28-29 October 2008 Snowstorm Case Review|
An early season Nor'easter produces up to 19 inches of snow across the North Country
IV. Mesoscale Analysis
Boundary layer temperatures represented the primary forecast challenge associated with this early season storm. Where and when temperatures would be cold enough to support snowfall during the event largely depended on the track of the low pressure system, which ultimately followed the Connecticut River Valley northward across New England. This particular track favored the western portion of the WFO Burlington forecast area, owing to the strong 850-700mb warm advection taking place to the east until the surface low center reached northeastern VT late in the afternoon on October 28th.
The observed sounding at Albany, NY (ALB) from 28/12z (Fig. 26) was representative of the above freezing near-surface conditions across much of the North Country early in the event. The ALB sounding at 12z also features a deep saturated layer with in cloud temperatures in the favorable dendritic growth zone (-12C to -18C). The wet-bulb zero level at ALB was around 4200', but around 1100' at Maniwaki, Quebec (Fig. 27).
Fig 26. Albany, NY sounding (ALB) at 12z October 28th. Click on image to enlarge.
Fig 27. Maniwaki, Quebec (WMW) sounding at 12z October 28th. Click on image to enlarge.
During the morning hours on October 28th, snow levels across the North Country fluctuated between 1500' and 2000'.
The meteogram at Saranac Lake (KSLK) is representative of the above freezing surface temperatures represent across much of the area through most of the daylight hours on 28 October (Fig. 28). Temperatures began falling appreciably after 2pm EDT.
Fig 28. Meteogram (time series analysis) for Saranac Lake, NY ASOS. Shaded region shows period of prolonged snowfall and reduced surface visibility. Click on image to enlarge.
Conditions began to change during the mid to late afternoon hours on October 28th. At 28/21z, mosaic composite radar reflectivity imagery (Fig. 29) had become much more widespread across northern New York, while also being affected by "bright banding" due to melting snow aloft in vicinity of the Champlain Valley. At this time, the surface low had reached the upper Connecticut river valley. At 28/21z, surface temperatures fell to 33F at KSLK and that marked the beginning of prolonged accumulating snow at that location. Visibility during the next 30 hours was generally below 3 miles in snow, and briefly down to 1/4 mile. The accumulating snow ended during the early morning hours on October 30th. Wind gusts had also increased and were near 30 mph at times between 29/04z and 29/06z (Fig. 28). Although falling mostly as rain, precipitation reached its heaviest at KBTV between 28/18z and 29/00z with a 6-hrly accumulation of 0.70".
Fig 29. Mosaic radar imagery at 21z October 28th. Also shown are surface observations and sea-level pressure (2mb contour interval). Click on image to enlarge.
Consistent with the 850mb warm advection (Fig. 12) and trajectory analysis (Fig. 13) shown earlier, a trough of warm-air aloft (or TROWAL) airstream evolved across northern New York late on October 28th and was coincident with the period of heaviest precipitation across the area, including heavy snow at the higher elevations of New York. The 28/12z run of the GFS shows the migration and eventual merger of two 700mb theta-e ridges associated with the warm conveyor belt on the western periphery of the low track by 29/00z (12h forecast). This formed the organized TROWAL during the afternoon and evening on 28 Oct. It appears that the easternmost theta-e axis was left from the stalled frontal boundary that past through New England several days earlier (26 Oct). This evolution is shown in Fig. 30 a-c.
Fig 30. The operational 28/12z GFS 700mb equivalent potential temperature (theta-e, K) and wind (kts) at (a) 28/12z (00h forecast), (b) 28/18z (06h forecast) and (c) 29/00z (12h forecast). A pronounced TROWAL extends north to south across far Northeastern New York at 29/00z.
A GFS model vertical cross-section at 29/00z (Fig. 31) shows a strong upward vertical motion signature (absolute magnitude greater than 20 microbars/s) within the TROWAL, likely a combination of large-scale warm advection and frontogenetic forcing mechanisms. The strong vertical motion also occurred through the dendrite growth layer (ambient temperatures of -12C to -18C) as shown in Fig. 31. The "folding" of the theta-e contours above the UVV max is indicative of conditional instability present in the 450-350mb layer over the northern Adirondacks.
Fig 31. The 12z GFS model vertical cross-section valid at 29/00z (12 hr forecast). Black solid lines are contours of equivalent potential temperature (theta-e). Dotted lines indicate negative omega, with warm (cold) color-filled areas corresponding to areas of ascent (descent). The -12C, -15C and -18C isotherms are shown as dashed black lines. This region is favorable for dendritic snow growth. The intersection of the plane of the cross-section with the ground is shown in the upper right. Click on image to enlarge.
An 88-D KCXX radar reflectivity cross-section taken from the Northern Adirondacks east-northeastward into north-central Vermont (Fig. 32) at 28/2053z shows the well-defined bright banding associated with melting snow aloft (maximized in the 950-900mb layer) across the Champlain Valley and along the western slopes of the Green Mountains. The RUC-derived freezing level at 28/21z slopes downward from near 800mb over north-central Vermont, to near 925mb (near ground level) across the Northern Adirondacks.
Fig 32. KCXX radar reflectivity (dBz) cross-section at 28/2053z, juxtaposed with 0h RUC model temperatures at 28/21z. The intersection of the plane of the cross-section with the ground is shown in the upper right. Click on image to enlarge.
The displacement of the bright banding feature downward toward the ground in and near the Champlain Valley is evident in this loop of radar reflectivity from 2058z through 2350z, as rain changed to wet snow with cooling temperatures aloft during this time period. Light snow accumulations would occur across the Champlain Valley during the late evening hours on 28 October into the early morning hours on 29 October as the surface low tracked northward into Quebec. Accumulating snow would also continue in the heavy snow accumulation area of Northern New York through the 29th with wraparound moisture and orographic ascent into the Adirondack Mountains. Additional snow would also occur along and downstream of the Green Mountains in Vermont with 850mb northwesterly winds of 35-40 kts through much of October 29th, before ending during the evening hours.