On February 12, 2020, a very strong arctic cold front swept through the upper tier of the central CONUS bringing blizzard conditions, dangerous wind chills colder than -50 F, wind gusts exceeding 60 mph, and crashing temperatures to portions of the Dakotas, Minnesota, Iowa, and Nebraska. Despite little snowfall expected with this front (generally between 1-2 inches), blowing snow and significant visibility reductions appeared likely should the existing snow pack be susceptible to being lofted and blown around. But prior to this impactful frontal passage, forecasters were left with a difficult decision: just how susceptible is the existing snow pack to become lofted into blowing snow?

One tool that could help forecasters answer this question is close examination of snow pack appearance on the 1.61 um Snow/Ice band offered on GOES-R ABI. It’s has been shown that the Snow/Ice band is sensitive to the amount of liquid water in a volume of snow and ice, i.e. its “water to ice crystal ratio” (CIMSS Blog example). We can apply this in operations by looking for “old, crusted over snow” from “new, fresh and fluffy snow.” In Figure 1, a swath of newly felled snowfall across eastern SD, southern MN, northern IA, into WI and MI can be seen as a lighter shade of grey compared to darker northern neighbors (you need to wait for bright white clouds to pull away to reveal the dark snow pack below). This lighter shaded area is where forecasters can more confidently delineate a fresher snow pack that may be more susceptible to being lofted and blown around. Lastly, this loop applies adjustment to the display range of the Snow/Ice band in order to more easily draw out this area, changing from the AWIPS default Min-Max of 0-100 to an adjusted Min-Max of 3-30.

As the cold front encroached upon Canadian border states of the Northern Plains overnight, close examination of the GOES-East Nighttime Microphysics RGB (Figure 2) revealed the exact location of the arctic cold front by way of rapidly advancing, arching area of low stratus towards the south embedded or underneath mid-upper level clouds moving northeastward. Higher clouds were associated with a weak mid level and surface wave moving east along the strong baroclinic zone charging southward.

Figure 3 annotates the location of the arctic cold front as well as METAR observations with the Nighttime Microphysics RGB. Closely scrutinizing satellite imagery for any subtle details can help forecasters latch onto synoptic and mesoscale features, particularly those that bring hazardous weather like this arctic front whose impacts started immediately after frontal passage. Rapid temporal tracking of this feature offered by GOES-East could give forecasters details like timing of onset to impacts, something very important to IDSS.

As the sun rose on February 12, 2020, forecasters at NWS Grand Forks were anxious to see if blowing snow could be viewed on satellite imagery as area observations, reports, webcams, and radar suggested (as it turned out the crust on snow pack might have been broken north of the aforementioned area of newly felled snow due to very gusty winds exceeding 60 mph aiding to the production of blowing snow). Luckily a GOES-East mesoscale sector was over the FGF area (thanks DMX!) during this time allowing 1 minute imagery to provide the most up to date satellite view available. A look at the Day Snow-Fog RGB (proven to be useful in monitoring blowing snow during the day) gave indications of horizontal convective rolls associated with blowing snow, evident moving out of southern Manitoba into northern North Dakota, but perhaps not as quite obvious for operational usefulness (Figure 4).

While the blowing snow plumes were somewhat noticeable in Figure 4, the default Day Snow-Fog RGB composite ranges within AWIPS doesn’t show features well in times of low light, i.e. near sunrise/sunset and near poleward locations in winter. Since all three of the components (0.86 um, 1.61 um, 3.9-10.7um) that make up the Day Snow-Fog RGB are sensitive to solar reflectance, we can adjust the composite ranges of this RGB to become more representative of the little solar reflectance available just after sunrise in this high latitude location. Shrinking the RGB composite ranges from the default (R: 0-100, G: 0-70, B: 0-30) to (R: 0-20, G: 0-15, B: 0-25) allows the forecaster to more clearly see swaths of horizontal convective rolls that make up blowing snow (Figures 5 & 6). Forecasters can adjust RGB ranges on the fly like this to make features easier to track, making it more operationally useful. Just don’t forget to readjust the range as more sunlight becomes available!

Figure 7 displays how blowing snow could be tracked on satellite throughout the day as dry arctic air scoured away clouds moving south and east near the front (stratus colored white/lavender as well as some higher cirrus as orange/red). Blowing snow extended all along the Dakotas and Minnesota border with blowing snow seemingly influenced by not only the Red River Valley in eastern North Dakota and northwest Minnesota (Figure 8), but also by the Coteau De Prairie/Sisseton Hills in eastern South Dakota and southwestern Minnesota (Figure 9). Significantly reduced visibility as noted by METARs can be matched with the blowing snow plumes seen funneling down the lower elevation of the Red River Valley (Figure 8) and Buffalo River Valley (Figure 9). Also, cessation of blowing snow can be noted within the Red River Valley as the horizontal convective rolls dissipated north to south in Figure 7. Lastly, blowing snow plumes could be noted advecting southward out of southeastern South Dakota into the relatively snow-free far northeast Nebraska towards the end of the loop in Figure 9.

