20200721

 Previous Forecast

Another MCS developed over northern Sonora last night and moved mainly along the border.  Outflows were able to trigger additional development in the Tucson area after midnight.  This is very similar to what some of the WRF runs were forecasting. (see previous discussion)

The WRFGFS and WRFHRRR did well predicting the secondary development from the MCS.  QPF was also quite good, especially with the MCS.  The WRFRR had locations and timing mostly correct, but not nearly enough precipitation.  I'll likely be switching the cloud MP scheme for RR runs soon as the Milbrandt-Yau is the better scheme for QPF.

Discussion

The center of the 500mb anticyclone is over northern Arizona, resulting in favorable easterly to northeasterly flow over southern Arizona.   Despite 5970m heights, temperatures are still not as high as they could be, like -4 to -5C.  

An easterly jet continues to be present across northern Mexico and provides favorable upper divergence over far northwestern Mexico.  This is part of the reason for the nightly MCS formation.

Tucson's skew-t has a nearly saturated profile, with a high PW of around 45mm.  A heavy rainer sounding.  CAPE is also pretty good, and I estimate it to be about 1000 J/kg.  There is a slight cooling below 850mb, but it won't take too much heating to get storms going.  Steering flow is optimum for steering storms from the high terrain of eastern Arizona into the lower deserts of southern Arizona.  Anvil steering flow is about 90 degrees different from mid-level flow, so anvil shading shouldn't be too much of a problem.

As Phoenix is close to the center of the 500mb anticyclone, winds are relatively light, but there is some favorable northeasterly steering flow.  There is a decent cap on top of a shallow mixed layer. Thus, it will take some work to get deep convection going.  

The WRF forecast from yesterday had moisture starting to decrease for western Arizona due to the surge weakening.  It appears the opposite has occurred, at least for now, due to another MCS-induced outflow surge.  PW has increased in Phoenix and to the south and is in the 45 to 50mm range.  Puerto Penasco is above 50mm.

Initializations

The NCEP runs are in for quite a challenge this morning as widespread clouds and precipitation continue over much of the southern 1/2 of the state.  The 6Z HRRR and GFS have initialized very poorly as by 12Z, the runs have almost no clouds over the state.  The 9 and 12Z RR initializations are much better and are fairly accurate.  All have the MCV initialized to some extent.

Day 1

Dry air advection does kick in this afternoon over western Arizona.  The 850mb dewpoint and PW all decrease at Phoenix, making it unlikely for any activity there.  Southeastern and southcentral Arizona look favorable as dewpoints manage to stay around 12C.

CAPE is moderate for much of SE Arizona and up towards the Rim, so these are likely to be the active areas today.

It looks like the capping inversion must still be with us.  Only limited activity is forecast by all the runs this afternoon.

There isn't much of a cap, but the mixed layer is shallow and not anywhere near the Level of Free Convection at 600mb.   Nevertheless, the forecast Skew-T is concerning as it's good in most other ways: CAPE, PW, and the wind profile.  Concerning because none of the model runs have any activity forecast for Tucson.

Scattered storms continue into the evening, mainly in far southeastern Arizona.  And that is pretty much it.

Day 2

Unless another MCS develops, the surge weakens and results in some drying.  850mb dewpoints are marginal, around 7 to 9C.

CAPE also decreases and is mainly less than 1000 J/kg.  It looks like only SE Arizona will have any chance of storms tomorrow.

A few runs do have activity in and near Tucson by later tomorrow afternoon.