July 10th - Still hot, but more widespread convection expected starting tomorrow

Overview:

Today:

The over-riding story for today will be the persistent near record or record high temperatures across the area. Moisture across the area is still somewhat lacking, with PWTR values at an 1" or less from central Pima county eastward. Overnight surface dewpoint recovery was minimal with values generally around 50 F. With some mid-level drying evident in the water vapor imagery to our northeast, convective initiation along eastern sections of the Rim is less likely today than it was yesterday. Without the aid of outflow from storms propagating southward off of the Rim, convective initiation locally will be a little harder to come by. Consequently, expect more isolated coverage of convection with the higher terrain of Cochise and Santa Cruz counties having the highest probability of rain. Further to our NW, convection may intitiate across the higher terrain near Prescott, in an axis of slightly better moisture to our west. Outflow and dust from these storms may be a concern for the Phoenix metro area towards sunset. As with the past several days, inverted "V" soundings with deep mixed layers will mean high based storms dominated by outflow, with strong gusty winds as the primary concern.

Tomorrow

Things get a little more interesting tomorrow as the tail end of an upper-level trough axis drops south through eastern Arizona. Models indicate more wide spread convection developing along eastern sections of the Rim just ahead of this feature. An axis of slightly enhanced northeasterlies at 500 hPa combined with slightly better surface dewpoints, should result in more coherent storms propagating southwestward into eastern Pima county. Strong winds are likely to be associated with these storms and I would not rule out convective organization with winds approaching severe limits. 

Looking Ahead

I still expect moisture and convection to increase over the weekend, but I am slightly less optimistic about the pattern than I have been. This is mostly because the upper-level low resulting from the anticyclonic wave break appears to stall over the Texas Pan Handle for a few days. Consequently, upper-level heights across the area do not drop as much as I had anticipated/hoped and we are unlikely to get a boost from significant synoptic-scale forcing. However, as the West Coast high shifts into the Great Basin, 500 hPa flow will turn to the east and moisture will deepen bringing back more classic Monsoon conditions into the area. Models are still suggesting that Sunday will be the most active day locally although there is plenty of time for this to change.

Today:

Large Scale Flow:

The large scale flow pattern remains largely unchanged with high pressure centered over southern Nevada and the trough that "picked up" Beryl now located over the Great Lakes.

This continues to place eastern Arizona is a region of broadly northerly flow, with a limited capacity for significant moisture return.

Zooming In:

Zooming in, the sounding from this morning shows precipitable water values near an 1" and lapse rates in the mid troposphere near dry adiabatic. The limiting factor in terms of instability continues to be dewpoints in the lowest 100 hPa of the atmosphere. Once again, a well mixed atmosphere would suggest dewpoints dropping back into the mid 40s this afternoon resulting in lifting condensation levels above 600 hPa by this afternoon. Winds in the 700-500 hPa layer would suggest storm propagation to the south/southwest. 

That sense of storm propagation is problematic for Tucson since convective initiation to our north/northeast is less likely in comparison to yesterday as subsidence and drying can be seen in the mid-level water vapor along the AZ/NM border this morning.

Additionally, surface dewpoints are several degrees cooler in this region as compared to yesterday. For instance, the dewpoints at Safford is 48 F this morning as compared to 57 F at this time yesterday.

Deeper moisture and greater instability is situated west of the area through central Pima and western Maricopa counties as indicated in the SPC mixed layer CAPE analysis this morning.

Convective initiation may occur in the higher terrain north of Phoenix on the northern edge of this plume of CAPE. Propagation of storms in this region will be limited due to proximity to the 500 hPa high pressure system, but outflow from any storms in the area may casue blowing dust in the northern Phoenix metro area. Throughout the region, convective inhibition values remain high indicating a late afternoon start to any activity today.

Most CAMs, including the UAWRF, indicate storms beginning in the late afternoon across Cochise, Santa Cruz, and Yavapai counties.

Given the thermodynamic profiles across the area, expect storms north of the border to quickly be overwhelmed by downdrafts associated with evaporative cooling. 

Downdraft CAPES should be in excess of 2000 J/kg will produce the potential for strong winds although with exceptionally high cloud bases, cooling associated with these outflows will be mitigated.

The HRRR ensembles highlight the main areas of wind concern in Yavapai county as well as Sonora, Mexico.

Tucson Metro:

Given the information above, I think that convection in the Tucson metro area is a low probability event. The one scenario is which I can envision rain in the metro area is in association with outflow from convection in Sonora reaching the area after sunset. Below, I've highlighted the outflow boundary forecast by the UAWRF to reach the area around 10 PM.

As the outflow reaches the area, the dewpoint should rise, and CAPE should be present as indicated in the forecast sounding below.

However, surface cooling has resulted in convective inhibition that is unlikely to be overcome by the outflow boundary, leaving the chance of rain at around 10%.

Tomorrow:

Large scale:

Things should become a little more interesting tomorrow as we get a little bit of synoptic-scale help. As mentioned in the overview, the tail end of a mid-latitude trough will drop southward across the eastern half of Arizona tomorrow. I have highlighted this feature on the GFS forecast of pressure on the dynamic tropopause where the trough is most prominent.

Cyclonic advection of potential vorticity ahead of this feature should provide a little in the way of synoptic scale lift. 

Zooming In:

Additionally, a tightening gradient on the eastern side of the high pressure should provide for enhanced NE flow at 500 hPa in eastern Arizona.

The moisture environment will also be a little better, especially in eastern sections of the Rim where PWTR values are expected to exceed 0.8" tomorrow afternoon with values 

With the expected surface heating and synoptic scale help, convective initiation is likely in the higher terrain in Graham and Geely counties by early afternoon:

By late afternoon, multiple gust fronts are possible across the area as highlighted in the 10m wind field in the UAWRF.

From and ensemble perspective, winds in excess of 30 knots are just about a given in any storms that do develop, with the possibility of gusts approaching severe limits and has resulted in the issuance of a marginal risk of severe by the SPC.

This scenario also yields the highest probability of measurable precipitation in the Tucson metro area that we have seen over the past week with probabilities of 30-50%.