Monday, December 10, 2012

Winter Weather Looms

It's another cold morning here near the Rillito Wash - looks like the low at house was 31F. This is the fifth or sixth morning this winter with lows of 32F or colder (some uncertainty because of max/min thermometer problem, before replacements came in the mail). The coldest morning so far was on November 12th when the low was 25F.

The annual Winterhaven Festival of Lights (see photo above) begins on Saturday the 15th. This is an event during which residents of the Winterhaven area (a couple of blocks south of here) decorate their homes with hundreds to thousands of lights and other Holiday displays - people throng to the area and hayride companies bring in horses and wagons. Of course there is a contest for best display and prizes awarded in a number of categories. No one has much concern about where all the electricity comes from and related environmental impacts. Regardless, an interesting aspect of the Festival is that it seems to act as a magnet that pulls cold weather systems into the Southwest. Some of the hayrides usually end up with wet and very cold riders. This year appears no different, as a strong short wave at 500 mb is forecast to dig down the West Coast during the week.

The GFS analysis for 500 mb at 00 UTC last evening (above) shows that the short wave of interest was south of Alaska at about 45 N. The operational ECMWF 500 forecast valid at 00 UTC on Friday the 14th (below) indicates a 5460m closed low along the southern California coast. The ECMWF and GFS are basically similar with their forecasts of this system, but there are some timing differences with the ECMWF apparently a bit faster.

Total precipitation forecast by the operational version of the GFS through 5 am next Saturday morning is shown below. If this forecast were to verify, it would be the first significant precipitation across southern Arizona since early September.

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