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28-29 October 2008 Snowstorm Case Review
An early season Nor'easter produces up to 19 inches of snow across the North Country
Overview | Surface | Upper Air: 850/700mb ; 500/300mb | Mesoscale Analysis
Jump to: 850mb | 700mb | 500mb | 300mb

III. Upper Air Analysis
(a) 850mb analysis
A broad region of 850mb cold advection accompanied the surface cold front across the north-central United States at 26/12z, aided by northwesterly 850mb winds up to 65 kt at Aberdeen, SD, and 50-60 kt elsewhere across Minnesota and the Dakotas.
Fig 9. The 850mb analysis based on rawinsonde observations and RUC background field at 12z October 26th. Height contours (solid lines) are every 30m. Isotherms (red, dashed lines) are every 2C, isodrosotherms (green solid lines) are analyzed every 2C at values of 8C or greater. Standard upper air station model is shown. Click on image to enlarge.
The strong band of 850mb cold advection migrated rapidly east and south before stalling across northeastern New York and Vermont at 27/18z. The 850mb baroclinic zone extended southward across eastern Pennsylvania, Virginia, the Carolinas, and southern Georgia at 27/18z. At this time, there was evidence of developing 850mb convergence in the mid-Atlantic region along the baroclinic zone (Fig. 10):
Fig 10. RUC based 850mb analysis valid at 18z October 27th. Wind barbs are shown in kts, solid lines represent 850mb geopotential height every 30m, dashed lines are isotherms every 2C, and green filled areas are 850mb dewpoint 10C or greater, contoured every 2C. Click on image to enlarge.
During the next 12-18 hours, an 850mb low would develop and deepen rapidly along the 850mb baroclinic ribbon and in vicinity of the initial area of low-level convergence. By 28/06z, the 850mb low was located over New Jersey (Fig. 11), with a noticable increase in the strength of the zonal temperature gradient along the east coast.
Fig 11. As in Fig. 10, except for 06z October 28th. Click on image to enlarge.
The low development and frontogenesis would also allow for the rapid increase in 850mb warm advection between 28/00-12z from New Jersey northward across New England which resulted in rapid increase in large-scale forcing for ascent and developing precipitation north of the low center. The warm advection would also keep thermal profiles relatively warm across the North County during the early states of the storm. As seen in Fig. 12, 850mb temperatures at 28/12z ranged from -2C to -3C across St. Lawrence County in the western part of the WFO Burlington forecast area, to +2 to +3C across the far eastern portion of Vermont.

Fig 12. As in Fig. 10, except valid for 12z October 28th, and with temperature advection shaded (Red for warm advection, blue for cold advection). Click on image to enlarge.
A TROWAL airstream (see mesoscale analysis section) would result in a prolonged period of 850mb warm advection and low-level ascent (through 29/06z) across northern New York as the 850mb low tracked northward and passed to the east of the heavy snow area in northern New York. Temperature advection gradually weakened as the 850mb low tracked northward through Quebec and as the storm became fully occluded and began to fill.
  • Click here for loop of 850mb height, temperature, wind, and dewpoint from 26/00z through 30/00z.
  • Click here for loop of 850mb height, temperature, wind, and shaded temperature advection from 27/00z through 30/00z.
The low-level ascent can also be seen in the NOAA HYSPLIT 40km ETA Data Assimilation system (EDAS40) trajectory analysis ending near Wanakena, NY at 28/20z (Fig. 13). The trajectories ending at 2-3 km show ascent of 1-2 km as they accelerate from northeast to southwest between 28/12z and 28/20z on the west side of the 850mb low track. The speed increase is particularly strong in the 3 km trajectory (green) late in the period from western New Brunswick to northern New York. This large-scale ascent corresponded to periods of heavy snowfall in the Adirondack region late in the afternoon on the 28th.
Fig 13. EDAS40 backward trajectory analysis produced with HYSPLIT model from the NOAA ARL website. Ending point is near Wannakena, New York. Star represents ending point at 20z October 28th. Green line is backward trajectory at 3 km. Markers are placed at 6 hour intervals (00, 06, 12, 18z). Click on image to enlarge.

