In addition to the potent short wave energy, the 850 hPa analysis places most of the North Country in favorable frontogenetic forcing just north of a warm front and near the closed 850 hPa circulation. From figure 3 below, the strongest warm advection lift is near the nose of the 40 to 50 knot jet at 850 hPa and associated with the tightening thermal gradient, which is highlighted by the yellow circle. This produced an area of moderate snowfall with rate around 1 inch per hour and surface visibilities between ½ and 1 mile during the mid-morning hours on February 25th, especially across central/southern VT.

Water Vapor, Sounding Data and Surface Analysis:

The water vapor (figure 4 below) during this event really helps to tell the story of our two distinct periods of precipitation. First area of precipitation impacted mainly the High Peaks in NY into central/southern VT with a period of moderate snowfall associated with very strong 850 hPa frontogenesis and low level jet. This was indicated by the colder/higher clouds tops on the water vapor image below departing the central/southern New England Coast. Meanwhile, the secondary feature was more subtle but you can see the convective nature to the cloud structure on the cusp of the dry slot, ahead of the approaching upper level low pressure. The combination of short wave energy, some mid level instability, and embedded mid level moisture, resulted in a period of moderate to heavy snowfall during the afternoon hours on February 25th. Furthermore, the right rear quadrant of a 150 to 170 knot polar jet analyzed over southern Ontario, created favorable upper level divergence and enhanced mid level ascent across our region. Surface visibilities were below ½ mile at time across many Automated Surface Observation Stations (ASOS), including Burlington, Massena, and Saranac Lake, with snowfall rates of 1 to 2 inches per hour. However, as the system raced into northern New England, snow quickly tapered off by evening, but travel was very hazardous with snow covered and slick roadways.

Figure 5 below shows the Buffalo, New York upper air sounding at 7 AM on 25 February. A few things of note here, first is the conditional unstable layer between 3 and 6 km above ground level with lapse rates of 7.0 C°/km associated with cooling aloft from approaching upper level low and mid level dry slot. This weakly unstable layer within the dendritic snow growth zone helped to enhance better flake size by increasing the lift, resulting in fluffier snow and higher snowfall rates during the afternoon hours. Also, note the very strong winds in the dendritic snow growth zone of 70 to 90 knots, which resulted in the snowflakes breaking down initially with needle-like structure, but these winds weakened as the upper level low pressure moved directly overhead by early afternoon on the 25th. Finally, these southwest winds did push a warm nose of near 0C air at 700 hPa toward our region, which combined with the weakly unstable layer did produce a very brief period of some graupel/snow pellets in parts of the area.

Figure 6 below shows the 3 hour surface evolution from 7 AM to 4 PM on 25 February 2022. Weak 1008 mb low pressure was located near Erie, Pennsylvania, while strong 1035 mb arctic high pressure was situated over northern Maine at 7 AM. By 1 PM primary surface low pressure was located near Rome, NY, while a secondary coastal system was developing over Cape Cod, as high pressure remained anchored across northern Maine. The collision between cold arctic air to our north and very warm/moist air just to our south, helped to promote favorable lift for a band of warm air advection snow prior to the upper level system arriving on Friday morning, especially across central/southern VT. Surface low pressure was racing off the New England coast by 4 PM, with building high pressure across southern Canada and the eastern Great Lakes.

Model Analysis:

Figure 7 below is the NAM 3km vertical cross section taken from near the city of Ottawa in the Province of Ontario, Canada (point A) to Worcester, MA (point B). The 2D frontogenesis (pink lines) shows two distinct areas of frontogenesis, but at different levels. First area is located over the St. Lawrence Valley is associated with favorable 700 hPa frontogenesis, while the second area is over eastern NY into southern VT at 850 hPa. The strongest omega (color filled) is between the 850 hPa and 700 hPa in this cross section and corresponds just above an area of enhanced 2D frontogenesis. The enhanced omega couplet was located just below the favorable dendritic snow growth zone (DGZ) at 9 AM on 25 February 2022, resulting in some finer needle-like flakes at times. As the initial lift associated with the 850 to 700 hPa warm air advection moved eastward by late morning, the combination of increasing winds between 700 and 500 hPA, along with a well defined dry slot nosing into southern/central VT, resulted in much finer flake size in this area. Meanwhile, as better dynamics, moisture, and weak instability arrived with the upper level system by early afternoon flake size and snowfall rates increased sharply again.

Figure 8 below shows the NAM-3km instantaneous precipitation rate and type of precipitation initialized at 7 AM on 25 February, valid for 9 AM on 25 February 2022. This clearly shows the higher progged snowfall rates (darker blue) over eastern NY into southern VT associated with initial surge of warm advection lift at 850 hPa, while a secondary area of enhanced snowfall occurred over southern Ontario into northern NY associated with favorable 700 hPa frontogenesis and approaching upper level system.

As the day progressed our atmosphere became slightly unstable in the 700 to 500 hPa layer, which resulted in a convective type structure to the precipitation fields. Figure 9 below shows pockets of progged heavier snowfall rates (darker blue) extending from the St. Lawrence Valley into northern VT, including the Champlain Valley. This produced snowfall rates of 1 to 2 inches per hour, with surface visibilities below ½ mile at times, creating very hazardous driving conditions across the North Country. The combination of better dynamics, moisture, and cooling aloft from the approaching upper level trough (increases instability), all came together to produce this burst of heavier snowfall rates and better flake sizes.

High Resolution Ensemble Forecast (HREF) 1 hour snowfall ensemble mean (Figure 10 below) highlighted the southern Adirondacks into central/southern VT as a favorable region for getting hourly snowfall rates up to 1 inch per hour on the morning of February 25th. However, as better dynamics arrived and interacted with weak instability and enhanced moisture from approaching short wave trough, snowfall rates of 1 to 2 inches per hour were progged by the HREF. This axis of expected higher snowfall rates with very poor visibilities extended from the St Lawrence Valley into central and northern Vermont during mid-afternoon hours on February 25th.

Radar and Impacts:

The final figure below shows precipitation evolution from 10 AM to 4 PM on 25 February across the North Country. Initially two bands of heavier snowfall were observed, with one across southern VT into New Hampshire and the second over northern NY. As the system evolved, drier air decreased the aerial coverage and intensity of the precipitation across parts of southern VTt. Meanwhile, additional heavy snowfall developed by mid afternoon from northern NY into VT, highlighted by the yellow returns on figure 11 below. These enhanced radar returns were associated with snowfall rates of 1 to 2 inches per hour, especially in the Champlain Valley between noon and 2 PM on the 25th and surface visibilities below ½ mile.

Overall a widespread 6 to 12 inches of snowfall occurred with this system in about a 12 hour window on February 25th. Many schools were already closed due to winter break, but given the high snowfall rates and poor visibilities, extremely hazardous road conditions were observed. The combination of these factors caused a 30 car pile on Interstate 89 near exit 17 in Milton, VT, during the late afternoon hours. Many other roads were snow covered and slippery for both the morning and evening commutes. The widespread snowfall was a welcome sight for many ski resorts and outdoor winter enthusiasts, as recent warm temperatures and rainfall depleted our snowpack.

Disclaimer: Figures 7, 8 and 9 courtesy of Tropical Tidbits web site: https://www.tropicaltidbits.com/