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Late-Season Snowstorm Set to Impact the Northeast U.S.

Tomer Burg • 18 April 2022 • Current Weather

Post Highlights

After a winter consisting of generally below-average snowfall in the Northeast U.S., followed by abnormally warm conditions through parts of February and March, winter is set to make a brief but notable return for parts of the region as a cyclone deepens while bringing in unusually cold air, resulting in heavy wet snow west of the storm track peaking in the Catskills and Appalachians.

This post will provide a moderately in-depth overview of what is causing this cyclone to develop and to produce heavy snow, and a look into potential snow totals and other non-snow impacts in the region.

Official NWS snow accumulation forecast (inch), valid as of this afternoon.

For those that are seeking a quick look into the forecast or less in tune with the meteorological analysis, this section provides a quick overview of what to expect with tonight's storm. This is not an official forecast; please reference your local National Weather Service office for more specific details for how this storm will affect your area locally.

Snow Impacts: Heavy wet snow is expected across the Catskills and Adirondacks, with over 4-8 inches of snow possible. Wet nature of the snow may result in power outages. Winter Storm Warnings are in effect for these regions.
Rain Impacts: Heavy rain is expected along and east of the I-95 corridor. While the storm will be quick moving, periods of heavy rain with localized lightning are expected to result in over 1-1.5 inch of rain, with heavy rain rates resulting in areas of flooding. A Flood Watch is in effect for Delaware & most of New Jersey.
Wind Impacts: Strong wind gusts are expected especially along the coast, with wind gusts over 40 mph from Delaware northeast and potentially over 50 mph in eastern Long Island, Cape Cod and coastal downeast Maine. This may be capable of producing coastal flooding.

Scroll down below for a more thorough analysis of the cyclone and the meteorology behind it!

GFS 500-hPa cyclonic vorticity in shading, 500-hPa height in thick black contours, 500-hPa wind in barbs, MSLP in gray contours, and simulated reflectivity in green shading.

The primary factors responsible for the cyclone are a trough and associated surface cyclone currently approaching the Mid Atlantic region, and a vigorous upstream trough approaching from the Great Lakes. As the upstream trough becomes increasingly amplified and negatively tilted, as indicated by a NW to SE orientation of the trough axis, the surface cyclone becomes positioned downstream of the Great Lakes trough in an area of cyclonic differential vorticity advection, providing forcing for ascent and deepening of the surface cyclone. Additionally, the cyclone will be located in the equatorward entrance quadrant of a strengthening upper-level jet streak, also associated with ascent. Both of these factors will contribute to the surface cyclone deepening from 1007 hPa to 991 hPa as it tracks through the region and is steered northward into interior New England. It should be noted that using the Bomb Cyclone calculator, this yields a deepening rate of 0.90 Bergeron, falling short of bomb cyclone criteria.

As the cyclone deepens with increasing forcing for ascent, heavy stratiform precipitation will develop northwest of the cyclone, which in conjunction with a cold air mass associated with the upstream trough merging with the main coastal cyclone will result in precipitation falling as snow northwest of the cyclone track, especially in higher elevations in Pennsylvania and New York. Meanwhile, a region of heavy rain and embedded thunderstorms associated with small instability aloft are expected along and ahead of the cold front following and south/east of the cyclone track, covering much of the I-95 corridor into New England.

HRRR 850-hPa wind (kt) and geopotential height (dam), valid at 2:00am EDT Tuesday.

As the cyclone deepens, the central pressure of the cyclone decreases, resulting in a strengthening pressure gradient between the cyclone and the surrounding environment. A stronger pressure gradient is associated with stronger wind, which will be strongest east of the cyclone with a low-level jet in excess of 90 knots. A surface inversion will prevent such intense winds from from mixing down to the surface, but especially with intense rain showers and along the coast where friction is decreased some of this momentum will mix downwards, resulting in strong gusts in excess of 40-50 mph especially along the coast. As the next section details, this low-level jet then becomes important for mesoscale processes such as where the heavy snow will fall.

HRRR 850-hPa temperature advection (C/3hr, shading), temperature (dashed contours), wind (barbs, knots), and frontogenesis (purple contours), valid at 2:00am EDT Tuesday.

