Monday, March 17, 2008

Cold Cutoff in the Southwest

Yet another success for the long-range models, especially theECMWF, which correctly forecast the development of the cold, midlevel low currently over the Southwest - beginning from about 5 to 6 days out.

The system produced more precipitation than I was expecting, given the very low precipitable water values. Here at the house, I measured 0.29" through 7 am this morning and light showers remain in the area. Low this morning was 33F.

Precipitation yesterday was from convective showers in the cold, unstable air. I did not hear any thunder here at the house. During midafternoon northern and western parts of the Tucson metro area had heavy showers of graupel (AKA snow pellets) and the ground was covered in some spots.

Perhaps some more of this today, before the system moves on east. It's hard to know how unstable it is this morning, since the somewhat critical Tucson upper-air sounding was missing both last evening and again this morning.

The NWS forecast this morning (see below) mentions the possibility of showers, thunder, and sleet. While showers accompanied by graupel may occur, the liklihood of sleet is extremely small, today or with any winter weather event in the Tucson metro area, since sleet results when raindrops fall through a layer of subfreezing air at or very near the surface. Definitions of graupel and sleet are also below.
Today: A chance of rain showers possibly mixed with sleet, mainly after 11am. Some thunder is also possible. Partly sunny, with a high near 53. South southeast wind 9 to 14 mph becoming west northwest. Chance of precipitation is 30%. Little or no snow accumulation expected.

From the Glossary of Meteorology:

snow pellets (Also called soft hail, graupel, tapioca snow.) Precipitation consisting of white, opaque, approximately round (sometimes conical) ice particles having a snowlike structure, and about 2-5 mm in diameter. Snow pellets are crisp and easily crushed, differing in this respect from snow grains. They rebound when they fall on a hard surface and often break up. In most cases, snow pellets fall in shower form...

sleet or grains of ice, generally transparent, globular, solid grains of ice that have formed from the freezing of raindrops or the refreezing of largely melted snowflakes when falling through a below-freezing layer of air near the earth's surface....

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