20210816

Previous Forecast

 A few strong/severe storms formed around Flagstaff and moved to the southwest, all the way to Yuma.  Other strong storms moved into the east side of Phoenix and then dissipated.

Most WRF runs didn't have this much activity, especially in western Arizona.  The only close one was the 9Z WRFRR, and I thought it had too much activity.  The various morning runs of the HRRR had no activity, so the WRF was in good company.

Discussion

The 500mb anticyclone is centered over Nevada while a trough drops down the backside over western Colorado, towards Arizona, while the weaker NM IT is still present, near Albuquerque.  An MCV is located over far southwestern Arizona.  Temperatures are quite cool over central Arizona as Phoenix reported a -8C.

At 250mb, the trough results in west to northwest upper flow, good for advecting anvils away from the storm motion.  There is also favorable divergence/difluence over northern Arizona.

PW has generally decreased since yesterday, and why wouldn't it as we were at a record high amount.  Phoenix has decreased to a still-high 48mm and Tucson at 40mm.  Tucson has a high MLCAPE at 1650 J/kg.  The vertical wind profile is good with westerlies aloft and a good mid-level ENE steering flow of 20 knots.  There are no low-level westerlies at the moment, so that is one negative.  At least it's not easterly.  There are minimal inversions, so it looks like it's going to be a big day at Tucson.

While not having a lot of MLCAPE, Phoenix has an extreme amount of elevated CAPE of 2100 J/kg!  Like Tucson, the wind profile is great, but they could use a little more low-level shear.  Yesterday, the surge was forecast to pick up by afternoon, so mixed-layer west winds should become stronger.  There is a capping inversion at 700mb, which will prevent any deep convection from initiating too early.  All systems go for lift-off.

Initializations

Storms in the Yuma area are dissipating, and elsewhere it is mainly clear except for a band of clouds across central Arizona.  GOES-derived CAPE is high from about Phoenix, out to western Arizona.  Cloud initializations were mostly accurate by all initializations.  The 6Z initializations missed the SW Arizona clouds and showers, but that shouldn't matter much as it's mainly clear there now.  The 6Z GFS had the NM IT located too far south, near El Paso.  PW was accurately initialized, except for the NAM.  Overall, initializations seem to be good.  WRFRR and WRFGFS are the favorites.

Day 1

By early afternoon, the gulf surge strengthens, resulting in increased moisture and a more favorable lower-level wind flow.  850mb Tds are very high at 15-17C over southcentral Arizona.  PW is in the 42-50mm range.

As one would imagine, with high 850mb Td and -7C aloft, CAPE is high to very high over central Arizona.  As seen earlier, GOES CAPE was very high all the way to the Colorado River valley, so the WRFRR might not have enough out that way.

The 21Z 500mb forecast has the CO trough moving into far NE Arizona, with temperatures around -10C.  Steering flow is northerly to northeasterly.

By late afternoon, all runs have strong to severe storms over the higher terrain of eastern and northeastern Arizona and strong storms developing over the higher terrain of southeastern Arizona.  Many runs also have severe storms in Yavapai County as well.

Tucson has one of the more impressive Skew-T that I've seen.  CAPE of around 2000 J/kg, some low-level shear, NW upper flow, and the PBL is deeply mixed.  Mid-level steering flow could be a bit stronger, though.  

Phoenix is also impressive as they are forecast to have 2500 J/kg of CAPE by late afternoon.  The PBL is also mixed deeply, but a weak cap will need to be broken.  Everything else looks good as there is a bit of low-level shear, and mid-level steering is a decent 10 to 15 knots from the northeast.

Widespread strong to severe storms covers much of southeastern Arizona by early evening.  Other storms are moving south towards the Phoenix area.  The model runs have different outcomes for the Yavapai storms as some runs dissipate them before making it into the Phoenix area, while others move them in and outflow boundaries start to interact from the activity in Pima and eastern Pinal Counties.

All model runs have the storms taking a more SSW route, and they end up in central and western Pima County by the early morning hours.

 Lightning flash rates are expected to be moderate to very high.  I would expect to see a fair amount of hail with storms.  Widespread severe winds are not expected as DCAPE was 900 J/kg from the morning Phoenix sounding.  However, isolated locations will likely see severe winds from wet microbursts.

Despite fairly fast storm motion, precipitation amounts could be above 2" for some central and southern Arizona areas.

Soil moisture is still high for many locations, so precipitation that falls is likely to mostly runoff.  Flash flooding risk is high.

Day 2

It's likely to be a down day for the lower elevations as the atmosphere will be worked over.  It still looks to be active for northern Arizona and along the Rim to the White Mountains.

CAPE is much lower, but still, a respectable 1300 J/kg at Phoenix.  The wind profile continues to be good, but there is a weak inversion at the top of the PBL, which will take some outflow boundary action to break it.  Not likely.

Tucson has only minimal CAPE, so no activity is expected there.