The snowy winter of 2013-14 continues its theme of frequent snow events, especially beginning on January 18, with a quick moving 1-2 inch snowstorm last night. More snow is on the way, however; after cold temperatures for the first half of the week, with highs in the 20s and lows in the single digits away from NYC and the immediate coast, a significant wet snowstorm is becoming increasingly likely on Thursday, followed by yet another light snow event potential next Sunday.
Today – Wednesday: Cold Continues, Sub-Zero Lows Inland
Posted above from left to right are the latest surface analysis and radar composite from WPC, and the initialized GFS 500 millibar heights and vorticity from NCEP MAG; the former is valid at 9z (4am), and the latter is valid at 6z (1am). Last night’s light snow event has mostly exited the region except for some snow showers remaining over Maine, with a weak surface low pressure off the coast of Maine east of the mid level shortwave trough centered over the Northeast US. Central Park recorded 1.2 inch of snow last night; this brings February’s snow total so far to 13.2 inches, and the winter to date snow total to 41.5 inches, which is nearly 26 inches above average to date.
A progressive and strong west-WNW flow in the upper levels continues, which will quickly carry the low pressure offshore while the high pressure over the Dakotas shifts eastward, covering the region later tonight into Tuesday and early Wednesday. With a cold air mass over the region, along with the high pressure providing the area with mostly sunny skies and fairly light NW-N winds, cold temperatures will continue through Wednesday. Highs today are expected to peak in the mid-upper 20s, with lows in the low-mid 20s. The more significant cold will come overnight, however, with widespread lows in the single digits away from NYC and the immediate coast. Tuesday night will be the coldest of the two, with the high pressure positioned overhead supporting mostly calm winds along with fresh snow cover leading to strong radiational cooling away from urban areas, with lows in the low 10s in NYC and the immediate coast falling into the single digits elsewhere and near/below zero in NW NJ/interior SE NY and the Pine Barrens in Long Island.
Another feature to make note of is a shortwave trough over northern California; this feature will continue to dive ESE, with precipitation developing ahead of this shortwave extending into the southeastern US by tonight into Tuesday. With the cold air mass aloft in place over the region, this will lead to another round of snow and some ice in locations such as North/South Carolina and parts of northern Georgia, Alabama and Mississippi. As the mid level trough departs the northeast US on Tuesday, the strong cold air mass will be pulled out of the region as well overnight, with temperatures in the NYC area warming up into the upper 20s on Wednesday, but with enough cold left over to support precipitation type as snow with the next upcoming storm system.
Wednesday Night – Thursday: Wet Snow Expected
The next storm in the long line of snow events will affect the region on Wednesday night into Thursday, with widespread snow likely to affect the region again. Unlike the last few events, however, there is somewhat higher uncertainty regarding the exact impact of this storm in the area, including borderline cold surface temperatures indicating a wet snowfall and the exact evolution of the low pressure track and intensity which will determine how much snow accumulates across the region.
6z GFS hour 54, at 12z Wednesday (7am), depicting the setup at 500 millibars in the initial development stage of the system, with the northern stream over Nebraska diving southeast to phase with the southern stream over Louisiana. The “kicker” shortwave can be seen over the NW US (image credit: PSU e-Wall).
Model Analysis: This storm will develop as a result of an interaction of two shortwaves, including the southern stream shortwave currently over northern California which will end up over the southeast US by early Wednesday morning, and a northern stream trough currently over the northeast Pacific which will enter the NW US tonight before diving southeast towards the southern stream on Wednesday. The two streams are then expected to phase over the southeast US, with the trough then likely to become negatively tilted, supporting a deepening low pressure tracking northeast parallel to the NE US coast. Another shortwave trough, which in the image to the left can be found over the NW US, will follow just behind the main system over the Midwest; this “kicker” shortwave will help to keep the system progressive, and depending on its proximity to the low pressure, may inhibit heavy precipitation and strong dynamics in the western end of the precipitation shield.
0z ECMWF at hour 96, for 0z Friday (7pm Thursday), depicting a closed mid-upper level low with a low pressure close to the coast. This run is the westernmost and warmest of all models (image credit: PSU e-Wall).
As of yesterday morning’s update, only the ECMWF depicted significant impacts consisting of heavy wet snow in the region, with the 6z GFS the only run of that model during the day to produce light to moderate wet snow over NYC, while the NAM and CMC were suppressed south of the area. With the 12z runs, the ECMWF and ECM Ensemble mean continued to depict a significant snowstorm affecting the area with the low pressure track near the 40N/70W benchmark, while the 12z CMC depicted over 16 inches of snow with a minimum pressure below 970mb; the 12z CMC was a significant outlier compared with the rest of the model guidance and even all of its own ensemble members, and has predictably backed down from its earlier output with last night’s 0z run. The GFS kept most of the precipitation offshore with earlier runs, but has generally trended towards a slower and further west placement of the shortwaves prior to phasing, along with a stronger northern stream, which resulted in a more significant tilt occurring earlier than previous runs and leading to a surface reflection of a further west and stronger low pressure. There has also been a trend on the GFS towards a somewhat weaker and slower kicker shortwave with recent runs.
Given that the GFS continues to catch up to the remainder of the model guidance, I am currently considering the GFS to be a suppressed outlier, with additional adjustments towards a stronger and further west storm likely. The main uncertainty is the intensity and track of the low pressure; the 0z ECM was the westernmost model with a track fairly close to the coast, resulting in heavy wet snow changing over to rain for most of the area, while the ensemble mean is slightly to the east. The ECM is significantly stronger with the phasing with a closed mid-upper level low tracking through the region; a recurring model bias to consider is an underestimation of the strength and southward extent of northern stream shortwaves digging south through the north central US, although in this case it appears the ECM is likely too far west, and for today’s update I am siding towards a low pressure track just west of the 40N/70W benchmark. The track and intensity of the low pressure, along with the placement and timing of the kicker shortwave, will be key features regarding how much snow falls in the area; due to marginal temperatures near the ground level and aloft, heavy precipitation and strong dynamics would be supportive of heavy, wet snow accumulations regardless of proximity to the coast, with a further west low pressure leading to a changeover to rain and a further east and/or weaker low pressure leading to less significant snow accumulations.
Preliminary Forecast for NYC Area: At this time, snow is likely to develop late on Wednesday night with temperatures in the 25-30 degree range, rising towards the morning hours as the snow potentially becomes heavy at times towards Thursday morning. Any potential changeover to rain would be likely to occur after Thursday morning; at this time, NYC and locations south/east would be favored for any potential changeover. The exact timing of precipitation remains uncertain, with snow ending between Thursday afternoon and the early overnight hours depending on the track and intensity of the system.
As this is a marginal setup, there is not enough confidence to predict exact snow accumulations at this time; as previously noted, the intensity and track of the low pressure, along with the evolution of the phasing of the north and south streams, the negative tilt of the trough and the timing of the kicker shortwave to the west, are all important factors to consider regarding the total snow accumulations in the area. With that said, given the likely intensity of the system, the potential exists for moderate to significant wet snow accumulations on Wednesday night into Thursday afternoon/evening. As the shortwaves involved with this system approach the US with better data sampling expected, the exact location of the heavy snow axis and the forecast accumulations will be gradually narrowed down over the next day or two. If there are any significant changes, an evening update will be posted today, otherwise the next full forecast update will be posted early on Tuesday morning.