28-29 October 2008 Snowstorm Case Review|
An early season Nor'easter produces up to 19 inches of snow across the North Country
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III. Upper Air Analysis
(c) 500mb analysis
The 500mb analysis at 26/00z (Fig. 20) shows northwesterly winds of 75-100 kt across the Dakotas and Montana with broadly diffluent flow downstream across the mid and upper Mississippi River Valley. The cyclonic shear side of the 500mb jet streak was associated with a very cold temperature minima of -40C across east-central Alberta, with the thermal trough extending southeastward into North Dakota.
Fig 20. 500mb analysis at 00z October 26th. Geopotential height (solid lines, every 60m), isotherms (red dashed lines, every 2C), and wind barbs at rawinsonde stations in knots. Standard upper air station model is shown. Click on image to enlarge.
By 27/00z, the 500mb trough dug strongly across the central Plains, and was associated with winds up to 115kt at Topeka, KS. The 12-hrly height falls also increased during the period 27/00z through 28/00z as the shortwave trough intensified (Fig. 21).
Fig 21. RUC based 500mb geopotential height (black solid lines, 60m contour interval), 12-hr height change (color filled, 30m interval), and wind barbs (kt) at 12z October 27. Click on image to enlarge.
The trough axis emerged off the southeastern U.S. coast between 28/00z and 28/12z contributing to strong differential vorticity advection (Fig. 22) and the coincident rapid deepening phase of the surface cyclone.
Fig 22. RUC based 500mb geopotential height (black solid lines, 60m contour interval), absolute vorticity (color filled, 10-5 s-1), and 700-400mb differential vorticity advection (blue solid lines, 10-5 s-1 ) at 12z October 28. Click on image to enlarge.
The system would become vertically stacked by 29/00z (Fig. 23), with the 500mb low passing across central New England. THe 500mb low was closed, and the broader 500mb trough exhibited a negative tilt from western Hudson Bay into New York and New England. The low then tracked northward into southern Quebec by 29/12z, with the North Country remaining within the broad circulation of the upper low and westerly to northwesterly mid-tropospheric winds through 30/00z.
Fig 23. As in Fig. 20, except for 00z October 29th. Click on image to enlarge.
(d) 300mb analysis
- Click here for loop of 500mb geopotential height (60m interval), absolute vorticity, and 700-400mb differential vorticity advection from 26/00z through 30/00z.
- Click here for loop of 500mb geopotential height (solid lines, 60m interval), 12hr height change (color fill, 30m interval), and wind barbs from 26/12z through 30/00z.
The 300mb analyses indicate a strong upper tropospheric jet in excess of 150kt across the Dakotas and northeastern Montana at 26/00z (Fig. 24).
Fig 24. RUC based 300mb analysis of geopotential height (solid black lines, every 60m), divergence (solid magenta contours, every 2x10-5s-1, and isotachs (color filled every 10kt above 60kt) at 00z October 26th. Click on image to enlarge.
This jet streak would result in significant amplification of the upper trough from 26/00z through 28/00z (see loop below). Thereafter, the strongest winds are located downstream of the trough axis along the east coast of the United States. This contributed to strong upper level divergence across the mid-Atlantic region beginning around 28/00z, and then northward across New York and New England from 28/15z through 29/06z (Fig. 25).
Fig 25. As in Fig. 24, except for 18z October 28th. Click on image to enlarge.
- Click here for loop of 300mb geopotential height (60m interval), wind barbs and isotachs (20kt contour interval above 60kt), and 300mb divergence (solid magenta lines, 10-5 s-1) from 26/00z through 30/00z.