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Climate: A Lake Effect-like snowfall on Long Island's North Shore

Climate articles:

During the evening of January 7, 2002, a significant local enhancement of snowfall near the Long Island Sound occurred. That evening, an upper level disturbance caused some very light snows over Long Island, with most accumulations being a half inch or less. Along the immediate north shore, a stationary band of snow formed and persisted for a few hours dropping up to 3 inches of snow in spots between Huntington and Stony Brook. It was not a major storm, and while not strictly speaking a pure sound-effect snowfall, it was clearly a sound enhanced event. In the narrow area effected, the vast majority of the snow that fell can be attributed to the local enhancement of precipitation by the Long Island Sound. The image at right shows radar estimated precipitation for the evening of January 7, 2002. Although the amounts were light, the image does indicate northeast to southwest banding over over the Sound.

Snowfall in most areas was very light, amounting over many hours to no more than half an inch. In the northern part of Smithtown 2.8 inches of snow fell during the entire day, of which nearly 2 inches fell between 8:30 and 10:30 PM local time. The NWS did not issue a snowfall statement for that day, but a handful of private weather observers did report the following:

                    Smithtown (north)2.8"
                    Centerport2.2"
                    St. James1.7"
                    Lake Ronkonkoma1.2"
                    Port Jefferson1.0"
                    Massapequa (south)1.0"
                    Hicksville0.6"
                    Oceanside0.3"
                    Mattituck0.3"

There were several factors involved in causing the enhanced area of snowfall. Meteorologist Eric Medeiros did a mini-case study of what happened that evening and took a look at the dynamics that caused the enhanced area of snowfall along the north shore. Here is his analysis:

Analysis of a localized snowfall on Long Island's North Shore

By Eric Meideros, Meteorologist

It appears that this event was a meso to sub mesoscale phenomena related in several ways to lake effect snowfall. The ideas in this mini-case represent a hypothesis built around the fundamental idea that frictional convergence in a conditionally unstable environment produces enhanced snowfall. We begin the analysis by looking at two key pieces of data:

  • The sea surface temperature over the Long Island Sound at that time was 5.5C (about 42F).
  • Delta T, or the difference between the sea surface temperature and the air temperature at the 850mb level was greater than 12C.

For lake effect snows, or in this case "sound effect" snowfall to be possible we usually look for lake induced instability. Since we know that the water temperature was near 5.5C, a good first place to look is at the 850mb level. The temperature at 850mb, as shown in this segment of the actual sounding data from OKX at 00z on the 8th, was around -7.3C at 0Z (7:00 PM local time). Click here to see more of the sounding data.

    -----------------------------------------------------------------------------
       PRES   HGHT   TEMP   DWPT   RELH   MIXR   DRCT   SKNT   THTA   THTE   THTV
        hPa     m      C      C      %    g/kg    deg   knot     K      K      K
    -----------------------------------------------------------------------------
     1000.0     12
      999.0     20    0.2   -0.1     98   3.82     10      8  273.4  283.9  274.1
      963.9    305   -1.7   -1.9     99   3.47      5     20  274.3  283.9  274.9
      927.7    610   -3.8   -3.8    100   3.14     15     18  275.2  284.0  275.8
      925.0    633   -3.9   -3.9    100   3.11     15     18  275.3  284.0  275.8
      892.4    914   -5.3   -5.3    100   2.89     35     15  276.7  284.8  277.1
      858.4   1219   -6.9   -6.9    100   2.67     40     10  278.1  285.8  278.6
      850.0   1296   -7.3   -7.3    100   2.61     30      8  278.5  286.0  278.9
      793.3   1829   -9.5   -9.8     98   2.30    305      9  281.7  288.4  282.1

This yields a Delta T of over 12C at 00Z which is considered conditionally unstable. Conditional instability implies that if a parcel of air is forced to rise it will continue rising once moist. In this case the sounding was already pretty moist through the column even without the addition of more moisture from the Sound. Examination of the actual sounding data from OKX at 00z on the 8th shows the same thing along with veering winds.

Click the image at the right to animate it and view a sequence showing about an hour's worth of nexrad images. In it you can see the formation of a rather high reflectivity band right on the north of shore of Long Island.

So we need to ask ourselves what type of situation would cause this somewhat intense band to form and sustain itself, but not continue inland? And why is the band not oriented in the direction of a normal Ocean effect band; that is, perpendicular to the shore or even parallel to the flow?

In this case the flow is from the NNE or NE..... if we disregard the surface wind, we see evidence of an effect that occurs many times along the long shore of lake Erie when the winds blow from the WNW. Wind over the water will turn less due to friction so the flow would tend to be more geostrophic. So, over the Sound we can assume (since there are no direct observations available) that it was from around 010 or 020, roughly NNE.

But that same wind when it crossed onshore onto Long Island would be affected by friction. The wind slows and turns in toward the direction of the pressure gradient, and in this case the pressure was lowest to the east. So the wind responds by turning from N or NNE over the water to N or NNW over the land and we get this type of situation . The vectors drawn on the image to the left help to illustrate how just this tiny turning of the wind due to surface friction caused some serious convergence right on the shore. Since the atmosphere was conditionally unstable and almost saturated, the air had no place to go but UP and it did, and it condensed and precipitated.

This effect frequently occurs along the LONG shores of the great lakes in lake effect events and often forecasts near Rochester or Erie bust by 2 or 3 or more inches for very minor 1-3" lake effect events simply because of this phenomena.