Rough Friday AM Commute But Even Rougher Forecast
Conservative Course Of Least Regret In Marginal Soundings
While the commute this morning undoubtedly left something to be desired, it was the Friday morning forecast that was a metaphorical 30-car-pileup, which ironically enough was a fortuitous break for area commuters.
Obviously the forecast for 3-6" of snowfall over northeastern Mass and much of southern New Hampshire verified as a mere 1-2" of sleet and assorted snow grains. The reason for this is quite simple. Here is the sounding, which is a profile of the temperatures in degrees celsius throughout the atmosphere, from Lawrence, MA at 1AM Friday morning.
Note the tiny area of the atmosphere near freezing at around the 850mb level (5, 000 feet), right as precipitation was forecast to transition to a period of accumulating snowfall for several hours through to the morning commute. This is also evident in the excerpt of the text data from the 850MB level below, as the temperature is pinned right at freezing at this level from 10PM through 4AM, while the rest of the profile is below freezing.
The forecast for 3-6" of snow was based off of guidance that implied that snowfall rates would be heavy enough to evaporatively cool this layer, while also allowing the snow to fall through it with enough haste to avoid melting. This was always a precarious position best reserved for instances in which the antecedent airmass is very cold, such as Tuesday's over performing snowfall. Ultimately, the flakes ended up being compromised enough by this layer that they were largely relegated to sleet and poorly developed snow flakes referred to as "grains", both of which accumulate extremely inefficiently. Whereas normal snowfall accumulates about 10" for every inch of liquid that falls, this type of frozen precipitation only accumulates approximately 3" for every inch of liquid, which is why an inch or two of sleet fell as opposed to between 3 and 6" of snow.
Final Grade: F