20190722

Previous Forecast
It was quite active to the west of Tucson yesterday afternoon and more so than I expected.  However, the WRFNAM and WRFGFS did a pretty good job.

Most of the runs struggled with the overnight activity in NW Mexico where a large MCS formed in Sonora, which is mainly the source of our clouds.  While most runs had large storms in Sonora, which was accurate initially, they mostly died off around midnight.  However, storms actually continued and moved out over the Gulf of Californa during the early morning hours, generating more debris clouds which are over us now.

Initializations
I might as well start with the bad part of the initializations which you probably already know, clouds.  Broken to overcast mid-level clouds are over much of southern Arizona along with a few areas of showers, which I experienced first hand on my bike ride into the UofA this morning.  There are areas of clearing over southeastern Arizona, but more clouds are moving in from southwestern NM.

The NAM is terrible.  There are almost no clouds initialized over southern Arizona.   The 12Z RR is quite a bit better, and the forecast by the 12Z WRFRR at 15Z looks pretty good except for southern NM.  However, much of the mid-level clouds dissipate soon after.

The 12Z GFS is better than the NAM but lacking clouds in south-central Arizona.  The WRFGFS also dissipates clouds throughout the morning.  The plot below shows the temperatures from the overnight runs. Tucson is already running a couple of degrees below the forecast.

The upper-air situation is also somewhat complicated.  I can see at least 2 troughs: one south of Douglas and the other in east-central NM which is most likely an MCV.  Satellite imagery reveals a large MCV over northern Baja and another MCV over east-central NM.

300mb shows the area of divergence over far northern Sonora that I mentioned yesterday and appears responsible in part for the Sonoran MCS.  So far, flow over Arizona doesn't seem to be divergent/difluent.  It is sure nice having the Empalme sounding operational!

The initializations generally captured the significant features.  The NAM had substantial PW errors of over 10mm in NW Mexico.  The other two initialized PW well. 
The 12Z NAM has too many errors to be of much use.  The other two have issues with clouds, and we'll just have to see how quickly it can clear off in real life. Confidence in the forecasts is pretty low today.

Day 1
The situation is much the same as was forecasted yesterday as the surge has weakened, but plenty of moisture is present over the state at 850mb dew-points are at or above 10C.  Low-level easterly flow in NM is also wet and has set up a convergence zone over southeastern Arizona and will be the focus for convective initiation later today.

CAPE is impressive with areas above 1500 J/kg.  Note the lower CAPE in the area of descending low-level easterlies in Cochise County.

Southeastern Arizona in under an area of weak upper difluence/divergence this afternoon which will aid in storm organization and propagation.

By mid-afternoon storms are mainly present over the high terrain over far eastern Arizona as well a few over southeastern Arizona.  The WRFRR had the least amount of coverage, but this may still be too much and/or too early.

There is an impressive amount of afternoon CAPE forecast for Tucson, around 1500-1900 J/kg.  The wind profile is good as well with 10-20 knot mid-level steering flow and low-level moist northwesterly flow, favorable for storm propagation.  The PBL is well mixed and not far from the LFC, which was around 600mb from the morning sounding.  Again, this is all assuming there is sufficient heating.

 By late afternoon (or maybe early evening), storms really get going, and a broken line is apparent in the WRFRR.  Some storms are very strong, producing possible severe winds, very heavy rain, and small hail.

Storms move into central Pima and Pinal Counties during the evening accompanied by very strong to isolated severe winds.

It is certainly possible (probable?) for a large haboob in Pinal and Maricopa Counties tonight.

What about storms for Phoenix?  Well, as always, it's a tough call.  The potential is there as CAPE is around 700-1400 J/kg and the vertical wind profile is good, but the PBL isn't mixed very deeply.  Typically, this situation needs multiple outflow boundaries from different directions intersecting over the valley for deep convection to form.  Perhaps very strong unidirectional flow will be enough, perhaps not.

The WRFRR does develop scattered storms around the valley while the WRFGFS does not.

Day 2 will follow later as well as an update with the results from the 15Z WRFRR.