A major ice storm impacted much of the Northeast US on 11 December – 12 December 2008 — many locations across that region experienced several hours of freezing rain, leading to widespread ice accumulations of 0.5 to 1.0 inch which brought down tree limbs and power lines (causing power outages for an estimated 1 million customers). After the storm, AWIPS images of the MODIS visible and near-IR “snow/ice” channel from the morning of 13 December 2008 (above) offered some clues as to the areal coverage of the more significant ice accrual. Since snow and ice are strong absorbers of radiation at the near-IR 2.1 µm wavelength,  the darkest areas on the snow/ice image are areas where the thickest accrual of ice occurred (although lakes and other bodies of water also appear as dark black features). Note that some of the darker ice-covered areas on the snow/ice image do not look as “bright” on the visible image as adjacent areas with deeper snow cover, since the ice (without a layer of snow on top) is generally more “translucent” or light gray in appearance than the brighter white snow cover.

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An unusually-early snowfall event occurred in parts of Louisiana and Mississippi on 11 December 2008. AWIPS images of GOES-12 6.5 µm water vapor images (above) revealed a well-defined axis of a deformation zone pivoting over the region that received the heaviest snowfall. The GOES sounder Total Column Ozone product (below) showed values in the 350-400 Dobson Unit range (bright green to red colors), indicative of the lowering dynamic tropopause associated with the intensifying disturbance.

MODIS visible, 11.0 µm IR, and 6.7 µm water vapor images at 19:19 UTC (below) showed a narrow banding structure in place over the area where moderate to heavy snowfall was falling at the time.

SSEC MODIS Today true color imagery from the following day (below, displayed using Google Earth) displayed a large area where snow remained on the ground. The highest snowfall accumulations included 9 inches at New Hebron in Mississippi and 8 inches at Amite in Louisiana. For locations such as Beaumont/Port Arthur in Texas and Lake Charles, Baton Rouge and New Orleans in Louisiana, this was the earliest snowfall on record.

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A minor “high wind event” was noted in Boulder, Colorado on 01 December – 02 December 2008, when  winds gusted as high as 48 mph during the night-time hours. GOES-13 6.5 µm water vapor channel images (above) showed the formation of mountain waves clouds over the Foothills region of Colorado, as the westerly winds began to increase at Boulder (located in the center of the images). Curiously, Boulder was the only location what experienced the strong winds — and their winds were light southerly until around 18:00 UTC on 01 December (below).

A comparison of GOES-12 and GOES-13 6.5 micrometer water vapor images (below) showed that a distinct warm/dry “hydraulic jump” signature formed immediately downwind of the highest terrain. Even though both GOES-12 and GOES-13 have a 4-km resolution water vapor channel, the better viewing angle afforded by the position of GOES-13 at 105º West longitude allowed a clearer depiction of the hydraulic jump (there was an outage of GOES-12 images during the 17:00-21:30 UTC period, due to a GOES-12 North-South station-keeping maneuver).

AWIPS images of the 1-km resolution MODIS and the 4-km resolution GOES-12 water vapor images (below) showed that the hydraulic jump was well-defined at that time, with one pilot report of moderate to severe turbulence at a flight level of 18,000 feet seen over the area of the warm/dry hydraulic jump signature on water vapor imagery.

Note that the position of the warm/dry hydraulic jump signature was slightly farther to the west on the MODIS water vapor image — this placed the hydraulic jump closer to the spine of the highest terrain as seen on an AWIPS-2 image of the topography (below). The large viewing angle of the GOES satellite does not allow as accurate of a placement of such mesoscale features (compared to the more direct viewing angle of an overpassing polar orbiter satellite).

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Hat-tip to blog reader Boris Konon for sending us this in an email:

Noted a large area of extremely cold cloud tops on a thunderstorm complex over northern Australia 17-23z 11/22.  So cold it went off our IR color table scale we use here at WSI for MTSAT!

MTSAT-1R Infrared (10.8 µm) images (above) did indeed show an area of convection that exhibited an unusually large canopy of cloud top temperatures colder than -80ºC (shades of violet to purple) on 22 November 2008. The coldest infrared brightness temperature seen with this particular thunderstorm complex was 174.78 K (-98.37ºC), just off the northern coast of Australia at 2030 UTC.

A comparison of MTSAT-1R Visible and Infrared images at 2030 UTC (below) showed that there were a number of overshooting tops within the area of cold cloud-top temperatures, as the convective system was beginning to move offshore across the Gulf of Carpenteria. This squall line was responsible for producing 25-50 mm of rainfall (BOM rainfall analysis) and wind gusts to 96 km/h (52 knots) at Bradshaw and 68 km/h (37 knots) at Gove (thanks to Jeff Callaghan, Australian Bureau of Meteorology for that information!).

A closer view using 1-km resolution NOAA-15 AVHRR Infrared (10.8 µm) imagery (below) revealed a bit more detail in the cloud-top temperature structure around 1956 UTC. Unfortunately, the image was marred by a large number of “noise” pixels — but the coldest reliable cloud-top infrared brightness temperature seen on this AVHRR image was 170.2 K (-102.95ºC), located within the farthest southeast cold pixel cluster (IR brightness temperatures of -100ºC and colder are enhanced in the yellow to brown colors at the end of the scale). If this cloud-top brightness temperature is accurate, it rivals what is believed to be the coldest satellite-sensed cloud top temperature value noted in the literature: -102.2ºC in Ebert and Holland, 1992.

Rawinsonde data from Darwin Airport at 0000 UTC on 23 November 2008 (below) indicates that the tropopause was at a very high altitude (around 100 hPa, approximately 16.0 km). Such a high and cold tropopause is not unusal in the tropics — especially in that particular region, as studies have shown that the average tropopause level is quite high and quite cold over the western tropical Pacific Ocean.

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