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.
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
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
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.
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
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
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.
Manager, Regional Weather Modeling Program
University of Arizona, Atmospheric Sciences