Snowfall across the Deep South

December 9th, 2017 |

GOES-13 Visible (0.63 µm) images [click to play animation]

GOES-13 Visible (0.63 µm) images [click to play animation]

GOES-13 (GOES-East) Visible (0.63 µm) images (above) showed a broad swath of snow cover from Louisiana to Virginia on 09 December 2017. Some notable storm total accumulations included 6.5 inches at Kentwood, Louisiana, 7.0 inches at Bay Springs, Mississippi, 12.0 inches at Jacksonville, Alabama, 2.0 inches at Century, Florida, 18.0 inches at Mountain City, Georgia, 7.0 inches near Roan Mountain, Tennessee, and 25 inches at Mt. Mitchell State Park, North Carolina. Daily record snowfall accumulations included a Trace at New Orleans, Louisiana, 5.1 inches at Jackson, Mississippi and 1 inch at Mobile, Alabama.

A closer view of GOES-13 visible images (below) showed the band of snow cover across Louisiana, Mississippi and Alabama. Much of the the snow melted quickly, due to warm ground temperatures and a full day of sun.

GOES-13 Visible (0.63 µm) images, with station identifiers plotted in yellow [click to play animation]

GOES-13 Visible (0.63 µm) images, with hourly surface reports plotted in yellow [click to play animation]

A more detailed view of the snow cover was provided by 250-meter resolution Terra and Aqua MODIS true-color Red-Green-Blue (RGB) images from the SSEC MODIS Direct Broadcast site (below). Note that snow cover was evident all the way to the Gulf Coast at Atchafalaya Bay, Louisiana early in the day.

Terra and Aqua MODIS true-color RGB images of the central Gulf Coast region [click to enlarge]

Terra and Aqua MODIS true-color images of the central Gulf Coast region [click to enlarge]

Terra and Aqua MODIS true-color RGB images, centered over Atchafalaya Bay, Louisiana [click to enlarge]

Terra and Aqua MODIS true-color RGB images, centered over Atchafalaya Bay, Louisiana [click to enlarge]

Terra and Aqua MODIS true-color images, centered over New Orleans, Louisiana [click to enlarge]

Terra and Aqua MODIS true-color images, centered over New Orleans, Louisiana [click to enlarge]

Terra MODIS true-color image, centered over Atlanta, Georgia [click to enlarge]

Terra MODIS true-color image, centered over Atlanta, Georgia [click to enlarge]

It is interesting to note that with the aid of reflected moonlight — the Moon was in the Waning Gibbous phase, at 59% of Full — the Suomi NPP VIIRS Day/Night Band (0.7 µm) was able to detect the area of deeper snow cover across southeastern Louisiana and southern Mississippi at 0741 UTC or 1:41 AM local time; this snow cover was then seen during the following morning on GOES-13 Visible (0.63 µm) imagery at 1440 UTC or 8:40 AM local time (below). A VIIRS instrument is part of the payload on the recently-launched JPSS-1/NOAA-20 satellite.

Suomi NPP VIIRS Day/Night Band (0.7 µm) and GOES-13 Visible (0.63 µm) images [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and GOES-13 Visible (0.63 µm) images [click to enlarge]

Snowfall in southern Texas

December 8th, 2017 |

GOES-13 Visible (0.63 µm, left) and Shortwave Infrared (3.9 µm, right) images, with hourly plots of surface reports [click to play animation]

GOES-13 Visible (0.63 µm, left) and Shortwave Infrared (3.9 µm, right) images, with hourly plots of surface reports [click to play animation]

The combination of lift from an upper-level trough and cold air behind the passage of a surface cold front  set the stage for accumulating snow across far southern Texas on 08 December 2017. As the clouds cleared, GOES-13 (GOES-East) Visible (0.63 µm) and Shortwave Infrared (3.9 µm) images (above) revealed a narrow swath of snow cover running northeastward from the Rio Grande River toward Corpus Christi — the highest snowfall total associated with this feature was 7.0 inches near Corpus Christi. Daily snowfall records included 0.3 inch at Brownsville and 1.0 inch at Corpus Christi.

A toggle between Terra MODIS true-color and false-color Red-Green-Blue (RGB) images from RealEarth (below) showed the southwestern portion of this band of snow cover (which appeared as darker shades of cyan in the false-color image).

