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A developing low pressure this morning resulted in more snow than expected, with thundersnow and localized totals up to 3-4″ north of NYC, while temperatures remained warmer than average, in the upper 30s to low 40s. The recent break from the cold will remain temporary, however, as a strong upper level low returns into southeast Canada, bringing another round of cold temperatures for next week but with the cold not as significant as the early January cold outbreak.
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Friday, January 17 Observations:
The week-long “January thaw” came to an end on Friday, with the last day of widespread warmer than average temperatures as a weak low pressure gradually approached from the west, leading to increased cloud cover later in the day and at night. Temperatures were slightly warmer than those of Thursday, peaking in the upper 30s to low 40s inland and the low to mid 40s elsewhere.
The highest temperature was 46 degrees in Newark, NJ and LaGuardia airport, and the coolest high temperature was 39 degrees in Sussex and Andover, NJ.
Recent Weather Recap: Active First Half of January
Since the last full update on January 3, the region has seen some of the most significant temperature swings in the last few years, ranging back and forth from near record warmth to record cold, the coldest temperatures seen in at least 10 years. After a moderate snowstorm on January 2-3 that produced a general 6-12 inches of snow across the area, temperatures peaked in the mid to upper 10s, the first time since 2009 that Central Park failed to reach 20 degrees. Meanwhile, a piece of the polar vortex over Canada split, and with a favorable upper level flow including a weak block-like feature near central Canada, the core of the cold air mass containing 850mb temperatures as low as -32C to -36C plunged into the Midwest on January 5-6, tracking east through the Ohio Valley and PA/NJ while moderating to near -25C to -30C. Frigid temperatures were observed on January 7; although actual highs were reached at 12am, daytime highs after 7am only peaked in the upper 0s to low 10s, which is anomalously cold for the area, along with strong winds leading to wind chill values near -10 to -20 degrees. Central Park set a record low of 4 degrees overnight, breaking the record of 6 in 1896 – the last time a record low temperature has been set there was on March 9, 1996, almost 18 years ago. Temperatures then made a quick rebound by the 10-12th as light snow fell on the 10th before a stronger system tracked to the northwest and brought temperatures surging back into the upper 50s-low 60s on the 11th.
Over the last week, an increasingly active pattern developed aloft following a temporary thaw as the strong upper level low retreated into northern Canada, generally characterized by a persistent ridge over the western US which has aided in the expansion of the severe drought over California, and a trough positioned near the central-eastern US along with a progressive flow as multiple shortwave troughs in quick succession dived southeast and amplified over the central US before moving through the region and offshore. The first of these frequent shortwave troughs moved through on January 13, when a weak wave of low pressure along the coast produced up to 1/4 to 1/2 inch of rain, and a gradual downward trend in temperatures began at that point, when high temperatures were still in the upper 40s-low 50s. The next trough and associated surface frontal boundary slowly made their way east on the 15-16th, when scattered rain/snow showers were observed but with no significant precipitation over the area, as the trough axis was elongated and only consolidated once too far east to result in significant impacts over the region.
The third system moved through the area today, and has proved to be more significant than anticipated. Several days ago, the ECMWF model depicted a stronger low pressure near the area leading to widespread moderate precipitation, but backed away from this depiction soon afterwards before both the ECM and GFS recently began to trend towards a weak low pressure developing near the area, with an axis of moderate snow over central PA to central NY state. Over the NYC area, the main expectation was for scattered rain/snow showers, primarily falling as snow in western parts of the area but with minimal accumulations overall, as precipitation rates were generally forecast to remain light, which along with a warm boundary layer would not support much snow accumulations. As the low pressure developed near coastal New Jersey this morning, intense snow squalls developed over northern New Jersey with additional squalls developing further south, with radar reflectivity values up to 35-40dbz with some of the squalls; thundersnow was reported multiple times over northeast NJ and NYC. The snow was generally wet, especially closer to NYC, Long Island and coastal CT, where little to no accumulations were observed, but with two axes of heavy snow in the area, one over far NW NJ and western Orange county with 2-3″ of snow, and another near Rockland, Westchester and Putnam counties into western CT, where 2-4″ of snow were reported, approaching 10 inches in localized spots in far NW CT.
Tonight – Wednesday: Cold Returns
The aforementioned mild pattern will continue in the short term, but with only a short duration of warmer than average temperatures left before the next return of arctic cold. Another Alberta Clipper-type low pressure system currently over the Ohio Valley produced a swath of light-moderate snow, but is currently drying out over the Appalachian mountains, with only isolated snow showers expected to pass the high elevations and reach the rest of the region. High temperatures are expected to peak in the upper 30s for most locations. A slight warm up is expected for Monday into the upper 30s to low 40s as a cold front approaches from the north, with isolated snow showers possible for interior parts of the area mainly in the evening, with the front moving through overnight, bringing the next round of cold into the region.
As a series of low pressures track into Alaska, ridging will persist over the western US and northeast Pacific, and is expected to temporarily cut off as an upper level low slides partially underneath the ridge into California. During the early month cold outbreak, there were some misconceptions regarding what the polar vortex is; it is not an object that can be visibly observed, but is rather an upper level cyclone associated with cold surface temperatures that always exists in some form, and is typically centered near the north pole region but occasionally splits and/or shifts away from the polar regions. A piece of the polar vortex is currently moving southeast over Canada, and will reach southeast Canada by Monday before shifting back north but remaining relatively close to the region. This will drag the cold air mass southeast towards the region, although the core of the cold will remain in Canada and will not be nearly as cold as January 7, with 850mb temperatures of at least -20C to -22C expected to cover the region for Tuesday night through Thursday. Despite a lesser magnitude than the early month cold, this will still be similar to typical cold outbreaks in the last few winters; highs on Tuesday will peak at 12am before cooling into the low-mid 20s in the daytime, with lows in the single digits expected away from NYC and the coast and Wednesday’s highs only reaching the mid 10s to low 20s.
Given the progressive pattern and the nearby upper level low, cold and dry conditions are generally anticipated, although a coastal low pressure is expected to develop near the Mid Atlantic coast on Tuesday as a shortwave trough swings through the region. The current model guidance depicts this system as developing too far east to produce widespread precipitation over the region; while some westward adjustments cannot be ruled out, this system is unlikely to produce much precipitation over the area besides snow showers, especially east and south of NYC.