20190914

Previous Forecast
Showers and storms were confined to mainly far eastern Pima and Cochise Counties yesterday, in line with most of the forecasts.  All runs didn't have enough activity in far southwestern NM.

Initializations
It is a complicated situation this morning as a large cyclonic circulation is located over eastern Sonora.  Satellite images indicate a couple of MCVs embedded in the circulation as well as extensive clouds and a few showers the stretch up into southeastern Arizona and southwestern NM.  As was forecast yesterday, 500mb temperatures have decreased over eastern Arizona and into NM as El Paso is -9C.

The upper trough is located over far southwestern Arizona wich results in quite strong upper winds over southern Arizona, and upper divergence/difluence. This pattern is favorable for organizing storms and blowing anvils away from the storm motion.

All the initializations look quite accurate and even have the clouds and few showers.   PW errors are minimal, even in the NAM.

Day 1
The forecasts from yesterday have verified this morning as a significant mid-level closed cyclonic circulation is approaching Arizona. Moisture has increased over the state from the east due to low-level easterly flow.   By early afternoon, PW from 30-35mm is present over much of the southern 1/2 of the state.  Strong low-level easterly winds are over southeastern Arizona though, which typically suppresses storms due to downslope flow.   Also, a weak surge gets underway, increasing moisture over southwestern Arizona.

This descending motion can be seen in the CAPE field as little or no CAPE over far southeastern Arizona.  While not impressive, CAPE is sufficient to support strong storms over the rest of southern and central Arizona.  One necessary component in an active day for the deserts is the presence of moderate CAPE over the higher terrain of the Rim and the White Mountains for storm initiation.

Widespread thunderstorms develop over the high terrain and move to the southwest this afternoon.

Yesterday, the runs were uncertain of the location of lower desert storms.  Today, I'm confident saying it's not going to be Tucson.  As the low moves closer, the 500mb temperatures warm back up to around -5C, due to the warm core nature of the low.

The situation is better for storms around Phoenix as there is no inversion at 500mb. While CAPE isn't that great, it should be enough for some storms.  A deep sub cloud layer should also result in very strong thunderstorm outflow winds. 

All morning runs move storms into Pinal and eastern Maricopa Counties by late afternoon.

A strong outflow boundary moves through Phoenix and a few storms pop up.  As mentioned earlier, CAPE was marginal so I don't expect a widespread outbreak of storms, and only the 6z WRFGFS had a big outbreak.

Even though I think it's unlikely, I wanted to CMA as the WRFGFS is forecasting a widespread severe outbreak for the Phoenix valley as the storms produce severe winds.  It will be interesting to see if the 12Z WRFGFS is the same, but I bet it isn't

All runs have strong outflow winds west of Phoenix this evening.  A few storms may form on the leading edge.

Day 2
The 500mb low moves into northeastern Arizona which results in unfavorable northwesterly flow/NVA/descending motion for much of southern Arizona except for the far southeast. 

Positives for tomorrow are CAPE and moisture remain over the state.

Storm activity is restricted to mainly the mid and high elevations tomorrow afternoon.

It doesn't look good for Tucson again, due to the strong mid-level inversion.

The Skew-T doesn't look as bad for Phoenix as there is some steering flow which may help push storms into the valley.  CAPE is moderate and the CBL is mixed deeply and the inversion isn't as pronounced.

A few storms get close.