20190822

Previous Forecast
All model forecasts were pretty good except for the 15Z WRFRR as it had a bit too much precipitation over SE Arizona.  The only issue was that all the forecasts didn't have enough rain for northern Chihuahua.

Initializations
Heights have come down over Arizona, but temperatures are still -5C at Tucson (-3C at SAN!).  However, there is cooler air in the vicinity as Flagstaff is -8C and ABQ reports -9C.  A weak trough is over western Arizona and another over eastern NM stretching down into Sonora.  A weak MCV is over southern Sonora while deep convection continues south of there probably due to the tail end of the trough.  The RR, NAM, and GFS have the main upper air features initialized well.  They also initialized the Sinaloa storms well too.  There is no Suominet GPSIPW data this morning, so I can't comment on the PW initialization, but other than that, they look good, and confidence is high.

Day 1
The forecast for a gulf surge to begin this morning verified as Yuma has a Td of 74F and Tucson is up to 63F.  However, it is still shallow as indicated by the 14Z sounding from Yuma.  The WRF forecasts from yesterday had a deeper layer of moisture, so this part of the forecast did not verify.  The shallow surge continues during the day, and 850mb dewpoints are marginal in the WRFRR.  The WRFNAM and WRFGFS are wetter and more like the forecasts from yesterday.  Without GPSIPW data, it's not possible to check to see how accurate the PW forecast is for the early part of each model forecast.  In any case, the WRFGFS is between the other two and has sufficient moisture for an active day.

The 500mb forecast has some less hot air over eastern Arizona, which will help steepen the lapse rate.  Light and variable steering level winds won't help move storms off the higher terrain though.

CAPE is very high along the border, which will support very strong to isolated severe storms.

The afternoon forecast Skew-T for Tucson has about as high as CAPE as we get around here.  Steering winds are poor, but with the PBL mixed close to the LFC, a strong outflow boundary is likely to be sufficient to trigger deep convection.

With the cooling of the PBL from the surge, storms take their time getting going.   By later in the afternoon, strong storms are underway mainly over eastern Pima and into Santa Cruz Counties.

The wetter WRFNAM has a broader coverage of storms.  My guess this forecast is overdoing it, but I can't rule it out.

All forecasts have little or no precipitation in Tucson while surrounding higher elevations may see up to 1.5" of rain.

While Phoenix doesn't have nearly as much CAPE as Tucson, it is sufficient to support some activity.  The big problem is the two inversions that will need to be overcome.  Will the moderate outflows from the south be strong enough?

Both the WRFGFS and WRFNAM develop isolated storms near the Phoenix area tonight.

The 15Z WRFRR has more activity than the 12Z, including storms in the Tucson area by late afternoon.

The 15Z WRFRR also has early morning storms for southeastern Arizona.  I've not had time to look at why this is the case.

Day 2
I haven't mentioned tropical storm Ivo yet.  It is forecast to be SW of southern Baja by mid-day tomorrow. While not very close to land, it does help continue the gulf surge and moisture increase into Arizona.

CAPE is moderate to high and is sufficient to support strong storms.

The 500mb temperature forecast for the ridge off the California coast is insane.  I have never seen such a large area of -2C (or warmer!) air.  A quite strong inverted trough is located over southern Sonora which results in weak easterly steering flow for southern Arizona. Storms may be able to move off of the higher terrain and into lower elevations.

The situation looks more favorable for storms in Tucson with some low-level shear, mid-level easterly steering, and upper winds blowing anvils 180 degrees away from the storm motion.  Good CAPE and PBL mixed nearly to the LFC.  What can go wrong? Hopefully, nothing.

Storms get a late start again and by late afternoon, are scattered around southeastern Arizona.

Activity continues into the evening, including Tucson.