20210812

 Previous Forecast

It was another active 24 hours for parts of the state, following the pattern of the past few days of an active afternoon for higher terrain (and Yuma!), evening storms around and west of Phoenix, and then early morning activity around Tucson and east of Phoenix.

Precipitation amounts were very heavy in some locations, like NE of Phoenix and just west of Tucson.  It's turning downright biblical!  "40 days and 40 nights".  This is 72 hours.

24 hours

The model runs were generally not very good.  The 6Z WRFGFS at least had some areas of heavy precipitation, though.  Some runs were able to again predict the early morning activity, but none had enough rain.  This high PW environment plus the weak IT wandering around can generate deep convection any time of day or night, and it's just not possible for the WRF to do well in this situation.

Discussion

The IT continues to hang around Arizona and is located somewhere around Phoenix.  Temperatures are moderate, around -6 to down to -9C at Vegas.  There is another IT near El Paso, which is more apparent on satellite imagery.

The 300mb anticyclone slowly moves back to the north, and winds have become light and variable.  A weak trough seems to be present over Arizona.

The Tucson 12Z Skew-T looks like yesterday, with a nearly saturated profile and mainly light and variable winds.  MLCAPE is low, but with some heating, this will increase somewhat.   Without any significant steering, afternoon and evening storms are likely to remain over the mountains.

Phoenix has an impressive amount of moisture and MLCAPE.  However, the sounding PW appears too high as the humidity sensor got wet (700-500mb). The Suominet PW from San Tan says only about 48-52mm, instead of 56mm.  This is confirmed from GOES PW, which is around 50-52mm.  Because of this error, MLCAPE is also too high.  Like Tucson, the wind vertical profile is mainly light.

According to the available Suominet senors, PW has generally decreased over the lower deserts and is in the 40-48mm range.  

Initializations

It's another difficult situation as a large arc of clouds covers much of northern and central Arizona, with a few embedded showers and storms.  The 6Z NAM wasn't cloudy enough over northern Arizona and too cloudy over southern Arizona.  The 6Z GFS did get the morning activity around Phoenix but was too cloudy for southern Arizona.  The 9Z RR was better, but still, a bit too cloud over far southern Arizona.  The 12Z RR was similar.  The best, at least for clouds, was the 12Z NAM.  It was missing the Phoenix area activity, though.  PW initialization errors were small except for the 6 and 12Z NAM. All runs basically initialized CAPE in the right locations.  I'd say the RR series looks the best.

Day 1

As mentioned earlier, moisture has decreased slightly since yesterday.  There is still more than enough for more activity, as 850mb dew points are still in the 13-16 range.

CAPE continues to be moderate to high for many areas.  >1000 J/kg has even reached the Vegas area, so maybe they'll get in on the action too.

The mid-level IT still doesn't move much.  This results in light and variable steering over the central part of the state.  Steering flow is favorable over NW Arizona to steer storms, once again, to western and perhaps, southwestern Arizona.  Temperatures north of the IT are also relatively cool, around -8 to -9C.  Temperatures to the south are quite warm, around -5C.

Thunderstorms take their sweet time getting going as by 22Z, not much is happening except for some isolated strong storms over the higher terrain.  I'm not surprised, as much of central/northern Arizona is cloudy this morning.  One exception may be SW Arizona, as some runs develop storms.

The late afternoon Skew-T forecast doesn't look very promising for either Tucson or Phoenix.  Tucson has only 600 J/kg of CAPE, as well as a significant inversion just below 500mb.  Steering winds are also light and variable.

Phoenix also has limited CAPE.  Steering flow is a bit better, and there is less of an inversion around 500mb.

I certainly don't get to talk about Yuma much, but it's worth mentioning today.  The vertical profile looks pretty good as the PBL is mixed quite deeply, and as always, plenty of CAPE.  Any outflow boundaries that come in from the north could trigger widespread deep convection.  Perhaps a repeat of a few nights ago?

By early evening, many runs have strong storms moving southwest towards the Colorado River valley.  A few storms roam around eastern Arizona and south of Tucson.  

Later in the evening, some runs do move/develop storms in the Yuma vicinity.  SE Arizona also remains active, with storms close to Tucson. 

It's possible that Tucson will see a few late-night storms.

The potential for late night/early morning activity for Phoenix too, but there is a fairly strong inversion that needs to be overcome at 700mb.  

Day 2

This is something I don't think I've seen before.  The Arizona inverted trough moves south and back into Mexico by early afternoon due to the building ridge north of the state and, perhaps, due to the circulation of the El Paso IT, which is now located in eastern NM.  What really stands out is the mid-level steering flow is now very favorable for moving storms off of the high terrain and into the lower deserts, much like back in July.  In addition, temperatures have cooled a little, with most areas below -6C.

A very moist airmass remains in place over the state, setting up what could be a very active day and evening for many parts of Arizona.

CAPE is moderate to high for much of southern and central Arizona.  CAPE is also sufficient to support high elevations storms.

By later in the afternoon, almost all runs have strong to severe storms over the higher terrain area of eastern and southeastern Arizona and northern Arizona.  Note the storms that initiate off the south end of the Chuska Mountains in NE Arizona.  This happened many times in July, and these storms could propagate all the way to south-central Arizona. 

Tomorrow evening, most runs continue to expand coverage of storms and move them quickly to the southwest and into the lower elevations of southcentral and southeastern Arizona.  

Phoenix has a slight risk for severe weather.  CAPE is greater than 1500 J/kg, and the vertical wind profile is excellent for propagating storms with excellent low-level shear and up to 25 knots of mid-level steering flow.  They are strongly capped, but with a strong enough OFB, it will break.

OFB interactions are forecast to occur, at least in some runs.

There is a slight chance of a major severe weather event for Phoenix.  This particular run is an outlier, so the probability is low.