Forecast models have trended farther northwest — and stronger — with a low pressure system, which is forecast to develop on a frontal boundary off the East Coast from late Thursday into Friday. This is the same frontal boundary which crossed the area on Wednesday Night and Thursday morning, brining the area rain and wind.
Models suggest that as the frontal boundary moves offshore, a secondary area of energy in the mid levels of the atmosphere approaches the area. This energy provides enough lift for precipitation, with sufficient moisture, with precipitation redeveloping to the west of the offshore front. After beginning as rain, precipitation is likely to change to snow — with accumulating snow possible during the morning commute on Friday
What is causing the system to bring us snow?
The offshore frontal boundary slows down quite dramatically as mid level energy approaches the area. This energy will provide sufficient lift for precipitation, and it takes on a negative tilt and strengthens while approaching the forecast area on Thursday Night into Friday morning.
While thermal profiles are warm at the surface initially, cooling the mid and low levels of the atmosphere will work with heavier precipitation rates to support a changeover to snow for many areas. Snow may fall and not accumulate for a period of time, before becoming heavier and accumulating while cooling the atmosphere.
Additionally, a very favorable upper level jet streak will be positioned in the Northeast US. Our area will be positioned in the right-entrance region of this jet streak, supporting enhanced precipitation to the northwest of the developing surface low. This could enhance snow into our area on Friday morning.
What uncertainties still exist moving forward?
Forecast models are still adjusting the exact positioning of the offshore frontal boundary and low pressure area. Adjustments to the positioning of these features will obviously have a huge impact on the forecast. A continued trend westward could mean higher snowfall amounts, while a trend eastward could mean lesser amounts throughout the area.
Additionally, while thermal profiles support snow aloft and in the mid levels, surface temperatures remain somewhat warm on all models. This suggests that even though light snow is falling, accumulations may be somewhat limited until temperatures begin to drop. Exactly when and how this occurs will be critical to snowfall accumulations.
A sharp cutoff is possible to the northwest of the surface low – in Northwest NJ, Southeast NY, and Connecticut. These areas may see very little snow, while areas to the east of NYC have the potential to see moderate snowfall accumulations.
Mesoscale models should help us iron out differences between solutions over the next 6 hours or so, giving us the ability to gain confidence in the forecast.
What is the expected timing and expected accumulations?
Rain is expected to begin later on Thursday Night, after midnight, and continue for a few hours into Friday morning. The rain will gradually change to snow throughout the area, with snow becoming steadier and heavier during the morning commute. During this time frame, accumulating snow is possible throughout the area. The morning commute may be significantly impacted by this snowfall.
Snow will continue through the morning hours and end during the late morning to early afternoon throughout the area. Accumulations remain uncertain and are highly dependent on exact frontal positioning and temperature — but generally, we favor 2 to 5 inches of snow in and around New York City. The potential exists for upward of 5″ on parts of Long Island and the Northeast NJ Coast. (updated at 6:00pm).