Dec 13, 2013: Significant Snow, Ice Tomorrow

Forecast Highlights:

SnowMapAnother cold day was observed across the region, with highs today reaching the low to mid 30s for most which is still at least 5-10 degrees below average. A low pressure currently developing near the south central US will move through the region on Saturday into Saturday night, resulting in the third snow event in the last 7 days as it produces widespread moderate snow, freezing rain and rain. Cold temperatures will return behind the storm, with another light rain/snow event likely on Tuesday.

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Thursday, December 12 Observations:

12.12.13A strong cold air mass remained in place over the region, with the coldest temperatures of the winter observed so far, the coldest since late November. Temperatures peaked early in the day in the mid 20s inland and upper 20s for the rest of the area, before slightly falling in the morning and reaching a secondary peak in the mid to upper 20s during the afternoon hours; these temperatures were generally 10-15 degrees below average. The highest temperature was 32 degrees in Montauk, NY, and the coolest high temperature was 25 degrees in multiple locations.



Tonight – Sunday: Moderate to Heavy Snow, Some Ice/Rain

The first major winter storm of the season will affect the region on Saturday into Saturday, as a quick moving storm system will produce widespread moderate to heavy snow, along with freezing rain and rain for parts of the region. The model guidance has been generally consistent over the last day, with high confidence regarding the overall impacts, but with some continued uncertainty with the specific details of the storm’s impact in the region.


The 18z run of the GFS at hour 24 (18z Sat), showing the main energies involved, the leading southern shortwave over Ohio, trailing southern shortwave over Texas, and the north stream digging south over Minnesota (image credit: PSU e-Wall).

f242Storm Synopsis: This storm will form as a result of the interaction of three energies; a northern stream disturbance currently just north of North Dakota, and two southern stream shortwaves, one currently near Texas and the other trailing behind it over Arizona. The lead southern shortwave will continue to track northeast into the Ohio Valley region as a surface low pressure develops near Kentucky and tracks into Ohio, spreading widespread overrunning snowfall into the northeast US region. Meanwhile, the northern stream will dig south over the Midwest region, where it will begin to interact with the leading southern shortwave as the trailing southern shortwave merges with the system, which along with a high pressure stationed over New England will lead to a transfer to a coastal low pressure near the Delmarva Peninsula, which will track northeast parallel to the coast while intensifying and producing widespread precipitation across the northeast US region.


Impact Analysis: Yesterday’s update noted that the most significant change has been with the model guidance trending towards a more robust southern stream shortwave, along with more digging of the northern stream, which leads to more interaction between the two energies and a stronger coastal low pressure developing closer to the coast. This solution has increased forecast precipitation totals over the region. Despite more consistency with the model guidance today, there remain some inconsistencies with the exact handling of the energies involved, which will lead to minor changes overall but with no substantial change anticipated. This setup generally favors a low pressure tracking just off the coast and into southeastern New England.

An arctic front is currently located just north of the area, and is slowly moving southward. As this front moves through the area, temperatures will slowly fall into the upper 10s to low 20s inland and low to mid 20s for the rest of the area by the morning hours. As the primary low pressure tracks into Ohio, widespread light to moderate snow will develop across the area generally after 10am, with temperatures holding steady in the low 20s inland and mid 20s across the rest of the area, possibly upper 20s in Long Island, as a strong high pressure over New England keeps cold air entrenched near the surface with a northeasterly wind. Steady light to moderate snow will develop by the early afternoon, and will continue to slowly intensify through the mid afternoon hours. By the evening, the snow will become moderate, heavy at times especially over northern NJ and SE NY, as the coastal low begins to intensify. With mid level temperatures warming up to near/above freezing, however, a changeover to sleet/freezing rain is expected over northern NJ/SE NY and rain for NYC and Long Island, along with the immediate coast of CT, with the latter locations changing over the earliest due to easterly winds near the surface as the coastal low makes its closest approach. The main challenge is forecasting the exact precipitation type between sleet, freezing rain and rain; the layer of above freezing temperatures is likely to be rather shallow over northwestern areas, which appears to support sleet, while locations in northeast NJ, SE NY and western CT are expected to remain below freezing at the surface but with a thicker warm layer, which would support moderate freezing rain with at least 0.1 to 0.2 inch of ice possible. At this time, I included these totals in the forecast, although this needs to be closely monitored tomorrow as slight temperature differences could mean more differences in the precipitation type and how much snow falls prior to the changeover.

