20190808

Previous Forecast
Since I talked about how poor the (WRF)NAM has been, now it decides to spite me by being one of the better runs recently.  It had a fair amount of activity over western Arizona but shifted a little too far north and missed the storms in Yavapai County. It was unable to forecast the blowing dust in Phoenix.  The WRFGFS was also OK.  The rest of the state only saw storms over the higher terrain, which was predicted well by all the runs.

Initializations
Westerlies continue to move into the SW US due to the trough along the west coast.  The high center has been pushed into Mexico, south of Douglas and just east of there, an MCV is spinning resulting in some clouds and showers.  This feature was initialized well by all the models.  A large area of thunderstorms is over the lower Gulf of California and appears to be forced by a large IT located over central Mexico which you can kind of see in the 500mb plot.  This feature is likely to play an important part in increasing thunderstorms for Arizona this weekend.  Only the RR had initialized the storms well.  If convection continues over the southern Gulf, a surge will likely be triggered.

The RR has the best initialization of PW, but it still has some 3-5mm errors.  The NAM has some locations in NW Mexico with over 10mm errors.  The WRFRR appears to be the best initialized with the WRFGFS close behind. 

Day 1
Moisture has decreased quite a bit over western and central Arizona as GPSIPW readings are generally in the mid 30mm range.  The forecast is for moisture to remain about the same as the weak flow continues up the Gulf of California into Arizona.

The surge, if you want to call it that, is shallow and at 850, dewpoints are only around 10C. 

This somewhat dry air results in only limited CAPE for much of western and central Arizona with only southeastern Arizona having enough to support more widespread storm activity this afternoon.

Tucson?  Not out of the question as there is a moderate amount of CAPE present.   Winds are not good as they are generally southwesterly plus there is a bit of an inversion on top of the mixed layer.  Upper winds are really strong thus anvils will shear off well in advance of storms and over Tucson. Still, maybe a weak storm or two in the valley.

The WRFRR continues storms into the evening over far southeastern Arizona.

Phoenix?  There is a bit of CAPE but probably not sufficient for storms even if there were outflows, which there are none forecast.  What's the deal with the winds?  Is this the second week of September?

Day 2

It looks like all the activity over the southern Gulf of California does trigger a surge as, by early afternoon. This results in quite strong low-level flow advecting moisture into southern Arizona.

Even the dewpoints increase at 850mb at some locations.  Eastern Arizona remains marginal at around 10C.

Arizona continues to have mainly unfavorable southwesterly winds over the state. The hot air of -6 to -5C remains over eastern Arizona.  I'm afraid it looks like another inactive day.

The WRFGFS had the most activity of any of the model forecasts, but none for the Tucson or Phoenix areas.  An MCS may develop along the border later in the afternoon.

Day 3

Day 2 forecasts are bad enough as it is so I rarely look this far out, but we need something to look forward to.  I mentioned the significant IT over central Mexico earlier and either a piece shears off, or the IT itself rotates around the high center and is over Sonora by Saturday morning. (WRFRR with GFS LBC @18Z)   

Very high CAPE is forecast to be in place over south-central Arizona.

The forecast Skew-T is excellent for a large outbreak.  CAPE is very high, there is good low-level shear, mid-level easterly flow increases as the trough approaches and upper winds are mainly southwesterly so anvils will blow away from the general storm motion.

Strong to severe storms develop during the afternoon hours over much of southeastern Arizona.