Early images from the NAM and RGEM models and lengthy NWS discussion:
The 12km NAM and RGEM go out 84 hours, so we will soon begin to see how they portray the beginning of the storm. The 1st and 2nd images from the RGEM and NAM are for 1 AM Monday. The 3rd image is the Euro for the same time. The 4th image shows the projected snowfall for Long Island (Islip) for the next 15 days as a running total from every Euro ensemble member.
Always see previous posts as well for more information and maps.
The National Weather Service covering Long Island and the Tri-State area currently thinks that western Long Island should stay all snow with some mixing for central and eastern Long Island. Here is their lengthy forecast discussion on the storm: Potential continues to increase for a winter storm Sunday night into Tuesday. Specific details on low track, precipitation type, and timing remain uncertain at this time. The 12z model cycle has shown some increased variability from previous 06z and 00z cycles. However, the overall thinking has not changed much for the potential winter storm Sunday night into Tuesday. There are several critical features that will ultimately determine the track, intensity, duration, and precipitation type. The main Pacific shortwave will be coming onshore of the west coast tonight into Friday. The deep closed low that swings across New England Friday will leave behind a confluence zone over the northeast since the low itself meanders along the coast this weekend. The main shortwave itself looks to open up as it approaches the Ohio Valley and then gets forced to dig southward around the confluence to its northeast. This forces secondary low pressure development along the Middle Atlantic coast sometime on Sunday night or Monday. The main shortwave then looks to amplify and deepen later Monday into Tuesday as northern stream energy dives within the backside of the trough. The timing of the phasing of these features will also important to the evolution and potential impacts from the deepening secondary low pressure system. There still is a relative clustering for the low to pass near the 40N and 70W benchmark around the Monday night timeframe. However, some of the guidance, including the ECMWF and UKMET have shifted the track of the low a bit further south from previous runs. The ECMWF EPS mean is also a bit further south and the probabilities of seeing greater than 3 inches of snow have actually decreased a bit from earlier runs. The ECMWF still remains one of the slower solutions compared to the rest of the guidance with much of the impacts Monday into Monday night. It should also be noted that the GEFS and CMC ensemble probabilities for greater than 3 inches of snow have decreased somewhat as well, which supports the increased spread on some of the members. There will likely be run-to-run changes for the next day or so. Adjustments are likely to continue as the PAC shortwave moves over the RAOB network in the next 24 hours and the model get a better sense of the strength of the confluence zone over the northeast. The latest forecast follows a consensus blend overall with the only significant change to adjust PoPs to likely late Sunday night into Monday night per collaboration with neighboring offices. Given the consensus track there does not appear to be much warming aloft to change precipitation over to a mix from snow. The mid level low centers also passing south of Long Island add some confidence to this idea. However, some of the models still show a dry slot at some point which could cut off snow growth aloft. Another factor may surround how much, if any, boundary layer warming occurs over Long Island. Have gone conservative with any significant boundary layer warming at this time due to the the consensus track and from a more northerly flow expected during the duration of the event. These mesoscale details will hopefully become clearer over the next few days. Following the consensus track, the precipitation type is mostly snow with snow/rain mix indicated at the immediate coast, mainly central and eastern Long Island.