20220716

 Previous Forecast

Strong to severe storms developed west of Tucson, and scattered storms with heavy rain were present over much of northern Arizona.

The WRFHRRR/RR did an ok job around Tucson but didn't have enough activity over northern Arizona.  The WRFGFS was better.

Discussion

It looks like a big day in/near Tucson and Phoenix, so I'll get right to the good part.  The Tucson sounding, finally, has a decent amount of MLCAPE, 1000 J/kg.  Another thing that stands out is the DCAPE, which is 1890 J/kg!  Winds could be better, as there isn't much shear, but steering flow is good, and anvil level winds are weaker, so anvils shouldn't go far out ahead of the storms.

Phoenix doesn't have quite as much CAPE, but it is still pretty good for 12Z.  The wind profile is similar to Tucson.  Like Tucson, DCAPE is very high, 1700 J/kg.  There is a high threat of strong to severe winds with storms.

Another MCS developed in Sonora last night and has resulted in an outflow-driven surge this morning.  Winds in southwestern Arizona are all out of the south, but dewpoints haven't come up too much.  Nevertheless, it's important to set up the surge flow so low-level winds will become westerly and provide low-level shear and inflow.  GPIPW is trending up, and GOES CAPE is also increasing over SW and SC Arizona.

The large mid to upper trough is over NW Mexico, so winds over Arizona are slightly difluent/divergent.  WV imagery indicates the trough is moving WNW.   On paper, it certainly looks like everything is coming together for a significant event.

Initializations

It's clear over Arizona, so cloud initializations are all good.  The 6Z HRRR was missing the Sonoran MCV, but the other initializations all got it.  Overall, all the initializations look good, and model confidence is high.

Day 1

By early afternoon there is a textbook low-level Gulf Surge wind field over southern Arizona.  PW isn't as high as it could be, but it is still enough.  The surge has pushed the dry air back to the Arizona border, at least for now.

The Gulf Surge wind field is also well established at 850mb, and there is a low-level convergence zone somewhere east of Tucson, so this is probably where deep convection will initially form.

CAPE is moderate to high over much of the southern 1/2 of Arizona.  Central Pima County seems to be the hotspot again today as CAPE is >1500 J/kg and is located near the maximum of GOES-derived CAPE.  Another hotspot is over the Phoenix area, where CAPE is >1000 J/kg.

The vertical wind field improves significantly for Tucson as low-level surge flow from the NW establishes by the early afternoon.  Shear is present as mid-level flow is ENE at 20-30 knots.  A surge often results in cooling of the lower troposphere and a capping inversion.  This does not seem to be the case today.

It's about the same story at Phoenix, except a weak capping inversion is present, so a strong OFB or OFB intersection will be needed to force deep convection.  CAPE is an impressive 1600 J/kg.  

Afternoon250mb flow is complicated and hard to decipher.  Generally, weak difluence/divergence is present over much of eastern and central Arizona.

Despite what appears to be a favorable afternoon verticle profile at Tucson, storms mainly develop over the higher elevations.  A few runs do have storms in the Tucson area, though.  Sometimes one shouldn't just go on the WRF deep convection forecast but use the Skew-T instead, and based on that, Tucson is likely to see strong to a few severe storms this afternoon. Severe storms are likely for much of the southern parts of eastern and central Pima County this afternoon.

It's not clear how the severe weather situation plays out for Phoenix.  Earlier runs developed deep convection by late afternoon to the east and moved it into the valley by early evening.

The 12Z WRFRR develops storms over and to the west of Phoenix this evening, with another line forming to the east.

The storms east of Phoenix move in later this evening.  Despite the details being fuzzy, the ultimate message is strong to severe storms at some point for Phoenix this evening.

One thing all runs agree on is that severe winds are likely with many of the storms.  This idea was reinforced by the morning DCAPE values from the soundings.  Sustained winds above 40 knots, with gusts above 60 knots.

The last 4 runs all have some amount of activity in the Phoenix area, as shown by the precipitation plot.

Day 2

There are two reasons for a probably down day.  The first is the old rule: a down day typically follows an active day as the atmosphere is worked over.  The second is that the surge weakens and allows the dry air in NM to penetrate much of eastern Arizona.