20210809

 Previous Forecast

Parts of SE Arizona got slammed by strong storms and heavy rain.  Tucson was out of luck as anvils from storms to the SW again put the kibosh on storms that tried to move in from the east around sunset.

Despite what seemed to be fairly accurate initializations, the run really struggled.  None of them were much good in Cochise County.  The 6Z WRFRR (lower left) was the least bad but had heavy precipitation in Mexico too far to the west.  

Discussion

The 500mb anticyclone center is elongated from Phoenix to Texas.  A pronounced inverted trough is located over SE Arizona and is responsible for widespread clouds, showers, and thunderstorms.  

The Phoenix sounding indicates that the storms south of town are supported by elevated CAPE of 1500 J/kg at 750mb.  I would assume these storms will continue to move towards Phoenix during the morning hours.  The IT can be seen in the 10 to 20 knot SE flow in the mid-levels.

The Tucson sounding exhibits the worked over "onion" profile, resulting in minimal MLCAPE.

Initializations

The NCEP initializations certainly have a big challenge getting the initial conditions correct today.  While not perfect, they all did an adequate job.  The 9 and 12Z RR were the most accurate as others were missing the ongoing storms south of Phoenix.  PW was initialized accurately except for the 6Z NAM and the 9Z RR, which was somewhat too dry over southern Arizona and into NM.    

Day 1

More than enough moisture is present this afternoon for all of the southern 1/2 of the state.  PW approaches 50mm in some areas.

CAPE is minimal over much of SE Arizona, but there is a strip of higher CAPE from Maricopa County into Gila County.  

The irradiance forecast has mostly clear skies along the northern parts of the higher CAPE, so heating should be sufficient for some storms to form.  

The IT is located over Pinal County this afternoon. It will result in ENE steering flow for that area, so storms that form over the higher terrain may be steered to the lower elevations of northern Maricopa County.

There is no model consensus regarding late afternoon and evening storms.  One extreme is the 9Z and 15Z WRFRR as they develop strong storms NE of Phoenix and moves them into Maricopa County this evening.  The 15Z HRRR is also similar.

15Z HRRR 6Z forecast

Most WRF runs have significantly less activity, so it seems to be a toss-up for what actually occurs.

 Some runs are developing early morning storms for central Arizona.  The Phoenix Skew-T indicates some elevated CAPE, which would support this idea.

Day 2

The IT slowly moves northward and is over northern Arizona tomorrow afternoon.  This puts much of southern and central Arizona in southwesterly mid-level flow and temperatures around -6C.  Generally, on the backside of a departing trough, convection is suppressed due to large-scale descending motion.

 

Moisture decreases, but there continue to be moderate amounts of CAPE.

The PBL does mix deeply by late afternoon due to more sun, but there is a cap at the top, likely to keep deep convection to a minimum.  Westerly flow also dominates the profile with dry air above the PBL top, also unfavorable.

Storms are likely to form in the more favorable area of NW Arizona and maybe a few over the far SE.

In many runs, Tucson doesn't look very good for storms either, as CAPE is low and winds are unfavorable.

A few runs are more active.  The 15Z WRFRR has more CAPE and a nearly saturated profile tomorrow afternoon.

This results in a lot of storms scattered about over southeastern Arizona.