20190923

Previous Forecast
A flash flood event is occurring in eastern Maricopa County.  Some locations have had over 4 inches of rain.

The 15Z Q3 product is about the same as the observed data.

The 15Z WRFRR and dev runs were quite active and predicted a lot of rain, but were just south, in Pinal County.

Initializations
As discussed yesterday, the morning activity was probably enhanced by the jet streak dynamics.  Flagstaff had 90 knots at 300mb.  Pretty good for early fall.  On the backside of the trough, winds are even stronger as Reno reported 115 knots.   This backside jet will continue to dig the trough to the south.

Winds at 500mb are also strong with 60 knots reported.   There is also a pronounced temperature gradient with -4 at Tucson and -16C at Reno which results in some cold air advection over southern Nevada and into western Arizona.  Initializations of the upper-air features are accurate.

It was a complicated situation at 12Z, as there were ongoing showers and storms, but the RR and NAM did a pretty good job.  The 6Z WRFGFS didn't have enough activity forecast for 12-15Z so won't be of much use.  NAM and RR PW initialization were too moist in northern Sonora and southern Arizona which is a little worrisome as the errors are around 5-8mm. 

Day 1
Supercell storms are already underway over western Arizona as of 16:30 and SPC has western Arizona in a "Enhanced" risk.  I don't know if that's ever happened before!  By the looks of the Yuma sounding, I can see why.

Strong low-level southerly flow continues into the early afternoon, advecting moist are into much of Arizona.

Extremely high CAPE is forecast for western Arizona for the late morning hours and into the early afternoon.  CAPE also increases over central Arizona as moisture increases.

Both the WRFRR and WRFNAM expand the coverage of the storms into the afternoon hours.  Storms generally remain north of I-10.  With a strong southerly flow feeding new deep convection, some rainfall amount could be extreme.

Note that the below graphic includes the morning activity over eastern Maricopa.

By late afternoon, the upper low is located over Vegas and puts northern and central Arizona in an area of strong upper divergence.  This will result in continued organized strong to severe storms over that area.

There is a lot of noise in the deterministic deep convection forecasts, so it's a bit hard to figure out when and where.  However, some of the runs consolidate convection into a broken line with embedded supercells.  This line moves eastward and impacts the Phoenix area by late afternoon.  Forecast reflectivities are extremely high, so some storms could produce large and damaging hail.

The late afternoon forecast Skew-T for Phoenix indicates very high CAPE, over 2000 J/kg.  Directional shear is minimal, but speed shear is good.  Probably no tornadoes, but as mentioned above, large hail is likely with some storms.  I do have to admit that I'm somewhat out of my element here as Arizona sees so few of these sorts of events.  I can't remember the date, but 10-15 years back, Phoenix had an extreme hail event and this kind of reminds me of that.

I'm seeing some severe winds, most likely from wet microbursts, for the Phoenix area too. 

This evening, the low is just to the SW of Vegas and seems to be digging even more to the west than was previously forecast.   It is also now nearly cut off from the main branch of the westerlies.  With this more westerly track, southern Arizona is in for a lot more activity over the next few days than was previously forecast.  I'll cover that in a second discussion.

The threat for strong to severe storms continues for much of southern Arizona into the evening hours as CAPE continues to be moderate to high.

It looks like Tucson will start getting some activity as CAPE is very high and mid-level temperatures have cooled.  Like Phoenix, directional shear is poor, but speed shear is good, resulting in some organization to the storms.

Of special concern is west of Phoenix where most runs are redeveloping storms.  A major flash flood event is possible.  Storms continue to display supercell characteristics into the late evening hours, so the threat for large hail, isolated severe winds, and very heavy rain continue.