The first weather system of April will affect the Southwest early next week. At this time the global models are bouncing around inconsistently on how strong the associated 500 mb short wave will actually be. The ECMWF has been weaker and farther north on the forecasts from 12 UTC versus those from 00 UTC. Shown above is the ECMWF 144-hour 500 mb forecast from 12 UTC yesterday morning, while the same forecast from 00 UTC last evening is shown below. Heights over southeastern Arizona are around 120+ m lower in the forecast below (note there is a 12-hour difference in the verifying time of the two forecasts).
The GFS ensmble forecasts from 00 UTC last evening also have a considerable spread for the Southwest - two of the 144-hour ensemble 500 mb forecasts are shown above and below. The stronger forecast (below) indicates 500 mb heights around 180 m lower near Tucson than the weaker forecast above. So, considerable uncertainty remains regarding the evolution of this system. It appears that we can expect cooler and windy conditions Monday afternoon and Tuesday, but the likelihood of showers and degree of cool-down remain uncertain - the GFS forecasts from 00 UTC indicated showers for southeast Arizona on only 3 of the member forecasts. So, it's wait and watch at this time.
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