20211005

 Discussion

Moisture has increased significantly since yesterday and is now around 30mm for most southern Arizona locations.  Showers and some thunderstorms are present over much of the state this morning as the moisture is interacting with the approach of the California closed low, which is now near San Diego.

Impressive upper divergence is mainly responsible for the shower and thunderstorms activity ahead of the low.

Surface-based CAPE is non-existent, but both the Tucson and Yuma 12Z soundings indicated 6-900 J/kg of elevated CAPE,  thus the ongoing thunderstorm activity, especially in SW Arizona.

Initializations

The NCEP initializations have their work cut out for them today, for sure.  To my surprise, all the initializations were fairly good at the placement of morning activity and clouds.  The best was the 12Z HRRR, as the other runs didn't have enough/any activity in SW Arizona.  All initializations had minimal PW errors.  The 12Z WRFHRRR appears to be the best initialization, but none are poor enough to exclude.

Day 1

So far, so good, from a model accuracy perspective as the band of clouds and showers moved across the state.  This band continues to move north, while activity continues over southwestern Arizona.   PW remains around 30mm into the early afternoon hours.  850mb dewpoints are highest in a band from far SE Arizona, along the base of the Rim, up towards Payson/Prescott.  A (near) surface low didn't seem to materialize over south-central Arizona, but moderate low-level SE flow is present in the area of higher dewpoints, resulting in a favorable vertical shear profile.

CAPE is quite high in the same areas, especially for this time of year.  Some areas are approaching 1500 J/kg.  Along with quite cool air in the mid-levels, some storms could have large hail.

By early afternoon, the mid/upper low begins to eject to the northeast, across NW Arizona, resulting in moderate to strong upper divergence/difluence over much of the state.  Everything is in place for a big severe event for parts of the state especially if there is some heating.   Satellite trends do indicate clearing over central and northern Arizona and the model runs generally have the right idea.

The afternoon vertical profile forecast is looking good for organized strong to severe storms in/near Phoenix.  There is both directional and speed shear, and >1000 J/kg of CAPE.   

Tucson doesn't look as favorable as there is limited directional shear and not much speed shear.  CAPE is also lower, but perhaps sufficient to support some storms.

The other hot spot for activity is eastern and far southeastern Arizona.  The directional shear is very good, as well as CAPE.

By mid-afternoon, strong to severe storms develop over southeastern Arizona as well as parts of Yavapai County.  RADAR reflectivity indicates large hail with some of the storms.  

Storms begin to develop in and around Phoenix by late afternoon, with some potentially severe storms.  Storms also move towards the Flagstaff area and continue in eastern Arizona.  Most runs are similar so forecast confidence is high.  

Besides hail, some storms could produce high lightning flash rates.  A tornado or two is also possible with a few of the storms, especially the ones north of Phoenix where shear is the best.

15Z WRFRR doesn't have as much activity for the Phoenix area.

Strong storms continue into the evening hours for far eastern Arizona and areas around Flagstaff.

The event is still on track to produce widespread moderate rain over northern Arizona, and some areas approaching 2".

202101004

 Discussion

A large mid and upper cut of low is slowly making its way towards the state, which will result in a transition-type event including the risk for severe weather over parts of Arizona tomorrow.  WV imagery and upper air plots place it SW of San Diego.  This has already resulted in an increase in moisture over the state as PW has increased to around 20mm for many locations in southern Arizona.  GOES-derived PW is increasing over Sonora as well, but no CAPE exists north of Sinaloa.

WV imagery indicates that the low may be a bit further to the south compared to the 250mb analysis.  This is also the case with the morning initializations as they have a position similar to the analysis.

Day 1

PW continues to increase slowly today, and by late afternoon, is in the mid 20mm range over southern Arizona.  

It's still dry at the lower levels, except for far SE Arizona.

Despite limited moisture and low CAPE, many runs manage to develop a few high-based showers and storms around the Tucson area later this afternoon and into the evening.

Low-level moisture continues to increase overnight and by early morning, there is an area of higher 850mb dewpoints from SE Arizona into central Arizona. This will result in scattered showers and storms for many parts of the state overnight.

Day 2

Now the fun begins!  The cut-off low has made its way to around San Diego by 12Z tomorrow, with temperatures as cold as -17C.

Moisture continues to increase and by 18Z, it is around 25-35mm for much of the southern 1/2 of the state.  In addition, a weak surface low is present over south-central Arizona, resulting in low-level southeasterly flow over eastern Arizona.  With the mid-level winds from the SW and low-level winds from the SE, there is going to be significant low-level directional shear.

The lower troposphere also moistens up, especially in a band from far SE Arizona to around Flagstaff.  This results in a band of moderate CAPE which is more than enough to support strong to severe storms, despite earlier widespread showers and storms.  Most of the model forecasts move the clouds and showers into NE Arizona during the morning hours, resulting in heating and recovery.

The forecast vertical profile exhibits classic severe weather and mini super-cell event for parts of Arizona later tomorrow afternoon.  CAPE is surprisingly high for the Phoenix area and the amount of directional and speed shear low topped organized severe storms are possible, with frequent lightning, hail, and some strong winds.

The various forecasts are in general agreement that southeastern Arizona and areas north of Phoenix are most likely to see strong to severe organized storms at some point tomorrow afternoon and into the early evening.  It's not possible to be more specific as the situation is so complex and dependent on how much activity occurs tonight, and on how much heating will be realized tomorrow.

48-hour precipitation amounts could be over 2" for the orographically favorable locations of northern Arizona.