20190830

Previous Forecast
Storm activity stayed away from the lower elevations, but they were quite active along the Rim and over southeastern Arizona.   One storm did make a run for the Phoenix valley around sunset but didn't make it.

All model forecasts were pretty good but generally didn't have quite enough coverage or rainfall. The RRx is now in the mix and subjectively, performed the best.

Initializations

The 500mb pattern is interesting for at least southern Arizona as the mid-level flow is northeasterly around 10 knots at both Tucson and Phoenix due to an inverted trough over northern Mexico.  Temperatures are not too warm as they are around -7C.

Satellite imagery indicates it's mainly clear over the model domain and confirms that there is a weak IT over northern Chihuahua/Sonora.  A weak MCV was noted just south of El Paso.   Model initializations of the cloud and upper air features were accurate.  Both the 12 and 15Z RR had minimal PW errors, and even the NAM was quite good over the CONUS.  It did have some moderate errors in Mexico.   There is no favored initialization today as they are all quite good.

Day 1

The 12Z sounding at Tucson didn't have much MLCAPE at around only 500 J/kg, but the troposphere below 500mb is quite moist with no significant subsidence inversions noted.  12Z PW was around 37mm but has been dropping since, which isn't very encouraging.  The vertical wind profile does look good with 10-20 knots of mid-level northeasterly flow and northwesterly upper flow, thus blowing anvils away from the storm motion.

A surge is forecast to strengthen, thus increasing moisture advection into southern Arizona.  I'm a little suspicious of this forecast as the PW trend at both Organ Pipe and Puerto Penasco has been flat to down over the last few hours.  

With this moisture advection, comes a corresponding increase in CAPE, becoming quite high over central Pima and into southwestern Pinal Counties.  Enough CAPE is present over the Rim and White Mountains to support deep convection, and with the favorable steering flow, the storms will move towards the lower elevations of southeastern Arizona.

Besides the good mid-level steering flow, slightly cooler air is present over eastern and southeastern Arizona, steepening the lapse rate slightly.  On paper at least, it looks possible for storms for the lower elevations of southeast Arizona today as long as the moisture increase does verify.

Scattered deep convection is underway over eastern Arizona during the mid to late afternoon and stronger storms near the border and into Mexico.  

Oh, oh.  In spite of no apparent mid-level inversions and not that much CIN, a cap develops on top of the mixed layer.  Otherwise, the situation is favorable for storms in the Tucson area.  DCAPE is a moderate 1000 J/kg so outflows could be strong enough to provide the lift needed to break the cap.

The 15Z WRFRR skirts storms around Tucson this evening.  Some of these storms look to be very strong, which is understandable with 1500 J/kg of CAPE.

The 15z WRFRRx does manage to fire off storms in the Tucson area as does the 12Z WRFNAM.

No model forecasts develop storms up to the northwest of Tucson this evening.  In spite of some storms coming close to the Phoenix area during the late afternoon, it's unlikely they will make it into the valley. The PBL isn't mixed very deeply plus the inversion on top of the mixed layer.  CAPE is OK, but strong outflows will be needed for deep convection, and those are not in the forecast.

Some of the runs do eject moderate outflow boundaries into Pinal county this evening thus some blowing dust is possible later in Pinal and Maricopa Counties.

Day 2

I've run out of time to look much at Day 2. The synopsis is favorable northeasterly steering flow moisture, and CAPE continues for storms to make it into the lower elevations.