The 18 December 2019 Snow Squall Event

The late afternoon of December 18th 2019 saw a line of heavy snow squalls pass through the area in advance of a cold front, resulting in a few snow squall warnings issued by our office. The squalls were quick-hitting, and in typical fashion put down less than an inch of snow, lasted only 10-15 minutes, reduced visibilities to less than 1/2 mile, and kicked up the wind for a short period of time during the heavy snow. The meteorological setup was almost classic in nature for this kind of event, with shallow convection in a single linear organized structure. The snow squalls moved in from the lower Hudson Valley by late afternoon, as noted from the KOKX base reflectivity that afternoon:

The first snow squall warning was issued for NYC and surrounding areas at 3:24pm valid until 4:15pm local time ahead of the squall line:

A quick look at the relevant surface observations from Central Park show (in yellow) the time, the quick reduction in visibility to a quarter of a mile, and the gusty W/NW winds (27kts = 31mph) as the squall passed through NYC.

In addition, the NYS Mesonet recorded 5-minute surface weather observations (university rooftop) during the event. The red arrow marks the passage of the squall line. One can clearly see the sudden drop in temperature (left panel, blue) and insolation (left panel, yellow), rapid pressure rise (right panel, maroon) and increase in wind speed (right panel, blue) during the squall passage.

The day before the event, forecasters at NWS New York, NY mentioned in the afternoon Area Forecast Discussion the possibilities of a snow squall event impacting the evening commute on the following day:

Meteorologically, several factors were at play on this afternoon that enhanced the likelihood of convective snow squalls. These included:

Favorable location on the cyclonic shear side of a 500mb jet:

An approaching cold front with an arctic airmass that follows:

By late in the afternoon, a surface cold front was approaching the area from the northwest. Surface temperatures behind the front were 15-20 degrees F colder, well below freezing. This surface cold front served as the forcing mechanism for the snow squalls.

Steep low level lapse rates:

Lapse rates were conducive to low and mid level instability by the afternoon, which helped to maintain the snow squalls as they approached and moved though the area. Note the difference in the 12Z OKX observed sounding and the 18Z HRRR below.

The snow squalls initiated across southeast NY, and moved across the lower Hudson Valley, NYC and Long Island during the late afternoon. Surface observations depict the cold air mass and westerly winds behind the cold front, typical of these events.

Snowfall amounts from the event were generally less than 1" around the area, with Central Park, NY (KNYC) picking up 0.7", Kennedy Airport 0.5" and LaGuardia Airport 0.2" with this event, per the 24-hr precipitation summary below.

As the squall progressed east toward Long Island, similar reduction in visibilities were noted by the NYS Mesonet cameras, shown here in Stony Brook, NY. The squall eventually weakened as it continued across eastern Long Island into the evening.

You can check out more local weather event write-ups from our National Weather Service New York, NY  Local Significant Event Summaries Page !