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Jan 31

Jan 31, 2014: Snow Likely For Monday

Forecast Highlights:

image1As the cold air mass exits the region, much warmer temperatures will briefly return for the weekend, rising into the low-mid 40s on Saturday and the upper 40s-low 50s on Sunday. The warmth will be short lasting, however; yesterday’s update noted a wave of low pressure likely to trend northwest on Monday, with increasing confidence for light to moderate accumulations spreading into the area, followed by a snow/ice to rain event on Tuesday night into Wednesday.

 

 


 

Thursday, January 30 Observations:

As a high pressure built into the region, positioned over the area early in the day, strong radiational cooling allowed for widespread temperatures in the single digits for the morning lows, falling to -6 degrees in Westhampton Beach, NY and -4 degrees in Sussex, NJ. Daytime highs were warmer, peaking in the mid to upper 20s for most of the area. More detailed temperature data will be added on Saturday.

 


 

Weekend – Monday: Warm Weekend, Moderate Snow Likely for Monday

With ridging developing over the southeast US and a low pressure expected to quickly track through the Great Lakes on Saturday, a continued south-SSW flow is expected with warm air advection removing the cooler temperatures over the region, leading to a warmer weekend. Mostly cloudy skies are expected on Saturday with highs reaching the low-mid 40s, while Sunday will be the warmest day with mostly cloudy skies, a westerly wind, a few isolated showers possible and highs reaching the upper 40s to low 50s across the area.

The main attention is focused on Monday, however, when the next moderate snow event is expected to affect the region. A shortwave trough currently over California will dive into the SW US, where with a progressive southwesterly flow, it will quickly track into the region on Monday, aiding in the development of a weak low pressure in the southeast US/lower Mid Atlantic. As of yesterday’s update, the majority of the models either kept the snow well to the south of the area, as with the GFS and NAM with little precipitation north of central Virginia, or brushed the area with light snow, as with the ECM and CMC. Yesterday’s update noted that taking into consideration recent trends, including models underestimating the strength of shortwave troughs entering the US along with a trend towards a slower frontal passage on Sunday, that the system was likely to trend further northwest with light-moderate accumulations spreading into NJ and NYC, possibly to the north of NYC as well. This trend has been reflected with most of today’s runs, with the GFS slowly edging further north with the latest 0z run depicting at least 1 inch of snow in NYC with heavier snow to the south, while the latest UKMET and CMC runs that are currently in progress as of the writing of this forecast are even more aggressive with the northern trend, depicting a period of moderate to even heavy snow in central NJ to NYC. The ECMWF, which has not had a good track record this winter, was interestingly the most northern model of the day, with the 0z run depicting up to 6 inches of snow while the 12z run slightly backed down but continues to support at least 2-4 inch accumulations.

0z GFS at hour 66, for 18z Monday (1pm), depicting moderate snow south of the area and light snow brushing NYC. The GFS has been gradually trending north and is likely to continue to do so (image credit: PSU e-Wall).

image1Given the overall trends, probability of snow affecting the area is higher, and has been increased to 50-60% in northern areas and 70-80% in southern areas. While some uncertainty remains with how far north the snow extends, the next factor to analyze is the placement and intensity of precipitation. Accounting for a slight north shift of the 12z model guidance, the strongest lifting and frontogenesis values are currently modeled to remain between a north Maryland to central NJ axis, just south of NYC, where the potential for moderate or even locally heavy snow exists but with some rain potentially mixing initially as surface temperatures take a little longer to cool down behind Sunday’s frontal passage. This would keep the moderate snow totals just south of NYC, with the NYC area over the precipitation gradient with snow accumulations decreasing further north. Given the overall setup and accounting for additional possible northward corrections on the GFS, my preliminary accumulation call at this time is for 2-4″ near and south of NYC, and up to 1-2″ north of NYC. Given the uncertainty in the outlook, these totals are subject to change with the next forecast update. Stay tuned for more information on Monday’s snow event.