(b) 700mb analysis
The 700mb analysis at 26/12z also showed an impressive area of cold advection and strong northwesterly winds (up to 80 kt at Glasgow, MT) associated with a shortwave trough digging southeastward into the central United States (Fig. 14).
Fig 14. As in Fig. 9, except for 700mb, at 12z October 26, and with dewpoint contours every 2C for values of -4C and greater. Click on image to enlarge.
The trough axis itself would become more sharply defined at 27/00z across Illinois, Missouri, and eastern Kansas, with a diffluent flow pattern eastward into the Tennessee Valley (Fig. 15).
Fig 15. As in Fig. 14, except at 00z October 27. Click on image to enlarge.
The shortwave trough axis reached the Tennessee Valley by 27/12z (Fig. 16), with an intense cross-flow temperature gradient (0C at HUN to -12C at BNA) and strengthening wind of 50-55 kt across the mid-Atlantic states downstream of the trough axis. The strong increase in across-flow horizontal temperature gradient in the vicinity of the trough axis was likely driven by an indirect vertical circulation in the exit region of the strong northwesterly mid-level jet streak, with warm air sinking to the south of the jet axis, and cool air rising to the north. The strength of the temperature gradient had also tightened from North Carolina to New York compared to the previous 12 hours as the strength of the upstream trough increased and amplified.
Fig 16. As in Fig. 14, except at 12z October 27. Click on image to enlarge.
At 28/00z, the 700mb trough axis had reached the Carolina Piedmont (Fig. 17), and was beginning to acquire a slight negative tilt. The jet axis was rounding the base of the trough from Jacksonville, FL to Morehead City, NC and the flow exhibited strong curvature off the Southeast Coast. The strong vorticity advection emerging off the Carolina Coast would contribute strongly to the explosive deepening phase of the surface cyclone shown in the surface section.
Fig 17. As in Fig. 14, except at 00z October 28. Click on image to enlarge.
There is also a coincident increase in 700mb horizontal frontogenesis from the mid-atlantic states northward beginning around 27/21z, which contributes to strong cloud top cooling and a baroclinic leaf structure in the IR imagery). This sequence in 700mb horizontal frontogenesis, cloud top cooling on IR, and expansion of composite reflectivity can be seen in Fig. 18. The northward expansion and increase in radar reflectivity occurs across the Chesapeake Bay, eastern Pennsylvania, and eastern New York occurs between 27/18z and 28/03z.

(a)700mb Frontogenesis 27/18z

(b)700mb Frontogenesis 27/21z

(c)700mb Frontogenesis 28/03z

Fig 18. RUC based 700mb horizontal frontogenesis (positive values shaded) at (a) 18z October 27, (b) 21z October 27, and (c) 03z October 28. 700mb geopotential height (solid black lines, 30m interval) and isotherms (dashed red, 2C interval) and wind barbs also shown. Corresponding IR imagery and mosaic NWS 88-D composite reflectivity shown to the right. Time of IR images are (a) 27/1815z, (b) 27/2132z, and (c) 28/0332z. Click on 700mb RUC-analysis images to enlarge.
  • Click here for loop of 700mb height, temperature, wind, and 2-D frontogenesis from 27/00z through 30/00z.
  • Click here for long-loop of IR imagery from 27/1215z through 30/0015z.
The rapid deepening of the low pressure system at low-levels resulted in a closed 700mb low after 28/06z and a decrease in frontogenetic forcing across central and northern New York. Much of the forcing during the daylight hours on the 28th and into the 29th was related to isentropic ascent and warm advection rather than frontogenetic forcing. The 700mb analysis at 28/12z is shown in Fig. 19.
Fig 19. As in Fig. 14, except at 12z October 28. Click on image to enlarge.

Overview | Surface | Upper Air: 850/700mb ; 500/300mb | Mesoscale Analysis


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