January 2022 - LZ

Synopsis

A major winter storm is expected to hit the Northeast primarily Saturday with a strong nor’easter developing and deepening as it tracks offshore. This will bring a swath of > 8″ accumulations from NYC through points north and east, with the bullseye likely over southern New England where snowfall totals could exceed 2 feet locally. Gusty north winds will accompany this storm, leading to some blowing and drifting snow. In line with most other forecasts this morning, my own forecast is for 7-10″ of snowfall in NYC but with the potential for a sharp gradient possibly cutting through the city itself, where the western part of the city could see below 6″ while eastern sections see closer to 10″ or more.

As with the last round of accumulating snow we had, antecedent conditions support enough cold air in place to result in an all snow event. With a storm track offshore enough so there should be no mixing, the primary uncertainty in snow totals arises from storm track. Model runs today should help us determine if the storm will track on/near the 40ºN 70ºW benchmark (more snow), or to the east (less snow). Overnight, model consensus has shifted west, supporting higher snow totals than were forecast at this time yesterday. If we see some consistency across runs today supporting this solution, confidence will grow in a significant snowfall for NYC. Mesoscale banding will bring bouts of intense snow with this storm, and the exact placement / duration of these features will determine where the highest snow totals occur.

GEFS ensemble plumes for total snow accumulation at LGA. The mean here is about 7″ with a significant spread between 3″-14″ between ensemble members.

Forecast Details

At the Surface

Models are projecting a couple shortwaves merging and developing as a coastal low off of the Carolinas later today and tonight. This storm is forecast to intensify and track a little east of the 40ºN 70ºW benchmark overnight, a position that historically signals the best snowmaking potential for NYC during coastal winter storms. The forward speed of this storm has picked up in model runs recently, but it is expected to be a strong storm with minimum pressures falling to the 960s by Sunday. A tight pressure gradient is expected between the intensifying low and high pressure to the north, which should cause gusty north winds to help advect cold air down to the coast.

Weather Prediction Center surface forecast valid for 7 AM Saturday

850 mb – 700 mb Level

A robust 850 mb low-level jet flowing onshore from the northeast is forecast. This should provide the storm with good moisture content, aiding the formation of snow. Strong 850 mb and 700 mb temperature advection and frontogenesis is also evident in all models, occuring during the morning Saturday. This should induce heavy snow bands to develop for a few hours early on Saturday. These bands could produce 1-3″ per hour snowfall rates, and as always, areas where these bands set up and persist the longest will ultimately pick up the higher end totals.

GFS model 850 mb relative humidity & winds at 7 AM Saturday. A strong LLJ flowing onshore from the northeast is visible.

500 mb Level

This storm will benefit from a deep, synoptic scale trough that will acquire a slight negative tilt. This trough will eventually form a closed low at this level. This will aid the low in intensifying by increasing divergence at this level, facilitating upward movement of air over the surface low, leading to lower surface pressures.

300 mb Level

The storm’s center is expected to lie downstream of an exit region of a jet streak, and also upstream of an entrance region of another jet streak to its north. This is ideal placement for maximizing divergence at this level of the atmosphere over/near the low’s center, and supports the development of a strong surface low.

Synopsis

A relatively fast moving offshore low is expected to bring this winter’s first round of accumulating snow in NYC. Fairly good agreement among different models suggests decent confidence of totals NYC in the range of 3-5″, and it is worth noting that trends have been towards a higher total in overnight model runs. In fact, both the Euro and GFS agree on about 5″ of snow for NYC by the end of this event, and I tend to agree with this end of the forecast range at this time.

Antecedent conditions with a shallow but decently cold air mass in place means the primary uncertainty arises from storm track. Subsequent model runs should help us determine if the storm will track on/near the 40ºN 70ºW benchmark, slightly inside (more snow), or to the east (less snow). The timing of the snow during the overnight hours into the early AM rush will help boost totals since we will have enough cold air in place. Moderate and potentially heavy snow is likely during a 3-4 hour window starting around 4 AM.