SnowMapLooking at a more region-wide scale, snow will first develop over Pennsylvania, New Jersey and southern NY on Saturday, first changing to ice/rain in southern areas before spreading northeast through eastern PA and all of New Jersey. Initially, the heaviest precipitation will fall under locations with temperature profiles near or above freezing, with moderate to heavy snow totals spreading into parts of north/east PA and NY state, but as the low continues to deepen, more precipitation will extend into locations with sub-freezing temperatures, leading to the heaviest snow axis setting up over central New England into Maine, where totals of at least 10-14 inches of snow likely. More uncertainties come into play over parts of northeastern PA into NY and northern New England, where the latest model runs have trended towards less of a shaft zone in between the primary low and the coastal low, although the possibility of an area of lower totals cannot be ruled out especially if there is more separation between the lows than currently anticipated, as the latest 0z high resolution RGEM run depicts. Another area of uncertainty is how far south the heavy snow axis shifts over New England; the GFS model is currently the most aggressive with mid level warming and depicts the heavy snow axis north of the above snow map, while the RGEM is the coldest with a further south heavy snow axis. The GFS is known to exaggerate mid level warming, and I sided towards a colder and slightly south scenario than what the GFS depicts, although the heavy snow axis may be subject to some revisions.


NYCSnowForecast for NYC Area: Light snow is expected to develop across the area mainly after 10am, which will continue through the mid afternoon temperatures with temperatures generally in the low 20s inland and mid to possibly upper 20s for the rest of the area. Towards 7pm, moderate snow is expected to cover the area, with heavy snow expected at times mainly over northern NJ, SE NY and Connecticut. The mixing line will continue to steadily advance northward, with NYC and Long Island likely to change over to rain around 10pm, while north/west suburbs of NYC change over to freezing rain by 11pm and interior northwest areas change over to sleet. Precipitation is generally expected to end towards 2-4am from southwest to northeast, with a few lingering rain/freezing rain showers possible afterwards.

At this time, the forecast is for 2-4 inches of snow in Long Island, 2-5 inches in NYC, 5 to 8 inches of snow over northern NJ with totals locally up to 10 inches, and 6 to 10 inches over SE NY and southern CT away from the immediate coast with locally higher totals possible. Ice accumulations of up to 0.10 to 0.15 inch are likely over northeast NJ into Rockland/Westchester counties and western CT.


There remain some short range uncertainties in the fine details that may have to be revised tomorrow. I slightly increased snow totals from yesterday’s outlook north/west of NYC, although one aspect to keep an eye on is how fast mid level temperatures warm up, with a slower warmup leading to a slightly longer duration of snow with more widespread 8+ inch snow totals possible over northern NJ. As noted earlier, the temperatures will also help to determine the exact precipitation type, as although the current placement of freezing rain is favored over NE NJ into far SE NY and western CT, this may slightly change around depending on how fast temperatures warm up and how thick the layer of warm air aloft is. These aspects will be monitored throughout the day tomorrow and if needed, some revisions may be made to the outlook. This is the final storm forecast; storm updates will be posted throughout the day on Saturday.


Beyond the storm: After temperatures warm up into the low to mid 30s on Sunday, melting some but not all of the snow/ice, cold temperatures will return on Monday with widespread lows in the 10s forecast and highs in the upper 20s-low 30s likely. A gradual warm up will begin on Tuesday as a low pressure tracks to the north, producing some light rain/snow with the highest probability of snow north of NYC, followed by a continued warm up into next weekend with highs in the 50s possible on Friday and/or Saturday before a cold front moves through with rain possible.

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