July 11th, 2024: Strong Thunderstorms in SE AZ Today to Kickoff Active Weekend

 Synopsis

An active day is expected across mainly Southeast Arizona with increased coverage of strong thunderstorms. Improved synoptics combined with steep lapse rates and significant DCAPE will promote the potential for strong outflow boundaries. CAMs are in good agreement that storms will initiate over the White Mountains and Mogollon Rim early this afternoon with organized storm clusters moving rapidly southwestward toward lower elevations (including Tucson) by late afternoon/early evening. Some storms will be capable of producing severe grade wind gusts which has prompted the SPC to issue a slight risk of severe weather today in SE AZ. Tomorrow into the weekend is expected to remain active due to a combination of sufficient low level moisture and brisk easterly flow aloft.

Current Conditions

SPC 500mb analysis this morning displays the 500mb anticyclone centered near the AZ/NV/UT Borders with brisk 20 to 30 knots northeasterly flow over Eastern Arizona.

SPC 500mb analysis as of 9:00AM MST this morning.

Poor visibility and air quality this morning as a low level blanket of smoke is observed via airport obs and satellite imagery across the lower deserts this morning.

Surface observations from the Tucson International Airport this morning courtesy of the NWS.

Moisture is making a gradual return with the 12z TUS sounding and SPC mesoanalyis showing pwats around an inch across Southern AZ.

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

12z TUS Sounding this morning courtesy of SPC.

Still very dry above 500mb, but marginally better moisture in the lower half of the troposphere compared to yesterday's 12z sounding. Not a lot of instability this morning with only around 200 to 300 J/kg of SBCAPE, but very steep lapse rates with mid level lapse rates around 8 degC/km. 

Today's Forecast

Synoptics


The 500mb anticyclone is expected to shift eastward toward the Four Corners throughout the day and into tomorrow with predominantly northeasterly mid level flow over AZ. A very subtle shortwave trough embedded on the eastern flank of the ridge is expected to dive southwestward throughout the afternoon. 

12z GFS 500mb cyclonic vorticity for 5:00PM MST this afternoon courtesy of Tropical Tidbits.

This feature is more pronounced via SPC 300mb analysis which shows a region of upper level divergence ahead of the embedded shortwave.

SPC 300mb analysis as of 10:00AM MST.

This feature will induce synoptic scale ascent which will aid convective initiation, enhancing mid level flow, and promote organization of storms coming off the higher terrain. 

Thermodynamics and Wind Shear

The 12z KTUS WRF-HRRR sounding forecasting pwats to remain just above an inch and surface dew points around 50 degF by the middle to late afternoon hours.

12z KTUS WRF-HRRR sounding for this afternoon at 4:00PM MST.

In addition, sufficient surface heating will allow a deep mixed layer (steep low to mid level lapse rates) to develop from the surface to around 500mb by this afternoon. An LCL near 600mb in combination with a significantly dry sub cloud layer will promote the potential for strong outflow boundaries (DCAPE between 1500 and 2000 J/kg). CAPE is expected to peak around midday (maximum around 1000 J/kg), but will decrease to between 500 to 700 J/kg by late afternoon as surface dew points drop to near 50 degF or less. 

There is a bit of vertical wind shear due to the embedded shortwave enhancing mid level flow with model soundings forecasting between 10 and 20 knots of shear between the surface and 500mb by late this afternoon. 

Timing and Location of Storms

Overall, the combination of a synoptic scale lifting mechanism, strong outflow boundary potential, and just enough moisture and instability will promote a decent chance of organized, strong thunderstorms across mainly Southeast Arizona this afternoon and evening. The 12z and 15z WRF-HRRR as well as recent runs of the HRRR are in good agreement regarding timing, location, and intensity of storms today. Storms are forecast to initiate along the White Mountains and Mogollon Rim by early afternoon (between 12PM and 2PM MST).

12z (left) and 15z (right) WRF-HRRR simulated reflectivity for this afternoon at 1:00PM MST.

By mid afternoon, storms are forecast to organize along outflow boundaries with the added synoptic scale support induced by the embedded shortwave. Also, expecting storms to initiate over the Sky Islands in SE AZ by mid afternoon.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity for this afternoon at 3:30PM MST.

