Friday, December 16, 2011

An Unsettled Weekend On Tap

First, a bit about the warmer temperatures that occurred last night. Overnight lows at many places in the metro were 10 degrees or so warmer than on Wednesday night. The Atmo WRF forecasts yesterday caught this warming quite accurately, as they brought in east winds during the night. At TUS and DM winds kicked in just after midnight, so that the lowest "morning" temperatures were at midnight, with temperatures jumping around before dawn and even going into the low 50s at times. Dewpoint temperatures plunged during the night, and, combined with the wind, meant there was no chance for a repeat of the heavy fog that occurred yesterday morning. Here at the house the morning low was 32F (28F with fog yesterday), and there is still a light frost on the roofs. The WRF forecasts stronger winds tonight, so we'll see if winds can keep the Rillito bottoms warm. Finally, the sun-tinted cumulus clouds in the view of the Catalinas (above) are scooting along quite rapidly on strong east-southeast winds, indicating a gusty day likely on tap.

I'm taking a look at the model forecasts made from the 0600 UTC data last night in this post, since the influence of the models' terrain resolution is nicely reflected in the various precipitation forecasts. The current 500 mb closed low malingers near northern Baja until it moves eastward across southern Arizona on Sunday. Above are the GFS (operational member) and NAM forecasts for total precipitation for the period ending at midnight Sunday night. The GFS (top), with its crude terrain resolution, focuses the weekend precipitation over the northeast third of the state with lesser amounts in the broad upslope (in the model) areas of the southeast and west. The GFS forecasts significant precipitation across the Navajo and Hopi Nations and the Little Colorado Basin, which is an area of downslope and minimized precipitation in most events. The GFS also forecasts up to a half inch of rain for the low elevation, metro Tucson area.

In contrast, the NAM (upper bottom), with its higher terrain resolution, forecasts little precipitation in the southeast part of Arizona and focuses the event along the upslope regions of central and northwest Arizona. The NAM forecasts a minimum of precipitation over the Little Colorado Basin and also along the main-stem Colorado River canyons. The NAM also forecasts a more significant storm for the mountains of northern Baja.

Finally, a look at Atmo's very high resolution WRF-GFS precipitation forecast - above, and for the same period. The main feature of interest in the WRF forecast is that heavier amounts are forecast in the upslope areas along and south of the border (quite different than the NWS operational models), while amounts at higher elevations are a mixed bag over southeast Arizona. Heaviest amounts are at the northern end of the GoC in the el Pinacate upslope region - indicating that low-level moisture from the GoC comes into play in the WRF forecast. The snow accumulation from the WRF is shown below - the model forecasts largest snow accumulations for this event to occur over the mountains to the south and southeast of Tucson and does little on the Catalinas. So, the details of each model's forecast are quite different, and it will be interesting to observe how things evolve during this next weather event. I admit to having a preference for the Atmo WRF and would predict that it will win the weekend contest of the models here in southeast Arizona.

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