Monday, February 13, 2006

Calling Inspector Morse

There hasn't been much of a thaw today except in the sunnier spots. Temperatures are maxing out mainly in the high 30s, but melting will be accelerating into this week. The model statistical outputs are all suggesting at least an approach to the 60 degree level by midweek.

Tonight and Tomorrow

For tonight, lows under partly cloudy skies will range from the upper 20s in the city to low 20s in the 'burbs. The warmup begins tomorrow with highs around 47 and partly cloudy skies.

Snow Intensity and Depth

The development of the storm which brought our snow over the weekend and record-breaking amounts to New York City and central New England will certainly provide a lot of material for future investigations. Detailed data on snowfall rates are hard to obtain, because snowfall measurement is very subjective and therefore not easily automated. I took a stab at analyzing the intensity of the storm over time with the chart on the right. It's based on the "hourlies" (METAR data, or aviation reports), from Washington National for the 36-hr period from early Saturday morning through early Sunday afternoon.

Time is shown on the horizontal scale. Bear in mind that distance along the axis is not directly proportional to time, because the observations are not evenly distributed in time. They are made every hour on the hour, and then whenever a change in conditions requires an update. Although Microsoft® Excel is the Swiss Army Knife® of data analysis, it is notoriously inept at handling unevenly distributed data points.

The vertical scale represents the type and intensity of precipitation. Positive values are snow reports, and negative values are rain reports. A magnitude of 1 is "light", 2 is "moderate", and 3 is "heavy". You can see that most of the morning had intermittent light rain, then light snow mixing in the afternoon. This became all snow by late afternoon, occasionally moderate through the evening. The real fun began after midnight, when there were 8 reports of heavy snow. By 9am, the steady snow was over, and there was a snow flurry in early afternoon (and possibly 1 or 2 more after the cutoff time in the chart).

I think an important factor in the higher snow depths which were recorded was not just the usual temperature-dependent frozen-to-liquid ratio, but also the very high rate of fall, which prevented the kind of settling which would happen over a longer period. Depths have visibly decreased a lot today, but I would suggest that this is more an effect of settling, rather than melting, given the temperatures we've had.

Chart ©, photo © Kevin Ambrose

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