Snow Squalls in Pennsylvania on 8 January 2020

By Dan Bikos and Bill Line

On 8 January 2020 numerous snow squalls moved across Pennsylvania leading to the issuance of multiple snow squall warnings by the NWS.  One of the challenges with this event is that it spanned the time period around sunrise.  Obviously radar and IR satellite imagery is not affected by this, but the Day Cloud Phase Distinction RGB, which has been shown to be quite useful for snow squall events, can only be used during the daytime.  One alternative to continue monitoring during this time period is to make use of the nighttime microphysics RGB at night.  This RGB product makes use of the 10.3 minus 3.9 micron band difference for the green component which discriminates between water (positive) and ice (negative) clouds at night.

The following 4 panel GOES-16 loop:

Upper-left: Nighttime Microphysics RGB

Upper-right: Daytime Cloud Phase Distinction RGB

Lower-left: MRMS 1-km composite reflectivity

Lower right:   IR (10.3 micron) band with METARs

First look at the Daytime Cloud Phase Distinction RGB in the upper-right, later in the loop when there is daylight this product does a good job in delineating which clouds are glaciated (green and also orange/red shades) versus water clouds (cyan/lavendar).  Compare the cloud tops that are glaciated with MRMS reflectivity and we see that there is good agreement in delineating the more intense snow squalls from the less intense regions of snow.

Now trace backwards in time these glaciated clouds through the sunrise period into the overnight hours and notice how they appear in the nighttime microphyics RGB in the upper-left.  The glaciated clouds appear tan to darker shades of red and also correspond pretty well with regions of higher reflectivity observed from MRMS.

We include IR imagery (with METARs) to illustrate only a loose correspondence between colder cloud tops and higher regions of reflectivity, the RGB products definitely do a better job since they can delineate ice versus water cloud tops.

Next, we’ll focus in on a modified Day Cloud Phase Distinction RGB product that allows you to see further towards the nighttime hours compared to the default product:

Upper-left: Nighttime Microphysics RGB

Upper-right: Modified Daytime Cloud Phase Distinction RGB

Lower-left: MRMS 1-km composite reflectivity

Lower right:   IR (10.3 micron) band with METARs and GLM Flash Extent Density

The modifications to the Daytime Cloud Phase Distinction RGB are the same as shown in this blog entry.

Compare the modified Daytime Cloud Phase Distinction RGB to the default in the previous loop and notice that you get more images towards the nighttime hours around sunrise.

We included the GLM Flash Extent Density data as an overlay in the lower-right panel, note that there was some lightning activity associated with some of the snow squalls which further increases confidence that these were high-impact snow squalls.

A social media post illustrates the significant impacts of these snow squalls.

Later the same day during the early afternoon, a snow squall quickly moved through Philadelphia, prompting the issuance of a snow squall warning by the local NWS office. The Day Cloud Phase Distinction RGB at 1-min temporal resolution provides an alternative/supplement to radar imagery for tracking the precise evolution of the snow squall (see animation immediately below). Considering the environment in which we are viewing this RGB, the combination of convective cloud elements, cloud top glaciation, and rapid motion all apparent in the 1-min RGB imagery indicate snow squall potential with this feature.\line\loops\20191218_dcpdrgb&loop_speed_ms=60

Nearby surface observations measured wind gusts to 46 mph in associated with the snow squall, and photos on social media show the approach of the snow squall into Philadelphia.


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