July 21st - Conditions should continue to improve through the weekend

Overview: 

Upper-level flow and shear patterns are finally favorable for marginally organized convective activity. However, moisture conditions are marginal for today. The NWS discussion made a great point about how the extreme temperatures we have experienced have allowed convective initiation to occur under less-than-ideal moisture conditions. The same should be true today. The limited moisture availability will likely mean a initiation later in the day. However, shear profiles today could mean that any storms may persist past nightfall.  Highest probabilities today/tonight will continue to be south and east of the Tucson area, although this activity could work into the metro area after sunset. Moisture conditions should improve tomorrow with more widespread and earlier initiation. Expect convection to fire up both along the Mongollon Rim as well as the higher elevation of Cochise County in the early afternoon. The activity should converge towards the immediate Tucson area as early as the middle of the afternoon. 

Today:

                                   12Z Tucson Sounding

The Good: Lapse rates in the 700-500 hPa layer are nearly dry adiabatic at 9C/ km. A rotating wind profile is evident in the 850-500 hPa layer with upwards of 25 knots of shear in that layer. The shear profile should create horizontal vorticity that will interact favorably with westward propagating outflow boundaries for the creation of new updrafts. It may also enhance updrafts in the region of greatest lapse rates.

The Meh: Forecast CAPE and DCAPE values are less than 1000 J/kg and lapse rates above 500 hPa are essentially moist adiabatic.

The Bad: Precipitable water values continue to decrease with just 1.14" available in this morning's sounding. Moisture in the lower boundary decreases rapidly up to about 820 hPa creating the potential for rapid drying of surface dewpoints in the afternoon. Our convective temp continues to be quite high at 108F, although we will likely easily best that mark today.

My Take: Based on the sounding, I think the probability of hazardous convection is somewhat minimal. Of course, with an inverted "V" sounding, gusty outflow winds are always possible, but I would not expect any significant or widespread severe level winds. However, unlike yesterday, I do think that any convection approaching the area from Cochise County has the potential to persist past sunset.

                           16Z Visible Satellite and PWTR

Sky conditions are clear over the region, setting the stage for another record hot day. The extent of greater than 1.2" of precipitable water has continued to recede to the west.

            12Z GFS 500 hPa Heights-Vorticity Valid at 18Z

The center of the 500 hPa ridge is situated over northern Arizona with a region of enhanced easterlies just south of the border. Note that the 500 hPa heights are greater than 5940 m placing us in the top 1 percent of values, which should lead to another record hot day.

HREF ENSEMBLES

     12Z HREF >30 knot wind probability Valid at 03Z/22

I have taken the time stamp from the HREF with the most widespread coverage of the greater than 30 knot wind probability in the Southwest. This does highlight the density of coverage is largest south of the border for today, but with a decent cluster of 40 knot winds just south of Tucson. The 03Z time stamp is indicative of the later than normal initiation.

12Z HREF 3hr MAX Precip and >1" Probability Valid at 03Z/22

Max Precipitation amounts in the immediate vicinity appear to be in 0.25-0.5" inch range. Precipitation accumulations appear to extend northward just east of Tucson along slopes of the Rincons and Catalinas.

UA WRF:

I will continue to focus on the HRRR based initializations as the GFS runs continue to be overly aggressive and the RR initialization shows activity well past midnight which also seems unlikely to me given the current conditions.

    12Z HRRR initialization PWTR and Wind Valid at 23:30Z

Convective initiation should start in Cochise County around 4 PM MST in the region marked by the black line.

12Z HRRR initialization PWTR and Wind Valid at 23:30Z

The outflow and PTWR front should be in Santa Cruz and extreme southeastern Pima County by 7:30 PM MST. At this point in time, the CAPE in the Tucson area should be minimal.

     12Z HRRR initialization TUS Sounding at 02:30Z/22nd

However, as the most outflow from the south pushes north, surface dewpoints/CAPE are forecast to increase.

12Z HRRR initialization PWTR and Wind Valid at 03:30Z/22nd

 

The moist outflow is forecast to push northward by 8:30 MST, which has the impact of significantly increasing CAPE

         12Z HRRR initialization TUS Sounding at 04:30Z/22nd

12Z HRRR initialization TUS Sounding at 04:30Z/22nd

Overenall, I would describe this as a plausible if not exactly probable solution. I do think this type of scenario is how to get convection into the immediate Tucson area, given the overall set up. 

Quick outlook for Saturday:

             12Z GFS 500 hPa Heights-Vorticity Valid at 18Z/22nd

The upper-level high pressure system should continue to shift to the Northwest, resulting in more favorable synoptic-scale conditions.

12Z HRRR initialization PWTR and Wind Valid at 21Z/22nd

Moisture availability should be improved with values approaching climatology. The combination of slightly cooler upper levels along with better moisture availability should result in significantly better instability.

Convective temperatures should be achieved by the early afternoon when CAPE values are expected to be between 1500-2000 J/kg. I would expect initiation to be well underway by 2 PM along the higher terrain both locally and along the Mogollon Rim. This will set up the possibility for multiple rounds of convection, increasing the threat of heavy precipitation accumulations with ensembles showing a greater than 10% chance of accumulations over an inch in the immediate Tucson vicinity.

12Z HREF 3hr MAX Precip and >1" Probability Valid at 03Z/23

There could also be widespread coverage of greater than 40 mph wind gusts with at least a 70% probability of wind speeds exceeding 30 knots.