CIRA Snow/Cloud Layer Product & VIIRS observations of the 12 Feb 2020 Blowing Snow Event

By Ed Szoke and Jorel Torres

On 12 Feb 2020 a strong cold front pushed southward across the Northern Plains and Midwest bringing dramatically colder temperatures and howling northerly winds, creating widespread blowing snow and blizzard conditions during the daytime hours of 12 Feb.  While dramatic, such conditions are not unusual for the Northern Plains, where forecasters have noticed that such widespread blowing snow actually appears in GOES-16/17 in some of the bands and RGB products.  For this case there are two excellent blogs out there on this event: one from Carl Jones of the Grand Forks, North Dakota WFO at https://satelliteliaisonblog.com/2020/02/13/arctic-cold-front-blizzard-feb-12-2020/ , the other by Scott Bachmeier of CIMSS at https://cimss.ssec.wisc.edu/satellite-blog/archives/35635.

Here we add to their excellent insight with a look at a couple of other satellite products.  One is the CIRA Snow/Cloud Layer Product, an RGB product that discriminates snow from clouds, but unlike other RGBs the snow appears as white.

Here is a loop of the CIRA Snow/Cloud Layer Product along with a zoomed-in loop of the blowing snow by the Day Snow-Fog RGB from 15-21Z, on 12 Feb 2020.

Here is a METAR time series for Grand Forks, ND.  The cold front comes by in the early hours of 12 Feb followed by a period of light snow and howling northerly winds.  By daybreak the clouds had cleared with visibilities quite low in the widespread blowing snow.

Corresponding animation of surface observations, from 15-21Z, 12 Feb 2020, can be seen below as well.

As noted in the two blogs referenced earlier, we are likely able to see the blowing snow from the snow cover because the blowing snow has smaller sized crystals with different reflectance properties, making it appear differently from the underlying snow in Band 5, which contributes to the two RGB products previously shown.

In addition to the high temporal refresh rate from GOES imagery, polar-orbiting satellites from JPSS can provide observations of surface features at high spatial resolution, in this case, blowing snow. Polar-orbiting satellites, SNPP and NOAA-20 contain the VIIRS instrument, comprising 22 spectral channels that exhibit 375-m and 750-m spatial resolutions.

The 1.6µm channel from NOAA-20 VIIRS (I-3) and the GOES 1.6µm (Band 5, previously mentioned above) are known for discriminating cloud phase: liquid water clouds (reflect at 1.6µm) versus ice clouds (absorb at 1.6µm). The 1.6µm spectral channel can also provide land/water contrast in the imagery, depict snow cover (i.e. black colors, since snow absorbs rather than reflects at 1.6µm) and observe blowing snow (i.e. greyish-white colors exhibiting varying reflective properties compared to snow cover).The main difference between GOES 1.6µm and NOAA-20 VIIRS 1.6µm is the spatial resolution: 1-km compared to 375-m, respectively. NOAA-20 VIIRS imagery, below, observes blowing snow at 1815Z, 12 Feb 2020.

Although polar-orbiters have a coarser temporal resolution compared to GOES, there were three polar-orbiting overpasses that observed the blowing snow event; 2 from NOAA-20, and 1 from SNPP. See video below; notice how blowing snow moves to the south throughout the animation. Observations were taken between 1815Z-1951Z, 12 Feb 2020.

 

Posted in GOES, POES, Satellites, Winter Weather | Leave a comment

Advected Layer Precipitable Water (ALPW) product for 5 February 2020 Severe and Flood event

Posted in Heavy Rain and Flooding Issues, POES, Severe Weather | Leave a comment

Thin fog over snow in southwest Kansas – 29 January 2020

A snow event in southwest Kansas on 28/29 January 2020 led to widespread snow amounts of 4 to 8 inches, with locally higher amounts (above 12″).  On the 29th, a thin layer of fog developed over the snow covered land.  Inspect the GOES-16 imagery:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/29jan20/4panel&loop_speed_ms=60

Upper left: Visible (0.64 micron) band

Upper right: CIRA Snow Cloud Layers RGB

Lower left: Day Snow Fog RGB

Lower right: Day Cloud Phase Distinction RGB

In the visible imagery, the fog is undetectable due to the lack of contrast (the snow cover and fog are the same color).

In the CIRA Snow Cloud Layers RGB, snow cover is white while  low-cloud/fog is yellowish-green.  Note how we can see through the fog since it is thin, the snow cover on the ground can be seen.

In the Day Snow Fog RGB, snow cover is red, low clouds / fog are light purple.  The transparency in this product is less than the previous RGB so that the snow cover under the fog is somewhat more subtle.

In the Day Cloud Phase Distinction RGB, snow cover is green, low-cloud/fog is cyan and there is sufficient transparency to view the snow cover under the thin fog.

The key takeaway point is to make use of RGB products to discern fog from snow cover, the visible imagery alone makes it much more challenging to make this discrimination.

As an example of data fusion, note this tweet from the NWS WFO in Dodge City which confirms the fog via web-cams:

 

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10 January 2020 – Southern Plains Severe Weather event with Elevated Mixed Layer (EML)

https://rammb-slider.cira.colostate.edu/?sat=goes-16&z=0&im=12&ts=1&st=20200110090117&et=20200110160617&speed=130&motion=loop&map=1&lat=0&opacity%5B0%5D=1&hidden%5B0%5D=0&pause=0&slider=-1&hide_controls=0&mouse_draw=0&follow_feature=0&follow_hide=0&s=rammb-slider&sec=conus&p%5B0%5D=band_10&x=5000&y=5000

Alternately, also available here:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/10jan20/band10&loop_speed_ms=60

 

ALPW Loop:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/10jan20/alpw&loop_speed_ms=120

 

Posted in Severe Weather | Comments Off on 10 January 2020 – Southern Plains Severe Weather event with Elevated Mixed Layer (EML)

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:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/8jan20/original&loop_speed_ms=120

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:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/8jan20/modified&loop_speed_ms=120

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.

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training\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.

 

Posted in Winter Weather | Comments Off on Snow Squalls in Pennsylvania on 8 January 2020