20190831

Previous Forecast
All runs struggled around Tucson as storms developed NE of town during the early afternoon hours which resulted in debris clouds and slight cooling.  This put the kibosh on any further development.  The model runs were too active in and near Tucson.  The WRFNAM was the worst.  The WRFRR had a lot of activity for far southern Arizona, but it was about 30-60 miles too far north.

Initializations
The 500mb pattern is mainly unchanged from yesterday as the high center is located near Vegas/San Diego.  Slightly cooler air is present over the state, ranging from -6 to -9C.  An IT/MCV is situated over the central Gulf of California.   RR and NAM have upper-level features and clouds initialized well.  The 12Z RR PW initialization isn't as good as it normally is as it's too dry over southern Arizona.  The NAM is too wet in Mexico.  15Z RR has only minimal errors and is the favored initialization.

Day 1
The PW trace for Tucson indicated that there was some moisture convergence at the time of the sounding as showers were moving through.  This resulted in the sounding being too wet as PW was 1.59 inches.  The PW trace shows a rapid decrease after 12Z with PW now around 1.3".  This is likely the source of the unusual 12Z WRFRR PW error in southern Arizona as GPSIPW spiked up to 1.75" at 11:45Z.  In any case, there is plenty of moisture about today, and weak moist advection continues into the afternoon, keeping PW around 35mm over the lower elevations.

Steering flow is good over much of southern Arizona as it is easterly around 10-20 knots as the high center is somewhere over northern Arizona.  Temperatures also look good at around -7C.

CAPE is low to moderate across much of southern Arizona while it's nonexistent for northern Arizona.  Hopefully, there will be enough to support deep convection initiation over east-central Arizona so those storms can move towards the lower elevations.

The 15Z WRFRR develops storms do over eastern Arizona during the afternoon hours, but I'm surprised to see that they die as they move to the WSW.  This is much less activity than the forecast from yesterday and less compared to the 12Z forecasts.  The WRFNAM is very active, and moves/develops strong storms over southern Arizona early this evening.  The WRFNAM was too active yesterday, so will that be the case today?  I hope not.

The 12Z WRFRR is similar to the 12Z WRFNAM.  Not a large outbreak in any case.

I don't really understand why the 15Z WRFRR is so inactive. The forecast Skew-T looks very good with a deeply mixed PBL, minimal inversion, >1000 J/kg CAPE, good steering, and low-level wind shear.  We'll have to see what the 15Z WRFRRx has to say.

The WRFNAM does keep some storms going into the evening.

The WRFRRx looks no better for southeastern Arizona.  Most activity dies as it comes off the White Mountains and only isolated storms are present over far southern Arizona later this afternoon and evening. 

I also looked at some of the HRRR runs, and they are similar to the WRFRR.  Little or no activity!

Day 2
Moisture increases over western and central Arizona, but low-level easterlies are forecast for southeastern Arizona, resulting in drying due to downslope flow.  As a result, CAPE is forecast to be quite high over central Arizona while far SE Arizona, it's quite low.

Mid-level steering flow is again excellent, and temperatures are favorable, around -7 to -8C.

Storms develop early and move towards Phoenix by late afternoon.

The potential is there as CAPE is around 1300 J/kg.  However, the PBL is not mixed deeply, plus there is an inversion.  It's going to take strong outflow boundaries to get it to go but what else is new?

The WRFRR moves storms around Phoenix and into Pinal and Pima Counties by late afternoon while the WRFNAM has strong/severe storms in Phoenix proper.