20200723

Previous Forecast
There was a bit more activity for the state over the past 24 hours.  There was quite a bit of rain south of Flagstaff as well as north of Tucson.  Storms popped up during the evening hours west of Tucson and Phoenix

The best performing run was the WRFGFS from 12Z, as it had precipitation more or less in the right locations.  It did especially well in western New Mexico, where widespread rains fell. Most runs were able to predict the evening activity in the Tucson and Phoenix area.

There is more to the story as early this morning, a large area of storms moved/developed over southeastern Arizona.  None of the above models predicted this event. The later 15Z WRFRR did, as well as some of the morning HRRR runs from yesterday.  I sent out a tweet with the updated forecast noting the possibility of AM storms.  The 15Z WRFRR was a bit early with storms and too strong, but these "morning surprises" are always tricky to forecast.

Discussion
Showers and some storms continue over much of southeastern Arizona, due to a continued increase in moisture as well as the forcing from a well defined inverted trough near Hermosillo.  PW is now a very high 48mm at Tucson and in line with what the WRFRR from yesterday had predicted.  I didn't think it would get that high!  There more where that came from as PW in Sonora is over 50mm. 
The west coast trough mentioned yesterday hasn't moved much and is still near San Francisco, resulting in mostly southwesterly 500mb flow over the state.  Temperatures are from -6 to a cool -9C.  The IT in Sonora was not well resolved by the upper air data, but I can sure see it spinning in the satellite imagery.

  The 12Z Tucson sounding data shows 1600 J/kg of MLCAPE.  Impressive!  However, winds are light and variable from 500mb to the surface.   The RADAR loop indicates that the thunderstorms over central Arizona are not moving much, and with such a wet and unstable air-mass, flash flooding is going to be a big problem.  Another area of storms is moving into far eastern Pima County at 16Z, steered by the Sonoran IT.

Initializations
This is going to be a rough day for the WRF runs due to all the activity at the model initialization times.  I'll start by looking at the 6Z runs.  The 6Z NAM, which now is a blend of RR and NAM, developed widespread strong activity during the early morning hours over southeastern Arizona.  It was too much and too early, but at least the morning storms ware there.  No morning storms developed in the 6Z WRFGFS. This will cause havoc with the later forecast, so I'm discarding it.    The 12Z WRFRR was actually quite accurate as it had the small MCS in far SE Arizona and then developed storms farther to the west during the first few hours.  It has the Sonoran IT and another IT/MCV associated with the MCS.  PW errors are trivial, and clouds were initialized well,  There is hope for forecasting later in the day!  The 12Z NAM was poor as it had only limited activity initialized, no Sonoran IT, and a strong MCV located over far southeastern Arizona.  All wrong.  PW was also initialized poorly.  WRFRR, you are our only hope!

Day 1
The surge continues to advect very high PW air into Arizona.  Phoenix increases to around 50mm by mid-day!  850mb Td is a very moist 15C over all of central and southern Arizona.  So we go from weeks of below-average PW to now way above (>90th percentile)!

The WRFRR seems to be doing a pretty good job with the storm activity as the CAPE plot shows the stable air in their wake.  From this image, it looks like the most active areas will be north of Phoenix later today.

Phoenix has enough CAPE to support strong storms.  PW is very high; thus, flash flooding potential is high.   The sub-cloud layer is not very deep, so outflows won't be very strong, but isolated wet microburst winds could be severe.  There is some steering flow thanks to the Sonoran IT, so storms that form in the high CAPE areas to the east should move westward.  Look at those upper wind!  Very atypical for the monsoon season. There are some significant negatives like the cooling outflow from activity to the south and anvil shading, which are significant issues.  Actually, the cooling outflow is already moving into the valley as of 1645Z.

By early afternoon, the Tucson area activity decreases while new storms develop over the higher terrain of the Rim and White Mountains.

Not looking good for the lower elevations of Phoenix as the cooling has created cap at the top of the mixed layer, and CAPE has decreased to only 5-900 J/kg.

Storms approach the Phoenix area by early evening.    My guess is no due to the lack of CAPE.

The 15Z WRFRR, which just became available, initializes storm OK and does develop some in the less modified air, just west of Phoenix, during the afternoon hours.  Maybe?

Will Mike Olbinski will get his haboob?  Even if storms do make it towards Phoenix, the PBL is not very deep, so organized outflow winds won't be that strong.

Now, back to the 12Z WRFRR for this evening.  Storms remain mostly over the higher elevations and don't generally develop in the valley.

Day 2
Confidence is low due to all of the complexity and uncertainty today.  The surge continues into tomorrow, and while PW has decreased, it's still more than enough.  The issue will be how well the atmosphere can recover from today's activity.

It looks like the favored locations will be central and eastern Pima County and up into parts of Pinal and southern Maricopa, based on the CAPE.

Arizona is caught between the west coast trough and the Sonoran IT, which makes its way to just south of the border.  Steering flow is again all over the place as it's pretty good over SE Arizona, but poor elsewhere.  Note the cooler air associated with the trough.  Definitely NOT a typical late July monsoon pattern!  It looks like September.

Well, this is interesting!  Tucson between 1200 to 1700 J/kg of CAPE by early afternoon as well as a favorable vertical wind profile with low-level westerlies, mid-level NE to SE flow, and light SW aloft.  Textbook tropical squall line profile.

Storms form over the higher terrain east of Tucson during the early afternoon hours.

By later in the afternoon, these storms increase in coverage and intensity and move across far southern Arizona in an organized squall line.  At the moment, confidence in this forecast is low.

Ths organized broken line of storms could continue through Pinal and into Maricopa County during the evening.  Again, confidence is low as one negative is that the Tucson area is going to be worked over from the ongoing activity.

15Z WRFRR is similar except it has only minimal activity around Tucson earlier.