20200812

 Previous Forecast

I had planned on writing a discussion yesterday, but I had issues I had to deal with.  The first was the RR sequence wasn't working and I spent quite a bit of time trying to rectify the situation.  Then, I got a call from my wife with horse trailer problems.  What I would have said is there was a chance for activity mainly south and west of Tucson where moist air and higher CAPE was present.  There ended up being some very strong storms over central Pima County which slowly moved towards the north where they died out as they approached I-8.  Finally something!

RR was missing for this plot, but the NAM and especially the GFS had predicted this activity.  Both weren't as strong as what occurred as rain estimates exceeded 2 inches.

Discussion

PW has increased over much of the state during the past 24 hours and is finally sufficient to support storms.  It's 40mm or greater over much of southwest and southcentral Arizona thanks to the surge event due to Elida and a large complex of storms in Sonora producing strong outflows.  

The 500mb map has the ridge suppressed to a location over far southern AZ and NM due to another trough over the NW CONUS.  Temperatures are quite warm, around -5 to -6C.

There isn't any CAPE on the Tucson morning sounding, but Phoenix has 1000 J/kg of elevated CAPE.  The surface to 500mb is mainly light and variable thus not much good for storm organization and/or movement.

Initializations

An MCV and associated clouds and a few showers have moved into far Southeastern Arizona.  All morning initializations did an OK job of getting these clouds in the right place.  Only the 12Z RR was able to initialize the MCV circulation though.  PW initialization was a different story.  Both the 6Z GFS and especially the 12Z NAM were too wet over far southern Arizona and into Sonora.  The WRFNAM will be questionable and probably produce too much activity.

Day 1

The surge decreases in intensity and by mid-day, low-level winds are mainly light and variable over Arizona and south over the northern Gulf of California.  Plenty of moisture is present over Arizona as 850mb Td is around 12C.  The circulation around Elida advects moisture northward along the Baja and eventually, some of this may work its way into Arizona.

CAPE increases and is low to moderate over much of central and western Arizona by early afternoon.

As is typical for the first day of a surge, the CBL has difficulty mixing deeply.  One difference is that there is only minimal cooling and no strong inversions on either the afternoon Tucson or Phoenix forecast Skew-T plots.  A strong outflow boundary could result in enough lifting for storms to form.

Model consensus is for scattered storms to develop later this afternoon over the high country of eastern Arizona and over higher elevations of southeastern Arizona.   In spite of CAPE being more than sufficient for storms in central Pima County, not much activity forms there, probably due to the cooling and wetting of the surface due to the previous day's activity.  

The WRFGFS does manage to develop a few storms in eastern Pima County this evening.  It could happen.

Day 2

Moisture generally decreases as the surge ends.  PW drops to the mid 30mm range over the lower deserts and 850mb dew-point temperatures are marginal, around 7 to 9C.

This results in mostly low CAPE across the state.  There should be enough to support some storms over the higher terrain.

And that is about what the WRFRR is forecasting for the afternoon hours.

Storms get close to the Tucson area, but CAPE is marginal  (300 J/kg)  so it's hard to say if they'll make it into the valley.  At least the wind profile has improved as there is some mid-level southeasterly flow around 15 knots.

The wetter WRFNAM has quite a bit of activity in Pima County by late afternoon and then spreading northward, impacting Pinal and eastern Maricopa Counties during the evening.  I find this unlikely due to the wet initialization, but who knows?  Phoenix needs something to look forward to keeping them going!