Horizontal convective rolls: a satellite signature of blowing snow and ground blizzard conditions

January 8th, 2015 |
GOES-13 0.63 µm visible channel images with METAR surface reports (click to play animation)

GOES-13 0.63 µm visible channel images with METAR surface reports (click to play animation)

An Alberta Clipper disturbance quickly moved through the north-central US during the day on 08 January 2014, leaving only light amounts of snowfall (generally 1 inch or less). However, very strong winds in the wake of the system (with gusts as high as 59 mph at Bismarck, North Dakota and 69 mph at Bullhead, South Dakota) produced ground blizzard conditions as the newly-fallen light, fluffy snow was lofted and organized into long horizontal convective roll features. GOES-13 0.63 µm visible channel images with overlays of METAR surface reports (above; click image to play animation) and overlays of cloud ceilings and surface visibility (below; click image to play animation) distinctly showed the widespread horizontal convective rolls, along with their effect on the weather as they moved near or over various locations.

GOES-13 0.63 µm visible channel images with cloud ceilings and surfaces visibilities (click to play animation)

GOES-13 0.63 µm visible channel images with cloud ceilings and surfaces visibilities (click to play animation)

One question that arises is: are these horizontal convective roll features clouds, or simply highly-concentrated areas of blowing snow, or perhaps a little of both? A comparison of Suomi NPP VIIRS 0.64 µm visible channel, 3.74 µm shortwave IR, and 11.45 µm IR images at 19:00 UTC (below) might shed some light on the topic. As seen on the GOES-13 visible images, many of the roll features were tall enough to cast a shadow — this suggests vertical mixing to the top of the boundary layer, which appeared to be about 1 km deep on the morning Bismarck ND rawinsonde report. A few sites reported heavy snow (reducing visibility as low as 0.15 mile) as a horizontal convective roll moved overhead — however, the 11.45 µm IR brightness temperatures were barely colder than -20 to -25º C for even the most well-defined roll features (so their ability to produce heavy snow seems dubious). On the 3.74 µm shortwave IR image, if supercooled water droplet clouds had formed at the top of the roll features, they would appear darker (due to the reflection of solar radiation off the supercooled cloud droplets) — but this is not the case.

Suomi NPP VIIRS 0.64 µm visible channel, 3.74 µm shortwave IR channel, and 11.45 µm IR channel images

Suomi NPP VIIRS 0.64 µm visible channel, 3.74 µm shortwave IR channel, and 11.45 µm IR channel images

Suomi NPP VIIRS true-color Red/Green/Blue (RGB) images from the SSEC RealEarth web map server (below) demonstrated the value of overlaying Google Maps information, for example to see which highways might be impacted by the larger and more well-organized horizontal convective rolls at that particular time.

Suomi NPP VIIRS true-color RGB images

Suomi NPP VIIRS true-color RGB images

Lake effect snow band from Lake Huron brings heavy snow to Pennsylvania

January 7th, 2015 |
GOES-13 0.63 µm visible channel images (click to play animation)

GOES-13 0.63 µm visible channel images (click to play animation)

McIDAS images of GOES-13 0.63 µm visible channel data with overlays of surface weather type (above; click image to play animation) showed a large lake effect cloud band that had formed over Lake Huron, moved inland across southern Ontario, and then became further lake-enhanced as it moved over Lake Erie and across northwestern Pennsylvania on 07 January 2015. The report of heavy (4-star) snow in far northwestern Pennsylvania was at Meadville — it reduced surface visibility there to 1.25 miles at times, and produced at least 6 inches of snowfall at that location. To the north of Meadville, 8.5 inches of snow were reported at Edinboro.

Suomi NPP VIIRS 0.64 µm visible image, with surface METARs, RTMA winds, and frontal boundaries

Suomi NPP VIIRS 0.64 µm visible image, with surface METARs, RTMA winds, and frontal boundaries

AWIPS images of Suomi NPP VIIRS 0.64 µm visible channel data are shown with overlays of surface METARs, RTMA winds, and frontal boundaries at 17:39 UTC (above) and 19:19 UTC (below). The RTMA surface winds showed that there was low-level convergence in the vicinity of the weakening cold frontal boundary/surface trough that was sagging southward and southwestward across the region (animation) — this convergence may have helped to maintain the cloud band as it continued to move southeastward across Lake Erie during the afternoon hours. Meadeville PA is station identifier KGKJ.

