20210806

Previous Discussion

That was one of the bigger busts of the season so far.  Very little activity occurred over the state, and the only large storms were just south of the border.  

 The WRFRR series was the worst as they were too active, especially around Tucson.  The WRFNAM and GFS were better, but still too much activity north of the border.

The HRRR wasn't much better, but at least it kept activity away from Tucson.

One error was the strength of the inversion at 500mb.  The 0Z sounding at Tucson indicated a much stronger inversion compared to the forecast.  CAPE was also not as high as was forecast.

Discussion

One part of the forecast that seems to have worked out is the call for increasing moisture.  PW has increased at both Phoenix and Tucson and is now in the 35-37mm range.   This has resulted in 1200 J/kg of MLCAPE at Tucson and about 900 at Phoenix.  The Tucson temperature profile looks better as the inversion at 500mb has weakened, and no significant inversions elsewhere.  The winds aren't that great as they are mainly light and variable, but there is a bit of east to southeast winds in the mid-level.  Upper-level winds are westerly, so anvils will move opposite of storms.

As mentioned yesterday, the NW CONUS trough has suppressed the monsoon ridge into southern Arizona.  Temperatures are high, around -4 to -5C over much of the southwest, so generally unfavorable for deep convection.  The inverted trough didn't move much and is located over south Texas and doesn't appear to become a factor for Arizona.  Excluding the somewhat favorable Tucson sounding, it doesn't look good for storms today.

Initializations

There is an area of debris cirrus over southern Arizona, while the rest of the state is clear.  GOES PW shows the areal increase in moisture over southern and eastern Arizona as the drier air retreats into NM.  6Z clouds were initialized well, but the 12Z RR had them too thick.  There are only 4 operational Suominet PW sensors in Mexico now, so it's hard to say how well PW was initialized there.  There are only minor errors at reporting stations, except for the 6 and 12Z NAM.  More US Suominet sensors are reporting now, as they must have slapped some bandaids and duct tape on their server.

Day 1

Yuma surface stations report strong southeasterly winds and dew points in the mid 70's again this morning, as the surge continues.  However, PW is only in the upper 30mm range, so the surge isn't very deep, at least that far west.   The highest values are east of there, from Pima County into southern Maricopa and Pinal.

The surge is forecast to remain strong and continue into the afternoon with the peak moisture from Pima Count up into the Phoenix area.  Most model runs continue to forecast 850mb dew points in the 14 to 16C range, so the potential is there.

Similar to the forecast yesterday for today, CAPE is very high from Pima into Pinal Counties by afternoon.  

The forecast Skew-T is a mixed bag for Tucson.  While CAPE is excellent, around 1500 J/kg, the boundary layer is not deeply mixed in any model runs.  The wind profile has strong low-level shear due to the surge, but only around 5 knots mid-level winds. Thus storm motion will be slow.  Will it be enough, though?  Tucson is going to need strong outflow boundaries to trigger deep convection.

Most runs mix Phoenix deeper, but some have a significant capping inversion.  The 12Z WRFRR is the best looking (so probably not correct) as the inversion is weak.  CAPE is good at around 1500 J/kg, but the wind field is poor with unidirectional westerlies.  Based on just the Skew-T forecasts, it doesn't look good for Phoenix.

All the morning runs have a nearly identical formation of strong to isolated severe storms over the higher terrain of southeastern Arizona this afternoon.

All runs have some sort of activity for Tucson by late afternoon.  The 12Z runs back off a little on the intensity, but a few storms could produce severe weather.

Even the HRRR gets in on the fun. (14Z run, valid 5pm)

The 15Z WRFRR showed up, and it has only a storm or two in the Tucson area.  The message is Tucson is right on the edge of activity, and it could range from not much to big storms.  What else is new???

Day 2

The trough continues to move eastward and, by early afternoon, is located over Nebraska.  This results in moving the center of the anticyclone to an unfavorable position in western Arizona.  Slightly cooler air moves into eastern Arizona.

The tail of the trough brushing by also ushers in much drier air over northern Arizona, and despite the surge continuing over southern Arizona, it dries out there too.

Ooof, this looks bad.  The mixed layer is shallow and dry air is entrained at the top of the layer.  Lower elevation storms look out of the question for the Tucson area.

Storms are limited to the higher elevations of southeastern Arizona tomorrow.