CAMs are in good agreement that storms will impact the Tucson vicinity by late afternoon (between 4PM and 7PM MST).

12z (left) and 15z (right) WRF-HRRR simulated reflectivity for this evening at 6:00PM MST.

The only way I could see somewhat limited coverage/intensity of storms in Tucson would be the influence of Mt. Lemmon. In some cases, outflow coming in from the northeast can be disrupted by Mt. Lemmon in which outflow boundaries can warm adiabatically due to outflow boundaries moving down the slope of the southern side. However, outflow boundaries can sometimes wrap around the Catalinas and Rincon's and converge over the Tucson valley which can enhance storm intensity and coverage. Albeit, considering CAMs are in good agreement with storms in Tucson today and considering the added support from the upper level shortwave, any negative impact from Mt. Lemmon is likely to be insignificant. 

By early evening (between 6PM and 9PM MST),  storms are forecast to develop west of Tucson in Central and Western Pima County. CAMs forecast increased intensity in these areas due to better thermodynamics.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity for this evening at 7:00PM MST.

Besides SE AZ, strong storms are likely over the higher terrain of Northern AZ from Flagstaff westward toward the CA and NV Borders. Based on current guidance, storms are unlikely in Phoenix but strong, dry outflow winds are possible this afternoon and evening that could be accompanied by widespread blowing dust.

Impacts

The primary impact from thunderstorms today will be strong winds as the SPC has SE AZ (including Tucson) in a slight risk for severe weather today. The SPC has a 15% chance of thunderstorm winds/wind gusts reaching severe criteria (greater than 50 knots) in SE AZ (including Tucson) this afternoon/evening.

SPC Day 1 Categorical Outlook.

SPC Day 1 Wind Outlook.

The 12z HREF forecasts a 90% chance of 4-hour max wind speeds in excess of 30 knots where dbz is greater than 20 this afternoon in SE AZ (including Tucson).

12z HREF probability of 4-hour max wind speeds > 30 knots where dbz > 20 for this evening at 6:00PM MST.

Due to brisk steering flow, a high LCL, and a dry sub cloud layer, excessive rainfall is unlikely. However, isolated flash flooding is always a possibility especially near any recent burn scars and poor drainage areas in urban locations.

Hail is also unlikely due to relatively low values of CAPE and a warm, dry sub cloud layer. However, hail cannot be completely ruled out within some of the strongest updrafts especially near Central and Western Pima county where there is better instability today.

Traveling conditions will be extremely hazardous especially along the I10 corridor in SE AZ, the I19 corridor, and the I8 corridor between Casa Grande and Gila Bend. Strong winds will likely produce widespread blowing dust in these areas reducing visibility. The evening commute in Tucson will also be impacted with the primary hazard being strong winds and blowing dust. 

As far as high temps are concerned, across the majority of the southern portion of the state (excluding Cochise county), an excessive heat warning remains in place through the end of the day today. Temperatures today around Tucson and other southeastern areas will likely be near or just above 100 degF with localized cooler temperatures around thunderstorms. Meanwhile, central and western Arizona will still be in excessive heat territory through Friday before a slight “cool down” over the weekend with high temperatures around Phoenix and through the deserts towards Yuma expected to be over 110 degF but unlikely to extend past 115 degF.

12z WRF-HRRR 2-meter temps for this afternoon at 4:00PM MST

Tomorrow through the Weekend

The deterministic GFS and ECMWF as well as their ensemble members are in good agreement that the 500mb anticyclone will become anchored over the Four Corners through the weekend with easterly flow aloft over AZ.

12z GEFS 500mb cyclonic vorticity for tomorrow through Sunday courtesy of Tropical Tidbits.

Moisture will also gradually increase into the weekend with models forecasting pwats to return to at or just above normal.

Storm coverage and intensity tomorrow will be highly dependent on if the atmosphere can recover from today's activity. CAMs forecast a decrease in coverage tomorrow but this will need to be monitored. 

Hesitant to go too much into detail regarding the forecast for tomorrow through the weekend as storm coverage and intensity will be highly dependent on day to day variability. In general, expect a mid grade monsoon pattern tomorrow through the weekend.