20160831

Previous Day

A bit more activity in eastern Phoenix than forecast by most runs.  The WRFNAM had the most and turned out to be quite good.

Initializations

It’s quite cloudy along with a few showers across far SE Arizona and into SW NM as the IT over Sonora seems to be providing some synoptic scale lift.  The NAM cloud cover was quite accurate while the other two runs did not have enough.  Elsewhere there were little or no clouds.

A broad IT continues to be located over NW Mexico and has moved further to the east into far SW NM.  There has been some warming of the mid levels recently and southern Arizona is around -6 to -7C @500mb.  Very warm over far southern Baja into mainland Mexico with -2C@500mb.  The NAM and GFS have the 700-500mb IT initialized well.  The consistent presence of the Guaymas upper air sounding data has certainly been a great help in initializing and verifying these sorts of features this summer.

There was quite a bit of deep convection over central Sonora last evening which has left behind a blob of wet air.  Unfortunately, the 12Z NAM is struggling with its IPW initialization as it’s quite a bit too wet in the area.  The GFS and RR are better.

Each initialization has a problem so model confidence is only moderate.  Also, no clear favorite except to say the WRFNAM has performed well recently and may be OK in Arizona as the IPW errors are in Sonora.

Day 1

Moisture continues to increase as easterly flow continues to transport wet air from southern NM into Arizona.  There was even a shallow outflow surge at Yuma where the surface dewpoint climbed into the low 70F range.

Looking at the first few hours of the runs, the WRFGFS and RR seem to have the best handle on the moisture as the WRFNAM is somewhat too wet around southern Arizona.  By early afternoon, moisture continues to increase and is quite high over much of the southern part of the state.

850mb dewpoint temperatures are as high as 14C over SE Arizona.

As Bob Maddox pointed out, mid level temperatures continue to slowly warm. Winds continue to be weak and become mainly southeasterly so less favorable for rim storms to propagate into the Phoenix area. Higher terrain storms that form over SE Arizona could move into the Tucson area.

Models are similar in predicting that the highest CAPE will be across SE Arizona.  

All three of the 12Z runs have scattered strong storms over SE Arizona during the afternoon.  This might be somewhat overdone due to the lack of model clouds. (WRFNAM below)

The Tucson afternoon vertical profile has some CAPE present along with a well-mixed PBL.  The inversion around 600mb, as seen on the morning 12Z sounding, continues and may keep deep convection in check.  This inversion is missing in the WRFGFS and RR.

Scattered storms are roaming around eastern Pima county by later in the afternoon.  With only 500 J/Kg of CAPE to work with, storms are not that strong.  Still, a wet microburst could produce some isolated strong winds.(WRFNAM below). The WRFNAM is in the middle with the WRFGFS having more activity while the WRFRR has less.

Activity quickly decreases around sunset.

Below is the experimental lightning flash density forecast.  It shows that the storms in Cochise county could have quite high lightning activity associated with them.

Day 2

CAPE/moisture continue to be sufficient to support deep convection over SE Arizona and up towards Yavapai county. The model runs are considerably different as the WRFNAM (below) has less CAPE than the WRFGFS.

In either case, activity shifts to the White Mountains and up along the Rim to north of Flagstaff.

Maybe some early evening activity over SE Arizona?