A quick-moving Northeast US shortwave and associated cold front brought a brief period of snowfall to higher elevation areas of W MD/E WV/S PA during the early morning overnight hours of 25 Jan 2022. Radar beam blockage and distance results in slightly degraded radar coverage in parts of this region. NWS Baltimore/Washington forecasters leveraged the NESDIS Snowfall Rate product to diagnose snowfall across the higher terrain in western areas of their forecast area during the evening. SFR is available in AWIPS at some NWS offices, and can also be found online here (and with more info) and here.

In the overnight near-term forecast update, the NWS forecaster wrote the following, with corresponding imagery included as Fig 1: “NESDIS snowfall rate product from Suomi NPP and NOAA-20 satellites from 0722Z and 0813Z respectively showed snow falling across the mountains west of Frostburg with liquid equivalent rates of around 0.05 in/hr. Another inch of snow is possible before snow ends later this morning behind Arctic frontal passage currently analyzed over southwestern PA.”

A version of the SFR product merges the satellite-based SFR with MRMS radar-based, helping to fill in gaps where radar coverage might be unavailable. That product surrounding the period of discussion is shown in Fig 2.

Observed snowfall reports later that morning confirmed up to a few inches of snow total had fallen in the region overnight. This event exemplifies how the NESDIS Snowfall Rate product can be leveraged to boost confidence in a nowcast/short-term forecast of snowfall, particularly in regions of less-than-ideal radar coverage.

Bill Line, NESDIS and CIRA

Strong northwesterly winds across North Dakota snowpack resulted in widespread blowing snow on 18 Jan 2022. The blowing snow reduced visibility considerably (less than 1 mile) in many areas, as was highlighted by NWS Bismarck, ND here (below) and here. NWS offices leveraged satellite imagery in their forecasts/analyses and communication of blowing snow during this event.

NWS Bismarck leveraged GOES satellite imagery to help shape their understanding of the blowing snow event and influence their forecast products. Specifically during the late morning, satellite imagery of blowing snow resulted in the addition of a county to the Winter Weather Advisory: “Quick update to add Divide County into the Winter Weather Advisory given satellite imagery suggesting significant blowing snow continuing from eastern parts of the county upstream into southern Canada.”

In the early afternoon, NWS/BIS provided a detailed update to the forecast/analysis of blowing snow, shaped by several observational data sources, including trends in satellite imagery: “Web camera trends suggest the lowest visibilities are more variable than ASOS/AWOS sensors alone would suggest, while satellite imagery suggests the most significant blowing snow is related to well-defined plumes that are occurring in Horizontal Convective Roles (HCRs). For the most part, those HCRs are relatively widely-spaced, leading to the variability in visibilities spatially, and temporally as the HCRs shift slightly with the background flow. Satellite imagery does suggest the most widespread blowing snow plumes are centered over Burke County and vicinity, where impacts are likely most significant. In the end, we continue to monitor trends for the need for any Blizzard Warning upgrades, but are holding off for now. Changes with this update cycle were mainly focused on observational trends through the afternoon hours, with no significant adjustments.”

GOES-East satellite imagery referenced in the discussion, specifically the default Day Snow-Fog RGB available in AWIPS, is shown in Fig 1. The plumes of blowing snow organizing into HCRs are easily diagnosed in the imagery streaming south across North Dakota. These are highlighted in the imagery as subtle difference in color (brighter red) compared with the background snow-covered surface, and the shadowing along the northern edges of the tall plumes.

As was introduced in this recent blog post, an attempt to further highlight areas of blowing snow, and associated HCRs, is made with an experimental Blowing Snow RGB (Fig 2). Regions of blowing snow, especially the HCRs, further stand out against non (or weaker)-blowing snow, snow-covered background.

Afternoon VIIRS passes provide a detailed view of the blowing snow plumes and HCRs via a similar experimental Blowing Snow RGB (Fig 3).

The afternoon NWS/BIS forecast discussion included further analysis blowing snow in satellite imagery. “Satellite shows well-defined blowing snow plumes embedded in Horizontal Convective Rolls (HCRs), which have been becoming somewhat more widespread in that corridor as temperatures fall to 0 F or below, increasing the ability for snow to be lofted, especially in areas where the heavy snow fell late last week and strong winds today eroded any crust….We will however continue to monitor the situation, especially since satellite trends do suggest some increase in HCRs and related blowing snow plumes recently, perhaps as the boundary layer depth shrinks a bit. Interestingly, those satellite images also suggest blowing snow is being transported as far south as Burleigh County, where the pre-existing snowpack on the ground is indeed sufficiently crusted to not be broken even with the winds today.”

NWS Grand Forks, ND also leveraged satellite imagery during this blowing snow event, discussing: “… along with horizontal convective rolls are producing snow showers and based on satellite trends, have increased in coverage, pushing into the central valley.” Downstream, NWS Aberdeen was monitoring satellite imagery for blowing snow: “Based off of satellite, we are seeing blowing snow in North Dakota continuing to push southeast. With the snow in our area crusted over, that should limit any lofting of snow from the strong winds…”

NWS/FGF further took advantage of satellite imagery by including it in their public messaging of the hazard (also below).

