20190718

Previous Forecast
Very little activity occurred over Arizona except for the higher elevations of southeastern Arizona.

The model runs struggled for the most part with the complicated situation and poor initializations and typically had too much activity.  The models had too much moisture and too much heating.

The best run was the 12Z WRFRR, and the worst was the 15Z WRFRR as it had too much storm activity.   It was surprising to me to see how different they could be and only separated by 3 hours.

Initializations
It was an unusual 500mb map this morning with a zonal jet and a ridge axis stretching all across the southern CONUS.  Southern Arizona is under 10-20 knot easterly flow, which is enough to steer storms off of high terrain towards the lower elevations.  A broad IT is located over NC Mexico and can be seen on satellite imagery

Something went wrong with both the 12Z RR and the 12Z NAM, so I have only the GFS to work with. I'll fall back to the 6Z runs plus wait a bit for the 15Z WRFRR to become available (it never did).   The various models initialize an inverted trough over the southern Gulf of California which I can't find in either the satellite imagery. If you use a little imagination, perhaps it can be seen in the upper air data.  There are scattered clouds over far southern Arizona, and these are initialized well by the 12Z GFS.   GFS PW initialization errors are minimal for both 6Z and 12Z.  Again, the NAM is very bad with huge errors in Mexico and significant errors in the SW US.  RR data is FTP'd from NCEP, and the data has failed to download as the FTP process times out.

Day 1
Over the past 18 hours, all of the runs have significantly decreased the amount of deep convection over SE Arizona today.  Why such a change?  I don't know yet and hopefully, I'll be able to figure it out.  The first clue is the sounding from Tucson.  While there is a fair amount of CAPE, it's all located above a weak inversion around 700mb.  My guess is the lower atmosphere still hasn't recovered from the moist and cool gulf surge from the past 36 hours.

The surge has weakened since yesterday but is there and continues to slowly increase moisture as by noon, south-central Arizona PW in the mid to low 40mm range.

In spite of all this wet air, CAPE is minimal.  This is also the case for the 6Z runs. 

The forecast Skew-T plot for Tucson tells a sad story of not one inversion, but two!  So it's a case of "too much of a good thing (moisture and associated cooling of the lower atmosphere)."

I was shocked to see that the recent runs have little to no deep convection anywhere in Arizona this afternoon!

The 6Z WRFGFS is a little more active with a stray storm or two along the border.

Day 2
It's even worse tomorrow as dry air starts to move in from the west and northwest.

Again, almost no activity.  I don't ever remember a surge like this where we end up with virtually no significant deep convection.