Mar 24, 2013: Some Snow Tomorrow

Forecast Highlights:

A late season snowstorm will affect the region tomorrow, continuing the colder and snowier than average pattern that has been in place for the last 10 days. Some snow is expected from this storm in the area, with most of the snow staying to the south. Afterwards, a colder than average pattern will persist through the rest of the month and into April with hardly any precipitation for the following week.


Storm Outlook: Some Snow, Limited Accumulations

The fourth widespread snow event of the month will affect the region on Monday, continuing the colder and snowier than average pattern in place. A low pressure currently near Kentucky will track into Ohio as a coastal low pressure intensifies on Monday afternoon and night, becoming the dominant low pressure as it gradually tracks ENE and away from the coast. Widespread precipitation is expected especially in the Mid Atlantic, but with less impacts towards the area and New England.

Model Overview: The model guidance has been variable with this storm, ranging from the NAM which, as with the last few storms, was a significant outlier in terms of snow amounts and was excluded from the forecast, to the ECM which has been consistently south and drier with less snow in the area. The latest NAM runs have come more in line with the rest of the guidance, but the most recent models appear to be trending weaker and southeast with the coastal low, a trend which if correct would result in much less snow than previously modeled across the area and into New Jersey. Based on earlier model runs, the low pressure was expected to be near 984mb not far off the NJ coast, with heavy precipitation spreading into south-central NJ resulting in significant snowfall of over 6 inches, while the area would be closer to the north end of the storm with lighter snow accumulations generally up to 3 inches. Based on the latest model runs coming in, the heavier banding would remain offshore, with less snow over south-central NJ and light snow or rain/snow in the area with totals under 1 to locally 2 inches.

Comparing today’s NAM and GFS runs to the latest observations at 3z, a clear trend has been for these models to show the primary low over Ohio a little too strong and too far southwest; the 12z NAM 15 hours out had the primary low over south central Indiana, when it was in reality over south central Ohio. Along with this trend, the secondary low was modeled a little too weak and too far west. In terms of the 500mb setup, the upper level low over Indiana was a little weaker and south than modeled, with heights ahead of the ULL a little too amplified, although the latest 0z run looks to have taken this trend a little too far compared to the latest observations. These observations suggest that while some models may have taken the southeast trend a little too far, there is a valid bust potential with the forecast regarding less snow than modeled over New Jersey into the NYC area.

Forecast Overview: Based on the latest trends and observations, I am siding closer to the less amplified models, with the NYC area north of the heavier precipitation. Evaporative cooling is likely to support precipitation type of mostly snow, perhaps mixing with rain in parts of the area under the light precipitation rates during the daytime. The main questions are how far north precipitation spreads, how much precipitation falls, and snow totals. Unlike the snowstorm last week, precipitation rates are not expected to be heavy in the area, and along with most of the event taking place during daytime, a marginal situation regarding accumulations is expected. Given trends with previous storms this month, I am siding towards the colder end of the model guidance, with highs mostly in the mid to upper 30s across the area. Boundary layer temperatures are expected to be marginal, and without the heavy snow rates needed for snow to quickly stick and accumulate, I am leaning towards the lower end of the accumulation ranges below the typical 10:1 snow to liquid ratios. Assuming 0.25″ of precipitation, accumulations would likely end up at 2 inches or less, mostly on non-paved surfaces.

The next question is how far north precipitation spreads, as a tight precipitation gradient is expected in the north end of the storm, which looks to set up close to the area. Relatively small differences with this gradient will make the difference between moderate-heavy wet snow as earlier runs suggested, and only scattered snow/rain showers as the 0z NAM shows especially north of NYC, which makes for a more difficult than usual forecast despite the time range. At this time, I am siding towards the gradient setting up near or just south of the area, with at least 0.15″ to 0.30″ of precipitation in NYC, decreasing to the north and increasing to the south. This would support accumulations of less than 2 inches across most of the area, with locally higher totals in Long Island and south of NYC. It should be noted again that due to the short range model differences and the tight precipitation gradient, confidence is a little lower than usual and there is a bust potential for totals lower or higher than expected.

Forecast for NYC Area: Light precipitation is expected to spread in from the southwest towards 6-10 AM, mostly focusing on NYC and further south/west, taking longer to spread into Long Island and southern CT. This is likely to fall as snow across most of the area, possibly mixing with rain initially. As the coastal low continues to develop south of the area, temperatures are expected to warm up into the mid to upper 30s, with a light rain/snow mix for the afternoon hours. By the late afternoon and evening, towards 5-8 PM, the focus of the precipitation in the area will shift into Long Island, where light to moderate snow, possibly mixing with some rain, is likely. To the north, lighter snow showers are expected in southern CT, also possibly mixing with light rain at times. Precipitation is mostly expected to end by midnight.

The current snow outlook is for up to 1 inch of snow in SE NY and southern CT and up to 2 inches of snow in northern NJ and NYC, with totals locally over 2″ in Long Island and south of NYC. Accumulations are mostly expected on non-paved surfaces. As with the previous parts of the discussion, it should be noted that confidence is lower than usual due to the short range changes in the models and the tight precipitation gradient in the northern end of the storm setting up near the area. Should the gradient end up north of the expectation, heavier snow totals of 2 to 5 inches would take place across most of the area. If the gradient ends up further south, however, even the current snow forecast may be a little too high. A few updates will be posted on Monday, comparing the storm to the forecast and making any necessary revisions to the forecast.

Longer Range Overview: Beyond this storm, a trough will remain over the region through March 31st, keeping temperatures cooler than average. The air mass will be stale, however, with temperatures gradually warming up, starting in the low to mid 40s on Tuesday, eventually reaching the upper 40s to low 50s by Friday into the weekend, with mid 50s also possible in parts of the NYC area, making it the first time temperatures in the 50s were reached since March 10-13.

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