A narrow, band, behind a a cut-off low brought surprise snowfall 2″ – 6″ for parts of Central Long Island and Central Connecticut, this morning. Even thundersnow was reported over at Islip, NY, with snowfall rates up 3″ per hour and visibility down to 1/8th of a mile. The rest of the area saw rain mix or change to some wet snow and sleet, when more intense rates of precipitation at times. This was due to colder air aloft, being dragged down to the surface. But little or no accumulation occurred.
So what caused this heavy snow band? One cause for this some frontogenetic forcing occurring behind the cut-off low. Another cause was instability. The OKX sounding was unstable with mid-lapse rates over 7.0 C/km. This produced convective banding with thunder over Central LI and CT :
Models showed this the last 3 days, showed that this band would form. But they were less consistent, on the timing, and placement. As well amount of dynamic cooling that would need to occur for snow to accumulate. Add on that it’s end of March, when it’s typically more difficult to get boundary temperatures to support snow. Especially along the coast. So considering those factors, it just wasn’t a likelyhood and a good move beforehand to forecast 2″-6″ for any specific area. Despite the outcome. However, we can use what we learned from it, to improve forecasts in the future.
This cut-off low also gave us heavy rains and thunderstorms, this past weekend. Many received between 1.50″ to 3.00″ of rain. As discussed in the previous blog post, a 50-60kt low-level jet move through area with pwats over 1.00″. This enhanced deep moist convergence, along a warm front over eastern parts of LI and CT most early Sunday morning. Causing elevated convection to train over these areas and increase rainfall totals to 3.50″ – 5.50″
Miguel Pierre Weather 