Notice in the previous section, the 850-hPa wind associated with the low-level jet has a strong southeasterly component into the NYC metro, but rapidly decelerates going into the interior Northeast. This occurs on top of a pre-existing temperature gradient aloft, resulting in the low-level jet enhancing that temperature gradient and resulting in strong warm air advection aloft. This enhancement of a temperature gradient is also known as frontogenesis. Recent literature such as Kenyon et al. (2020) has shown a linkage between frontogenesis maxima and heavy banded precipitation, especially snow banding.

HRRR cross-section through the snow band, valid at 2:00am EDT Tuesday.

One limitation of maps like the previous one is their horizontal, 2D nature, while the atmosphere is a 3D fluid. One way to get a better sense of the vertical structure of frontogenesis is through cross-sections, which the one above slices NW to SE through the heavy snow band. Generally, it can be seen that strong ascent as denoted by dark blue shades (indicating negative omega) is well correlated above frontogenesis maxima as denoted by the purple contours. Especially on the left side of the cross section, a frontogenesis maximum exists at 700-hPa, with strong ascent above it, collocated with the placement of the heavy snow band.

To get a sense of the type of snow expected with this band, notice that the ascent is maximized above the frontogenesis maximum, in between the two dashed red contours denoting the temperature range between -12C and -18C. This region denotes the dendritic growth zone (DGZ), which when collocated with strong ascent and high saturation (not shown) favors the growth of dendrites, which are lower in density and generally favor higher snow to liquid ratios (i.e., above the standard 10:1) when not disrupted by other factors such as wind. In this case, however, marginally cool surface temperatures near freezing may lead to aggregation and alongside relatively warm ground temperatures result in lower snow to liquid ratios than would've otherwise been the case had this occurred earlier in the winter.

Notice that just west of the band a region of subsidence exists in the lower levels, as denoted by gray shading indicating positive omega. While heavy preciptiation bands are associated with strong ascent, this is often offset by strong subsidence (descent) inland of the band, resulting in poor snow growth with lower ratios and lighter snow rates. This accordingly enhances the horizontal gradient in snow accumulations between the band and locations just inland of it.

HRRR modeled 10:1 snow to liquid ratio snowfall (inch).

The image above shows the HRRR model forecast snow accumulation, using the standard 10:1 snow to liquid ratio (meaning that 1 inch of liquid precipitation equals 10 inches of snow). This map would indicate widespread snow accumulations upwards of a foot of snow from northeast Pennsylvania into the Adirondacks, and appreciable snow accumulation even in lower accumulations. However, this is not what is expected to actually accumulate.

For one, consider that 2-meter temperatures will be near freezing in high elevations and warmer in lower elevations. This will result in an enhanced gradient in snow accumulations with lower ratios below 10:1 in lower elevations. Snow ratios are also dynamic factors, as described in the previous section, and can have large variations both spatially and temporally within a single event. While sun angle will not be a factor as the bulk of the snowfall will occur overnight, relatively warm ground temperatures may also result in light snow rates accumulating at a slower rate than melting on contact with the ground, while heavier snow rates would more efficiently accumulate. All of these factors generally support ratios below the standard 10:1, especially in lower elevations.

HRRR modeled positive snow depth change (inch).

One way to mitigate this issue is to use the positive snow depth change product available on many model sites. This essentially takes the hourly change in snow depth, and accumulates only positive increase in snow depth for each hour. Thus by accounting for factors that affect changes in snow depth, this yields a more realistic picture of how much snow may actually accumulate on the ground. Notice that snow totals are lower than the previous map, especially in lower elevations, generally for the reasons previously discussed.

3km NAM nest modeled positive snow depth change (inch).

Another high-resolution model within range is the 3km NAM nest model. The Environmental Modeling Center (EMC) recommends using positive snow depth change for NCEP models besides the HRRR and RAP models, as these models factor in sleet and freezing rain directly into the snow accumulation fields, thus artifically inflating displayed snow totals. For the NAM and NAM nest, however, positive snow depth change routinely underestimates snow accumulation, sometimes by large amounts, which may be a result of a warm ground bias. This is an important caveat to keep in mind when looking at snow accumulation from the NAM.

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