20190913

Initializations
Mid-level flow is light and variable over Arizona and temperatures have cooled slightly around -6 to -7C.  There appears to be an inverted trough over NW Mexico according to the upper air data.  Some cyclonic circulation can be seen in the cloud field.  300mb upper air data indicates an upper trough off the coast of Baja and this feature is likely to play a part in Arizona's weather over the next few days.

All morning initial conditions look accurate, even the NAM PW, so forecast confidence is high.

Day 1
It's a fairly unusual situation today as the moisture in southeastern Arizona increases due to southeasterly flow instead of a gulf surge.

Afternoon CAPE isn't great, but it appears to be sufficient to support storms over southeastern Arizona.

Scattered storms form mainly over the higher terrain by mid-afternoon.

The positives for Tucson this afternoon is 6-800 J/kg of CAPE and a well-mixed and deep convective boundary layer.  However, steering flow is weak so storms will have a hard time moving into the lower elevations.  One or two could form on the outflow boundaries though.  The sub cloud base is quite deep, so storms could produce very strong winds.  Another negative is the pronounced warm layer around 500mb.

I mentioned the upper trough earlier, an by afternoon, it is providing upper divergence/difluence over southern Arizona could result in a bit of organization.  The flow will shear anvils off to the northeast, but this won't matter much as there isn't any mid-level steering.

Day 2
The weather situation becomes much more interesting tomorrow as the upper trough is located over far western Arizona resulting in upper divergence and difluence over much of the state. There is even a bit of a jet over far southeastern Arizona into southwestern NM.

Moisture continues to advect into Arizona, mainly from the east.  PW increases across all of southern and central Arizona and even makes it as far north as Flagstaff.  All runs have similar forecasts so there is confidence in this moisture increase.

There is a corresponding increase in CAPE, which results in significantly more potential thunderstorm activity.

There is quite a bit of model spread regarding the 500mb pattern for tomorrow.  Generally, all runs agree on moving an inverted trough to somewhere south of Arizona which results in easterly to northeasterly flow.  They have significant differences in the strength of this flow and as there is no obvious strong IT in Mexico, so I went with the weaker WRFRR.  One feature they do agree on is the cooling at 500mb over much of the state.  The WRFRR forecasts around -7 to -9C which may be due to some cool air on the tail of a weak trough to the north.

Thunderstorms develop over the high terrain of the Rim and White Mountains as well as southern Arizona during the early afternoon hours.  Steering flow is favorable for moving these storms into the lower deserts.

Tucson's wind profile is mostly good with upper-level southwesterly winds blowing anvils away from the storm motion.  Steering winds are favorable for propagating storms off of the higher terrain, but low-level shear is lacking.  The CBL is mixed deeply so storms could go nearly any time from mid-afternoon onward.

It's difficult to say when and where for storms tomorrow as each run is a bit different, but I guess that storms move/develop over the lower deserts by late afternoon and continue into the evening.  Bad news for the UofA football game!