Aqua MODIS true color imagery (above) revealed a long smoke plume originating from a large wildfire that was burning northwest of Los Angeles on 03 December 2006 (500-meter resolution image). Strong northeasterly Santa Ana winds (gusting to 33 mph at nearby Thousand Oaks, CA) were helping to create an environment favorable for rapid wildfire growth. Farther to the north, haze was evident over parts of the Sacramento and San Joaquin Valleys, where unhealthy particle pollution levels were present — the Santa Ana winds appeared to be ventilating some of this haze from the interior valleys, blowing it westward over the San Francisco region and out across the adjacent Pacific Ocean.

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Occluded strong systems will occasionally display features that suggest the presence of a TROWAL, or a TROugh of Warm air ALoft. The midwestern snowstorm of 1 December 2006 was no exception. The pressure analysis below over the Infrared (channel 4 from GOES 12) data, which data has been stretched, shows an occluded system in east-central Illinois. Other features of note include the obvious snow band in Missouri, Kansas and Oklahoma that is speckled by the heat of unfrozen lakes.

A signature of a TROWAL is a thin tongue of warm air north of the occluded system. Conceptual models of storms, such as those by Carlson (warm and cold conveyor belts) suggest warm air off to the east of the storm in the warm conveyor belt. The GFS 700-mb Theta-E analysis below, for example, shows a narrow tongue of warm air stretching along the southern Lake Michigan shoreline. This corresponds well with the position of strong echo returns from radar.

The inference to draw from these images is that warm and moist air drawn north in the TROWAL air stream helps to sustain precipitation very near the dry slot. Consider the water vapor loop below, that shows redevelopment of precipitation over southern Lake Michigan, precipitation that subsequently moves over the western shoreline. Where is the moisture source for this developing precipitation? Strong forcing of the coupled jets will produce vigorous ascent, but unless an airstream, such as a TROWAL airstream, supplies moisture, the ascent will not yield heavy precipitation. During this storm, heavy snowfalls over eastern Wisconsin suggest the presence of a TROWAL airstream that is confirmed in thermodynamic fields (above) between the surface and 500 mb.

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An impressive winter storm (which featured a TROWAL airstream) brought snow, sleet, and freezing rain to much of the central US during the 30 November to 01 December 2006 time frame — maximum snowfall totals included 8 inches in Texas, 15 inches in Oklahoma, 16 inches in Kansas, 18 inches in Missouri, 18 inches in Illinois, 17 inches in Wisconsin, and 17 inches in Michigan. The extensive swath of fresh snow cover was quite evident on the 01 December MODIS true color image (above; snow cover and clouds appear white) and false color image (below; snow cover and ice crystal clouds appear as shades of red, while water droplet clouds appear white). Note that if you fade between the MODIS true color and false color images (Java fader applet), you can see that there is also an area of ice-covered ground across parts of northern Missouri and southcentral Iowa (just north of the main snow band) — those areas received a significant glazing of ice due to freezing rain 2 days earlier (as discussed in the 30 November blog posting).

Also of interest is the rapid melting along the periphery of the southwestern portion of the snow cover (especially across Texas and Oklahoma) during the day on 01 December (QuickTime animation of GOES-12 visible images).

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A fast-moving winter storm was responsible for widespread snow, sleet, and freezing rain across much of central US on 29 November 2006; several counties in southeastern Iowa reported 1/8 to 1/4 inch of ice accumulation from the storm (MODIS false-color composite image). AWIPS MODIS images from the following day (30 November 2006) show the extent of the ice-covered ground: the area affected by the ice storm in southeastern Iowa appears to be bare ground on the visible channel image (above), but note the dark signal over that same region on the Band 7 (2.1µm) near-IR channel (below) — this dark signal confirms the presence of a coating of ice. This ice glaze was not apparent on the visible channel image since it had very little impact on the albedo of the ground surface, but the ice coating is strongly absorbent in the 2.1µm portion of the electromagnetic radiation spectrum (which yields a dark signal on that particular channel). Just to the north of the large patch of ice-covered ground is a narrower feature over east-central Iowa which exhibits a somewhat darker signal on the 2.1µm image; this is due to cloud shadows being cast onto the surface from the high cloud layer that was present (note that those shadows were also apparent on the visible channel image). An examination of the corresponding 11.0µm IR window and the 3.7µm shortwave IR images (Java applet) shows that there was very little thermal contrast between this ice-covered ground and the adjacent bare ground over the remainder of Iowa (due to the presence of a cold arctic air mass over the entire region on that day).

The remainder of the ice-covered ground (across southcentral Iowa into northern Missouri) was revealed by MODIS true color and false color image composites on the following day (01 December 2006), as clouds from a departing winter storm cleared from that area.

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