Sprinkles Or More Numerous Showers?

The early runs of the WRF model at Atmo last night have produced considerably different forecasts as another 500 mb short wave, in the southern branch of the flow, develops and moves across Arizona tomorrow and Saturday. Both the NAM and GFS bring a leading vorticity maxima north-northeastward across Arizona, followed by the main short wave on Saturday. Above graphic shows precipitation forecast by the WRF-NAM through 11 pm MST Friday night (18 April) and the same forecast from the WRF-GFS is shown below. The forecasts are quite different and leave the forecaster facing a dilemma - will the initial impact affect west-central Arizona or will it focus on southeast Arizona and the White Mountains? Note that the GFS version produces measurable rainfall over much of Pima County while the NAM version has forecast almost no rainfall for the county. Both versions forecast some additional, light showers on Saturday, mostly east of the Tucson metro area.

The difference results primarily from the initial influx of increased moisture associated with the leading portion of the short wave. The models forecast the most significant showers to be associated with the leading part of the short wave and the GFS version also forecasts development of higher values of CAPE across southeast Arizona. The situation appears to be one in which the different models have minor differences in the forecasts of development of the short wave in the southern stream of the flow and of the northward advection of moisture as this feature takes shape. The situation is further complicated by the current dryness in the southern stream. I see that the new NWS NAM forecast run this morning  shifts the main impact of this event to southeastern Arizona as the main part of the short wave passes. So each different model run seems to come up with slightly different forecasts. It appears to be a situation which will require careful monitoring of the PW tomorrow - so a tough forecast due to uncertainties that persist as the event draws near. The new WRF forecasts this morning should be interesting also.