Suomi NPP VIIRS true-color and false-color RGB images [click to enlarge]

Suomi NPP VIIRS true-color and false-color RGB images [click to enlarge]

Farther to the north, another southwest-to-northeast oriented band of snow cover was seen on Terra MODIS true-color and false-color RGB images (below), stretching from San Antonio to Austin to College Station. The highest snowfall total there was 5.0 inches (NWS Austin/San Antonio summary),

Terra MODIS true-color and false-color images [click to enlarge]

Terra MODIS true-color and false-color images [click to enlarge]

Alaska’s first -40º temperature of the 2017-2018 winter season

November 19th, 2017 |

NOAA-18 Infrared Window (10.8 mm) image, with surface identifiers and air temperatures plotted in red [click to enlarge]

NOAA-18 Infrared Window (10.8 mm) image, with surface identifiers and air temperatures plotted in red [click to enlarge]

Alaska’s first (official) surface air temperature of -40º or colder for the 2017-2018 winter season was reported by the Cooperative Observer at Chicken (-43ºF) on 19 November 2017. A NOAA-18 Infrared Window (10.8 µm) image at 0320 UTC (above) showed cold air drainage into river valleys, with the coldest infrared brightness temperatures around -40ºC/-40ºF (darker blue color enhancement). Chicken is located about midway between Eagle (PAEG) and Northway (PAOR), where 03 UTC surface air temperatures were -17ºF and -24ºF, respectively. However, PAEG reached their minimum temperature around 11 UTC after additional hours of cloud-free radiational cooling.

An automated RAWS site at Chicken reached a minimum temperature of -34ºF at 1120 UTC — the dew point at that time was -42ºF. However, a MesoWest map (below) shows that the RAWS tower is located on a small hill (at an elevation of 2060 feet) — and the Cooperative Observer instrument shelter was likely located in the lower elevations of the settlement.

MesoWest map showing the location of the Chicken RAWS site [click to enlarge]

MesoWest map showing the location of the Chicken RAWS site [click to enlarge]

For comparison, note the 2011-2012 and 2010-2011 winter seasons.

Diagnosing Snow Depth over Montana using GOES-16

October 3rd, 2017 |

GOES-16 Visible Imagery (0.64 µm), Snow Ice Near Infrared Imagery (1.61 µm) and Shortwave Infrared Imagery (3.9 µm) all at 2157 UTC on 3 October 2017 (Click to enlarge)

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing

A modest early-season storm has produced a snowfall over north-central Montana and southern Alberta. This image (source) shows total precipitation, with a minimum axis over northern Montana that has an echo in the Visible Imagery above: Whereas most of the snow cover is bright, a region west of Cut Bank MT shows a greyer view, suggestive of less snow on the ground.

GOES-16 includes a channel that senses reflected solar radiation at 1.61 µm and this is a wavelength at which snow strongly absorbs radiation. Thus, snow-covered grounds (and cirrus clouds) appear dark in the 1.61 µm Channel, but very bright in the Visible (0.64 µm). Clouds made up of water droplets are bright in both channels during the day. A Toggle between just the Visible and the Snow/Ice Channel is shown below to highlight regions of snow where clouds are not present over northern Montana and southern Alberta. Note that snow can be inferred in these regions using only the Visible Imagery because rivers — still ice-free in early October — stand out very well in the Visible.  Snow is also apparent in the Mountains of northeastern Wyoming:  Bright in the visible, Dark in the Snow/Ice channel.

Note that the localized minimum in precipitation, that shows up somewhat dark in the visible, is comparatively bright in the Snow/Ice Channel.  In addition, the 3.9 µm Infrared Imagery at the same time shows a region of relative warmth:  above freezing surrounded to the north and south by sub-freezing brightness temperatures.  All three channels — the visible, snow/ice and shortwave IR suggest a relative minimum in snow from Cut Bank westward to the Rocky Mountains.

GOES-16 Visible Imagery (0.64 µm), Snow Ice Near Infrared Imagery (1.61 µm) at 2157 UTC on 3 October 2017 (Click to enlarge)

GOES-16 Baseline Products include a Land Surface Temperature Product that is shown below from 2247 UTC. Temperature warmer than Freezing are diagnosed in/around Cutbank and to th west, with subfreezing temperatures to the north and south. Previous to that time, the Cloud Mask diagnosed clouds over the snowcover and the Land Surface Temperature did not produce a value. This toggle shows the Cloud Mask at 2227 and 2237 UTC — clear skies (black) expand over the snow cover in those ten minutes.

GOES-16 derived Land Surface Temperature, 2247 UTC on 3 October 2017 (Click to enlarge)


================= Added 4 October 2017 ====================
The snow on the ground and clear skies allowed for cold temperatures. The Land Surface Temperature Baseline Product from 1047 UTC, below, shows isolated sub-zero values between Cut Bank and Havre. The morning low in Havre, which had a record snowfall from this storm, was 8.

GOES-16 derived Land Surface Temperature, 1147 UTC on 3 October 2017 (Click to enlarge)