentially including parts of the area.

Tonight Into Thursday: Significant Late Season Snowstorm
The snowstorm that brought up to 3-4 inches for the interior parts of the area yesterday was significant for this time of the year, however this storm will be an even bigger snow producer. There continues to be uncertainty with the models, with the GFS still steady with a northern solution and the NAM steady with a colder and further south scenario, however despite the differences with these models, New York City is still expected to see snow tomorrow regardless of whether it accumulates or not, and the interior parts of the area may end up with 4 to as much as 7-8 inches of wet snow by Thursday morning.

Model Analysis: This winter, we have had many cases where 24 hours away from the storm there were still differences with the models, and this storm is yet another one of these cases. For the last few days, the GFS model has consistently been north of the other models, showing mostly rain for New York City and even showing plain rain for the interior parts of the area. The highlight runs today were the 12z and 18z NAM, which showed a significant snowstorm for the northern parts of the area. The 12z ECMWF is also colder than the GFS, and the GGEM is in between the NAM and GFS.
The NAM has not had a solid handle on this storm yet, and at this time I consider its earlier runs to be too far south, which kept New York City with mostly, if not plain snow. The GFS, at the same time, is north of every single model and even its own ensemble mean, and while I do not think that the storm would end up as far south as the NAM has it, I do think that the GFS may adjust slightly south tonight to result in a slightly colder scenario. Its 18z run did shift slightly south of its 12z run, however it remains to be seen whether it was just a temporary jump or if it is taking a southern trend. The latest run of the NAM has also trended north, showing a warmer solution for New York City with 6 hours of plain rain, which is still a little south of my thinking but it has trended north from its runs this morning.
At this time, I used a blend of the NAM/GFS, expecting the 850 mb line to end up near New York City/northeastern New Jersey, which would bring rain to these areas for a while in the evening and early overnight hours and keeping the storm as entirely frozen for the interior parts of the area, however it is possible that the storm ends up slightly colder of what I used for tonight’s update. Stay tuned for updates tomorrow afternoon and evening that will discuss any potential changes for the remainder of the storm.

Storm Forecast:
Looking at the latest radar, precipitation is already moving into Pennsylvania. Most of this is virga, or precipitation that is not reaching the ground, however by at least 3-6 AM, precipitation is expected to begin reaching the ground west of NYC. Temperatures are currently dropping across the area with dew points also dropping, which will likely result in evaporative cooling once precipitation moves in, allowing the storm to begin as snow for most of the area. Due to mild surface temperatures, no accumulations are expected in NYC by tomorrow morning, however especially in the interior parts of the area, up to an inch of snow may be possible by tomorrow morning.

Tomorrow: As 850 mb temperatures remain below freezing, snow will continue north and west of NYC in the morning, however warmer surface temperatures will result in rain mixing with the snow by the morning into the early afternoon hours for Long Island, New York City and portions of northeastern New Jersey with the interior areas seeing plain snow. By the middle of the afternoon, however, the 850 mb line will begin to reach New York City, and a changeover to plain rain is expected for New York City by noon-2 PM, getting as far north as northeastern Bergen County and southern/central Westchester county. Parts of NW NJ should also change over to a period of plain rain, however for these areas, most of the storm will bring frozen precipitation.
Due to mixing and rain, no accumulations are expected during the day for Long Island and New York City with light accumulations less than 1 inch for southern/central Westchester and parts of northeastern New Jersey. For the interior parts of the area, a wet snow will fall for most of the day, mixing with sleet and rain late, with at least 1 to 3 inches of snow expected.
Tomorrow Night: As the low pressure moves off the coast near southern New Jersey, moderate precipitation will continue to affect the region, though with the 850 mb line still north of NYC, rain is expected to continue there as well as in NYC and northeastern New Jersey. The interior parts of the area will continue to see snow/sleet mixing with rain at times, eventually changing over to snow. By 2 AM, the storm is expected to begin moving out of the area, and by Thursday morning, a few snow showers are expected.
Forecast Snow Totals:

By the time that the storm ends, at least 3 to 7 inches of snow are expected for the interior parts of the area, with a trace to 1″ for the immediate NYC area except for NYC itself, which should see little or no accumulations. The scenario I am going with would support a mainly rain storm for NYC other than non-accumulating snow falling in the morning, but even if this storm was to produce more snow, during the day the snow would barely stick with surface temperatures still above freezing, snowfall rates not heavy enough, and the sun angle also an issue. If the overnight event was to bring snow to NYC, there would likely be at least light accumulations, but places further inland would still see more significant accumulations.
For Bergen County and southern Westchester/Rockland counties, I am expecting up to 1-2 inches of snow, which will mainly accumulate with the front end of the storm tomorrow morning and towards the end of the storm tomorrow night. The interior parts of the area should see mainly snow, mixing with sleet and rain at times, though at least 3 to 7 inches are still expected, locally up to 8″ in the higher elevations towards Orange County.
The map is a moderate confidence one, and areas of uncertainty include central Pennsylvania, where the accumulations may be shifted a bit south. Accumulations may also be needed to be revised in parts of the area north/west of NYC, but that will be discussed below. I also went with the northern solutions for accumulations in New England, and the northern end of the 4-8″ zone may be shifted slightly south.
Other Scenario: For tonight’s update, I sided with the northern model solutions. There continue to be colder models, however, and should this scenario verify, the interior parts of the area would see plain snow with 4 to 8 inches expected, places such as Bergen County and southern Rockland/Westchester would see 2 to 4 inches of wet snow mixing with sleet and potentially rain at times, and New York City would see a trace to 1-2 inches of snow with a brief period of rain in the evening, and otherwise a mix of rain, snow and sleet. At this time, the more likely solution is the one discussed previously, and the snowier solution is losing support. While I’d give a low probability for such a scenario happening, I will keep an eye on the observations tomorrow in case such a scenario becomes reality, which is unlikely but a possibility.
There is still some last minute uncertainty with the forecast, and it may not be completely nailed down until we reach the observation range while the storm is ongoing tomorrow. Stay tuned for storm updates that will be posted tomorrow in the afternoon and evening hours on this storm.
Longer Range: Either tomorrow’s update or Friday’s update will discuss in more details the coming storm potentials on Sunday and Tuesday/Wednesday and how they may affect the region.

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