Arizona WRF Discussion: 20210811

Previous Forecast

Insanity! It was one of the bigger days of the monsoon season so far, especially for SW Arizona. Yuma area PW peaked at 59mm just before the storm moved through. In Tucson, storms moved over the NE side for hours, resulting in a 6.15" and a 5.58" at The Lakes at Castle Rock. I even got in on the action in Paradise, 1.55". A gauge just on the other side of Silver Peak got 3.31". This resulted in big flows in normally dry washes, which up until now, I never saw more than a trickle.

The WRFNAM and GFS were terrible. Little or no precipitation around Tucson and almost none in southwestern Arizona. The GFS did move storms into west-central Arizona, though, but not into southwestern AZ. The WRFRR was much better for Pima County as it did forecast some big accumulations and did a reasonable job moving storms into western Arizona. It was unable to keep them going into southwestern Arizona.

Discussion

This is certainly a strange-looking 500mb map with an anticyclone centered over the far NW CONUS and another off the east coast. Arizona is still under the influence of the inverted trough, which has hardly moved since yesterday and is located over far western Arizona.

The 300mb map is also one of the more unusual ones I've seen. Closed circulations all over the place!

Is it possible for another day of heavy rain in southcentral and southern Arizona? It sure looks that way as it's cleared out over much of southern Arizona. Storms continue east of Phoenix, but it's even clearing there too. I'm not sure how representative the soundings for Tucson and Phoenix are, as storms were active at 12Z. Tucson still has 600 J/kg of MLCAPE and a nearly saturated profile. Steering winds are light from the WSW, but that didn't matter yesterday. It won't take much heating for storms to go again, but they are likely to remain over the higher terrain with weak steering.

Phoenix looks similar.

Initializations

As mentioned above, it's rapidly cleared this morning. All initializations are struggling as the 12Z RR has too thick clouds over NW Arizona but looks pretty good for southern Arizona. It was also able to initialize the area of storms around Phoenix. The NAM, also too thick over much of eastern Arizona and is missing the Phoenix area activity. The 6Z GFS has the Phoenix area activity but was a bit too cloudy over southern Arizona. 9Z RR was missing. PW errors are larger than usual due to ongoing activity and are about 5-8mm at some Arizona locations in all initializations. With all the issues, model forecast confidence is low.

Day 1

It's going to be a challenging forecast day, but I'll give it my best shot. A surge continues to advect very moist air into Arizona, with 850mb dew points a very moist 14-16C this afternoon. The surge signal (SE winds) is strong even at 850mb.

It does appear that the large circulation of Kevin got close enough to trigger the surge. TS Linda is following behind Kevin and appears to be on the same track.

Western Arizona looks to be worked over, but central and southeastern Arizona have moderate to high CAPE, so those areas could be active today.

Storms struggle to get going, and only scattered storms are present over eastern and southeastern Arizona this afternoon. I would have expected more, but maybe the atmosphere is just too worked over, and of course, the day after a big day is usually down.

Tucson has a decent amount of CAPE by late afternoon, but a cap is at the top of the mixed layer. The wind profile isn't much good, as it's become mainly light and variable, so storms won't come off the mountains. The surge is still going strong with moist WNW flow below 700mb. The morning LFC is only 800mb, and the below forecast has the mixed layer above it, so I don't know why there isn't more deep convection. Something must have changed since 12Z, or the LFC was calculated too low.

CAPE is fairly low at Phoenix but may be enough to support some activity as long as there is an OFB trigger. However, this looks unlikely.

Both the 12 and 15Z WRFRR develop a few storms south of Phoenix this evening.

Some activity continues into the late-night hours.

Due to the relative lack of storm activity, perhaps it will be clear enough in some areas to see the Perseid meteor shower.

Day 2

The lower troposphere dries out a little tomorrow as the surge weakens, but more than enough moisture is present for storms, which are likely to produce heavy rain.

CAPE is again moderate to high over parts of southern Arizona. It looks like far NW Arizona may also see storms.

It looks like another day for mid to high elevation storms as steering flow is weak, but CAPE is high. Tucson is mixed well by late afternoon, so storms are possible.