July 13th, 2024: Scattered High Terrain Storms Today With Increasing Activity Tomorrow

 Synopsis

The 500mb anticyclone has finally shifted east near the Four Corners and will remain anchored in that region maintaining easterly/southeasterly flow aloft over Arizona today and tomorrow. Today will likely be a near repeat of yesterday with storms remaining mainly confined to the higher terrain of Northern AZ and near the International Border. Increased moisture and instability will promote an increase in storms tomorrow, but there is some uncertainty regarding storm coverage in lower elevations. 

Current Conditions

An outflow boundary coming off the Mogollon Rim late last night initiated a few short-lived showers and isolated thunderstorms in the Phoenix metro. No measurable precip at Sky Harbor, but a few gauges across the northern Phoenix metro measuring between a trace and 0.10 inches. This outflow boundary initiated cells all the way down to Yuma by early this morning!

Visible satellite imagery this morning displays mostly clear skies across AZ and scattered elevated convection across California.

Visible satellite imagery overlaid with GLM flashes at 9:45AM MST this morning courtesy of College of Dupage.

This elevated convection is associated with a region of cyclonic vorticity advection and upper level divergence induced by an upper level low off the CA Coast.

SPC 300mb analysis as of 9:00AM MST this morning.

Meanwhile, SPC 500mb analysis displays the mid level anticyclone centered just north of the Four Corners with brisk 15 to 25 knot easterly/southeasterly flow aloft over Arizona.

SPC 500mb analysis as of 10:00AM MST this morning.

There is a very tight moisture gradient between Southwestern and Southeastern AZ with moisture rapidly increasing from east to west.

SPC mesoanalysis of precipitable water as of 9:00AM MST this morning.

The 12z TUS sounding this morning displayed a very dry atmosphere with 0.66 inches of precipitable water and no CAPE.

12z TUS sounding courtesy of the SPC.

The only real good signature from this sounding is the mixed layer below 500mb.

Today's Forecast

The 500mb anticyclone will remain centered near the Four Corners today with easterly flow aloft over Arizona. Even though brisk mid level flow is a favorable mid level wind pattern, the synoptics are still relatively neutral as there isn't any embedded shortwave to induce ascent.

Similar to yesterday, CAMs forecast a diurnally driven pressure gradient to develop between the thermal low over the western AZ deserts and a surface high over western Texas. This will induce easterly surface winds advecting dry air from the east into SE AZ. Looks like another localized surface convergence boundary will develop between the drier easterly winds and moist westerly winds induced by the Gulf of CA. The 12z and 15z disagree on the location of this convergence zone with the 12z farther east near Tucson and the 15z west of Tucson (similar to yesterday).

12z WRF-HRRR 2-meter dew points for this afternoon at 3:30PM MST.

15z WRF-HRRR 2-meter dew points for this afternoon at 3:30PM MST.

In addition, the 12z and 15z KTUS WRF-HRRR soundings are in disagreement regarding instability this afternoon with the 12z forecasting near 800 J/kg of CAPE and the 15z forecasting no CAPE.

12z KTUS WRF-HRRR sounding for this afternoon at 3:30PM MST.

15z KTUS WRF-HRRR sounding for this afternoon at 3:30PM MST.

I'm leaning toward the 15z WRF-HRRR solution as it is more reflective of the thermodynamic conditions as of 11:00AM MST.

Overall, expect isolated to scattered showers and thunderstorms mainly near the International Border in Santa Cruz and Southeastern Pima Counties this afternoon. I cannot completely rule out an isolated cell or two near Tucson this afternoon but probabilities are below 20%. Any storm that develops has the potential to produce lightning, gust winds, and brief heavy rainfall. 

Other than that, should be another active day in Northern Arizona from Flagstaff westward with CAMs forecasting an organized cluster of storms to develop this afternoon. CAMs even forecast this cluster of storms to make it into Las Vegas this evening. 

High temps will be a couple degrees cooler in the lower deserts this afternoon (but still quite hot) with high temps between 100 and 105 degF in SE AZ and near 110 degF in South-Central AZ. 

12z WRF-HRRR 2-meter temps for this afternoon at 3:00PM MST.

Tomorrow's Forecast

The 500mb anticyclone will remain anchored near the Four Corners maintaining easterly/southeasterly flow aloft over Arizona. 

12z GFS 500mb cyclonic vorticity for tomorrow at 11:00AM MST courtesy of Tropical Tidbits.

CAMs forecast pwats to increase to between 1 and 1.5 inches across the lower deserts (increasing from east to west) with CAPE also increasing to between 1000 and 1500 J/kg tomorrow.

12z WRF-HRRR total precipitable water for tomorrow at 12:00PM MST.

12z WRF-HRRR max CAPE for tomorrow at 3:00PM MST.

The combination of increased low level moisture and CAPE will lead to an uptick in activity. This morning's CAM solutions are less aggressive with storms in the lower deserts compared to yesterday's 12z as shown below.

12z yesterday (left) and 12z today (right) WRF-HRRR simulated reflectivity for tomorrow at 7:30PM MST.

 

The main reason for this trend is this morning's CAM solutions are forecasting a delayed convective initiation over the White Mountains and Mogollon Rim around late afternoon tomorrow and forecast outflow to move through the lower deserts well after dark. If this were to verify, the development of CIN would make it difficult for outflow boundaries to initiate storms in the lower deserts. This trend will need to be monitored closely with this afternoon's and tomorrow morning's CAM solutions.

Overall, expect an uptick in storms tomorrow especially over the higher terrain with storm coverage in the lower elevations likely dependent on the timing of convective initiation tomorrow afternoon.