Saturday, December 24, 2016

Snow - How Low Will It Go?

The toughest forecast issue this morning is that of how low the snow level will fall early tonight, and whether parts of metro will see some Christmas Eve flakes. I've looked at the SREF plumes from SPC, mainly because the GEFS plumes basically go for just rain at the airport, and there's more detail and spread available from the SREF's 26 members. Plot above shows total QPF at TUS, while plot below shows total snowfall at the airport. Looks like about half the members forecast at least a few flakes at the airport.

I've looked at the 06 UTC WRF forecasts for the evolution of the soundings at TWC and don't see a very great signal for wetbulbing. During the late afternoon and early evening, the depth of vertical motion is good for developing at least moderate precipitation, but low-levels are too warm for much wetbulbing. Later, around 08:00 to 10:00 pm, the near surface becomes very favorable for wetbulbing, but the depth of saturated layer becomes shallow and doesn't appear favorable for good snow growth. My cloud physics background is not very good, but I don't find signals in the WRF forecasts that would make me go for snow at airport. Although, obviously, some of the SREF soundings must forecast more favorable structures. Another watch and see what actually happens situation.

The 06 UTC runs of the WRF don't have snow turned on, but the 00 UTC run of the WRF-GFS does. The forecast below is from that run and shows total snowfall through 06:00 am MST tomorrow morning. That forecast keeps snow out of the metro. The model forecasts a significant event for the Baboquivari Mountains and Kitt Peak, as well as for our three local Sky Islands. If this forecast verifies things could become a bit dicey during the night on I-10 toward Benson and Texas Canyon, and also on Highway 83 south toward Sonoita.

I've been thinking a bit about where there might be interesting photo ops as this event unfolds. Best place to watch the front roar in might be Kitt Peak - open today but closed tomorrow. Current forecast shown on their website doesn't mention snow. Gates Pass might be another possibility, with good views of the approaching front. Mountains, saguaros, and snow cover should be easy tomorrow morning around the fringes of the Sky Islands. Oracle, Madera Canyon, and also the Greaterville/Box Canyon Road should be interesting right after sunrise. But, Saguaro NP East has much easier access. 

Regardless, we have a very interesting Christmas weekend on tap. Have a great Holiday.

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