Snow bands accompanied by gusty winds led to localized and brief periods of significantly reduced visibility across upstate New York during the afternoon of 05 Dec 2019. A few of the snowbands advancing south of Syracuse prompted the issuance of Snow Squall Warnings by NWS BGM. Area webcams confirmed the hazardous driving conditions (see tweets here and here). The first warning and radar imagery associated with one of the earlier snow squalls is shown in Fig 1.

Rare snow squall warning south of Syracuse. This is almost like a severe thunderstorm warning in the sense that it is a relatively short lived, and intense burst or line of snow, much like a thunderstorm is. pic.twitter.com/mrvDW0cE5h

— Stacey Pensgen (@WHEC_SPensgen) December 5, 2019

The Day Cloud Phase Distinction RGB can be useful in identifying bands of potentially heavier snowfall early in their development, as well as in tracking said bands in time with radar imagery (or in the absence of adequate radar imagery). One-min GOES-16 imagery was available over the region for this event, providing near-real-time satellite information.

GOES-16 Day Cloud Phase Distinction RGB imagery highlighted clouds associated with earlier snow squalls accelerating southeast away from Lake Ontario between Rochester and Syracuse (Fig 2). Most of the scene appears as cyan color, signifying primarily low-level, liquid cloud tops given near equal moderate-high contributions (higher reflectance) from 0.64 um (green) and 1.6 um (blue), with low contribution (relatively warm brightness temperatures) from the IR window (red). The snow squall, however, appears as a localized area of bright green given a lack of contribution from the 1.6 um (blue) component, due to ice at the cloud top, in contrast to the surrounding stratus deck. The region associated with the snow squall also appears to have weak convective elements per the high spatial detail contribution from the 500 m 0.64 um band. While light snow was observed to be falling across areas under the low stratus (cyan areas), the area with obvious glaciation and convective elements had the heaviest observed snowfall and gusty winds, and resulting reduced visibility.

Also plotted in Fig 2 are GOES-16 Derived Motion Wind vectors, capturing a 30-40 knot progression of the squall. These speeds are similar to the observed wind speeds at the surface.

Underlaying the Cloud Top Phase derived product from GOES-16 allows for a cloud top phase determination to be output along with the RGB information, when sampling in AWIPS (Fig 3). In this case, the derived product is indeed indicating ice where we expected based on the RGB interpretation, and supercooled liquid droplets in the cyan zones.

A loop of the Cloud Top Phase product shows the progression of the “ice” area (red), though maybe not the full areal extent as can be diagnosed from the RGB (Fig 4).

Later in the afternoon, another area of apparent significant cloud top glaciation is observed developing over Lake Ontario, expanding as it accelerates southeast and onshore (Fig 5). Subsidence in the immediate vicinity of this weak area of convection is also observed via cloud clearing.

The CTP product again also captures the area of increasing cloud top glaciation (Fig 6).

Bill Line, NESDIS