Thundershowers This Afternoon?

There are a number of interesting things evolving in the weather situation as a fairly strong, Fall-like trough approaches the west coast. Once Hurricane Manuel weakened, the storm moved rapidly eastward into north-central Mexico and on toward southwest Texas. Manuel has produced a significant disaster in Mexico, with over 100 deaths being reported now and many more missing in the floods and landslides. Manuel moved off in exactly the opposite direction of the early NHC forecasts after the storm re-formed south of the GoC. Graphic below shows CG lightning strikes for the 12-hours ending at 11 pm MST last night - storm activity north of Manuel and from near the Continental Divide eastward.

This morning's 12 UTC sounding at TUS (above) indicates that PW is currently less than an inch; however, the SPC diagnostics forecast some CAPE this afternoon. The ugly, strong inversion just above 600 mb remains and is a significant deterrent for deep convection, as is the very dry upper-troposphere. The early WRF forecasts are quite similar for both versions this morning - below is PW forecast by WRF-NAM on the 5.4 km grid valid at 7 pm this evening. Note that the model brings higher PW in from the east and south, with values well over an inch by evening.

The models weaken the inversion very substantially (may be over-done) and forecast storms for eastern Pima County this afternoon and evening. Above is WRF-NAM forecast on the 1.8 km grid of composite radar echoes valid at 2 pm MST this afternoon. The WRF models forecast today to be the most active day, with only some light shower activity for eastern Pima County tomorrow. The outcomes will depend upon the low-level moisture advection that actually occurs and whether the inversion weakens later today. On Saturday night and Sunday the models forecast a significant dry out from the south-southwest to march across the state in advance of the Pacific trough - below is PW forecast on the 5.4 km grid valid at noon on Sunday the 22nd. The color bar to right indicates that PW is forecast well below 10 mm across most of Arizona on Sunday. The models also forecast a very significant wind storm with the Pacific trough, and I'll cover that in a separate post.