20210718

 Previous Forecast

Little or no activity was present for Tucson (until this morning) or Phoenix.  NW Arizona was the hotspot for storms yesterday.

The WRFRR runs were quite accurate, followed by the WRFGFS.  However, it was lacking the central-western Pima County storms.  The WRFNAM was bad, as usual.

The 15Z HRRR was also fairly accurate.

Discussion

The big story this morning is the MCS moving across Tucson.  It's moving along fairly fast, so rainfall amounts have generally been less than an inch.  It was somewhat unusual as it appeared in Northern Sonora about 7Z and was not obviously associated with any previous storms (MCV) from the afternoon and evening.  I could find no features in the upper-air maps either.  The western CONUS is dominated by a large anticyclone-looks like summer!  The center stretches from around El Paso up to Salt Lake City, resulting in mainly E to SE 500mb flow for Arizona.  There is an interesting feature that I initially missed, which is an inverted trough over eastern NM.  It's responsible for the ongoing deep convection there and is favorable to move across southern Arizona later.

Tucson, again, had an impressive amount of morning MLCAPE with 1500 J/kg.  None of that matters now, as it's all been wiped out due to the morning storms.  However, the vertical wind field did give me a clue to the reason for the MCS formation.  300mb and above winds were strong, peaking at 40 knots, as a jet streak moved through, associated with a north moving upper-level IT.  This provided the upper-level divergence/difluence for the MCS formation and propagation.  My guess is with this support, the MCS will keep going for a while, maybe making it to Phoenix?  

The various model runs had jet streak a little too far to the west.  This is the WRFRR forecast for 12Z this morning.  Who knows if the model runs would have predicted the MCS if the streak was in the correct location.

There is enough CAPE at Phoenix to support the approaching MCV.  You can also see the winds associated with the upper jet streak.

Initializations

It goes without saying that the MCS is going to play havoc with the WRF runs for today.  The 6Z runs had no clue.  It started to kind of show up on the 9Z WRFRR, but it wasn't nearly strong or widespread enough.  The 12Z RR was better but still not strong or widespread enough.  I was surprised to see that the NAM, of all initializations, actually did a good job.  I guess a broken clock can occasionally be correct.  Moisture has generally decreased, by a lot in some cases, over western Arizona but is impressive elsewhere as PWs over an inch have made it into southern Utah.  Initializations were quite good, especially considering the MCS in southern Arizona.  Even the NAM wasn't as bad as it usually is but still had some 4-5mm errors in western Arizona.  The 15Z WRFRR just started to run, and it did a better job initializing the MCS.  However, it's too far west, and during the first few hours of the model run, it moves it too far west.   Overall, I have little confidence in the model runs today.  The least bad is the 12Z NAM and 15Z RR.

Day 1

Maybe western Arizona has dried out, but central and eastern Arizona sure haven't.  A weak surge is underway, advecting very moist 50mm PW air into south-central Arizona.

The WRFNAM (I can't believe I just wrote that!) moves the MCS near Phoenix by mid-day, which is too slow.  In its wake, the atmosphere will be cooled and stabilized.

Despite not having enough activity associated with the MCS, the 12Z WRFRR apparently has a realistic representation of the CAPE, as it's quite low over much of south-central Arizona.  CAPE is again favorable for NW and SE Arizona.

The WRFNAM develops storms over much of the higher elevations by late afternoon, while little or no activity is over southern and central Arizona.  This makes sense to me.

The 15Z WRFRR is similar in regards to the timing and location of storms.  The remnant MCS is moved too slowly and too far west.  This error shouldn't matter too much as it's close enough and has the same result of stabilizing central Arizona.

The WRFNAM keeps storms going over NW and SE Arizona into the evening hours.

The WRFRR is a lot different for NW Arizona as there is no activity there this evening.  Like the WRFNAM, it keeps storms going in SE Arizona.  The storms might be too far west as that far eastern Pima County may be too stable.  It all depends on how quickly it can clear out.

Both runs develop another MCS over southeastern Arizona tonight, apparently in response to the NM IT that moves over the area tonight.

Some very heavy rain is predicted with this MCS-over 3" at some locations.

Soils become saturated, so there is likely to be widespread flash flooding.

There is a chance that late night/early morning activity could develop around central Arizona, as the air mass is extremely moist, around 50mm of PW at Phoenix.  While there isn't a lot of CAPE, there could be enough to support storms.  Any storms that do form will produce extremely heavy rain.

Day 2

As has been the case recently, the Day 2 forecast uncertainty is sky-high.  One thing that seems certain is that there will be enormous amounts of moisture over central and western Arizona.  Some runs have PW approaching 60mm!

CAPE is low over SE Arizona in the wake of the MCS and perhaps in south-central Arizona, depending on the overnight activity.  Perhaps the White Mountains and Rim will be the active areas tomorrow, but that is just a guess.

The potential for storms is there for both Tucson and Phoenix.  Will there be enough heating and/or outflow boundary interaction to trigger deep convection?  I don't know.  The wind profile for both is excellent with low-level shear, mid-level southeasterly winds, and weak upper winds.

The WRFRR says no.

The WRFNAM has a bit more high elevation activity but nothing lower down.