20200817

Previous Forecast

It was the most active day in quite a while, perhaps the most active this season.  Strong to severe thunderstorms moved from the higher terrain of Gila county southwestward across Pinal and Central Pima Counties.  Additional storms threatened Phoenix from the north and northwest, and a big haboob and other storms moved into southwestern Arizona during the evening.

The 12Z WRFNAM was the best run as it had the most activity.  It also had the dryline in the correct location, which was just NW of Tucson proper, as was made evident by the QPE product above.  The NAM haboob also verified.  All model runs missed the convection to the west of Phoenix that continued into western Arizona.  All runs did not produce enough precipitation as there were some localized amounts above 2 inches.

Discussion

The monster 500mb anticyclone continues to intensify and is likely around 6000m now.  In the past 30 years or so, I can remember maybe 2 or 3 times that 500mb heights reaching 6000m.  It's happened at least twice this summer and a few times over the past few years.    The center is northwest of Vegas, resulting in moderate to strong ENE flow over much of Arizona.  Temperatures have cooled another degree and are a favorable -7 to -8C.  

Surface dewpoints are in the 50s over much of the southern 1/2 of the state, and as high as 70 in Yuma.  GOES derived PW shows dry air over far SE Arizona and also over northern Arizona.

There is a bit of CAPE present over far western and southwestern Arizona.

The Phoenix 12Z sounding indicates only minimal CAPE, but the residual boundary layer is well mixed to above 500mb.  Moderate to strong (for the monsoon season) east to northeast flow exists from around 600mb and above.  One issue is that with this stronger unidirectional flow aloft, anvils will move out ahead of storms and perhaps interfere with downstream deep convection.  This won't be a problem if CAPE doesn't increase as there won't be any deep convection to generate anvils!

Initializations

Both the 6Z NAM and GFS have a moderate moist bias for western Arizona.  Maybe this isn't such a bad thing as the WRF has had a low bias with it's QPF?  The 12Z also has about a 5mm wet bias in that area but is fine elsewhere.  The RR has minimal errors.  I'd say the RR has the best initialization, but the WRFRR has had its problems recently.  I'll favor the 12Z WRFNAM.

Day 1

Downsloping easterly flow dominates most of the southern 1/2 of the state. Despite marginal 850 dewpoints, this is unfavorable for deep convection.  

Despite this somewhat drier air, CAPE isn't as low as I would have expected.  There is plenty over the higher terrain, so storms are likely there.  CAPE is also moderate along the Colorado River, so that may be another area of activity later today.

Scattered storms develop over the high country this afternoon and try to move into the lower elevations of central Arizona.

The forecast Skew-T at Phoenix has only 500-900 J/kg of CAPE.  Typically, over 1000 is needed for any significant activity.  Winds are also unidirectional and without any low-level shear.  Positives are that the boundary layer is mixed relatively well, and only a weak inversion is present.

Both the WRFNAM and RR manage to develop some scattered weak storms in the vicinity of Phoenix this evening.

The WRFNAM moves these storms to the SSW and has a bit more activity in general.

It looks likely that Phoenix will have extremely strong winds and another dust storm as DCAPE is around 1800 J/kg.  As the dust source region is typically Pinal County, it probably won't be as bad as yesterday, but winds are predicted to be stronger and maybe even isolated severe.  This outflow boundary may produce another haboob westward down I-10 or I-8.

Other storms develop over parts of southeastern Arizona and even threatens the Tucson area.  CAPE is only a few hundred J/kg, so thunderstorms are not expected in Tucson proper.

Day 2

Subsidence and the downslope easterly flow keep much of eastern Arizona dry.  Wet air still is present over the western 1/3 of the state, and perhaps this is sufficient for activity there.

CAPE turns out to be sufficiently high to support deep convection over the higher terrain of the Rim and White Mountains.  Extremely unstable air is over the northern GofC!

Scattered storms develop over the high terrain and move toward the Phoenix area by late afternoon, but CAPE is minimal, so they die out as they move into the lower elevations.