Thursday, December 31, 2020

Stormy End Of Year For Much Of Country

Mostly clear skies over Tucson and the western portions of the Catalinas this morning at 8:15 am MST. Down at bottom is view of a slightly skewed snow stake at Forest Lake, Arizona (along RIM west of Heber), where there is a bit of snow cover.

The 500 mb chart for 12 UTC this morning (above) shows a large closed low over northeastern Mexico - this system is bringing nasty weather to much of Texas this morning - both heavy rains and snow. Note that another short-wave trough is over California and Nevada - this feature is digging toward Arizona, but is extremely moisture-starved. The two GFS 500 mb forecasts below are valid at 5:00 pm today and at 5:00 pm on New Years Day tomorrow. Too bad that the trailing feature is so dry. Although the 1320 UTC IR image (third below) shows that there is a fairly large shield of high cloudiness with the second system that may bring us periods of clouds today and tonight.

This morning's hazards  map for the US (above) shows a variety of stormy weather on-tap for much of the northwest and south-central parts of the country. The dark gray areas over Arizona indicate poor air quality.

Finally, I show two interesting, 12 UTC sounding plots from this morning, associated with the Mexican closed low. Above is at Chihuahua, Mexico, and indicates a substantial snow event underway. However, the sounding below (from Del Rio, Texas) has a warm layer (> 0 C) from 850 to 700 mb - this would likely melt the snow falling from above. However, below the warm layer is cold layer extending down almost to the surface. This cold layer has the potential to freeze rain drops falling from above, producing sleet (now called ice pellets) at the surface. Don't know exactly what fell down there but there were NWS statements warning of sleet as well as a wintry mix of snow, ice, and rain. 

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