Blocking continues to dominate the flow pattern over and near North America. Shown above is the NAM 500 mb analysis at 12 UTC this morning (Saturday 4 May 2013). A Rex configuration is present over the eastern Pacific and an Omega pattern is currently over the East. In the West, the weak, closed low and associated vorticity maximum over Idaho and Oregon, are forecast to move southwestward with time. This movement ejects the current closed low at the west edge of the analysis field eastward across the Southwest - see earlier post.
As this first short wave moves across Arizona, the WRF-GFS early run this morning forecasts several bands of showers to develop. Above is the forecast of composite radar echoes valid at 11 pm tomorrow (Sunday) night. This shower activity is high-based and low-level moisture content (see concurrent forecast of precipitable water below) remains limited. Through Monday morning the model forecasts only trace to extremely light rainfall to reach the surface. The air is so dirty that we really need a general rain to wash the dust out. When I scan my weather logbook I find that the last time there was measurable rainfall here the house during May was in 2009 on the 22nd when 0.41" fell.
By 84-hours (valid 5 pm MST next Wednesday - above) the NAM model forecasts little change in the overall pattern at 500 mb. The eastern block at that time has a more Rex configuration, while the Pacific block has a bit of an Omega appearance. So the pattern remains blocked up with translating short-waves remaining at high latitudes.
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