2190907

Previous Forecast
Generally, the forecasts didn't have enough activity over central Pima and northwest of Phoenix.  One lone storm managed to pop up in Phoenix proper yesterday evening.

Initializations
The series of troughs are knocking down the 500mb ridge, and the center is now ill-defined and spread from eastern Arizona over into Arkansas.  Temperatures are still -5 to -7C in our neck of the woods.  There appears to be a weak IT over Sonora which seems to be responsible for the widespread storm activity over southern Sonora and Sinaloa.  A weak trough in the westerlies is moving into western Arizona from California.  A wide band of mid-level clouds stretches north from that activity across much of central Arizona.  All initializations are struggling with the clouds, and none have the coverage close to what was observed.  Maybe they will dissipate here soon, but it's not looking like it according to recent satellite images.  PW was initialized well except for the NAM where it was too wet in Mexico.  Only the 12Z WRFRR has the storm activity initialized well.  I'd say the 12Z RR has the best initialization.

Day 1
PW is forecast to be in the 35-40mm range for the lower deserts, and 850mb dewpoints are a favorable 12-16C over southern Arizona.

CAPE is moderate to high over southern Arizona by early afternoon.

Storms develop over far SE Arizona and into Mexico during the afternoon hours.  Scattered thunderstorms develop over the Rim too.

Tucson has quite a bit of CAPE by late afternoon, and the boundary layer is mixed fairly well.  Winds are not favorable for steering storms from the higher elevations though.  Nevertheless, a good outflow or two should be enough to trigger storms in the area.

Strong outflow boundaries move into the valley during the late afternoon/early evening resulting in widespread storms for the Tucson area.

There is a chance that some storms may be severe with damaging winds and very heavy rain.

Storms will have a high flash flood potential as some may produce 2-3 inches of rain with isolated amounts even higher.

A few storms are possible in Pinal county during the evening.

The model forecasts are extremely divergent for the Phoenix area.  The WRFRR has moderate to high CAPE forecast for this evening.  Winds aren't favorable plus the PBL isn't mixed very deeply.  It will take significant outflow boundaries to trigger storms.  The WRFNAM has almost no CAPE while the RR has approximately 2000 J/kg.

The WRFRR does have strong outflow boundaries moving into eastern Maricopa later this evening.  Will they be strong enough?

The WRFRR does manage to develop storms later this evening in and around the Phoenix area.

The 15Z WRFRR is now in, and it also has widespread storms for Tucson.

Strong/severe storms develop in Pinal County on the outflow boundaries from the previous storms.

As in the 12Z WRFRR, a few storms manage to develop over the Phoenix area by later in the evening.

Day 2
Storms are limited during the day, but Tucson may see storms again during the evening hours.