March 13, 2017: Analysis for Tomorrow’s Blizzard

The winter of 2016-17 has featured many swings, with relatively little snow and a notable absence of significant cold air, with the exception of a single week in early February. The onset of meteorological spring on March 1st appears to have instead brought about the late onset of meteorological winter, as the last two weeks have largely featured anomalous cold and even a light snow event last week. The most significant snowstorm of the winter is expected to affect the Northeast US over the next day and a half, producing upwards of 1 to 2 feet of snow accompanied by blizzard conditions and coastal flooding.

Today’s forecast analysis focuses on the meteorological factors driving the evolution of this snowstorm, along with a detailed impact analysis. The first two sections describe the meteorological aspects of this storm in detail; skip to the bottom of this post for a public-oriented impact analysis.


 

Synoptic Overview: Widespread Snow Expected

Before we delve into the specifics regarding exact snow accumulations, it is important to get an overview of the big picture first, in order to get a sense of what is expected to occur and why.

 

500-hPa vorticity and geopotential heights valid at 00 UTC Tuesday (8 PM EDT this evening). Notice the north stream trough over Wisconsin and the primary shortwave trough over Mississippi.

The development of this storm comes primarily as a result of the interaction between two cyclonic vortices over the central-eastern United States. The shortwave trough currently over the south-central US emerged over land on Saturday evening, March 11, before quickly progressing southeast downstream of an amplifying ridge over the western US and upstream of an antecedent upper level low situated over southeast Canada.

 

300-hPa wind and geopotential height from the 4km NAM, valid at hour 18 (1200 UTC Tuesday, 8 AM EDT).

As this shortwave trough begins to interact with the trough farther north, it will acquire a negative tilt tonight, as implied by a northwest to southeast orientation of the trough axis, and begin to track northeast up the coast. Strong upward vertical motion is expected off the Southeast US coast in association with strong cyclonic vorticity advection downstream of the trough, along with strong upper-tropospheric divergence in the right-entrance quadrant of a powerful 300-hPa jet streak, which will aid in facilitating the development of a surface low pressure offshore, tracking northeast up the coast as it rapidly deepens.

These forcing mechanisms for upward vertical motion support widespread development of moderate to heavy precipitation across the region. Despite the time of the year in mid-March, when temperatures on average are warmer and the frequency of snow events decreases, the cold air mass currently over the Northeast US in association with the aforementioned upper level low is sufficient to result in snow as the dominant precipitation type away from the coast across much of the region.

 

4km-NAM forecast wind at 850 hPa, valid at 1500 UTC Tuesday (11 AM EDT).

Fine-tuning precipitation types will be somewhat of a challenge, given that the surface low tracks in close proximity to the tri-state area. As the coastal low quickly deepens with a tightening pressure gradient, an anomalously strong southeasterly to easterly low-level jet will develop near the area, with onshore winds up to 80 knots advecting warm air aloft into the area. This warm air advection initially will be sufficient to lead to a layer of above freezing temperatures aloft, perhaps resulting in sleet in parts of Long Island, coastal Connecticut and NYC, although any such period in NYC will likely be brief as the mid-level low deepens and pulls away, with a strong northerly wind on its backside advecting cold air aloft into the area.

The greatest impact of precipitation type on snow accumulations is expected over Long island into coastal SE Connecticut, east-central New Jersey and perhaps coastal parts of NYC. In addition to a longer duration of warmth aloft, surface temperatures will likely warm to near or just above freezing, likely resulting in a period of sleet and rain during the mid to late morning hours before a changeover back to snow. As will be discussed in the next section, heavy snow rates are still anticipated, although this will somewhat limit the maximum snow potential.

 


 

Mesoscale Overview: Intense Snowfall Rates Expected

The factors mentioned above support widespread moderate to heavy snowfall across much of the region, although exactly where the heaviest snow falls depends on more mesoscale, or localized, factors, such as low-level frontogenesis, which has been shown to correlate with the development of intense snow bands in nor’easters. Model guidance, especially lower-resolution models such as the GFS and the CMC, often struggle to resolve the exact positioning, orientation and intensity of such banding, although in recent cases high-resolution models such as the 4km NAM and RGEM have done a sufficient job at resolving the evolution of the banding, even if not exactly nailing down the correct placement.

In order to forecast snow accumulations, both the duration of snow and the intensity of snow need to be considered. In this case, the duration of the event will be relatively short, as the shortwave trough quickly swings through the region before becoming closed off and ultimately stalling over Maine, with only a 12-18 hour duration snow event for most locations. As such, intense snowfall rates are needed in order to get high snow accumulations, and as discussed further below, such snowfall rates are in fact expected.

 

4km-NAM forecast 700-hPa frontogenesis, valid for tomorrow morning and afternoon.

Frontogenesis is formally defined as the enhancement of a density gradient, although in cases such as this one it can be similarly defined as the enhancement of a temperature gradient. Prior research has correlated strong frontogenesis with intense snow bands. In locations of maximized frontogenesis, strong upward vertical motion is likely to lead to the formation of an intense snow band, and especially if the maximum ascent occurs in conjunction with the layer of dendritic growth zone (DGZ), such a setup would result in a band of intense snowfall rates and high snow to liquid ratios. Outside of the band, subsidence results in poor snow crystal growth and lighter snowfall rates, thus leading to an enhanced snowfall gradient relative to what model snow maps tend to show.

