Dec 5, 2011: Snow Possible Wednesday Night

Widespread fog was observed across the area this morning, and although the cloud cover slightly cleared by the afternoon hours, temperatures were colder than expected today, peaking in the upper 50s to lower 60s inland, mid to upper 50s in the immediate NYC area, and the mid 50s in Long Island and southern CT. As a cold front slowly approaches the region tomorrow, temperatures will slightly warm up, with showers expected throughout the day and into Wednesday morning. Looking forward, the model guidance is starting to lock onto a single solution, showing a storm affecting the area on Thursday as has been mentioned over the last few days. What has not been modeled, however, was the potential for snow in parts of the area.


Tomorrow's Outlook:

With the cold front approaching the region, slightly warmer temperatures are expected across the area. Cloudy skies are expected throughout the day with occasional showers, likely amounting up to 1/4 inch, and SW/SSW winds are expected as well. The potential is there again for morning fog across the area. High temperatures are expected to peak in the upper 50s to lower 60s from NYC and further west, and the mid to upper 50s in Long Island and S CT.


Wednesday - Thursday: Coastal Heavy Rain, Interior Snowstorm

Throughout the last few days, when the model guidance has been inconsistent with the set up, the potential was there for a storm to affect the region; first it appeared that this would be on Wednesday, but two days ago Thursday became the more likely day for the storm. The model guidance has not handled this storm well; two days ago, the GFS model showed Thursday's storm way north into Canada before quickly trending towards a storm affecting the region that day. Up until yesterday, the closest scenario to a snowstorm was on the GFS, which itself was inconsistent ranging from a big rainstorm to a rain/snow event. With an overall unfavorable pattern, it had appeared that a snowstorm was unlikely. This year has proved many times that snowstorms can affect the area under unfavorable set ups, such as late January and March, and it appears that this may once again be the case for parts of the area.

The models today all showed a coastal storm affecting the area, the first sign of agreement from the models with this storm. Even so, details still vary from each model and with every run; the GFS keeps the storm mostly offshore with light rain in NYC, the ECM has a light snowstorm in NYC, the CMC also has light rain, and the UKMET supports a warmer solution. The NAM shows a significant snowstorm, but its details have changed, as the 12z (late morning) run supported heavy snow across most of the area, the 18z (afternoon) run supported heavy snow from NYC and further north/west and heavy rain east of NYC, and the latest 0z run was even warmer, with almost an inch of rain in NYC followed by 3 hours of light snow, while NW NJ sees a heavy snowstorm with 6+ inches. While there are still inconsistencies with the models, it is becoming increasingly clear that the storm will affect the area, and is not expected to track inland.

Overall, there are two main possible scenarios at this time; the map to the left illustrates these scenarios. A third possible scenario is where the storm stays mostly offshore, although that scenario is less likely. In the first scenario, portrayed by the 0z NAM, NYC sees a mostly rain storm with over 1 inch of rain possible, followed by back end light snow, while northern NJ sees rain changing to snow with accumulations, becoming higher towards NW NJ. The second scenario shown by the 18z NAM/12z ECM supports some rain to start the storm, followed by a rain/snow mix in NYC and snow across northern NJ with light to moderate accumulations.

This set up is a very complicated one and the track has to be in the right place for NYC to see a snowstorm out of this; a small shift west and NYC sees heavy rain, small shift east and NYC sees rain/snow showers. The middle scenario favored by the 12z/18z NAM would support a few hours of moderate-heavy snow due to favorable dynamics, but with this set up, it is more likely that the higher accumulating snow potential stays inland than over NYC. For now, I went with a scenario in between the 18z and 00z NAM, with snow inland bringing light to moderate accumulations, rain to rain/snow in NYC with light accumulations possible, especially N/W of the city, and mostly rain in Long Island and a rain to rain/snow mix in southern CT. This scenario is still subject to change due to uncertainty, and tomorrow's forecast may either change the forecast to heavy rain to light snow in NYC, rain in Long Island, and a rain/snow mix across the rest of the area, or could show a snowier outcome especially for NYC, SE NY/N NJ, and southern CT where light to moderate accumulations would be possible. Stay tuned for more information on this storm and how it will affect the area.


Friday - Weekend: Colder Temperatures

The expectation has not changed for a colder air mass to move in behind this storm. With 850mb temperatures dropping to near -10 degrees celsius, high temperatures are likely to drop into the upper 30s to lower 40s across most of the area in the weekend, with widespread lows in the 20s as well. More information will be posted on the colder temperatures this weekend throughout the next few days.