Feb 23, 2012: Unexpected Snow/Sleet Tonight

Forecast Highlights:


- Snow Finally Returns; 1-3 inches possible in northern areas tonight
- Temperatures struggle to get above 40 degrees on Friday
- Briefly windy Friday night; chilly, dry and mostly sunny for the weekend
- Next larger storm possible late next week


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With a warmer air mass still covering the region, today brought mild temperatures once again, with most of the area seeing highs in the lower to mid 50s inland and the upper 50s to lower 60s across the rest of the area. In a typical winter, these temperatures would be considered unusually warm, but these much warmer than average temperatures have been so frequent this winter that they are barely as notable as they typically would be.

Despite the current warmth, much cooler temperatures will return tomorrow. The model guidance struggled once again today with handling the storm for tonight, and unexpectedly changed the outlook to shift the snow zone tonight much further south, with accumulating snow and sleet expected in the northern half of the area tonight.


Tonight - Friday Night: Sudden Change In Forecast


Forecasting storms in the short range is typically supposed to be an easy task; the models are supposed to have converged on a single solution with minor differences, the radar and observations are generally close to the models, and a single forecast can be made with high confidence. The models, however, have had a terrible handle on this storm, and even now, literally hours before the storm is about to begin, are still not sure how to handle the storm. Two days ago, the potential was there for parts of the area to exceed 60 degrees; with the latest changes, not only will temperatures struggle to even pass 40 degrees, but apparently an accumulating snow/sleet event is hours away for the northern parts of the area.


Storm Analysis: The original question yesterday was regarding the location of the warm front and how far north/south it would end up. With the storm currently south of what yesterday's models showed, it is evident that the warm front will stay to the south of the area, with the tight gradient from 45 to 70 degrees in a short distance instead taking place towards southern New Jersey and Delaware. With the warm front staying to the south of the area, temperatures will stay in the 30s to mid 40s across the area tomorrow. The biggest change has been with the location of tonight's precipitation, however; the models earlier today had the moderate snow focused from about east central New York towards Massachusetts and southern New Hampshire/Vermont, while bringing a 40 degree rain for the area. In a matter of just 2 runs, less than a day before the storm will start, a significant south trend took place on most of the models, most notably the NAM and CMC, which with their latest runs show a brief period of heavy snow/sleet for most of northern New Jersey, southeastern New York, and southern Connecticut. With the short range models, the HRRR and RR also keep the storm south with the heaviest accumulations in southern Connecticut, with 2 to 4 inches, while the RUC is sticking with a northern scenario which appears to be too far north, as NYC barely even sees any rain on that model. The GFS is currently the northernmost model, but it was the slowest to catch up to the south trend with the storm, and also does not have a pronounced transition to a secondary low as the NAM has, which appears to have a better handle on the overall formation of the secondary low as will be discussed in more details below.

The second part of the forecast is regarding the formation of the secondary low. The GFS currently has the least emphasis on this development, although it continues to slowly trend towards a stronger signal of a secondary low developing earlier. The 18z NAM was the most bullish with this development, showing moderate to heavy rain for the area late on Friday evening, quickly intensifying into a major northern New England snowstorm by Saturday. The 0z NAM run slightly backed down, although it still develops the secondary low earlier than the GFS, with the CMC somewhat similar to the NAM as well. At this time, it does not appear that a strong storm should quickly develop in time to bring significant impacts for the area, although locally heavy rain is possible on Friday evening. This aspect of the storm though is still slightly uncertain as the models once again do not have a good handle on this, despite this development barely 24 hours from now, although huge changes are not expected with the Friday evening outlook.


Forecast For NYC Area: This is probably the most difficult forecast I've made this winter, especially due to the conflicting signals from the models for the storm despite it being only hours away. As a result, this is not a high confidence forecast, although after reviewing the set up, I have at least some level of confidence in the changes in the forecast.

Precipitation should start to fall across the area towards at least 2 AM west of NYC and 2-3 AM east of NYC. When the storm starts, temperatures will still be slowly dropping, ending up in the upper 30s to lower 40s across most of the area, starting out as light rain as a result. Moderate to locally heavy precipitation is expected with the front end of the storm around 3-6 AM, and temperatures should quickly cool down at the surface while slightly cooling down at 850mb as a result of the heavier precipitation, allowing for frozen precipitation to fall in the northern half of the area. Although the 925mb and 850mb layers are below freezing, there is a small warm layer above 850mb, meaning that sleet will also be involved in the forecast for most places that see frozen precipitation as the snow partially melts while moving through the warm layer and refreezes as sleet closer to the surface. At this time, I am thinking that the northwestern parts of the area towards extreme NW NJ to interior SE NY, and interior southern Connecticut will have at least a brief period of a moderate-heavy burst of snow and sleet tonight, with the heaviest precipitation likely towards northern Orange county into interior Connecticut, where the heavier precipitation may fall as mostly, if not plain snow. Should the heavier precipitation fall as snow, the snow totals will be enhanced in those areas compared to other places further south where sleet will limit the total accumulations. Some sleet may mix in south of the areas mentioned above, although any sleet should be brief with no accumulations. Towards the morning hours, temperatures aloft will begin to warm while remaining steady in the mid to upper 30s across most of the area, with a changeover to rain for most places except for Orange county, which may see sleet and/or freezing rain last into the morning hours.

Snow/sleet amounts also cannot be predicted with high confidence, as the accumulations will depend on how much snow and sleet each area sees along with how far south/north the main band of precipitation ends up. Based on the scenario mentioned above, I am expecting a swath of 1 to 3 inches of snow and sleet to extend from extreme NW NJ and Orange County, NY to interior SE NY and interior Connecticut. Amounts may end up locally higher in interior southern/central Connecticut and parts of Orange County. There is still some uncertainty with the accumulations, and if this forecast does bust, this map would likely be too far south, with the actual result a bit north of the map with a less snowy outcome.

Going into tomorrow afternoon, most of the rain will end, with temperatures remaining steady or slightly rising into the upper 30s inland and the upper 30s to lower 40s across the rest of the area. As the cold front approaches, the secondary low will begin to develop to the south of the area, helping to enhance the precipitation as it approaches the area. Light to moderate showers, locally heavy, are expected to redevelop around 5-6 PM while reaching the area, with the light to moderate rain lasting through about 10-11 PM. By 12 AM, the rain should end across most of, if not all of the area, with dropping temperatures and clearing skies. Windy conditions are expected to develop behind the cold front; while the exact intensity of the wind is uncertain and depends on the development of the secondary low, with a tight pressure gradient setting up, it is possible that winds may gust near to above 40 mph across parts of, if not most of the area for a brief time late Friday evening. Occasional breezy conditions should last overnight, with winds gusting again up to 40 mph on Saturday.

With all of the uncertainty, this forecast is still not a high confidence one. As the northern half of the area is near the southern border of the rain/snow line, slight changes with the north/south location of the band will also change the forecast precipitation types and the snow/sleet amounts expected. It is possible that a slightly further north scenario verifies, in which case amounts would be slightly lowered and temperatures would be slightly raised. An update reviewing what happened with the storm overnight as well as the latest forecast for the evening showers and the wind intensity will be posted on Friday morning.


Longer Range Brief Overview:


Chilly temperatures will continue through Monday, with highs generally in the lower to mid 40s. A weak low pressure will move through the Northeast on Tuesday, but this low pressure has trended weak enough to the point where it barely brings in any cold air, and the next storm develops further north and moves east, bringing more rain and warmth for the area. As should be expected, there is still uncertainty, but at this time a rain event still appears more likely than a snowstorm for the late week.