Thursday, July 31, 2014

Continued From Below - Challenging Day For Forecast Models

Around 7 am MST this morning the Kitt Peak webcam (above) showed a light shower to the north. Visible satellite image from about same time below indicates widespread debris cloud over much of northern Mexico and the Southwest. So while moisture is on the increase, big question for today will be how long the thick cloud cover will persist.

The morning sounding from TWC on campus (above) continues to indicate a fairly dry surface layer, with cloud bases at about 600 mb. Winds aloft are currently light and variable below 300 mb. The latest NAM however forecasts two centers within the 500 mb anticyclone to become better defined by evening - one near the Big Bend in northern Mexico and one over southern California. Southern Arizona ends up in the circulation around the western lobe of the anticyclone, while a fairly distinct trough separates the two centers. Picture may be complicated by any MCVs left behind by the MCS over Sonora.

I have looked at both the NAM and GFS driven variants of the early WRF model forecast. The two versions have dramatically different forecasts for eastern Pima County. Both versions seem to allow too much sunshine too soon. The WRF-GFS forecasts storms in eastern Pima County during mid to late afternoon, but the NAM version does not. Interestingly, the NAM version does not convect much until during the night and early morning hours, when storms move into much of Pima County from the north. Above is WRF-NAM forecast of composite radar echoes valid at 1 am on August 1st. The accumulated rainfall forecast by the WRF-NAM through 6 am MST tomorrow morning is shown below. The bulk of the rainfall over Pima County is forecast to occur after midnight - a very interesting forecast by the model. At this time I would favor the NAM version, given the adjustment of the 500 mb anticyclone that is underway, the heavy morning cloud cover, and its delay of storms for the metro area until tonight. But, will be of interest to hear what Mike Leuthold finds in the new runs from this morning.

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