28-29 October 2008 Snowstorm Case Review|
An early season Nor'easter produces up to 19 inches of snow across the North Country
II. Surface Analysis
Several days preceding the heavy snow episode, on 25-26 October, a strong frontal system and an antecedent moist air mass brought rainfall accumulations ranging from 1-3 inches across the North Country. The NCEP/Hydrometeorological Prediction Center's (HPC) surface analysis at 00z on October 26th (date/time format noted as 26/00z through the remainder of this review) shows an occluded front extending from north to south across northern New York southward to the mid-Atlantic coastline (Fig. 2). This front would stall just east of New England, and would become a factor in the organization of the precipitation of the subsequent storm system, as described in the mesoscale analysis section of this write-up.
A relatively mild air mass remained in place behind the occlusion across the North Country; the high temperature on October 26th and 27th at Burlington (KBTV) was 59oF and 55oF, respectively. Meanwhile, a cold front was moving quickly southeastward across the northern Plains states, and would play a key role in the development of the storm.
Fig 2. Portion of HPC surface analysis for 00z October 26th. Isobars are every 4 mb. Standard meteorological frontal symbols and surface station model apply.
By the evening hours on October 26th (27/00z), the cold front had moved to the eastern Great Lakes region southwestward across the Tennessee River valley and into central Texas (Fig. 3).
Fig 3. As in Fig. 2, except for 27/00z.
The front pushed eastward to the East Coast during the morning hours on 27 October. A weak frontal wave (1013mb) was analyzed at 27/15z near the southeastern Virginia and northeastern North Carolina border (not shown). The low pressure area would deepen slowly as it shifted to near Cape Hatteras, North Carolina at 28/00z (Fig. 4). Meanwhile, to the north, the surface front pushed eastward and became quasi-stationary across central New England.
Fig 4. As in Fig. 2, except for 28/00z.
A period of rapid deepening would ensure over the next 12 hours as the surface low tracked northward along the frontal zone to just south of Long Island at 28/12z (Fig. 5). During this 12 hour period, the lowest analyzed sea-level pressure value associated with the system dropped from 1007 to 991 mb.
Fig 5. As in Fig. 2, except for 28/12z.
The low pressure system began the occlusion process as it tracked northward during the daylight hours on the 28th. The low center reached the upper Connecticut River valley at 28/21z (Fig. 6).
Fig 6. As in Fig. 2, except for 28/21z. Click on image for enlargement.
The surface low would continue to deepen as it tracked to near Sherbrooke, Quebec at 29/00z. The low pressure system migrated slowly northward across Quebec reaching peak intensity around 29/09z (Fig. 7), and then slowly filling during the daylight hours on 29 October. A pressure trough analyzed just north of the St. Lawrence valley at 29/09z (Fig. 7) would sweep southeastward across northern New York later that morning and was associated with a prolonged period of steady snowfall following the passage of the low during the day on 29 October.
Fig 7. As in Fig. 2, except for 29/09z.
The low pressure system associated with the heavy snow episode met the criterion typically used for explosive cyclogenesis, that is, an average deepening rate of at least 1mb/hr over a period of 24 hours. As shown in Figure 8 below, the 24 hour pressure change associated with the analyzed minimum HPC surface pressure reached -25 mb at 29/03z.
Fig 8. Plot of minimum sea-level pressure (solid line) and 24 hour pressure change (dashed line) versus time, from 27/15z through 29/21z. Sea-level pressure values are based on 3-hourly HPC surface analyses. Click on image for enlargement.
A loop of HPC surface analyses from 26/00z through 30/00z is available here.