Since yesterday morning, the global models have trended wetter for the end of the month. Graphics here show GEFS QPF plumes for TUS from 00 UTC last evening (above) and from 06 UTC last night (below). By 06 UTC last night the ensemble members forecast a precipitation event Monday late into Tuesday the 28th. The POPs for TUS are essentially 100%, with only one member not forecasting at least 0.01" for the airport. However, the spread for QPF is very large - ranging from 0.00 to 1.50 inches. So much uncertainty remains.
The big driver for the wetter forecast seems to be the intrusion of a pronounced atmospheric river (AR) into southern Arizona. The 00 UTC WRF-GFS forecast of precipitable water (second below on the 5.4 km grid) valid at 05:00 am MST on the 28th shows this distinct feature.
Here are two WRF-GFS forecasts for total precipitation though 11:00 am on the 28th The top is from the 00 UTC run on the 5.4 km grid (compare with the week-long forecast shown in previous post to get a feel for how quickly the model forecasts have changed). Below is the 06 UTC forecast on the 1.8 km grid. It is important to note that the 06 UTC GFS operation run forecasts half an inch at airport but the high-res WRF version reduces this to a tenth of an inch.
End of month becoming more interesting, especially if the wetter trend continues between now and then.