20210807

Previous Forecast

Another day, another bust, at least for Tucson.  Storms developed around Tucson during the afternoon hours, with some of them being severe warned.

It wasn't quite as bad as the previous day, as WRF could correctly predict the look of the 0Z Skew-T for Tucson and the outflows that moved in around sunset.  However, it didn't predict the widespread strong storms to the west, which resulted in anvils moving over the valley and literally squashing new and existing deep convection.  Textbook anvil suppression.  The cloud camera movie was spectacular as deep convection was boiling over Mt Lemmon with multiple storms and many Pileus clouds, then the anvils came over.  Around 6pm, the outflow boundary moved in, but nothing happened.  

Discussion

A mid-latitude trough has suppressed the 500mb ridge over the western CONUS. The axis from somewhere over NW Mexico to Oklahoma resulting in westerly mid-level flow over Arizona.  Temperatures over Arizona are quite warm at -4 to -6C.  

The Tucson Skew-T has only minimal MLCAPE as there is some weird looking dry layer at 850mb.  The mid and upper troposphere is dominated by westerlies, and there is a subsidence inversion around 500mb.

Phoenix looks much the same, except there is a bit of MLCAPE.

Initializations

It's clear over all of the state, and PW is mainly 25-38mm over southern Arizona.  PW initialization errors were quite high in the 6/12Z NAM and the 6Z GFS, so it looks like the RR are the initializations to go with today.

Day 1

It doesn't look good for most of the state as the anticyclone moves back into SW Arizona with -5C temperatures.  However, the tail of the trough brushes SE Arizona with temperatures around -7C to -8C, so this area seems favorable for storms today/tonight.

A surge continues to import moist air into south-central Arizona, and 850mb dew point temperatures are quite high at around 14C.

The CAPE forecasts by the various runs are considerably different.  The 12Z RR is the lowest but sufficient to support storms over far southern and southeastern Arizona.

The 9z WRFRR seems to be more in line with the GOES-derived CAPE.

Just about all runs agree that only far SE Arizona and SW NM will see storms today.

The potential is there for Tucson (900-1400 J/kg of CAPE), but the mixed layer isn't that deep, and there is a cap present, so it looks unlikely that storms will form.

Scattered storms are likely to continue into the evening, mainly along the border.

Day 2

The 500mb anticyclone is centered somewhere over eastern Arizona, which results in more favorable southeasterly flow, at least for southern Arizona.  Slightly cooler air also is forecast to be present over southeastern Arizona and southwestern NM.  Another mid-latitude trough is moving through the NW CONUS, keeping the ridge suppressed.

It's quite the battle between dry air to the north and very moist air just to the state's south.  The dry air is winning despite a strong surge continuing.  

It appears that SE Arizona will be the only active area tomorrow.