Compare the above RGBs with single-band VIS and IR imagery in Figs 4 and 5, respectively. While the presence of blowing snow/HCRs can be realized given the shadowing in the VIS and slight temperature difference in the IR, the exact location and extent of blowing snow, especially non-HCR areas, is much more difficult compared to in the multi-spectral, RGB images.

Bill Line, NESDIS and CIRA

Low, thin clouds and fog developed across northern Illinois snowpack overnight into the morning hours of 13 Jan 2022. Comparison of single band imagery with multispectral Imagery and level-2 products remind us of the advanced methods we have for analyzing low clouds and fog in complex scenes, in this case, during the day. One-minute imagery was available over the region during the period, which is quite valuable when monitoring the real-time evolution of low clouds near TAF sites.

Starting off with visible imagery, the scene is encompassed with a lot of light gray to white, representing both snow and cloud cover, diagnosed by stationary and non-stationary movement, and shadows (Fig 1). Near the IA/IL border, some movement is noted representing likely low clouds, but the delineation between low clouds and snow cover is difficult.

While Geocolor establishes a more familiar scene from space, including adding color to non-snow surface features, it doesn’t do much to help to separate clouds from snow cover (Fig 2).

IR Window imagery helps capture the higher (colder clouds), but again doesn’t help much with differentiating potential low clouds from the similar-temperature snow surface (Fig 3).

By combining the VIS, IR and Snow/Ice bands, we are left with a picture that separates snow cover (green) from low (liquid) clouds (cyan, light blue) from high (ice) clouds (pink/red) in the Day Cloud Phase Distinction RGB (Fig 4). High detail is maintained in the features with the inclusion of the 0.5 km VIS. The reflectance components can easily/quickly be adjusted in AWIPS (lower max) to draw out features during low light situations. The nearly stationary and semi-transparent low clouds across northwest Illinois, which could not be diagnosed in previous imagery, is easily observed in the RGB against the snowy background.

Similarly, the Day Snow-Fog RGB combines the Veggie Band, Snow/ice Band, and Fog Difference to separate snow cover from clouds (Fig 5).

The FLS Probability level-2 product, now available in AWIPS, combines satellite and NWP information to give a probability of MVFR, IFR, and LIFR conditions (and Cloud Thickness). During the day, the areal extent of coverage matches well with appearance in imagery, though with reduced spatial detail and some difficulty in multi-layer cloud scenes (Fig 6).

One method for incorporating the quantitative fog products into AWIPS workflow is to underlay the three fog probability products in your typical Cloud imagery display. In the example shown, sampling the area will not only show the RGB readout information, but also the probabilities of the various restrictions, for that location (Fig 7). This type of display allows one to view the imagery but still get the benefit of the quantitative information without taking away valuable screen real estate, while also helping to connect the quantitative product to the imagery.

Bill Line, NESDIS and CIRA

Previous blog posts and presentations have introduced the ability to diagnose plumes of blowing snow in GOES ABI imagery. In particular, the feature becomes apparent in the Day Snow-Fog RGB, primarily due to contrast of the plumes with surrounding environment in the Snow/Ice band and Fog Difference, and shadowing. Recent snowfall and strong winds resulted in a widespread blowing snow event across southern Saskatchewan/Manitoba into North Dakota on Jan 8. Surface observations and webcams reported considerable visibility reductions within the region of blowing snow as detected by ABI and VIIRS, including some areas to less than 1 mile. The blowing snow was pointed out by Carl Jones (NWS/FGF) on Twitter:

Blowing snow plumes 💨 💨 💨 finally showing up in North Dakota. Thick batch coming out of Saskatchewan. #ndwx #skwx pic.twitter.com/ItSY57Audv

— Carl Jones (@Wx_Jones) January 8, 2022

An experimental Blowing Snow RGB captured the blowing snow well across the region, as a shade of gold to dark yellow compared to the red snow-covered background and blue of clouds. Some plumes of blowing snow appear to develop into HCRs at times (as was documented in this paper), and have shadows associated with them in the imagery. Recall, this experimental RGB is similar to the Day Snow-Fog RGB available in AWIPS, but with the higher resolution 0.64 um band replacing the 0.87 um band, and with ranges tweaked to better highlight the blowing snow feature.

Compare the Blowing Snow RGB in Fig 1 with the Day-Snow-Fog RGB in Fig 2.

A similar experimental Blowing Snow RGB can be applied to VIIRS 375 m I band imagery, providing excellent spatial detail (Fig 3 and Fig 4). The individual plumes/HCRs of blowing snow can better be observed, as well as more subtle areas of blowing snow. Combining NOAA-20 and S-NPP, an animation of four VIIRS swaths can be created within a period of ~2.5 hours. Of course, ABI has the advantage in showing the longer evolution of blowing snow with time. Utilized together, one gains an ideal understanding of the feature and its evolution.

Bill Line, NESDIS and CIRA