The first significant storm system of 2014 affected the region over the last two days, with widespread snow accumulations between 6-12 inches across the area. As the system departs, however, frigid temperatures will remain in the region with tonight’s lows in NYC likely the coldest since January 2004. A temporary moderation will occur on Monday with some rain and interior ice, followed by a rapid flash freeze and even colder temperatures for next Tuesday (Image credit: PSU e-Wall).
Tonight – Saturday: Frigid Temperatures Continue
Throughout late November and most of December, the polar vortex and some of the coldest temperatures in the northern hemisphere were found over Canada, a change from recent years when the cold was often centered near Europe or Asia. With a lack of blocking, the cold generally failed to drop southward to a significant extent, only partially dipping into the region with the two mid December snowstorms when temperatures were colder than expected across parts of the region. Over the last few days, however, a weak block-like feature has been placed near Greenland, suppressing the polar vortex over southeast Canada which along with strong northerly winds allowed for strong CAA to drag the frigid temperatures down into the area with well below normal temperatures. The progressive pattern will keep the cold short lived, but with even more anomalous cold to come next week, as will be discussed in the next section.
4km NAM run at hour 14 depicting tonight’s low temperatures, with widespread near to below zero degree readings across the area. This run is likely a little too cold over NYC (image credit: PSU e-Wall).
For tonight, with the high pressure overhead, decreasing winds are expected tonight which along with mostly clear skies will allow temperatures to fall to near or below zero across many interior locations, likely approaching -8 degrees in NW NJ and interior SE NY. The main question is how cold Central Park will get, as a low below 5 degrees would be the coldest temperature since January 2004. Central Park has not reached zero degrees since January 1994, almost 20 years ago. Typically, northerly winds are favorable for colder temperatures in NYC, with a light NW wind expected tonight. The light wind speed, however, is likely to work against Central Park reaching zero degrees with the Urban Heat Island keeping temperatures in the city several degrees warmer than the suburbs, where more radiational cooling is expected, leading to temperatures near or below zero. At this time, Central Park is expected to fall to near 3-5 degrees. Temperatures are expected to approach record lows in the NYC climate stations, with records for January 4th close to zero degrees. Temperatures will quickly rebound on Saturday into the mid 20s across the area with partly sunny skies and a light SW wind as the trough departs the region.
Sunday – Thursday: Rain, Then Major Arctic Cold Outbreak
18z GFS at hour 60, at 6z Monday (1am), depicting the rapidly deepening low pressure over western PA and the anomalous arctic air mass over the north central US (image credit: PSU e-Wall).
A very active beginning to next week is expected, and involving a fairly anomalous scenario including the polar vortex and the core of the cold over Canada spilling southward into the north central US with the aid of weak blocking currently present over Canada, leading to an extreme arctic outbreak covering the majority of the central and eastern US. As the polar vortex reaches the US/Canada border and becomes neutrally tilted on Sunday night, a wave of low pressure will develop near the southern US, tracking towards western Pennsylvania and NY while beginning to rapidly intensify. Temperatures aloft will be above freezing across the majority of the region, preventing snow from falling again, but with near freezing surface temperatures possible initially especially over the interior Northeast in parts of PA/NY and possibly NW NJ at the beginning of the event, which may lead to some freezing rain. Periods of moderate rain are expected overnight with a southeasterly flow leading to temperatures warming up overnight, ranging from the mid 30s in western area to the mid 40s in NYC and low 50s in Long Island/CT.
The main highlight of the event comes towards Monday, as the core of the cold air mass will have plunged into the Midwest, with 850mb temperatures over the Midwest as low as -32C to -36C. This type of air mass has not entered the US to such a widespread and southward extent in the last few years. The low pressure over the region, meanwhile, will continue to rapidly intensify as it enters Canada, deepening to nearly 948mb by Monday as it becomes vertically stacked over the eastern Hudson Bay. Along with ridging in the western US shifting to the east, this will pull the polar vortex to the east with the frigid air mass over the Midwest surging eastward into the northeast US region, leading to very strong cold air advection with 850mb temperatures cooling from 10C to near -20C in just 18 hours. At the surface, temperatures in the NYC area will start out in the morning ranging from the upper 30s in western areas to mid 40s near NYC and low 50s in Long Island, and will quickly fall into the mid-upper 20s by the late afternoon and evening. With the strong push of cold air, enough moisture is likely to be left in place to support snow showers at the back end of the cold front on Monday afternoon.
Monday night into Tuesday are expected to bring some of the coldest temperatures the region has seen in the last few years as the anomalously cold air mass moderates, but still enters the region with widespread 850mb temperatures between -20C and -30C, which is colder than the majority of recent arctic outbreaks. A rapid flash freeze is expected on Monday night, as temperatures quickly fall into the negative single digits in western areas and the low to mid single digits elsewhere, possibly the upper 0s near and east of NYC, with any leftover rain from Monday expected to freeze into black ice. A very tight pressure gradient is expected as well with a strong westerly wind, leading to wind chill values near -5 to -20 degrees. Anomalously cold high temperatures are expected on Tuesday with continued strong westerly winds, as despite partial sunshine, highs are only expected to peak in the mid 0s inland and upper 0s to low 10s for the rest of the area.
As with the theme of the progressive pattern, however, the coldest temperatures will remain short lasting, and are expected to moderate fairly quickly as well. With the trough departing as a more zonal flow develops across the US, temperatures overnight will struggle to fall below zero degrees for most locations before rebounding into the low-mid 20s on Wednesday and the low-mid 30s on Thursday. A weak low pressure may produce some snow and/or rain across the area on Thursday and Friday, although at this time the probability of a major snow event appears fairly low. A pattern relaxation is then expected heading into mid January, with a more zonal flow developing across Canada as well with the polar vortex likely to retreat northward for an extended period of time resulting in temperatures returning to near or above average in the region. More information on the longer range outlook will be posted with tomorrow’s update.