Mar 9, 2013: Mid-Late Week Storm Overview

[notice]This post is the first part of tonight’s update, which was separated into two parts. The first part is a review of the mid-late week storm, and the second part to be posted shortly is a forecast update.[/notice]

Mid-Late Week Storm Brief Review

Over the mid-late week period, the area was affected by a long duration storm, with two rounds of precipitation. The first was associated with a coastal low on Wednesday night, while the second was with an inverted trough on Thursday night into Friday. Forecasting these two rounds of precipitation proved to be difficult with variable performance of the model guidance, resulting in less snow than expected with the first storm and more snow than expected with the second storm.



Radar at 11:38 PM on 3/6 from the NWS, showing light-moderate snow east of NYC. The northern end of the precipitation field was south of the expectation.

Originally, the coastal low was expected to produce light to moderate snow accumulations; this was based on a compromise of the GFS/ECM solutions, with the ECM among the southern to show light snow accumulations across the area, and the GFS the most bullish with a major snowstorm. The northern end of  the precipitation shield ended up more south than what most models indicated, with hardly any precipitation north and west of NYC. Even during the day of the storm, most models were still too far north with the precipitation; for example, the 12z GFS showed just under 1/4″ in NYC by 4 PM and over 0.10″ across all of northern NJ, while very little precipitation fell north of I-78 through 4 PM.

The majority of the models had at least light precipitation across the entire area, but the NAM especially had difficulties with handling this event; its 6z run on the day of the storm showed nearly an inch of liquid equivalent precipitation in NYC from the coastal; in reality, no measurable precipitation was recorded in Central Park. Less snow than expected fell across many parts of the region, including the area where up to 2 inches, locally higher, fell in Long Island and parts of CT.

Radar at 6:18 AM on 3/8 from the NWS, showing heavy snow over Massachusetts and moderate snow in the NYC area. More precipitation fell than expected across the region.

The second part of the event was on Thursday night. The inverted trough signal generally began showing up more persistently on the NAM/GFS models on 3/5, although at that point it only appeared to be a light to moderate snow event focusing over the Northeast, north of the area. The NAM was the only model to consistently depict significant snow totals of over 10 inches across the region, including the area, although a few CMC/RGEM runs did show significant snow totals as well, while the ECM also had higher totals in parts of the region than the GFS. Typically in cases like this, the GFS tends to underestimate precipitation amounts, although the NAM’s poor performance with the coastal even during the day of the event, along with other storms throughout the past month and its usual wet bias, especially made its solution appear to be a significant outlier. Even when the event started, it still appeared to be a moderate event at most for most of the area, with totals higher than 6 inches expected in parts of southern CT and SE NY. In the end, the GFS ended up significantly underestimating precipitation rates and amounts, especially heavy snow banding over Massachusetts into Connecticut and parts of the NYC area, while the ECM was closer to reality but still to some extent underdone with precipitation totals; the NAM was correct in its depiction of significant snow totals, but still had several errors regarding mid level temperatures and axis of heaviest snowfall.

The NAM was inconsistent with its handling of both rounds of precipitation in its medium to long range; aside from its typical wet bias, the NAM was repeatedly too aggressive with mid level warming, first by warming 850mb temperatures above freezing across the region during the peak of the coastal early on 3/7, then with the inverted trough by changing all of New England and most of the NYC area to ice/rain early on 3/8 when in reality these locations continued to see plain snow. Along with this trend, the NAM was too far west with the heavy snow axis, initially showing it over central NY and NE PA, gradually trending east at one point until its 3/7 0z run, 24-36 hours before the event, when it showed 12-18 inches of snow across all of northern NJ into the Hudson Valley and eastern NY state with lower totals in New England. During the last 24 hours before the event it correctly trended to show the highest totals over MA/CT with less significant mid level warming.

With the second part of the storm, snow totals generally ended up between 4 and 10 inches across most of northern NJ and Long Island, and 8 to 16 inches in southeastern NY and southern CT. The highest total reported in the area was 15.5″ in Chester, NY (Orange county), with 15.0″ in Colchester, CT (New London county).

Poll: NYC Area Weather ran a poll leading up to the storm asking about the outcome of the storm. Below were the final results:

What impact will NYC receive from the midweek storm? (34 votes)
2 votes – nothing, storm stays south
5 votes – light rain/snow showers, under 1/2 inch
9 votes – significant storm, more rain than snow
18 votes – significant storm, more snow than rain

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