August 8th, 2024: Localized Flooding Risk in Southeast Arizona Today

 Synopsis

Arizona is between an upper level low over the Northern Gulf of CA/Baja California and an anticyclone over Eastern New Mexico/Western Texas this morning. Ample amounts of moisture and associated buoyancy will lead to a chance of showers and thunderstorms in Southeast Arizona capable of producing heavy rainfall rates and localized flooding concerns this afternoon and early evening. SPC has a marginal risk (5%) of severe grade wind gusts across much of Arizona today as well. CAMs forecast additional storm activity overnight associated with an embedded shortwave moving northward into Southeastern Arizona. 

Tomorrow will likely be a down day due to overworked thermodynamics from overnight convection and subsidence in the wake of the embedded shortwave. 

Current Conditions

At 9AM MST, visible satellite imagery overlaid with RAP 500mb analysis displayed an upper level low over the Northern Gulf of CA/Southern Baja, a broad subtropical ridge stretching from Nevada into the Southern Plains, and multiple MCVs across Arizona and Sonora.

Visible satellite imagery overlaid with RAP 500mb analysis and glm flashes at 9AM MST this morning courtesy of College of Dupage.

These MCVs/embedded shortwaves are associated with the mid level clouds and isolated showers and thunderstorms across the region early this morning. 

Unlike yesterday, skies are mostly clear this morning across Southeast Arizona with less stubborn debris clouds to deal with. In fact, should have several hours of mostly clear skies this morning/early afternoon due to weak subsidence (induced by differential AVA) in the wake of an MCV moving northward. 

No shortage of moisture and buoyancy today as indicated by the 12z TUS sounding which measured 1.80 inches of precipitable water (well above the 90th percentile based on sounding climo), a surface dew point of 67 degF, and over 1000 J/kg of Mixed-layer CAPE.

12z TUS sounding this morning courtesy of the SPC.

 However, the vertical wind profile is relatively unfavorable with hardly any wind shear and weak easterly mid level flow. 

Today's Forecast

Synoptics/Dynamics

The synoptics and associated dynamics will become gradually increasingly favorable throughout the afternoon and into the evening hours as a shortwave embedded in the northeast quadrant of the low over the northern Gulf of CA moves northward into Southeast Arizona.

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

Initially early this afternoon, southeasterly/easterly mid level flow will be quite weak (on the order of 5 to 10 knots) which will allow slow storm propagation off the higher terrain. As the embedded shortwave approaches, mid level flow will enhance to around 15 to 20 knots out of the southeast later this evening.

12z GFS 500mb cyclonic vorticity valid at 11PM MST this evening courtesy of Tropical Tidbits.

This feature could also allow some nocturnal shower and storm activity to persist across the region tonight. 

Thermodynamics

CAMs are in good agreement that the best chance for showers and storms in the Tucson vicinity will be late this afternoon/early evening (between 4PM and 7PM MST). Both the 12z and 15z KTUS WRF-HRRR sounding forecasting between 500 and 1000 J/kg of MLCAPE this afternoon as shown below.

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

With sufficient buoyancy, relatively deep moisture, and DCAPE near 1100 J/kg in combination with weak steering flow, my concern is for the potential of localized flooding and even wet microbursts (will provide more details regarding precip in the next section). Overall, the thermodynamics are favorable for thunderstorms in the Tucson area this afternoon and evening.

The highest probability for storms in Phoenix would be during the early evening hours when outflows are forecast to move through the area. The 12z and 15z WRF-HRRR aren't very bullish with thermodynamics during this time as the 12z and 15z KPHX forecast sounding only showing around 300 J/kg of MLCAPE and between -50 and -150 J/kg of MLCIN.

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

It will be a race against developing nocturnal CIN in Phoenix, so outflows would need to be sufficient enough to breach CIN in order for storms to initiate.

What to Expect/Impacts

In general, it's looking to be quite an active day across Arizona. CAMs are in good agreement that storms will initiate over the White Mountains, Mogollon Rim, and Sky Islands of Southeast Arizona during the early afternoon hours.

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

Weak southeasterly steering flow will allow storms to gradually propagate off the higher terrain of Southeast Arizona and CAMs forecast convection to move into the Tucson vicinity by late afternoon/early evening.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 5:30PM MST this afternoon.

 
There's some uncertainty and disagreements among CAM solutions regarding storm coverage in the lower deserts of Pinal and Maricopa Counties (including Phoenix) this evening with the 15z WRF-HRRR being the more aggressive solution.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 9:30PM MST this evening.

Storm coverage in Phoenix will be primarily dependent on the trajectory and efficiency of outflow boundaries coming off the Mogollon Rim and from convection in Southeast Arizona.

In addition, CAMs forecast some overnight activity across the region which is associated with the aforementioned embedded shortwave moving northward into the region.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 10PM MST tonight.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 1AM MST tonight.

The SPC has much of Arizona (including Tucson and Phoenix) in a marginal risk (5%) for severe grade wind gusts today. 

SPC Day 1 Wind Outlook.

This seems reasonable as DCAPE should be just sufficient enough to produce strong(ish) outflow boundaries, and this is also backed up by the 12z HREF which has 50 to 70% conditional probabilities of 4-hour max wind speeds in excess of 30 knots (where dbz > 20) in Pima, Pinal, and Maricopa Counties this afternoon and evening.

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

However, what concerns me the most is the potential for excessive rainfall and localized flash flooding, particularly in Santa Cruz and Southeastern Pima County. The 12z HREF has 70% probabilities of 3-hour precipitation in excess of one inch, and a 10% probability of 3-hour precipitation in excess of three inches within a 40km radius late this afternoon in those areas.

12z HREF ensemble mean 3-hour precipitation and 3-hour probabilities of rainfall in excess of one inch (brown contour) and excess of three inches (red contour) valid at 5PM MST this afternoon courtesy of the SPC. 

Driving conditions will be extremely hazardous today especially along the I10 corridor between Wilcox and Casa Grande as well as the I19 corridor between Tucson and Nogales. The Tucson evening commute could also be impacted with the main risks being reduced visibility from blowing dust and localized flooded roadways.

Tomorrow's Forecast

Models forecast somewhat of a down day tomorrow which seems likely as Southern Arizona will be dealing with subsidence in the wake of the embedded shortwave and overnight convection will likely over work the thermodynamics. Expect a delayed storm initiation tomorrow and activity likely confined mainly to the higher terrain and near the International Border.