20190730

Previous Forecast
Showers and storms were present over much of southeastern Arizona yesterday and into the evening, much as was forecast by the WRF.  Strong storms formed over the Rincons during the late afternoon and looked like they were going to make a run for Tucson, but only the Marana area received any significant precipitation.  I don't know what the deal is with them receiving repeat big storm while most of the rest of the Tucson area has seen very little activity.

All the runs turned out to be pretty good for southeastern Arizona except they didn't have enough activity for Yavapai County.  The WRFGFS was the best of the bunch.  Showers and a few storms continued into the morning hours with some activity around Phoenix.  Both the WRFGFS and WRFRR had some weak activity in the general vicinity.

Initializations
While reviewing the satellite imagery, I noticed a swirl in the area of cloudiness over far SE Arizona. Looking back at the last 36 hours of imagery, it was the MCV mentioned yesterday that formed the night before last.   The area of clouds are present in the initializations, but the circulation is not, which isn't surprising.  A large IT is located over the northern Gulf of California with small areas of cyclonic flow moving around the periphery.  The models have initialized the large feature well, but the small scale circulations are missing. Clouds are initialized well in both the GFS and RR.    All initializations have some significant PW errors over Arizona and down into Mexico.  The NAM and GFS are pretty bad while the RR is the least bad; thus it's the favored initialization today. 

Day 1
Both the morning soundings from Tucson and Phoenix had almost no CAPE. Tucson had a strong inversion at 850mb due to rain-cooled air near the surface. I was surprised to see a flash flood watch from the Tucson NWS for this afternoon and evening.

It is plenty moist as most lower elevation locations have 40-50mm of PW today.  Low-level easterly flow has strengthened in New Mexico, and fortunately, the southern 1/2 of the state is still pretty wet; thus, there is no drying from the east.

850mb dew-points are relatively high, but nothing too extreme (>15C).  Southeasterly flow in NM is moderate even at 850mb.

The CAPE measured by the soundings at 12Z looked pitiful, but by early afternoon, it increases to a decent 500 to 1500 J/kg over much of southern Arizona.

All three 12Z runs are consistent in developing widespread storms over southeastern Arizona this afternoon.

By early afternoon, Tucson is well mixed, 6-900 J/kg of CAPE, and a favorable wind profile with good mid-level steering flow.  Just based on the forecast Skew-T, it looks likely for storms in parts of town this afternoon.

However, as is so frequently the case, the strongest storms skirt around the valley.  WRF QPF generally has less than 1.5" of rain, but there could be a storm or two the produce more.   Does the forecast warrant a flash flood watch?

Scattered weak storms move into Pinal County during the late afternoon.  Based on the amount of forecast CAPE, this looks about right.

The Phoenix late afternoon Skew-T forecast plot has only limited CAPE, but winds are favorable, and the PBL is mixed quite deeply.  My guess is that like last evening that a few showers/weak storms develop when the outflow boundaries move through.

Stop the presses!  The 15Z WRFRR is now available, and it is quite a bit more active.  It has slightly more CAPE and stronger outflow boundaries that move into the Phoenix area during the early evening. 

They are sufficiently strong for some storm formation in parts of the Phoenix valley.  The 15Z WRFRR has frequently been the best run of the morning suite but can sometimes be too active so we'll have to wait and see.

The Mexican IT move slowly to the NW and is over the far northern Gulf of California overnight.  This is close enough to keep some activity going into the early morning hours.

Day 2
The Mexican IT begins to lift out over western Arizona as a west coast trough moves closer.  This results in quite strong mid-level flow over the state.

PW continues to increase and is very high by morning.  With the influence of the trough plus the extreme moisture, showers and storms could occur just about any time.  Flash flooding looks much more likely than today.

The 15Z does have morning activity over parts of southern Arizona, which is certainly possible.  As no other runs have nearly this much activity, I don't think it's too likely.

If the above happens, it will negatively impact later development and results in only scattered storms.  If not, it could be a very active day as even the most conservative run has areas of moderate to high CAPE for much of the state.

Impressive CAPE is forecast by early afternoon for Tucson with around 1600 J/kg with relatively strong mid-level flow and some low-level shear.  Flash Flooding is likely with a PW of 47mm, that's for sure.  Storm motion will be moderately fast (for the monsoon season) thus I don't expect any extreme rain amounts.

Assuming it's sunny and early morning activity is limited, strong storms develop early and are widespread over southeastern Arizona by early afternoon.  Some areas may see over 2" of rain.

It is possible Phoenix may see some storms later in the afternoon, but forecast confidence is low.