Suomi NPP VIIRS 0.64 µm visible image, with surface METARs, RTMA winds, and frontal boundaries

Suomi NPP VIIRS 0.64 µm visible image, with surface METARs, RTMA winds, and frontal boundaries

Suomi NPP VIIRS 11.45 µm IR channel images (below) showed that the coldest cloud-top IR brightness temperatures over both the Ontario and Pennsylvania portions of the band were -36º C (lighter green color enhancement) — at London, Ontario (CYXU), embedded towering cumulus (coded TCU EMBD) were reported at both 18 UTC and 19 UTC.

Suomi NPP VIIRS 11.45 µm IR channel images

Suomi NPP VIIRS 11.45 µm IR channel images

The Terra MODIS Sea Surface Temperature product around 18:11 UTC (below) showed that Lake Erie water temperatures were still as warm as the middle 30s F (blue) on either side of the blacked-out lake effect/lake enhanced cloud band — so moving an arctic air mass with 850 hPa temperatures colder than -20º C or -4º F over that water yielded a sufficient “delta-T” value to promote further enhancement/growth of the snow-producing cloud band which originated over Lake Huron.

MODIS Sea Surface Temperature product

MODIS Sea Surface Temperature product

Strong winds in parts of northern Scotland and southern Norway

January 2nd, 2015 |
EUMETSAT Meteosat-10 6.25 µm water vapor channel images (click to play animation)

EUMETSAT Meteosat-10 6.25 µm water vapor channel images (click to play animation)

A band of strong westerly/northwesterly winds moved across northern Scotland and southern Norway on 02 January 2015 (surface analyses), with a number of locations reporting wind gusts in excess of 50-60 knots. The highest wind gust seen was 64 knots at the Gullfax Platform (ENGC) in the North Sea at 16 UTC, with a wind gust to 63 knots at Oslo Fornebu airport (ENFB) at 21 UTC; in the morning hours, an aircraft was blown off the runway at Stornoway Airport, Isle of Lewis, in the Outer Hebrides of Scotland (EGPO). Ferry services between Kristiansand and Denmark as well as those between Sandefjord and Sweden were cancelled due to the strong winds. EUMETSAT Meteosat-10 6.25 µm water vapor channel images (above; click image to play animation; also available as an MP4 movie file) showed that most of the stronger wind gusts occurred beneath a ribbon of drier air aloft, in which instability rain/snow showers (and even thunderstorms) were developing.

A Suomi NPP VIIRS true-color Red/Green/Blue (RGB) image at 10:57 UTC visualized using the SSEC RealEarth web map server (below) showed a more detailed view of the instability showers and thunderstorms as they were developing over the North Sea. Along the west coast of Norway, thunderstorms were reported at Haugesund, Ålesund, Florø, and Bergen.

Suomi NPP VIIRS true-color image

Suomi NPP VIIRS true-color image

Rare low-elevation snowfall in the eastern Mojave Desert

January 1st, 2015 |
Aqua MODIS 0.65 µm visible channel image and False-color Red/Green/Blue (RGB) images

Aqua MODIS 0.65 µm visible channel image and False-color Red/Green/Blue (RGB) images

A cold storm system moving through the Southwest US on 31 December 2014 produced significant snowfall in many of the higher-elevation mountain ranges of California, Baja California, Nevada, and Arizona (up to 20 inches at Mountainaire AZ), but also left lighter amounts of snowfall at some low-elevation locations of the eastern Mojave Desert where snowfall is considered to be quite rare (NWS Las Vegas Public Information Statement) | Event  Summary). This event marked the first time that snowfall had been recorded during the month of December at Needles, California.

As clouds began to clear over the region on the following day (01 January 2015), areas which still had snow on the ground could be seen using satellite imagery. On a comparison of Aqua MODIS 0.65 µm visible channel and false-color Red/Green/Blue (RGB) images at 20:33 UTC or 1:33 PM local time (above), snow cover that appeared as shades of white on the visible image also appeared as darker shades of red on the RGB image.

As the day progressed, the sun had the effect of melting the lighter amounts of snow cover, as seen on GOES-15 (GOES-West) 0.63 µm visible channel images (below; click image to play animation). However, due to the presence of the unusually cold air mass, new records for coldest 01 January daily maximum temperature were set for Phoenix (46ºF) and Tucson (41ºF).

GOES-15 0.63 µm visible channel images (click to play animation)

GOES-15 0.63 µm visible channel images (click to play animation)