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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GOES-12 and GOES-13 visible images (above) showed the development of a narrow band of terrain-forced “standing wave clouds” over extreme northeastern Minnesota on 21 November 2008. Surface wind barbs (plotted in cyan) indicated that the surface winds were generally from the northwest at speeds of 10 knots or less across the region; however, the cloud motions suggested that the northwesterly winds at higher altitudes were a bit stronger. This northwesterly wind direction was perpendicular to the higher terrain of the “North Shore Ridge” — where elevations rise to 2000-2100 feet — which runs from southwest to northeast across the Arrowhead Region of northeastern Minnesota (topography image courtesy of Rick Kohrs, SSEC).

The fact that a thin shadow was cast on the surface along the northern edge of the cloud band indicated that this cloud feature was either fairly deep, or was located at a fairly high altitude. Note that a cirrus plume can be seen that was apparently being sheared off the northern portion of the main standing wave cloud band, which was then carried south-southeastward across Lake Superior by the stronger winds aloft. AWIPS images of the MODIS visible, 11.0 µm IR window, Cloud Top Temperature, and Cloud Phase at 16:22 UTC (below) indicated that a significant portion of the aforementioned cirrus plume was colder than -30º C, with the Cloud Phase product indicating that Ice cloud was present (pink color enhancement).

Vertical cross sections of RUC13 model fields (below, courtesy of Dan Miller, Science and Operations Officer at the Duluth MN National Weather Service forecast office) did a fairly realistic job of depicting a deep pocket of upward vertical velocity (Omega, purple contours) within the 800-300 hPa layer that was providing the forcing for the standing wave cloud band — and as moist layers (Relative Humidity greater than 50%, green shading) passed through the deep pocket of Omega, a standing wave cloud band formed that could then seen on satellite imagery. The higher-altitude moist layer arriving at the later time periods seems to correspond to the layer that produced the cirrus plume — and this higher layer was at temperatures colder than -30º C, in agreement with the temperatures seen on the MODIS IR image and Cloud Top Temperature product.

A closer view using 250-meter resolution MODIS true color and false color imagery from the SSEC MODIS Today site (below) actually depicted two separate standing wave cloud bands, with the high-altitude cirrus streaming off the upper portion of the bands showing up quite nicely. One could also see that many of the smaller lakes across northeastern Minnesota were either completely frozen or were in the process of becoming ice-covered.

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