20210721

 Previous Forecast

Another active day, especially for NW Arizona, again.  A few strong storms popped up after sunset in the Tucson area.

The model initializations struggled to get the morning activity initialized correctly in western Arizona as they typically had too much.  This resulted in a cloudier and more suppressed air mass in western and northwestern Arizona.  In real life, NW Arizona received good heating and was much more active than the forecast.  The WRF performed well around Tucson as nearly all runs were very accurate, developing storms after sunset. 

Discussion

The big story is the large inverted trough that is now making its way to the west from southern Texas.  This is the strongest inverted trough I've seen in years.  Winds are highly divergent over NW Mexico.

The 500mb anticyclone is located over the Four Corners and has quite warm (-5 to -3C) air associated with it.  Mid-level flow is light from the east for much of the state.

This is something you don't see every day, a 71F dew point at Tucson.   Another thing you don't see every day is the enormous amount of mixed-layer CAPE, 1900 J/kg. There is only a minor inversion at the top of the residual PBL, and with some heating, it could be a very active day.  The wind profile is pretty good with 10-15 knot mid-level steering.  One negative is the anvil level winds are also easterly at 30 knots, so anvils will blow out ahead of storms.  Another negative is the dry air advection continues over southeastern Arizona, and Suominet stations show decreasing PW.

PW is still high in central and western Arizona, ranging from 45-50mm, but has also decreased over the last 24 hours.   Much of NM is dry and clear, and  PW is less than 24mm.

Initializations

As seen above, there are widespread clouds over western Arizona, but for once, there is no precipitation.  Much like yesterday, the various WRFRR runs incorrectly initializes and develops showers and storms over western Arizona.  Not just a few showers, but a large area.  It is also very smoky again, and it could be thick enough to reduce heating, especially over eastern Arizona.  The 6Z NAM and GFS were pretty good as they had minimal or no precipitation and scattered to broken clouds.  PW was initialized well except for the NAM, where it was too moist in Arizona.  All RR initializations are suspect due to the major errors in morning convection.  Perhaps it will be OK for the eastern 1/2 of the state, but I don't trust it elsewhere.  I looked at the HRRR, and it did develop a few showers, but nothing like the WRFRR.  The 6 and 12Z NAM is OK, but the GFS is the favored initialization.

Day 1

The dividing line between the very moist air is along the Pima/Cochise County line.  It also appears that there is moisture convergence to the west of this line, and as mentioned before, these backdoor drylines can act as a focus for convective initiation.  

A weak and shallow surge helps keep PW high over western Arizona.

There is quite a difference between the 6Z and 12Z runs concerning the amount of CAPE forecast for this afternoon.  The 6Z WRFGFS has very high CAPE over south-central Arizona.

The 12Z WRFRR has quite a bit less CAPE, and the dryline is west of Tucson.  CAPE is still sufficient to support strong storms.

Deep convection initiates right around Tucson by early afternoon.

A broken line of strong to severe storms moves into western Pima County later in the afternoon.

The morning WRFRR series of runs are quite similar to the WRFGFS, except they develop stronger storms to the east of Phoenix.

I don't trust the WRFRR for western/central Arizona as it had such a poor initialization in that area and is probably more stable in the WRFRR than in real life.  Both 6Z WRFGFS/NAM runs expand storms to the north and maybe even into the Phoenix area.

The WRFGFS Skew-T looks good for storms as the PBL is deeply mixed, and there is more than enough CAPE. 

The 12Z WRFNAM has considerably less CAPE and not as well mixed PBL, so there is a lot of uncertainty with the Phoenix forecast.

Storms decrease into the evening hours with nothing in southern Arizona.

Day 2

Here we go!  By Mid-day tomorrow, the IT is located over west Texas while the high center continues to be over the Four Corners.  Moderate easterly steering flow is present over southern Arizona.

A jet streak approaches southern Arizona from NM at 250mb, putting the area in favorable upper difluence/divergence.

Low-level flow from the Gulf of California increases and pushes the drier air back into NM.  There is a strong surge signal even at 850mb. 

There is a big discrepancy between the runs on how much moisture and CAPE there will be by tomorrow.  The WRFRR has less of both.

I don't know which is more accurate.

As with the CAPE forecast, the forecasts for storms are significantly different between runs, so confidence in when and where is low.  All I can say is that there will be strong to severe storms scattered around much of the mid/high elevations of the state during the afternoon hours.  The WRFGFS is the most active.

The WRFRR and NAM focus activity on southeastern Arizona.

By early evening, all hell breaks loose over the lower elevations of south-central Arizona in all model runs.   Here's a sample from three of the runs...take your pick.

The model Skew-T forecasts are insane.  They are uniformly in the 1500-2000 J/kg for CAPE, and just look at the depth and strength of the low-level winds!  

The situation is similar in Phoenix.

Most runs have storms in the Phoenix area later in the evening.

All the severe weather we typically see in Arizona is likely tomorrow: severe winds, hail, intense lightning, and the most dangerous, flash flooding.  The flash flooding risk is particularly high, and the burn areas could see catastrophic flooding.  48-hour total precipitation.

The situation is worse by Saturday as rain and storms continue.  72-hour total precipitation.