20200814

 Previous Forecast

It was a much more active day over southern Arizona, and storms managed to form as far north as just west of Phoenix.  There were some areas of over 2" of precipitation.

WRF really struggled as it didn't have nearly enough activity.  I've seen this issue before where on "marginal" days, it can significantly underpredict precipitation.  The WRFRR continues to be especially bad for some reason thus, I'm going to give a higher priority to the 12Z WRFGFS.  The HRRR morning runs had some activity west and southwest of Phoenix, so there may be something wrong with the WRF configuration when it comes to the RR.   The WRFNAM generally had the right ideas but was too light, as did the WRFGFS.  All runs missed the activity that developed west of Phoenix.  

I've been keeping an eye on the GOES derived CAPE product, and yesterday, it had an area of CAPE which corresponded pretty well with the regions of thunderstorm activity.  

Discussion

The center of the 500mb anticyclone is centered over Arizona, resulting in weak midlevel flow and temperatures from -5 to -7C.  The NW CONUS trough is on the move and will allow the anticyclone center to move well north of Arizona over the next few days.  Finally!

The Tucson morning sounding data looks pathetic as winds are light and variable all the way to 250mb, and Mixed Layer CAPE is practically 0.  The good news is there are no significant inversions, except for the morning radiation inversion.  The residual PBL from yesterday is very deep and goes almost to 500mb.

The GOES CAPE product shows no CAPE over the state, so no repeat of yesterday.

Initializations

Skies are mostly clear for Arizona except for a dissipating band over southeastern Arizona.  All initializations did a pretty good job with cloud locations, but the NAM clouds were too thick.  PW initializations have only minor errors, including the NAM.  The end result is all the initializations look quite accurate, thus there is no favorite, except to say the WRFRR will probably not have enough activity.

Day 1

Dry air dominates northern and central Arizona, while marginal moisture for storms is present over southern Arizona.  Very moist air isn't far away, just across the Arizona/Sonora border.

This results in mainly low to no CAPE over the state.  High CAPE is lurking just south of the border, just waiting to jump the wall. 

I can't totally rule out any activity around the Tucson area as the afternoon Skew-T has a bit of CAPE and a decent wind profile.  The convective boundary layer is also mixed very deeply,  nearly to the LFC (550mb).

Isolated thunderstorms develop along the border by late afternoon.  This is about the only area of significant activity.

The WRFGFS has a bit more activity farther north.

Day 2

Finally, there is some real action on the horizon.  There is a tropical system organizing south of Cabo this morning.  Overnight, it is positioned in a favorable location to trigger a Gulf Surge, which will begin to advect very moist and unstable air into Arizona.

By midday, the Surge continues to advect wet air into the state.  This may be only the second classic Gulf Surge we've had this summer.

CAPE is high to extreme much of southwestern 1/3 of the state.  As is typical with the first day of a surge, a significant cap likely exists.

I don't think "significant" describes the intensity well enough.  How about "extremely stout"?

The question to answer is if/where this cap can be overcome.  It's usually somewhere in southeastern Arizona.  The cap does erode away during the afternoon at Nogales as the CBL deepens, so it looks like a good chance for big storms by late afternoon.  The wind profile is favorable for steering storms from the higher terrain.

  

Strong to severe storms develop along the border and southward in northern Sonora by late afternoon.

Could the Tucson area see any activity?   The cap becomes weak by late afternoon, but the CBL is not mixed anywhere near deep enough.  It will take a strong outflow to trigger deep convection.

Day 3 preview.