Previous Day
The trend of the previous 24 hours of less activity continued and it turned out that most of the runs even at 12Z forecast too much activity.  However, it was a tricky situation with the dry/wet boundary location being difficult to exactly forecast.  It turns out the the WRFRR did do quite well both in Tucson and Phoenix.  It had only a moderate outflow through Phoenix and kept the thunderstorms just to the east of Tucson.   Areas just east of Tucson had strong storms as well as east of Phoenix.

I'm a bit short on time this morning thus I can only do a quick look.  NAM IPW initialization is excellent and OK in the RR and GFS.  Most of the eastern two thirds of the state is clear and all three models are the same.   The mid level inverted trough is now quite weak and broad and is located over Sinaloa somewhere and it appears the models have it initialized OK.  All three initializations are good.

Day 1
Plenty of moisture around with a weak/shallow surge underway.  Yuma's Td is in the 60's, but the IPW is only 29mm.  It appears that much of SE Arizona has sufficient deep moisture to support storms today as that area has 10C Td and above.

This results in quite a bit of CAPE available.  The models have some large differences between amounts with the WRFGFS (below) being the middle of the road.

Mid level steering is weak at 10-15 knots, but from a favorable northeasterly direction.  Mid level temperatures, especially in NM are quite warm though, around -5C.  So like yesterday, it looks most favorable for storms where moisture/CAPE is higher and mid level temperatures are slightly cooler.

I've also started plotting divergence at 250mb, but the field is a bit noisy as is typical with trying to plot a synoptic feature using a mesoscale.  In any case, areas of divergence are present over Sonora and into eastern Arizona.

The WRFGFS (below) and WRFRR are similar in developing storms over the higher terrain by early afternoon.  The WRFNAM is much more aggressive and has much more activity, which is the less likely solution.

The Tucson forecast vertical profile is marginal as there isn't much CAPE and as Bob mentioned, inversions in the mid levels.  However, the shear profile is a bit better with low level NW winds, mid level easterlies, and SW aloft.  The best way to get storms in the Tucson area is with this wind profile and the issue is typically, NW winds dry out the PBL.  As a weak surge is continuing, hopefully the low level air mass to our NW will continue to moisten.

By late afternoon, storms converge on SE Arizona from the NE and E.  Note that convection is all around Tucson and not in the valley proper.  Typical!

The inverted V profile is not as large today so the risk for widespread strong outflows will be less.  However, localized strong/severe winds may be associated with individual storms.  The mixing height is not nearly as high across much of the state as it was in previous days.

There is little or no CAPE available in the Phoenix area and no activity is expected there.

Day 2
Moisture continues to increase due to the ongoing gulf surge and by the afternoon, Td@850 has increased across much of the central and western parts of the state making this a much more active day than today.

The mid level circulation becomes more easterly across much of the state as the 500mb high moves north.  Still, quite warm air is present with mainly -6C at 500mb.

The WRFGFS (below) is still the middle of the road model and forecasts a decent amount of CAPE across all of southern Arizona.  This will very much help sustain storms that come off of the high terrain to the east and northeast of the lower deserts.

As typical on a surge day, it takes awhile for storms to get going.  By late afternoon, areas of deep convection are moving off of the White mountains into SE Arizona.

By late afternoon, the Tucson vertical profile is mixed deeply and nearly ready for deep convection.  CAPE isn't great, but the steering and directional shear profiles are good.

It's a much different story in Phoenix as the surge has cooled the PBL and it would take strong outflow intersections and convergence to get things going here.  Like Tucson, the wind profile is excellent.

All three 12z runs move strong storms into eastern Pima county by early evening so it looks likely that Tucson will see storms in and around the valley tomorrow.

Mike Leuthold
Manager, Regional Weather Modeling Program
University of Arizona, Atmospheric Sciences