Threat of Strong to Severe Storms from Tucson Westward Today

Overview:

Mixed-layer moisture values have increased relative to yesterday following some activity last night which will allow for significant CAPE in excess of 1000 J/kg to develop from eastern Pima county westward by early afternoon. Meanwhile, an upper-level cyclone will continue to move slowly westward across Chihauhau and Sonora Mexico. North of this feature, east-northeasterly flow has increased to 20-30 knots which will lead to 0-6 km shear value in excess of 30 knots across much of the area as surface winds become westerly in response to diurnal heating. The combination of CAPE and shear will support the development of strong to severe storms. CAMs are indicating development across the Sky Islands surrounding Tucson az early as 2 PM. However, more widespread and organized activity is expected in the late afternoon-early evening as an inverted trough axis north of the upper-level low in Mexico approaches the area. In Phoenix, the potential for activity will depend on the intensity and timing of outflows from Rim activity and Pima county. Storm potential will be maximized in the event of outflows from both the north and south reaching the Phoenix metro area concomitantly. 

Activity on Wednesday will be heavily dependent on the evolution of convection today. Broadly speaking, I would expect a downturn in coverage and intensity of activity as the inverted trough axis passes to our west and shear values diminish. However, we will have to keep on eye out for the possibility of convectively driven mesoscale vorticies propagating northward/northwestward through Mexico on the eastern side of the upper-level low which should be near the Gulf of California tomorrow. The UAWRF is suggesting that such a feature will be near the international border south of Tucson by tomorrow evening, which should enhance storm coverage and intensity particularly south of the Tucson metro area.

Today:

Large Scale:

The inverted trough is clearly visible in the morning's 500 hPa mesoanalysis extending northward from Chihauhau, Mexico into protions of eastern New Mexico. Further to the north, a broad area of high pressure extends from northern Arizona eastward into Oklahoma. The gradient between these features is enhancing the easterly to northeasterly flow across southern Arizona with 500 hPa wind speeds of 20-30 knots.

500 hPa Mesoanalysis Valid at 9 AM MST

Meanwhile, this morning's water vapor image shows that dry air that influenced the region yesterday has passed to our west. Further south, the large plume of water associated with Tropical Storm Fabio is evident, although the flow regime is currently not conducive to produce any moisture surges into southeastern Arizona from the TS.

Upper-level Water Vapor Valid at 9 AM MST

Zooming In:

 Soundings from Tucson and Phoenix indicate increasing moisture from east to west with precipitable water values of 1.44" and 1.78" respectively. Both sounding show sufficient CINH to delay intiation of storms in valley locations until mid-late afternoon. While mixed layer moisture looks slightly better in Phoenix, the mid-level flow is stronger in Tucson which is closer to the center of the upper-level cyclonic circulation. Drying above 500 hPa in Tucson is associated with exceptional DCAPE values on the order of 1800 J/kg which should provide for strong deep outflows. In Phoenix a moist mid-level is mitigating DCAPE values somewhat and may be somewhat probematic in terms of convective initation from outflows later in the day.

12Z KTUS (left) and PHX (right) soundings 

Precipitable water values indicate a large gradient from west to east across the region with moisture transport vectors showing that the gradient is likely to strengthen over eastern Pima county into the early afternoon. This gradient should define the eastern edge of the threat area for today, with the greatest likelihood of excessive rainfall being defined by the confluence of the trough axis location this afternoon and the axis of deepest moisture from south-central Pima county northwestward.

PWTR Mesoanalysis valid at 10 AM MST

 

The Weather Prediction Center has issued a slight risk of excessive rainfall for today that covers all of Pima county, most of Pinal county, as well as southern Maricopa county.

WPC excessive rainfall outlook

Today's Forecast:

Forecast soundings from the UAWRF indicate that CINH should be overcome by surface heating by around 1 PM by which time CAPE values are already in the 1500-1700 J/kg range. Mixed-layer moisture is significantly better than yesterday, although mid-level moistening as the upper-level trough axis approaches results in a decrease in DCAPE values to around 1100 J/kg.

UAWRF forecast sounding for KTUS valid at 1 PM MST

At this time however, the inverted trough is still suffuciently far east that any focus for convective intiation is still likely to be the higher terrain around Tucson.

UAWRF 500 hPa wind field forecast valid at 1 PM MST

This is consistent with the UAWRF and HRRR forecasts of early storms developing across the higher terrain in the immediate vicinity of Tucson.

UAWRF forecast reflectivity valid at 1 PM MST

By 3 PM, activity has increased in coverage and moved through the Tucson metro area as well as most of Santa Cruz.

UAWRF forecast reflectivity valid at 3 PM MST

Outflow from this activity cools the surface, creating significant CINH and reduced CAPE in its wake. 

UAWRF forecast sounding for KTUS valid at 3 PM MST

The intensity and location of activity that should develop as the inverted trough moves closer to the region early this evening will depend on recovery of the boundary layer. Consequently, areas that avoid convection early in the day will be more susceptible to stronger organized convection this evening when synoptic scale forcing and shear will be maximized. The 5 PM sounding at Tucson shows significant CIN and and reduced CAPE right as the trough axis approaches the region.

UAWRF forecast sounding for KTUS valid at 5 PM MST

UAWRF forecast 500 hPa winds valid at 5 PM MST

Consequently, the UAWRF focuses convective activity this evening, from Tucson northward into Pinal county, where the boundary layer remained largely uncontaminated by early convection. 

UAWRF forecast reflectivity valid at 9 PM MST

HREF ensembles mimic this northward shift in convection with the highest probabilities of heavy rain shifting from near the international border in the 2-5 PM timeframe....

HREF 3 hr max QPF and prob > 1" valid for 2-5 PM MST

to central Pima, Pinal, and Maricopa counties in the 5-8 PM timeframe.

HREF 3 hr max QPF and prob > 1" valid for 5-8 PM MST

My Take:

Satellite trends as of 11:30 AM indicate that the CAMs may be too early in initiating convection along the higher terrain this early afternoon as cumulus buildups have yet to start in earnest across the region. Later initiation should favor stronger more organized development of convection as maximum CAPE, shear, and forcing will be better aligned. Regardless, decent mid-level flow and significant instability will support the threat of strong to severe storms this afternoon, although the development of a coherent MCS will likely depend on timing of initation.

SPC convective outlook

Wednesday:

I am not going to spend a lot of time discussing Wednesday as I think so much depends on today. In general, things should be quieter as the upper-level cyclone has largely moved to our west. However, I did want to point out that the UAWRF shows an MCV approaching the region tomorrow late-afternoon or early evening.

UAWRF forecast 500 hPa winds valid at 5 PM MST Wednesday

This feature serves as a focus for development near the international border.

UAWRF forecast reflectivity valid at 5 PM MST

From and ensemble perspective, the early day subsidence and lack of a synoptic-scale focusing mechanism leads to a diffuse threat of storms initiated in the higher terrain in the 5-8 PM timeframe.

HREF 3 hr max QPF and prob > 1" valid for 5-8 PM MST Wednesday