Wednesday, December 29, 2010

How Much Precipitation By Tomorrow Morning

Clouds in the Tucson area have thickened in the past hour or so and precipitation has started in the Catalinas. A key question for short-term and small scale forecasts is how long the highway up to Summerhaven will remain open. It will undoubtedly be closed sometime in the next few hours and some who might be tempted to drive up into the snow this afternoon may well end up stranded on the mountain.
I've grabbed some products from the model runs this morning. The top graphic is NAM forecast precipitation for period ending at 1200 UTC tomorrow morning (30 December 10). The NAM forecasts a general precipitation event, with some amounts in southeast Arizona reaching over an inch. The middle image is total precipitation forecast by the Univ. of AZ WRF-NAM high-resolution model from this morning's run. The WRF-NAM forecasts a very distinct difference between the storm's impacts on the mountains versus low elevations.Note that only a couple of tenths of an inch are predicted in the Tucson metro area and in much of the Santa Cruz Valley - probably due to forecast strong downslope winds across the higher terrain to the southwest. In the southeast part of state the model continues to forecast maximum precipitation to occur in the Catalinas.
Since people down here rarely have to face snow-covered roads, there are several other very important forecast issues related to driving conditions. For example, will Highway 83 become snow covered and icy at the high points between I-10 and Sonoita (this is a particularly treacherous stretch of highway even in fair weather)? Will all the roads become icy in the southeast Arizona grasslands? Will I-10 become snow covered on the high terrain between Tucson and Benson; over the pass called Texas Canyon? east of Wilcox? The WRF-NAM forecast of snow accumulation through tomorrow morning (bottom) keeps the accumulations at high elevations and says "No" to all the questions above - except of the course the one about the Catalina Highway.
So we are now in the observing phase of this storm. Note - just had a call from my son who is at a resort north of Durango CO at about 8,000 ft elevation and he reports that snow is piling up at 2 to 4 inches per hour there! Second note - it's taken me so long to put this post together that I see that it has started to sprinkle here at the house.

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