The first December in 3 years to feature a wintry pattern continues with a colder than average pattern across the region with occasional storminess, as following interior lake effect snow last night, temperatures today struggled to rise into the mid to upper 20s for most locations. Temperatures will remain well below average, in the 10s/low 20s for lows and 30s for highs, as the next significant storm approaches the region with widespread snow, ice and rain expected for Saturday into Sunday.
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Wednesday, December 11 Observations:
Behind Tuesday’s wave of low pressure, which produced at least 1-4 inches of snow across most of the area, colder temperatures and drier conditions returned into the region, with partial sunshine and high temperatures reaching the upper 20s to low 30s inland and low-mid 30s for the rest of the area; average high temperatures are generally in the low to mid 40s. The highest temperature was 35 degrees reported in multiple locations, and the coolest high temperature was 29 degrees in Port Jervis, NY.
Short Range: Cold Continues Through Friday
With mostly clear skies and light winds, temperatures have already began to rapidly fall in typical radiational cooling spots such as interior NW NJ/SE NY and the Pine Barrens in Long Island, with Westhampton Beach down to 17 degrees as of 6 PM. Partly cloudy conditions will continue tonight with lows in the low 20s in NYC and 10s across the rest of the area, likely falling into the upper single digits towards parts of NW NJ and interior SE NY. With southwesterly winds expected for Friday, slightly warmer temperatures are also anticipated, reaching the upper 20s inland and low to mid 30s for the rest of the area. A dry arctic front will move through overnight bringing northerly winds, but with widespread cloud cover preventing temperatures from falling below the upper 10s to mid 20s across the area.
Saturday – Sunday: Widespread Snow, Ice, Rain Expected
The next major weather event is expected by Sunday as a broad yet fast moving system enters the region, producing widespread precipitation with accumulating snow, ice and rain expected. The model guidance has generally been variable with the handling of this system, depicting a drier and further south storm yesterday, but has since reversed this trend with more signals showing up for a stronger storm. While there remains uncertainty with the fine details in the setup, confidence is high that the area will experience its third accumulating snowfall event over the recent 7-day span.
The 18z run of the GFS at hour 48 (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).
Storm Synopsis: This storm will form as a result of the interaction of three energies; a northern stream disturbance currently over western Canada, and two southern stream shortwaves, one currently near southern California and the other entering the northwest US. 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, leading 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: The storm is expected to produce widespread precipitation across the region, ranging from moderate-heavy snow to areas of ice and rain near the coast. The key aspect to the storm is the handling of the mid-upper level setup which will ultimately reflect to the conditions observed near the surface. Relative to past storms, the model guidance has generally handled this storm well, but have still had some issues with the handling of the shortwave energies involved, especially with the leading southern shortwave and the northern stream. A bias models occasionally display is to underestimate the southern stream, and it appears yesterday’s southern trends may have been partially a result of this as today’s guidance has been generally trending more robust with the southern stream. The north stream is also often a struggling point for the models, with a recent example being the late November storm where the medium range GFS significantly underestimated how far south the northern stream would dig, and thus failing to depict a major storm. In this case, the handling of the north stream has not been overly inconsistent, although it has been trending south with the last few runs, which along with the more robust southern shortwave is leading to a stronger surface low reflection with heavier precipitation and more significant impacts. Additional minor corrections in the handling of these energies is expected with Friday’s runs, which may lead to additional small shifts in the low track and sensible impacts in the region; at this time, I am siding towards a moderately amplified low tracking close to the coast, although this may be subject to slight changes with Friday evening’s update.
