August 17th, 2024: Strong Thunderstorms Possible Across Southeast Arizona Today

 Synopsis

The broad synoptic scale upper air pattern today is characterized by a mid latitude trough off the West Coast, the 500mb anticyclone in New Mexico, and favorable southeasterly flow aloft across Arizona. Improved thermodynamics will promote an increased chance for more widespread showers and thunderstorms across Southeast Arizona (including Tucson) this afternoon and evening. There is a chance for storms in Phoenix, but will be dependent on the strength and trajectory of outflow boundaries from convection to the south/southeast. Main impacts from storms today will be strong, gusty winds, blowing dust, and brief heavy rainfall.

Expect decreasing storm coverage tomorrow due to weaker mid level flow and less favorable thermodynamics.

Current Conditions

12z GFS 500mb analysis this morning displays an impressively deep mid latitude trough (especially for this time of year) off the West Coast, the mid level anticyclone over New Mexico, and southeasterly/southerly flow aloft across Arizona.

12z GFS 500mb analysis this morning courtesy of Tropical Tidbits.

This is a relatively favorable synoptic scale upper air pattern for Arizona due to marginally brisk mid level flow associated with the stronger height gradient between the ridge to our east and the trough to our west.

Thermodynamics this morning aren't very impressive with the 12z TUS sounding measuring near normal moisture, modest lapse rates, and similar magnitudes of CAPE and CIN (around 300 to 400 J/kg).

12z TUS sounding this morning courtesy of the SPC.

Today's Forecast

Synoptics/Dynamics

The synoptic scale upper air pattern remains on the favorable side for convection in Southeast Arizona today due to southeasterly/southerly flow on the order of 15 to 20 knots. There isn't any dynamic focusing mechanism like an embedded shortwave or inverted trough. However, I don't see anything synoptically to inhibit or limit convection today.

Thermodynamics & Wind Shear

Visible satellite imagery as of 10AM MST shows mostly clear skies across Arizona, so shouldn't have any issues with surface insolation and steepening of low and mid level lapse rates. There's marginally more moisture to work with today, so buoyancy should be much higher this afternoon. The 12z and 15z KTUS WRF-HRRR soundings forecast between 750 and 1000 J/kg of Mixed-Layer CAPE and minimal CIN by the middle of this afternoon as shown below.

12z (left) and 15z (right) KTUS WRF-HRRR soundings valid for 3PM MST this afternoon.

With significant mixing of the boundary layer, expecting some inverted-V signatures by late this afternoon with DCAPE reaching around 1500 J/kg. This will promote the risk of strong outflow boundaries and strong wind gusts from thunderstorms. Vertical wind shear isn't quite as impressive as it was yesterday, but still expecting around 10 to 20 knots of vertical speed shear which could allow some marginal organization of storms propagating off the higher terrain. 

In general, thermodynamics are favorable for showers and storms across Southeast Arizona (including Tucson) this afternoon and evening.

Both the 12z and 15z WRF-HRRR are showing storms in the Phoenix vicinity this evening after sunset (will show evolution in the following section). Both the 12z and 15z KPHX WRF-HRRR soundings forecast between 500 and 1000 J/kg of MLCAPE along with around 1500 J/kg of DCAPE.

12z (left) and 15z (right) KPHX WRF-HRRR soundings valid for 7PM MST this evening.

Somewhat similar thermodynamics compared to Tucson, but a major difference is there is forecast to be a bit more CIN this evening which could cause some problems with inhibiting ascent.

What to Expect/Impacts

Overall, expecting an active day across Southeast and possibly South-Central Arizona. CAMs are in good agreement that convection will initiate over the higher terrain to the south and east of Tucson by early afternoon. 

12z (left) and 15z (right) simulated reflectivity valid for 3PM MST this afternoon.

Storm movement will be from south/southeast to north/northwest, and CAMs forecast vigorous outflow boundaries to move into the Tucson vicinity by late afternoon which should initiate storms in the area.

12z (left) and 15z (right) 10-meter winds valid for 4PM MST this afternoon.

12z (left) and 15z (right) simulated reflectivity valid for 4:30PM MST this afternoon.

 I always look to find any reasons why storms wouldn't develop in Tucson for days like today, and the only thing that could cause some issues would be anvil shading. However, the KTUS model soundings didn't show any CIN prior to when storms are forecast to move into the vicinity, but something to keep in mind. In general though, expect a busy afternoon and evening in the Tucson vicinity.

The area I am most concerned about later today is the I10 corridor between Tucson and Phoenix where the UA WRF is forecasting very strong outflow winds. 

12z (left) and 15z (right) 10-meter winds valid for 5:30PM MST this afternoon.

12z (left) and 15z (right) 10-meter winds valid for 7:00PM MST this afternoon.

There are some differences between the 12z and 15z regarding the forecast magnitude of winds with the 15z being more aggressive. Either way, expect extremely hazardous traveling conditions along the I10 corridor between Tucson and Phoenix with wind gust as high as 50 to 60 mph. Main hazard would be reduced visibility from blowing dust, and considering the forecast magnitude of these winds, a Haboob (dust storm) is possible.

After sunset, the 12z and 15z WRF-HRRR as well as hourly morning runs of the HRRR are showing storms in the Phoenix vicinity as shown below.

12z (left) and 15z (right) simulated reflectivity valid for 8:00PM MST this afternoon.

This is the best UA WRF signature I've seen for the Phoenix area this season thus far. As shown in the previous section, KPHX model soundings do forecast a bit of CIN during this evening, but if strong, effective outflow can make it into Phoenix there should be plenty of buoyancy for storm development. 

Main threat from storms today will be strong, gusty winds and blowing dust with the 12z HREF showing 50 to 70% probabilities of 4-hour max wind speeds in excess of 30 knots (where dbz>20) in Cochise, Santa Cruz, and Eastern Pima and Pinal Counties this afternoon and evening.

12z HREF 4-hour max wind speeds and probabilities of wind speeds in excess of 30 knots where dbz>20 valid for 8PM MST this evening courtesy of the SPC.

Storms will also be able to produce brief heavy rainfall, but any excessive rainfall/flash flooding threat will likely remain isolated due to brisk steering flow, high LCLs, and a dry subcloud layer. Also, wouldn't be surprised for a few of the strongest updrafts to produce nickel to golf ball sized hail.

Tomorrow's Forecast

The large scale synoptic pattern becomes a bit more unfavorable tomorrow as the aforementioned mid latitude trough lifts northward into the Pacific Northwest weakening the mid level height gradient and decreasing mid level flow across Arizona. 

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

Models also forecast some drier air moving in from the east into Southeast Arizona as shown below.

12z WRF-HRRR total precipitable water valid for tomorrow at 11AM MST.

With weaker steering flow and drier air leading to less buoyancy, expect a decrease in storm coverage and intensity tomorrow with convection likely remaining confined mainly to the higher terrain.