The loop above shows the frontogenesis forecast from the 4km NAM model. Initially, there is a single axis of frontogenesis over central PA into the Catskills associated with warm air advection ahead of the mid-level shortwave. As the coastal low begins to rapidly deepen with a strengthening low-level jet, a secondary axis of frontogenesis develops over New Jersey on Tuesday morning and quickly progresses north, passing through the tri-state area around 12-15 UTC (8-11 AM EDT). This axis then merges with the initial frontogenesis axis, leading to a pivoting band of frontogenesis over east-central New York into Vermont over much of Tuesday afternoon and evening. Research by Jaymes Kenyon shows the frontogenesis patterns associated with pivoting snow bands near east-central NY, which is similar to the forecast for this event, further supporting the modeled depiction of an intense snow band setting up over east-central NY. Taking higher snowfall ratios into account, this band is likely to be associated with snowfall rates near or above 3-4 inches per hour.

Based on the latest high resolution models, the maximum frontogenesis axis is expected from central PA into the Catskills, the mid-upper Hudson Valley, and into western MA and Vermont, which is generally where the heaviest snowfall rates are likely to occur for the longest duration, resulting in higher snowfall accumulations. Over the tri-state area, the initial frontogenesis band during Tuesday morning, especially combined with strong low-level moisture and warm air advection, is expected to be associated with very intense precipitation rates, although with somewhat lower snowfall ratios owing to a relatively warm temperature profile, likely resulting in snowfall rates around 2-3 inches per hour between 8 AM and 11 AM EDT, locally up to 4″/hour. Along with a relatively quick progression of the band through the area, this is likely to keep snowfall amounts in the area slightly lower than locations farther inland.

 

Snow Forecast Analysis

The latest model derived snow maps generally range from PA to Maine with the highest snow accumulations, and some of them depict the highest accumulations over northern NJ. This is where caution needs to be exercised with interpreting 10:1 snow to liquid ratio snow maps; these maps factor in precipitation that falls as snow, without accounting for ratios which tend to vary both spatially and temporally over the course of a snow event, and often struggle to properly handle areas of precipitation type transitions. Some snowfall maps incorporate the Kuchera ratio scheme that accounts for variations in snow to liquid ratios, although these maps still struggle to accurately resolve the effects of intense snow bands and the subsidence surrounding them.

This forecasts attempts to account for the axis of heaviest snow by compositing the region of maximum forcing for ascent and manually adjusting for snow to liquid ratios. The previous section suggests the highest snowfall rates are likely to occur over PA into the Catskills, the mid-upper Hudson Valley, western MA, and Vermont. Taking temperature profiles into account, which will be more favorable for high snow to liquid ratios farther north into the Catskills and Vermont with the absence of a mid-level strong warm layer, along with the pivoting of the frontogenesis axis near eastern NY, the highest snow totals are likely over the Catskills into portions of NE PA and Vermont, where around 22-28 inches of snow are likely to fall.

Over the NYC area, as previously mentioned, the initial passage of the frontogenetically-enhanced snow band on Tuesday morning will be associated with intense snowfall rates, but will move fairly quickly through the area, followed by a changeover to sleet and rain in Long Island and possibly parts of NYC. The quick motion of this band and lower ratios relative to locations farther north will limit snow amounts to generally below 16-18 inches for most of the area except for NW NJ and SE NY, where higher ratios and a longer duration of banding will result in higher snow amounts.

 


Impact Analysis

Based on the above analysis, the latest forecast is for 8 to 12 inches in eastern and central LI, 12-16 inches in central NJ south of I-80 into NYC and coastal Connecticut, and 16-20 inches over most of northern NJ and Southeast NY.

The main area of uncertainty at this time involves the inland extent of mixed precipitation, which will result in the southeast snow gradient shifted either west or east, and the exact position of the maximum axis of frontogenesis, which may result in the heavy snow band being shifted slightly east or west. If this forecast does not verify, the most likely bust scenario at this time is if the low tracks farther west than forecast, which would result in more sleet and rain falling farther inland and lowering the snow totals in the map above by about ~4 inches.

The primary impacts with this storm, as discussed in the next section, will be heavy snowfall, coastal flooding and strong winds.

Snow Impact

Snow is expected to develop around 1-2 AM tonight, then gradually intensify until 8 AM, when widespread moderate snow is expected. By that point, at least 4 to 7 inches of snow are expected near and west of NYC, with 1 to 4 inches east of NYC. Between 8 AM and 11 AM, an intense band of heavy snow is expected to move through from south to north, lasting for about 1-2 hours at any given location. This band will likely be associated with snow rates over 2-3 inches per hour, but slightly lower in eastern Long Island where sleet and rain will likely begin to fall. This band should mostly move out by 12 PM, with precipitation type mixing with sleet in most of Long Island and possibly NYC. Snowfall rates should slowly subside in the following hours, with snow mostly tapering off after 2 PM Thursday.

In the event that the forecast does not verify, the most likely bust scenario is for an earlier onset of sleet in NYC, NE NJ and southern CT, around 10-11 AM, which would place a limit on the snowfall rates, although the overall timing would not change by much.

Wind Impact

This system will also be associated with strong winds and blizzard conditions. As the coastal low deepens and the pressure gradient strengthens, low-level northeasterly winds will intensify, peaking near 30-40 mph sustained over much of Long Island, and 20-30 mph elsewhere. Stronger gusts in excess of 30-40 mph inland and 40-50 mph in Long Island are expected, which will lead to blizzard conditions at times during the peak snowfall rates, reducing visibility to 1/4 of a mile or less. These strong wind gusts may also result in coastal flooding, especially in northern shores along northern Long Island and coastal New Jersey.

Leave a Reply