TROWALs in a storm

December 1st, 2006 |

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.

Stretched IR from 12z 1 December 06

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.
700-mb ThetaE from GFS

Radar from 12z

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.
Animated Gif of WV imagery

Heavy snow from Texas to Michigan

December 1st, 2006 |

MODIS true color image

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).

MODIS false color image