A considerable winter storm system brought bitterly cold temperatures, strong winds, and snow to a broad swath of the US during the week leading up to Christmas 2022. A potent shortwave trough embedded in strong northwesterly flow dug southeast across the Rockies on 21-22 Dec, east through the central plains on 22 Dec, and lifted east/northeast toward the east coast on 22-23 Dec. Accompanying the shortwave trough was a sharp cold front that dropped surface temperatures dramatically along its path. For example, the temperature at DIA dropped 37F in 1 hour (42 to 5F)! The average daily temperature at DIA on the 22nd was -15F, which was the 2nd coldest day on record at DIA. Thanks to NWS Boulder for these statistics.
GOES-East water vapor imagery provided a great depiction of the shortwave traveling across the country (Fig 1). The shortwave, or local vorticity max, can be diagnosed in the imagery as an area of compact cyclonic flow in the moisture field and clouds, and tight couplet of drying/descent (warm BTs; upstream of trough axis) and moistening/ascent (cooler BTs; downstream of trough axis). Contours of 500 mb wind speed capture the strong jet that was associated with the trough, with a core of 120+ knot winds! Additionally, contours of surface temperature overlaid highlight the progression of the cold front in association with the trough, with freezing temps (darkest blue) clearing Texas. From NWS Marquette’s water vapor analysis later on the 22nd: “Increasingly negative-tilt mid-level trough now moving into the central CONUS as noted on the latest water vapor imagery and its associated 160+ kt upper-level jet rounding the base of the trough will cause rapid deepening of the storm system over the central Great Lakes over the next 24-48 hours.”
An alternative view of the trough is in the Airmass RGB, which includes the reddening signal of high PV/stratospheric air descending deep into the troposphere and where cyclogenesis is occurring, which is quantified by an overlay of 1.5 PVU pressure contours (Fig 2).
The progression of the front south across the central and southern high plains can be visualized in GOES-East IR imagery, leveraging a grayscale color map with max and min values selected to represent warmer BTs as dark gray and colder BTs as light gray (Fig 3). This can quickly and easily be done in AWIPS by modifying the colormap range. The surface obs provide the degree of temperature drop across the front, while the satellite imagery provides rapidly updating and spatially fluid information about the exact location of the front.
Focusing on the front progression across southern Colorado, and leveraging 2-min updating imagery, we see the front backing west across the plains and into the eastern mountains, and then slowly creeping into the mountain passes (Fig 4).
Further south across Texas during the day on the 22nd, we leverage a TrueColor/IR image combination to capture the cold air progression in the context of clouds and land features (Fig 5). The IR overlay is semitransparent, gray scale colormap, ranging from black on the warm side of the front to white on the cold side.
Strong winds blowing over fresh snowfall across the central and northern US plains resulted in widespread blowing snow, which was captured in satellite imagery given clear skies within the arctic airmass. Previous posts on this blog discuss blowing snow detection with ABI and VIIRS imagery (use the search box on the right). An experimental blowing snow RGB has been tweaked over time to provide a product that attempts to highlight regions of blowing snow, including blowing snow that has transitioned into HCRs, across a broad range of geography, seasons, and time of day. A zoomed out view of the RGB during the day shows how widespread the blowing snow was during the day (Fig 6). Important features include blowing snow (orange or bright pink), snow cover (darker red), bare ground (green), clouds (purple or blue). The RGB is fairly similar to the Day Snow Fog RGB, but more clearly reveals and isolates the blowing snow signature by leveraging the 500 m VIS and and focusing the recipe on lower-reflectance signatures, which adds more contrast to the image.
Zooming in on a couple of regions, we highlight areas of intense blowing snow, confirmed by surface observations, occurring over western Nebraska, western South Dakota, and eastern Nebraska into Iowa (Fig 7-9). Within these plumes of blowing snow, visibility was often reduced to below 1/2 mile.
In fact, the Omaha, NE NWS office leveraged the Day Snow Fog RGB for Blowing Snow detection during the event: “The clouds are pushing southeast of the area, but we are still seeing quite a bit of blowing snow on our GOES16 RGB Day Snow-Fog curve. And in open areas, visibilities are still being reduced to 1 to 3 miles, based on surface observations and DOT plow cams.” See below for a comparison between that, and the experimental Blowing Snow RGB.
A similar RGB can be created with VIIRS imagery at 375 m (Fig 10). Notice that the regions of blowing snow become brighter when positioned near the edge of the VIIRS swath.
Figure 10: 22 Dec 2022 VIIRS experimental Blowing Snow RGB.
Now a few zoomed in looks at the VIIRS RGB over Nebraska, South Dakota, and Iowa. In particular, notice individual plumes of blowing snow are more well defined and can be diagnosed little better, and shadows cast by the plumes are also more apparent. Below are a couple of regional comparisons between ABI and VIIRS of the RGB (Fig 11 and 12).
The method above, of course, leverages VIS, NIR, and LWIR channels. At night, however, we must rely on IR imagery to isolate the feature. A method that seems to work fairly well (though not as effective as the daytime technique) is the single band IR imagery with a grayscale colormap and range focused around that of the surface (Fig 13). Again, these tweaks to the colormap range are simple to make in AWIPS from event to event. The plumes of blowing snow, developed into HCRs, exhibit a few degree cooler BT compared to the clear/background surface. In the animation, the region of blowing snow appears more uniform in Iowa, organizing into the HCRs over Illinois and eastward.
Widespread Blowing snow continued into the day on the 23rd from the upper Midwest southeast across the Ohio River Valley region within a broad region of surface winds gusting over 40 mph.
Numerous NWS offices leveraged satellite imagery to detect and track the plumes of blowing snow. See a few examples below, with AFD snippets and accompanying Blowing Snow RGB imagery over the region.
Aberdeen, SD: “Blowing snow RGB sat pix indicate that blsn is gradually ending over the James valley, and has pretty much ended over the Missouri valley. Will maintain the blizzard warning after 00Z for the Coteau and wc MN, since winds will continue to howl over the area well into the evening. However, will allow the warning to expire for the James valley come 6pm CST.”
Davenport, IA: “Looking out the window, the peak gusts are containing instantaneous white outs, while the steady condition is above that level. The worst conditions are visible on satellite, with convect roll streamers of blowing snow showing up, again, mainly over the central and northern CWA.”
Grand Forks, ND: “Satellite observations indicate a few prominent blowing snow plumes (horizontal convective rolls) creating significant travel difficulties and whiteout conditions. These plumes are located as follows:…”
“A few other areas of blowing snow plumes do exist in the region, however are not substantial from satellite observations. This doesn’t mean there isn’t visibility issues outside the above listed areas, just that they aren’t strong enough to register a signal on satellite. Observations outside of these plumes appear to indicate areas of blowing snow still existing, with visibility between 1 and 5 miles at times. Within the aforementioned stronger blowing snow plumes, blizzard conditions will continue through the rest of the afternoon until sundown.”
Bill Line, NESDIS/STAR