The active weather pattern that ended 2013 will continue into the beginning of the new year, starting with a moderate to significant snowstorm on Thursday into Friday that may lead to near blizzard conditions in parts of the region, accompanied by near record cold with lows near to below zero expected away from NYC and the coast. A rain event is likely early next week, followed by another strong cool down (Image credit: PSU e-Wall).
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Tuesday, December 31 Observations:
The colder air mass that entered the region on Monday continued to spread over the region, while the core of the cold remained just north of the US/Canada border. A weak clipper tracked through the region, leading to increased cloud cover with isolated snow showers, mostly north of the area, while high temperatures returned to below average levels in the upper 20s-low 30s.
The highest temperature was 34 degrees in JFK airport, and the coolest high temperature was 28 degrees in multiple locations.
Today – Friday Morning: Snow, Wind, Frigid Temps
The new year of 2014 has started in NYC under mostly clear skies, 26 degrees, and a wind chill of 16 degrees, with mostly to partly sunny skies expected to continue into today with highs in the upper 20s to low 30s. The dry conditions will remain short lasting, however, as the new yeas begins with a moderate to significant snowstorm on Thursday and Friday, followed by near record cold on Friday into Saturday.
6z GFS at hour 0, showing initialized conditions, representing the main players in this system: the southern shortwave over Texas, central shortwave over Montana, and another northern stream energy over western Canada (Image credit: PSU e-Wall).
Storm Synopsis: There are three shortwave troughs involved in this system; a southern stream shortwave currently over Texas, a shortwave over Montana, and another northern stream shortwave over western Canada. The Montana shortwave will continue to track southeast today, picking up but not fully phasing with the vorticity over Texas as it gains a more neutral tilt. Meanwhile, the second north stream shortwave over Canada will dig into the central US on Thursday, approaching and later phasing with the shortwave over Tennessee leading to a deepening of the low pressure system. Initially, two surface lows will be present, one off the coast and another through the southern Ohio Valley, which later merge off the coast of New Jersey not far southeast of the 40N/70W benchmark and continue to quickly deepen as the consolidated low tracks towards Newfoundland.
Model Analysis: Short range changes have occurred in the forecast, which will be discussed in more details. Updates over the last few days have generally sided with a moderate snow event, but noted the potential for a more significant event cannot be ruled out until the fine details have been narrowed down, as this is a complex setup where minor changes can have more significant impacts on the exact scenario and snow totals. As of yesterday’s update, there was a minor trend towards a less significant system which further decreased confidence on the outlook. Over the last 12-24 hours, there have been minor but significant changes in the mid level setup towards a stronger and more amplified shortwave over the central US, more digging of the Canadian shortwave, and a weaker kicker shortwave that later enters the western US which also supports a more amplified system overall. These trends have reflected in the surface level with a stronger and slower surface low pressure closer to the coast, with widespread moderate to heavy snow developing overnight. These trends appear to be supported by latest observations over the western US. Taking this into consideration, as well as the possibility for snow ratios above 10:1 to 15:1, confidence is higher regarding a significant snowstorm than as of yesterday’s update.
The latest model guidance is not fully on board with a major snowstorm, however; the storm is now within the range of the shorter high-res models, with a split between the stronger and snowier high resolution models, such as the NAM, SREF and RGEM, and the slightly weaker and drier global models, such as the ECM, GFS, CMC and UKMET. Among the former group, the NAM depicts at least 1.20″ of liquid-equivalent QPF (Quantitative Precipitation Forecast), and the RGEM through hour 54 and the SREF mean with at least 0.70″. Among the latter group, the ECM depicts at least 0.30″, GFS with about 0.50″, CMC with 0.30″, and the UKMET with slightly less precipitation than the CMC. The NAM is known for its wet bias, and in this case is likely exaggerating precipitation totals, and at this time is being considered a wet outlier. The CMC, meanwhile, has been among the last models to catch up to the system, having depicted only 1-3″ of snow in NYC as recently as yesterday’s 12z run, and along with the UKMET are considered as dry outliers. At this time, I am siding with totals closer to the RGEM/SREF, with liquid-equivalent precipitation ranging from 0.60″ in western areas to just over an inch in eastern areas. The next factor to consider is snow ratios, which could be above the standard 10:1 ratio which further increases snow totals. With strong cold air advection occurring on the back end of the storm as frigid Canadian air surges into the region, a saturated temperature profile between -10C and -20C is expected during the peak of the system late on Thursday night into Friday morning, with a somewhat isothermal layer in the low-mid levels supporting snow to liquid equivalent ratios above the standard 10:1, likely around or slightly above 15:1 especially towards the end of the storm. This would support at least 6-7″ of snow in western areas to 10-14″ of snow towards eastern parts of the area.
