August 21st, 2024: Active Afternoon Across Southeastern Arizona If Clouds Can Clear Out

Synopsis

CAMs forecast an active afternoon and evening across much of Southeastern Arizona as an inverted trough moves northward across the region. The amount of buoyancy and associated storm coverage and intensity across the area will be primarily dependent on mid level cloud cover throughout the day. If clouds can clear by early afternoon, strong storms are possible across Southeast Arizona (including Tucson), but if clouds persist then there will likely be much less activity. Storms that develop have the potential to produce strong winds, blowing dust, and heavy rainfall that could lead to flash flooding. 

Current Conditions

At 9:45AM MST, visible satellite imagery and lightning detection displayed scattered showers and thunderstorms in Southern Arizona (south and west of Tucson) associated with a robust MCV.

Visible imagery overlaid with glm flashes courtesy of the College of Dupage.

The broad synoptic scale mid level pattern remains quite busy with a mid latitude trough off the Pacific West Coast, a 594 dm anticyclone in Eastern New Mexico, and an inverted trough/closed low to the southwest of Arizona.

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

There are a couple of vorticity lobes of interest embedded within the inverted trough/closed low. The first is moving northward across the border and is associated with this morning's shower and storm activity to the south and west of Tucson. Farther to the south, another vorticity lobe is rapidly moving northward and could influence storm chances later today in Southeast Arizona.

SPC mesoanalysis this morning shows no shortage of instability across the lower deserts with SBCAPE estimates in the 1000 to 2000 J/kg range thanks to ample low level moisture.

SPC mesoanalysis of SBCAPE and SBCIN as of 9AM MST.

Today's Forecast

Synoptics/Dynamics

The broad synoptic scale mid level pattern remains relatively favorable across Southeastern Arizona due to southeasterly flow across the region. 

As of 10AM MST, there's a vort lobe crossing the Border just west of Nogales producing widespread clouds and embedded showers and thunderstorms across Eastern Pima and Santa Cruz counties.

Visible imagery overlaid with glm flashes and 500mb RAP analysis as of 10AM MST courtesy of College of Dupage.

CAMs don't appear to be picking up on the extent of this activity, and show the vort lobe farther west than reality. I'm concerned this will allow cloud cover to persist into the early afternoon hours and potentially leave localized subsidence in its wake.

Thermodynamics

In general, afternoon/early evening storm chances in Tucson will be highly dependent on the persistence of morning cloud cover and shower activity. Both the 12z and 15z WRF-HRRR forecast impressive thermodynamics by this afternoon, but is due to the model forecasting mostly sunny skies during the middle of the day as shown below.

12z (left) and 15z (right) WRF-HRRR global horizontal irradiance (GHI) valid for 1PM MST this afternoon.

And this is why the 12z and 15z WRF-HRRR KTUS soundings are forecasting between 1000 and 1300 J/kg of Mixed-layer CAPE, minimal CIN, and near 1500 J/kg of DCAPE this afternoon.

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

If clouds can clear by early afternoon, then should be able to generate enough buoyancy for strong thunderstorms. 

What to Expect/Impacts

I know I sound like a broken record, but these morning clouds will dictate the coverage and intensity of storms in Tucson today. If clouds can rapidly clear out by midday, then it will be very active across the region. The 12z and 15z WRF-HRRR are extremely aggressive with both runs forecasting strong convection in the Tucson vicinity by mid to late afternoon as shown below.

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

There is a chance for storms in the Phoenix area this evening, but will likely be dependent on activity in Southeast Arizona.

15z WRF-HRRR simulated reflectivity valid for 8PM MST this evening.

SPC has Southeastern Arizona (including Tucson) in a marginal risk for severe grade wind gusts.

SPC Day 1 Wind Outlook.

This seems reasonable based on the 12z HREF having 70% probabilities of 4-hour max wind speeds in excess of 30 knots (where dbz > 20) this afternoon.

12z HREF 4-hour max wind speeds and probabilities of winds in excess of 30 knots where dbz>20 valid for 7PM MST this evening courtesy of the SPC.

There's also a risk for flash flooding with the 12z HREF indicating 30 to 50% probabilities for 3-hour rainfall over 1 inch for much of Southeastern Arizona as well as Northern Arizona. Greatest chances for excessive rainfall in Southern Arizona would be this afternoon/early evening with some isolated areas possibly receiving up to two inches. Therefore, the Weather Prediction Center has placed much of Arizona under a slight risk of excessive rainfall today. 

I would anticipate hazardous traveling conditions across the area with main hazard being reduced visibility and localized flooded roadways.

However, these probabilities and risks are really under the condition that clouds clear by midday to generate enough buoyancy for strong storms to occur.

If clouds persist in the Tucson vicinity through the early afternoon hours, I would expect minimal activity. It's all dependent on those darn clouds!!

We shall see...