Analysis of GOES-East imagery during this event provided information on antecedent conditions as well as precise tracking of the arctic cold front and blowing snow causing blizzard conditions behind the front. This information was used directly in operations helping refine the boundary of blizzard warnings and winter weather advisories, social media messaging, as well as IDSS support to core partners of the National Weather Service.

Carl Jones, NWS Grand Forks, ND

Ideal conditions for the development of heavy freezing spray developed across the coastal waters of the northern Gulf of Alaska, including Cook inlet, on the back edge of a low pressure system during the afternoon/evening of 10 Feb 2020. Analysis of GOES-West water vapor imagery reveals the associated upper level trough exiting to the east as the next system approaches from the west (Fig 1).

Given the expected surface conditions, NWS Anchorage issued a Heavy Freezing Spray Warning for the associated offshore waters zone (Fig 2).

During the late afternoon and early evening of the 10th, surface observations indicated the development of gusty winds, temperatures well below freezing, and rough seas; all conditions necessary for the development of freezing (salt water) spray. Gusty winds and temperatures well below freezing were reported at Stations AUGA2 and AMAA2 (Fig 3), with wave heights of 10-15 ft reported at 46080. This is the region within which heavy freezing spray was expected and likely occurred based on analysis of VIIRS and ABI satellite imagery.

Given the relatively high latitude of the region, three VIIRS passes (one from NOAA-20, two from SNPP) were available during the day within 2.5 hours of each other. The five 375 m I bands from SNPP VIIRS for 2123 UTC are shown in Fig 4, centered over the region of freezing spray. A modified gray scale color table was created to focus on the reflectance values of the spray. The higher spatial resolution of the VIIRS imagery (vs GOES) captured the phenomenon in enhanced detail, allowing for easier diagnosis as to where freezing spray was occurring at that moment. The spray is observed as a region of relatively high reflectance (lighter gray) vs lower reflectance open sea extending from station AUGA2 through Cook Inlet and station AMAA2 into western portions of the broader Gulf of Alaska. Viewing VIIRS 375 m channels I1 – I3, it is obvious that the spray is most apparent in channel I2 given the relatively high reflectance of the lofted sea particles over the very low reflectance ocean surface.

Now viewing I4 and I5, the spray has a higher brightness temperature (darker gray) in the I4 (3.9 um) channel vs I5 (11.45 um) channel as a result of added reflectance component during the daytime due to scattering of the airborne particles. The brightness temperatures are similar in areas of clear sky with no spray over the ocean. Taking the difference between the two channels provides a clear view of where the spray is occurring. Sea current patterns are also apparent in bands I4 and I5, and are differentiated from the sea spray, particularly in band I4 where the spray has a higher brightness temperature, and currents have a lower brightness temperature.

Figure 4: 2123 UTC 10 Feb 2020 SNPP VIIRS I bands 1-5 and band 4 minus 5 difference (left to right, top to bottom). Higher res: I1 (0.64 um), I2 (0.865 um), I3 (1.61 um), I4(3.74 um), I5 (11.45 um), I4 – I5. Color table: low reflectance and warmer brightness temperatures is dark gray, high reflectance and cooler brightness temperatures is light gray. For the I4-I5 difference, dark gray represents a greater positive difference, light gray is near 0 difference.

Imagery from the three daytime VIIRS passes (2038 UTC from NOAA-20, 2123 UTC and 2306 UTC from SNPP) provides a sense of evolution of the spray during the day at high spatial resolution (Fig 5). Clouds appear to have developed within the region of spray by 2306 UTC.

GOES-West Full Disk sector provided high temporal resolution imagery over the region of lofted sea particles. At such a high latitude, the imagery spatial resolution is degraded (pixel area increases by roughly 4x), but still useful for detecting the spray in a broad sense. Given our analysis of the VIIRS imagery, we take a look at the 1 km (at nadir) 0.86 um band from GOES-West, and are able to track the evolution of the sea spray through the afternoon (Fig 6). A similar color table is used to that presented with the VIIRS imagery.

The CIRA Snow/Cloud product was very effective at capturing the spatial extent of the sea spray without needing to make any modifications (Fig 7). The integrity of the RGB for tracking other features is, therefore, maintained.

The ability to detect and track sea spray could be useful to NWS forecasters in verifying forecast products such as a Heavy Freezing Spray Warning, and for issuing new forecast products.

Bill Line (NESDIS and CIRA), Louie Grasso (CIRA)