 

Tuesday – Wednesday: Snow, Ice Rain Returns

A more active pattern will be underway across the US by this time frame with shortwave troughs frequently emerging out of the southwestern US, which with a progressive southwesterly flow in the upper levels end up reaching the northeast US region. The next system to affect the region will be on Tuesday night into Wednesday, as a strong shortwave over the southwest US enters Texas on Tuesday while gaining a neutral tilt, but with anticipated confluence over the US/Canada border leading to the shortwave flattening out as it enters the NE US, which along with a high pressure placed to the north of the low pressure leading to a transfer to a secondary low pressure near the coast. The main uncertainty remaining at this time is how much snow and ice affect the area.

0z GFS at hour 108, for 12z Weds (7am), depicting the end of the heavy precipitation period. This model keeps the heaviest snow in the Hudson Valley, but a further south secondary low would lead to more snow/ice in the area (image credit: PSU e-Wall).

f108Storm Analysis: Over the last day, the model guidance has not changed significantly with the overall synoptic setup for this time frame, and significant changes are unlikely at this time. The main aspects to watch for, however, include the location of the transfer to the coastal low pressure, which will influence how much frozen precipitation the area receives. With connection to the Gulf of Mexico, significant amounts of moisture will be pulled into the system, supporting heavy precipitation rates in the Ohio Valley into the northeast US region; as the system reaches the area, the most favorable dynamics are likely to be placed between central NJ into the central-northern Hudson Valley and further east into southern New England, where the heaviest precipitation is currently likely to fall in the region. Initially, temperature profiles are likely to be cold enough to support snow for at least most of the area at the onset of precipitation towards 11pm-1am on Tuesday night, but with mid level warm air advection occurring as the 850mb low takes longer to transfer to the coast than the surface low pressure, leading to a changeover to rain near coastal areas and freezing rain north/west of NYC where low level cold air will take longer to erode. The main challenge at this time is to determine the timing of the changeover and how much snow falls until that point, as the aforementioned dynamics support heavy snow between at least 2am-7am for locations that remain with plain snow.

Over the last 2-3 days, the model guidance has generally trended northwest with the low compared to the initial modeled depiction of plain snow in the area, with the low pressure now expected to track into the eastern Ohio Valley before transferring to the coast. The more recent runs, however, have slightly trended south with the location of the transfer to the coastal low, especially the GFS which in its earlier runs brought the primary low well into NY state but has slightly backed to the south. With similar setups compared with this system, occasional model biases include the low pressure transfer occurring further south than originally modeled, and my preliminary track outlook at this time is a primary low moving over Kentucky and eastern Ohio before transferring to a coastal low pressure near east central NJ, which then tracks NE into eastern Long Island and Cape Cod. This type of a setup would support at least a 2-4 hour period of snow for most locations outside of the lower Hudson Valley and interior CT; given the heavier precipitation entering towards the end of this time frame, moderate snow rates are possible especially north and west of NYC prior to the changeover, supporting at least light to moderate snow accumulations. More significant accumulations would occur towards the lower Hudson Valley and interior CT, however, where heavier snow is likely to last for a longer period of time before changing over to freezing rain by Wednesday morning. The main areas of uncertainty at this time include the potential for a further south low transfer than currently forecast and a slightly weaker primary low, which would support a longer duration of snow prior to the changeover and more widespread moderate accumulations in NYC and further north/west.

 

Preliminary Forecast: Light snow is forecast to develop from SW to NE towards 11pm-1am, steadily increasing in intensity as temperatures range from the mid 20s in NW NJ/interior SE NY to the low 30s near the coast, with at least a 2-4 hour period of snow likely for most locations except for Long Island, which is likely to change over to rain faster than other locations. Precipitation will gradually intensify and become moderate to heavy after at least 1-3am, when the mixing line will begin to move north with a changeover to rain likely in NYC and the coast and freezing rain/sleet further inland, where the changeover to rain is not anticipated until at least Wednesday morning. Most of the rain is likely to fall prior to 12pm, with lighter showers persisting afterwards as highs range from the mid 30s inland to the low 40s near the coast.

As the storm is 4-5 days away, some uncertainty remains with the specific details of the storm, although at this time, the preliminary outlook is for at least minor accumulations in NYC and Long Island, light to moderate accumulations (generally 2-6″ range) north/west of NYC into southern CT, and moderate-heavy accumulations (generally over 4-6″) towards interior NW NJ and SE NY. As previously noted, however, some changes are still possible, including a shift towards a further south secondary low pressure which may increase snow and ice totals in the area. More information will be posted with Saturday’s forecast discussion.

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