Alaska’s first -40 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 -40 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)

Using GOES-16 to view clouds over snow

May 1st, 2017 |

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

A late-season snow storm dropped a band of heavy snow over Colorado, western Kansas and western Nebraska on 29-30 April 2017. In the visible image above from sunrise on 1 May 2017, it is difficult to guess where the cloud features sit on top of the snow (Click here for a visible image with a map), even with the knowledge that they are casting shadows in this early morning imagery. GOES-16 includes a 1.61 µm channel, however; radiation at that wavelength is absorbed strongly by ice — either in the form of cirrus clouds, or snow, so that reflectance is small over ice features. The toggle below between the 1.61 µm “Snow/Ice” Channel and the 0.64 µm “Red Visible” channel shows ice and snow as dark. Clouds that are made up of water droplets are highly reflective in the 1.61 µm and in the 0.64 µm channels; such water clouds (there are only a few of them!) show up as very bright against the dark background of snow in the 1.61 µm channel.

Note in the toggle above that shadows are much darker in the 1.61 µm channel. Why?

Atmospheric scattering is stronger at shorter wavelengths in the atmosphere; there is more scattering of 0.64 µm radiation than of 1.61 µm radiation. In the shadow regions, more 0.64 µm radiation than 1.61 µm is being scattered back towards the satellite for detection. Shadows in the 0.47 µm “Blue Visible” band should be even less distinct. Non-annotated versions of imagery are available here for 0.64 µm and here for 1.61 µm.

During the subsequent late morning and early afternoon hours, the edges of the long swath of snow cover were seen to melt quickly — due to heating from the high May sun angle — on GOES-16 Visible (0.64 µm) and Snow/Ice (1.61 µm) images, below. The Snow/Ice images helped to highlight bright cumulus clouds (composed of supercooled water droplets) drifting southeastward across the snow cover.

GOES-16 Visible (0.64 µm, left) and Snow/Ice (1.61 µm, right) images [click to animate]

GOES-16 Visible (0.64 µm, left) and Snow/Ice (1.61 µm, right) images [click to animate]

Animations of GOES-16 Visible vs Snow/Ice images from the previous day (when the southwestern portion of the swath of fresh snow cover first became evidentt as clouds from the parent storm departed) are available here: Animated GIF | MP4.

Winter storm from the Upper Midwest to the Northeast US

March 15th, 2017 |

GOES-16 Water Vapor (6.9 µm) images, with hourly plots of surface weather [click to play MP4 animation]

GOES-16 Water Vapor (6.9 µm) images, with hourly plots of surface weather [click to play MP4 animation]

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

A winter storm produced heavy snow across parts of the Upper Midwest on 13 March 2017, and then merged with subtropical jet stream energy to help develop an intense and rapidly-deepening storm which affected much of the Northeast US during 14 March15 March (WPC storm summary). GOES-16 ABI Water Vapor (6.9 µm) images covering the 13-15 March period (above) showed the development and motion of this complex storm. .

A closer view of the Northeast US on 14 March (below) displayed some of the complex banding structures associated with the deepening storm, and also showed the sharp gradient of precipitation type along the coastal areas. Notable weather impacts included a storm total snowfall of 48.4 inches at Hartwick, New York, a new all-time 24-hour snowfall accumulation of 31.1 inches at Binghamton, New York, 0.4 inch of freezing rain accumulation at Chesilhurst, New Jersey and a wind gust of 77 mph at Plum Island, Massachusetts.

GOES-16 Water Vapor (6.9 um) images, with hourly surface weather symbols [click to play animation]

GOES-16 Water Vapor (6.9 um) images, with hourly surface weather symbols [click to play animation]

Some interesting features were also seen in the wake of the large Northeast US component of the storm. For example, a GOES-16 Mesoscale Sector provided 1-minute interval  0.5-km resolution Visible (0.64 µm) imagery of lake effect snow bands streaming southward off Lake Michigan on 14 March, which produced heavy snow in parts of southeastern Wisconsin, northeastern Illinois and northwestern Indiana (below).

GOES-16 Visible (0.64 µm) images, with hourly surface plots [click to play MP4 animation]

GOES-16 Visible (0.64 µm) images, with hourly surface plots [click to play MP4 animation]

GOES-16 6.2 µm, 6.9 µm and 7.3 µm Water Vapor images (below) revealed widespread mountain waves downwind of the southern Appalachians on 15 March.

GOES-16 Water Vapor images: 6.2 µm (top), 6.9 µm (middle) and 7.3 µm (bottom) [click to play animation]

GOES-16 Water Vapor images: 6.2 µm (top), 6.9 µm (middle) and 7.3 µm (bottom) [click to play animation]

These mountain waves were responsible for a few pilot reports of moderate turbulence, two of which are highlighted below.

GOES-13 Water Vapor (6.5 µm) image, with pilot report of turbulence [click to enlarge

GOES-13 Water Vapor (6.5 µm) image, with pilot report of turbulence [click to enlarge]

GOES-13 Water Vapor (6.5 µm) image, with pilot report of turbulence [click to enlarge]

GOES-13 Water Vapor (6.5 µm) image, with pilot report of turbulence [click to enlarge]