Showers are occurring along the Borderlands and extend west-northwestward across Yuma and into the Mojave Desert. Above graphic shows Yuma composite radar echoes at 7:30 am MST this morning (Monday, January 26th). The 12 UTC sounding from TWC this morning (skewT plot below) shows a very substantial increase in PW this morning, with current values above 6/10 of an inch and higher values to the south. However, low-levels remain fairly dry at this time. Dry layer above 600 mb will probably moisten-up as winds aloft veer to the south during the day.
The 500 mb closed low west of Baja has been inching west and northwestward during the night. The NAM 500 mb forecast from 12 UTC this morning (above) moves the low well west of northern Baja by 5:00 pm MST this evening and indicates only weak, positive-vorticity-advention over southeast Arizona. Various models had been quite consistent during last couple of days, forecasting a fairly significant precipitation event for most of southern Arizona. However, the latest, short-term forecasts have come in considerably drier.
The WRF-NAM forecast run at Atmo from 06 UTC actually forecasts the event to slip-slide away to the west, with light precipitation also to the east over Cochise County and southwest New Mexico - leaving a large area of no rainfall centered about over Tucson (below, showing accumulated rainfall through 5:00 am MST tomorrow morning). The new 12 UTC forecast from the NAM (valid through same time tomorrow) is at the bottom and now indicates at best very light showers for the metro area.
These zig-zags in the model forecasts as the event has drawn near are disconcerting. Today will apparently be a day for careful monitoring of the observations and radar data as the event unfolds.
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