20190803

Previous Forecasts
It was a bit more active for higher terrain areas of the state compared to the previous day.  Generally, the model forecasts underestimated the amount and intensity of activity.  One glaring error was in central Pima County as some severe storms developed during the late afternoon.  The 15Z had a little activity there but wasn't nearly enough.  It did poorly with the storms in Yavapai County too.   Generally, all the runs didn't have intense enough storms, and something with the model configuration is struggling in this regime. 

The WRFNAM was a bit better with northern Arizona precipitation amounts and coverage while completely missing the central Pima County activity. 

Initializations
The 500mb anticyclone is sprawled out over the Four Corners region resulting in generally weak southerly flow over the state. Yesterday's forecast called for mid-level easterly steering flow for today.  It is around 1C cooler, so at least that is an improvement.  The wind field indicates that perhaps there is a weak IT in the Phoenix area, but I didn't see anything on the satellite imagery.

Model initializations are generally good with the RR taking the prize for the best as it had the lowest PW errors.

Day 1
Moisture appears to be sufficient to support storms as PW is around 35 to 40mm as weak and shallow southerly flow up the Gulf of California continues.

850mb doesn't look as good as Td temperatures are marginal at around 10C.  I'm also concerned about the dry air advection over central Arizona decreasing available moisture in the convective boundary layer.

This drying is responsible for the lack of CAPE in Maricopa, Pinal, and parts of Pima Counties.  Tucson is right on the edge for activity, so it will be a problematic forecast there.  (what else is new)  All model runs are similar to the placement of this wet/dry boundary.  The WRFRR has the lowest amount of CAPE over eastern Arizona, but it's still very high.  The WRFGFS and WRFNAM forecast CAPE to be over 2500 J/kg!

Both the WRFRR and WRFGFS forecast light northeasterly flow by early afternoon resulting in some movement of storms off the higher terrain of east-central Arizona towards southeastern parts.  Temperatures are slightly cooler too, at around -6C.

 The various runs are similar as they develop strong storm over the high terrain of eastern and southeastern Arizona this afternoon.

It's looking marginal for Tucson as CAPE is forecast to only be around 300-600 J/kg.  Plus, 250mb winds are strong and will shear anvils off from activity to the SE over the valley, much like yesterday.

Storms continue to slowly move to the southwest and by early evening, develop/move into Cochise county and maybe far eastern Pima county. 

The WRFGFS and WRFNAM forecast storms developing over SE Arizona rather than moving in from the northeast.

Tucson might get lucky as a few storms might develop as strong outflows move into the valley.  It looks more likely there will just be strong winds and some cooling.  Storms that do form may produce very strong to isolated severe winds as the PBL is so deep.

The 15Z WRFRR is now available, and it's similar to the 12Z run as it develops afternoon storms over both far southern Arizona and eastern Arizona.   By early evening, storms move/develop in Cochise County.  That really looks like the hotspot for later today.

Tucson might get lucky as a few storms manage to form as strong outflow moves through.

Day 2
The mid-level flow continues to be light and variable for most of the state.  500mb temperatures continue to be around -5 to -6C.

850mb moisture decreases slightly and is marginal for storms for low and mid-elevations.

This slight drying results in only low to moderate CAPE, so thunderstorm activity is expected only over the higher terrain.  However, there is little confidence for the forecast as there is quite a bit of spread as the WRFGFS has considerably more CAPE/moisture.

All three runs do manage to develop a few storms in far SE Arizona tomorrow evening.