As previously noted, an arctic cold front will move through on Friday night, with a strong cold air mass firmly entrenched over the northeast US region on Saturday morning as a strong high pressure will be placed over northeastern New England with widespread temperatures in the 10s in the Northeast and 20s across the NYC area. With temperature profiles cold enough, snow is expected to develop across the entire area by the mid-late morning hours on Saturday, especially after 10am. Moderate snow is expected, especially progressing towards the late afternoon and evening with mid level warm air advection. By the evening, a surface low pressure will develop just off the coast and begin to intensify, enhancing precipitation over the area and into New England. The mid level low will take longer to transfer to the coast, however, which signals an expected changeover to sleet/freezing rain for most interior locations and rain for NYC and the coast, with the latter locations expected to change over the earliest with an easterly flow at the surface. The main uncertainty at this time is regarding the snow rates during the late afternoon and how much temperatures warm at the mid levels, as well as how much precipitation is left over behind the changeover; at this time I am siding towards moderate rain continuing in Long Island and NYC through the late overnight hours and mixed precipitation falling inland, but failing to change over to plain rain as low level cold takes longer to erode, although some additional changes are possible which will have an impact on the forecast accumulations.
On a more region-wide scale, the overrunning snow during Saturday is expected to focus over Pennsylvania, central-northern New Jersey, and parts of southern NY state, producing at least moderate accumulations. As the coastal low develops, heavier precipitation is expected from the NYC area into New England and Maine, with widespread moderate to potentially heavy snow spreading across the Northeast. As with every storm, there are locations that receive more or less snow than forecast, although at this time range it is often difficult to narrow down these locations with high accuracy. Generally speaking, however, locations under the coastal low especially with any enhancement from banding may receive more snow than forecast, which based on latest projections appears to be along an axis roughly from northern NJ into SE NY and interior central New England. Northwest of the heavy precipitation shield associated with the coastal low, there may be an axis that receives less snow than forecast in between the weakening primary low and intensifying secondary low, which has happened with numerous recent transfer scenarios such as February 8-9, 2013 in the northern Hudson Valley into north New England, and December 29, 2012 in parts of north NJ into the lower Hudson Valley. Latest indications suggest this may be placed along an axis from northeast PA into east central NY and parts of VT/NH. As this storm is still 2 days away and slight changes with the handling of the setup are still anticipated, these zones are subject to change and may not be that significant, although the locations under the axis of heavy snow, currently forecast to set up from the lower-mid Hudson Valley into interior New England, will continue to be monitored for possible revisions.
Forecast for NYC Area: The latest forecast is for snow to develop on Saturday morning, generally around 10am, with temperatures in the low-mid 20s inland and mid-upper 20s for the rest of the area. Temperatures will fail to rise much during the day as snow gradually intensifies and becomes moderate by the afternoon and evening. With a coastal low developing south of the area, heavier precipitation is expected to develop in the evening into early overnight hours, but warming mid level temperatures will lead to a changeover to rain for NYC and Long Island and sleet/freezing rain, possibly mixing with snow, for the rest of the area. Precipitation is expected to continue through the late overnight hours before ending.
At this time, the preliminary forecast is for at least 1-3 inches of snow in Long Island, 2-4 inches in NYC, and 4-8 inches for the rest of the area, locally higher especially towards NW NJ, southeast NY and interior CT. The highest snow totals in the region are currently likely from the lower-mid Hudson Valley into interior New England, with the potential for at least 8-12 inches of snow or higher. Freezing rain is expected to be a concern as well especially for locations north/west of NYC, with the possibility for light ice accumulations.
As previously noted, there are still some slight uncertainties with the forecast, and additional changes are possible. Assuming the current scenario verifies, snow totals may be underestimated for northern and western parts of the area. It is possible that a somewhat less amplified scenario verifies, however, in which forecast snow totals would roughly remain the same but with less intense snow rates and a longer duration of snow, along with less precipitation falling as rain/sleet/ice before ending. This would also lead to lower snow totals over New England than currently forecast. Conversely, there is also the possibility that an even more amplified scenario verifies, in which case snow accumulations would be lowered for the area with more freezing rain/sleet inland and rain elsewhere as the heavy snow axis shifts further inland, although at this time this appears to be a lower probability scenario. The final storm forecast will be posted on Friday evening, with storm updates posted throughout the day on Saturday.