20200908

 Discussion

An unusually strong trough is dropping south into Arizona this morning and will interact with a little moisture that has moved into the state from Mexico.  

Initially, I wasn't going to write a discussion as moisture was forecast to be limited, but that has changed.  Moisture increased to 36mm overnight and is currently still quite high, at 28mm.  Phoenix has increased to around 28mm too.  MLCAPE is very impressive at 2200 J/kg, but there is a gradual inversion from 850 to just above 700mb.  There isn't much shear to speak of, but as you go north, it improves considerably.

One of the big unknowns today is all the smoke.  There is so much over central and southern Arizona that it is acting like a thin overcase of normal clouds.  We use the NASA GEOS5 AOD product in WRF, but it was unable enough reduction in solar irradiance.  This reduction is likely to keed temperatures a few degrees below the forecast highs, which may impact any possible convective development.

Day 1

Strong southerly flow ahead of the approaching trough continues to advect moderate amounts of moisture into southern Arizona.  The front continues to push south, ressulting in mosture convergence along the boundary. 

CAPE also increases and is very high in a narrow stripe, just ahead of the front.  We'll see in a little bit what the cap/inversion looks like.

It's going to be close at Phoenix!  The cap slowly erodes away during the early afternoon, resulting in a narrow window where deep convection and severe storms are possible.  CAPE is a high 1800-2400 J/kg, along with some directional and speed shear.  The slight reduction in surface heating due to the smoke may make a difference between storms and no storms, at least in the Phoenix area.

My guess is that no storms will form as none of the dertimistic runs have any.  Add in the issue with the heating, and that makes it even more unlikely.  Still, with the above sounding, one can't rule them out.  Storms do develop just north of Phoenix and over far southeastern Arizona during the afternoon hours.

 A few runs develop deep convection just to the east of Phoenix.  I took a look at the HRRR and it was very similar.  the HRRRx was more like the WRFGFS and WRFNAM which kept deep convection farther to the northeast of Phoenix.

Strong to severe storms continues over far eastern Arizona into the evening.

Day 2

The trough becomse a cutoff closed low over northern Arizona.

The front stalls out over southeatern Arizona, and ahead of it, moisture and CAPE continue to be sufficient for some storm activity, especially in southwestern NM.