Arizona WRF Discussion: 20190909

Previous Forecast
It was the worst bust of the summer, at least for Tucson. The forecast was for widespread strong to isolated severe storms and heavy rain/flash flooding on Saturday night. Every model run had almost the same result, which gave me high confidence in the forecast. During the afternoon hours, the forecast was accurate as storms were to the east of Tucson and forecast temperatures and PW were tracking observed. By late afternoon, the PW began to drop while forecast PW increased. The other problem is that no storms developed to the west of Tucson as the forecast had multiple outflow boundaries converging over the valley. The outflow from the east did come through, right on time, but it was insufficient to trigger deep convection. Unfortunately, the critical piece of information, the 0Z sounding from Tucson, is missing. Scattered storms did develop over the city later in the evening, but nothing that strong. I also looked at the HRRR runs, and they also busted as they were similar to the WRF runs.

The 12Z sounding from the next morning indicates a strong subsidence inversion just above 500mb and if this existed at 0Z, must have been part of the reason.

A discouraging bust in an overall disappointing season.

Overnight, storms developed in the Tucson area with 2.91 on Mt Lemmon. The WRFGFS and WRFRRx had thunderstorms in the general area.

Initializations
The mid-levels of the atmosphere are dominated by a west coast trough which results in generally westerly flow over the state. Unfortunately, the trough is not advecting any cooler air as 500mb temperatures are still -6 to -4C. There is another weak trough from west Texas down into NW Mexico. Widespread scattered cumulus is present over much of southern Arizona and into Mexico. I don't see any issues with either the upper-air or cloud initializations. PW was initialized well too.

Day 1
It was a pleasantly cool and humid morning here in Tucson. The dewpoint is around 65F, and cumulus bases are well below the mountain tops. Looks and feels like Hawaii or Mexico!

The morning Tucson sounding has a lot of MLCAPE, over 2000 J/kg, so the potential is there for intense storms. The LFC is quite low too, but the subsidence inversion is strong, and the air above is very dry.

The very moist lower atmosphere continues into the afternoon as 850mb dewpoint temperatures are as high as 18C.

The abundant low-level moisture results in moderate to very high CAPE over much of southern Arizona.

Just for fun, here's the afternoon forecast Skew-T for Yuma. CAPE is insane at above 3000 J/kg, and the wind profile has significant directional shear. However, the cap is extremely strong and won't be broken. This is going to be the case for much of the state, but not quite to this extent.

The convective boundary layer is much deeper at Phoenix, but still strongly capped. CAPE isn't all that great either.

The Tucson Skew-T looks a bit better as there isn't much of a cap to speak of so some deep convection may develop in southeastern Arizona.

All runs do develop a few storms in southeastern Arizona by late afternoon into the evening hours.

In Bob Maddox's blog posting this morning, he mentioned the chance for nocturnal storms again tonight. This certainly looks possible as CAPE is exceptionally high over Maricopa and Pinal Counties this evening.

Holy crap, look at that CAPE! It won't take soo much to get this airmass to form deep convection, but will it?

All runs do develop early morning storms for areas in and around Phoenix. The WRFNAM is the most active. These nocturnal storms can produce high amounts of precipitation and flash flooding.