Storm Timing and Impact: Precipitation is already present over the northern Ohio Valley in the form of light snow spreading eastward along a frontal boundary extending to the low pressure exiting the Rockies. Overrunning light to moderate snows will spread into New York state and south-central New England during early Thursday morning, with the surface frontal boundary likely setting up just south of the NYC area, with temperatures ranging from the low-mid 20s inland to mid-upper 20s elsewhere, while temperatures further south in New Jersey and the Mid Atlantic rise into the low 40s. As there remains separation between the northern stream shortwave over the northern Ohio Valley and the Gulf of Mexico moisture over the southeastern US, Thursday morning’s radar imagery will depict a large dry slot covering the southern Ohio Valley into Pennsylvania and New Jersey, which will fail to fill in until the evening hours; this dry slot, however, alone does not imply the storm will fail to produce as much snow as expected. Throughout the day on Thursday, the axis of snow will remain focused over NY state into south-central New England, with moderate snow accumulations expected. Over northern parts of the area, 1-3 inches are expected, with less than 1 inch over north NJ, NYC and Long Island. The main uncertainty rests with coastal development, however, as the coastal low develops off the coast with the possibility of some precipitation enchancement near NYC, Long Island and possibly northeast NJ. Should this occur, accumulations of up to 1-2 inches would be possible by Thursday evening, but with mixing with rain possible in coastal Long Island at times as temperatures warm up into the low 30s.
The peak of the storm will take place overnight across the region. As the strong mid level shortwave becomes neutrally tilted near Tennessee and phases with the northern stream energy from Canada, the low pressure will intensify over Kentucky and West Virginia to at least 1004 mb, which is a change from yesterday’s expectation as it now appears the western low will be as strong as, if not a bit stronger than, the developing coastal low pressure, which then crosses through Virginia and tracks towards just south of the 40N/70W benchmark overnight. With the strong mid level shortwave trough moving through the region with widespread strong synoptic lifting, widespread moderate to heavy snow is expected to develop over the locations that were previously under the dry slot, from west to east, starting out with moderate snow developing over Pennsylvania by 4-7pm and moderate-heavy snow over the NYC area by 6-9pm. As this occurs, with the low pressure deepening as a result of the aforementioned phase, a tightening pressure gradient is expected, leading to a strong northerly wind developing at 15-30 mph across most of the area, locally higher towards Long Island and southern CT. This is expected to lead to near blizzard conditions over Long Island and CT with blowing snow leading to low visibility. Additionally, a frigid cold air mass will be located just north of the US/Canada border due to the displaced polar vortex as a result of weak blocking near Greenland, with the strong northerly winds leading to strong CAA as the frigid temperatures surge southward into the region, leading to a flash freeze in the Mid Atlantic from the 40s to the 10s/20s and a changeover to snow, while the NYC area sees temperatures falling into the 10s overnight with moderate-heavy snow, and likely the upper single digits in NW NJ and interior SE NY. Along with the strong wind gusts, wind chills down to 0 to -10 degrees are expected. Snow is expected to taper off from west to east towards 8-10am on Friday.
With the aforementioned factors, the highest snow accumulations across the region are expected over eastern NY state into southern New England, where at least 8 to 16 inches of snow are expected with the possibility of localized higher totals over 16 inches in southern New England. Over the NYC area, as discussed in the previous section, accounting for current forecast precipitation totals and snow ratios, at least 6-10″ of snow are expected over northern NJ and NYC, 7-12″ over southeast NY, and 8-14″ in Long Island and southern CT. There remain some uncertainties in the exact setup, however, including the strength of the primary low over Kentucky/West Virginia as well as the track and strength of the coastal low overnight, which influences how heavy precipitation will be overnight and how far inland the heavy snow will extend, as well as the exact snow ratios. At this time, I am siding with higher totals than the global model guidance,although some minor changes are still possible to this outlook, and the potential for totals to end up lower than the current forecast remains valid if the short range models are overdoing the development of the system. One potential margin of error includes the possibility that the strongest lifting ends up south of the area, with the heavier overnight snows staying further south or east.
Forecast Summary for NYC Area: Light to moderate snow is expected to develop on Thursday morning over southeast NY and south-central CT, persisting throughout the day with highs in the mid-upper 20s and 1-3 inches of snow accumulating. NYC, north NJ and Long Island will be drier, especially northern NJ, with up to an inch possible with locally higher totals up to 2″ not out of the question in NE NJ and NYC, with highs in the upper 20s. By the overnight hours, moderate snow will develop, falling heavily at times, between at least 6-9pm, continuing through the overnight hours before ending by 8-10am on Friday. Strong northerly winds are expected at 15-30 mph, which will lead to near blizzard conditions over Long Island and southern CT, which along with temperatures falling into the 10s for most and single digits in NW NJ/SE NY will lead to wind chills in the 0 to -10 degree range by Friday morning.
At this time, snow accumulations of at least 6-10″ of snow are expected over northern NJ and NYC, 7-12″ over southeast NY, and 8-14″ in Long Island and southern CT with totals possibly locally higher over southern CT. As previously mentioned, however, there remain some uncertainties in the outlook, and totals could slightly change. This is the final in-depth discussion for the storm; an evening update will be posted with any revisions to the forecast, and storm updates will be posted throughout the day on Thursday into Friday morning.