Ingredients for heavy snow

Ingredients for heavy snow for the RLX CWA from Nor'easters

Nor'easters are synonymous with heavy snowfall over the Northeastern United States. Many kids who grew up along the eastern seaboard pursued a career in meteorology due to the love of snow, heavy snow at that. There is one sure fire way to get snow recorded in feet of snow in the megalopolis section of the United States and that is Nor'easters. The definition of a Nor'easter is: "A cyclonic storm occurring off the east coast of North America. These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage.Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas." -www.weatherquests.com

Nor'easter's bark is much worse than its bite, at least in the Charleston, West Virginia forecast area. After review of nearly 30 years of Nor'easters from 1959 thru 1988, few storms brought widespread six inches or greater of snowfall to the county warning area (CWA). Of the 25 Nor'easters from 1959 to 1987, only four produced regnant snowfall totals of 12 inches and greater. Six produced snowfall amounts between 6 to 12 inches, while the remaining 15 storms produced less than warning criteria snow. If accumulating snow is to occur within Charleston's CWA, it usually doesn't come in the form of Nor'easters, rather low pressure systems that track across the Tennessee Valley. The goal of this article is to increase awareness of Nor'easter's impact to improve winter verification scores.

Table 1 ranks the top seven Nor'easters from the late 1955 through 1987

After careful inspection of Nor'easters that provided the greatest aerial extent of warning criteria snowfall across the forecast area, comparisons were drawn to provide forecasters insight during the watch / warning decision process. Heavy snow warning criteria for our forecast area is six inches in 24 hours or five inches in 12 hrs. Due to the historical time frame, data used was bounded to surface charts, 850mb, and 500mb analysis.

One compelling character of Nor'easters that brought over a foot of snow to the Charleston, WV forecast area was a strong cut off 850mb low that tracked east of the Blue Ridge. One sign meteorologists look for in prediction of heavy snowfall is the track of the 850mb low. Heavy snow typically falls 1.5 degrees to the left of the 850mb low track. In addition, heavy snow occurs more frequently with 850mb lows possessing a northeast movement. With cut-off 850mb lows tracking across the Interstate 81 corridor, this places a good portion of the Charleston CWA in a synoptically favored sector for heavy snow. Temperatures at 850mb in the high snow producing systems ranged from -5C to -10C across the state of West Virginia. Colder temperatures (closer to -10C) where found in the Parkersburg vicinity with the warmer readings across southern West Virginia. Heavy snow typically bisects the -5C isotherm.

Of the top seven snow producing storms, no strong signals were gleaned from 500mb charts. Nearly 75% of the systems had an open wave or an open wave that became closed low. The top storm (Feb 10-12 1983) had a closed low at 500mb tracked across West Virginia. Information can be learned from following the path of the absolute vorticity maximum. The top seven snow storms (where average snowfall amounts exceeded 12 or 24 hour warning criteria), the maximum vorticity passed across southeastern Kentucky, southwestern Virginia, or eastern Tennessee. Another correlation to heavy snow in the mountain state from Nor'easters is a Greenland block in the 36 hours that precede coastal low development. A Greenland block is a cut-off upper level high pressure centered near Greenland. This setup allows cold air to remain entrenched over the northeastern United States and depending on the strength, the mid-Atlantic as well. Given the latitude of West Virginia and the meso-scale phenomena known as the warm wedge, heavy snowfall of a foot or more is uncommon across the lowlands. This is not surprising since cold air is typically displaced with lows tracking up the coast, due to the warm wedge feature. The warm wedge is a lee trough that develops on the windward side of the Appalachians due to easterly flow at the surface. With a down sloping component and compressional warming, boundary layer temperatures climb above freezing. However with a strong low level cyclone at 850mb, winds have a northerly component negating the warm wedge.

The January 1978 storm had a closed 850mb low tracking up east of the Appalachians from the Gulf Coast states. It peaked out at 132dm off the New Jersey coast. During the February 1983 winter storm, the 850mb low tracked first across eastern Texas then northeast to Mississippi then east of the Appalachians. At the point of peak intensity it reached 135dm just off the Virginia shoreline. The February 1979 storm possessed different characteristics as it was a negatively tilted open wave at 850mb that tracked over the central United States then strengthened off Virginia coast to a closed low.

The second highest storm in terms of aerial coverage of warning criteria snowfall was January 1964. Much like the January 1979 and February 1983 Nor'easters, a closed 850mb tracked from the gulf coast state northeast east of the Appalachians. January 1964 was the strongest system between the years 1955 to 1988 with an occluded surface low, which passed over West Virginia. The 850mb low reached peak intensity as it tracked into New England, the lowest analyzed height contour was 117dm at 00Z on the 14^th. The CWA was positioned between the -5C and -10C isotherm, while the low tracked northeast across western Virginia.

After careful review of surface plots during Nor'easters some parallels were drawn. The first is high pressure of 1020mb or greater over the mid-Atlantic while the surface cyclone is moving across the states of Louisiana and Mississippi. With this synoptic setup, cold air remained in place across West Virginia supporting snow rather than rain. There were several instances where a 1036mb to 1044mb anticyclone built south into the Dakotas as the surface low intensified as it traversed up the eastern seaboard. This served up a reinforcing shot of cold air from the Great Lakes due to a northerly wind component. North winds allow for cold advection at the surface supporting frozen precipitation, not to mention this counteracts the warm wedge. High pressure positioned over northern New England and southern Quebec was another indication noted in warning criteria Nor'easters.

What this means for us:

As stated above Nor'easters do not typically bring warning criteria snowfall to the Charleston forecast area. As stated earlier, the 23 cases between 1955 and 1987, four produced widespread snow greater than a foot. Six storms generated six inch snowfall for more than half of the forecast area. Of the cases that warranted a heavy snow warning, similarities were uncovered to more accurately predict heavy snowfall across the CWA. Below is a list of meteorological parameters historically are associated with Nor'easters that muster warning criteria snowfall in the Charleston CWA.

1. Upper level anticyclone over Greenland 36 hours prior to coastal cyclogenesis

2. Surface high pressure greater than 1036mb over New England

3. Closed 850mb low tracking up the leeward side of the Appalachians

4. The -5C isotherm at 850mb resides on the northwest side of the low.

5. Path of the absolute vorticity maximum passes south of West Virginia.

6. An open wave 500mb trough passes to our south where curvature changes from cyclonic and anti-cyclonic.

7. Rapid height falls with a cyclone tracking east of the Appalachians.

By clicking here you will find a composite surface low chart that plots the track off all Nor'easters where the average snowfall eclipsed 12 inches. The surface low track isn't the main feature to key on for heavy snowfall rather, if the criteria listed above is satisfied. If continuity exists between numerical models of the seven items above, forecaster confidence would be high to issue watches and warnings with increased lead time.

For the individual events a graphical plot of the storm total snow can be found here.