August 23rd, 2024: Storm Coverage Mainly East of Tucson Today

 Synopsis

A mid latitude trough is expected to deepen along the West Coast today causing broad mid level flow to turn southwesterly across Arizona. Another embedded shortwave is expected to lift northward across extreme Southeast Arizona and combined with sufficient thermodynamics will promote a chance of showers and thunderstorms mainly east of Tucson this afternoon and evening. Main impacts from storms will be strong winds, hail, and brief heavy rainfall. SPC has Southeast Arizona in a marginal risk for severe grade wind gusts and hail today. 

Today's Forecast

Synoptics/Dynamics

The broad synoptic scale mid level pattern is characterized by a deepening mid latitude trough off the Pacific Coast, a high amplitude ridge over the Plains, and brisk southwesterly flow across Arizona.

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

Zooming in, there's an embedded shortwave expected to lift northward across extreme Southeast Arizona this afternoon and evening. This feature will act as a dynamic forcing mechanism for synoptic scale ascent. 

In addition, as the trough deepens along the west coast today, the mid and upper level height gradients will tighten allowing for increasing mid and upper level flow across the region and therefore enhancing vertical wind shear. 

Thermodynamics & Wind Shear

Visible satellite imagery this morning displays mostly clear skies across the region, so shouldn't have any issues with surface insolation and consequent steepening of low and mid level lapse rates. Both the 12z and 15z KTUS WRF-HRRR soundings forecast between 750 and 1000 J/kg of Mixed-layer CAPE, and minimal Mixed-layer CIN by the middle of this afternoon.

12z (left) and 15z (right) KTUS WRF-HRRR soundings valid for 2PM MST this afternoon.

DCAPE is forecast to be a bit lower today, but still high enough to produce locally strong outflows. 

The forecast vertical wind shear is somewhat characteristic of a Type IV Monsoon Pattern due to more of a westerly component to the winds associated with a deepening mid latitude trough upstream. Expect magnitudes of effective bulk shear on the order of 20 to 30 knots today across Southeast Arizona which could aid in marginal organization of storm clusters. There's also a bit of directional shear between the surface and 3km, so an isolated supercell or two cannot be ruled out.

What to Expect/Impacts

Overall, convection will remain confined mainly to extreme Southeast Arizona this afternoon and evening where the best dynamic forcing is expected. SPC has Southeast Arizona (including Tucson) in a marginal risk for severe grade wind gusts and hail today. The 12z and 15z UA WRF solutions keep the majority of storm activity just to the north and east of the Tucson metro this afternoon and evening as shown below.

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

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 5PM MST this afternoon.

So I would expect the majority of activity to remain over the higher terrain to the north and east of Tucson today. However, a few cells are possible in the Tucson vicinity which could produce strong winds, isolated large hail, and brief heavy rainfall.

The 12z HREF has 70 to 90% probabilities of 4-hour max winds speeds in excess of 30 knots in extreme Eastern Pima and Pinal Counties as well as Cochise and Graham Counties this afternoon.

12z HREF 4-hour max winds speeds and probabilities of wind speeds in excess of 30 knots (where dbz > 20) valid for 5PM MST this evening courtesy of the SPC.

Excessive rainfall seems unlikely at this time due to brisk steering flow, but isolated instances of flash flooding are possible across the region, especially over the higher terrain where storms may train over an area for multiple hours. The 12z HREF has 30% probabilities of 3-hour precipitation in excess of one inch in extreme Eastern Pima and Pinal Counties as well as Cochise and Graham Counties later today as shown below.

12z HREF 3-hour mean precipitation and probabilities of precipitation in excess of one inch valid for 5PM MST this afternoon courtesy of the SPC.

I would anticipate hazardous traveling conditions especially along the I10 corridor between Tucson and the AZ/NM Border. If convection occurs in Tucson, the afternoon/evening commute could be impacted with main threats being reduced visibility and localized flooded roadways, especially in poor drainage areas.