End Of Week Storm

First - the diurnal temperature swing yesterday was around 55 F - low here at house was again 28 F and high at airport reached 83 F - which was a record high for the date.

The short wave at 500 mb that will dig southward during the coming 48 hours is currently over western Canada and the Pacific. Upper-air data this morning indicate that temperatures in this wave are currently quite cold - at minus 39 C or colder. The early WRF-GFS model run from Atmo this morning extends through 11 pm MST Friday night the 20th of December.

Above is WRF-GFS forecast of composite radar echoes valid at 1 pm MST on Friday afternoon, while graphic below shows the model's forecast of total precipitation through 11 pm Friday night. The model forecasts a broad maximum of precipitation (through late Friday night) to occur from north of Phoenix, southward across central and western Pima County, and into northwestern Sonora.

This morning's run of the NWS NAM model is considerably different than the midnight run of the WRF-GFS. That model forecasts very light rainfall through 11 pm MST Friday night across most of southern Arizona except for the far southwest and the southeast corner.

The surface plot forecast by the WRF-GFS for noon on Friday indicates a cold, rainy, and dreary day for metro Tucson.

Final note is that the 500 mb spaghetti plot from the GFS model ensembles (below) valid at only 72-hours continues to show largest uncertainty from the east Pacific across the Southwest. This means that timing and duration of the coming weather event may be a bit difficult to forecast exactly.