August 18th, 2024: Convection to Remain Confined Mainly South and East of Tucson Today

 Synopsis

Widespread overnight storm activity across Southeastern and South-Central Arizona will lead to a decrease in diurnally-driven convection today. The broad synoptic scale pattern remains similar to yesterday with a mid latitude trough off the West Coast, the 500mb anticyclone in New Mexico, and southerly/southeasterly flow aloft across Arizona. Strong heating of the surface and sufficient boundary layer moisture will allow modest buoyancy across Southeastern Arizona this afternoon and evening. CAMs forecast storm coverage to remain mainly over the higher terrain, but isolated storm activity is possible in the Tucson vicinity this evening. Primary impacts from storms today include gusty winds, blowing dust, and brief heavy rainfall.

The synoptics become increasingly unfavorable tomorrow due to subsidence leading to a decrease in storm coverage.

Last Night's Cliff Notes

Last night...well that was interesting. Storms fired up across the Tucson vicinity between 9PM and midnight providing quite the bolt show, spotty brief heavy rainfall, and gusty winds. Looking at obs/reports this morning, looks like accumulated rainfall was highest farther to the north near Flowing Wells, Casas Adobes, and Oro Valley where between 0.25 and 0.5 inches of rain was measured. There was also a 62 mph wind gust reported west of Oro Valley last night. 

Scattered showers and storms eventually made it to Phoenix during the early morning hours with obs reporting precip between 0.10 and 0.25 inches across the area. Dynamics associated with an MCC in Sonora was primarily responsible for this overnight activity, but neither the 12z or 15z WRF-HRRR as well as the raw morning runs of the HRRR yesterday picked up on nocturnal storm chances.  I've seen this before though as CAMs tend to have a difficult time resolving impacts in Arizona from MCS activity in Sonora.

Current Conditions

As of 10AM MST, visible satellite imagery overlaid with 500mb RAP analysis displays a band of mid level clouds and embedded showers and thunderstorms across Central and Northern Arizona.

Visible satellite imagery overlaid with RAP 500mb analysis as of 10AM MST courtesy of the College of Dupage.

This region of clouds and showers is associated with a couple of MCVs moving northward coupled with upper level divergence within the right entrance region of a 300mb speed max over the Southern Great Basin.

SPC 300mb analysis as of 10AM MST this morning.

Zooming out, the broad synoptic scale upper air pattern remains similar today with a high amplitude ridge to our east, a deep mid latitude trough to our west, and southerly/southeasterly flow aloft across Arizona.

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

The 12z TUS sounding was unavailable today, and the PHX sounding is contaminated by convection so will use SPC mesoanalysis to show current thermos. 

I assume low and mid level lapse rates are messy this morning as indicated by low quantities of MLCAPE and large quantities of MLCIN across the region via SPC mesoanalysis.

SPC mesoanalysis of MLCAPE and MLCIN as of 10AM MST this morning.

Today's Forecast

Synoptics/Dynamics


The broad synoptic scale 500mb pattern will remain consistent throughout the day with southerly/southeasterly mid level flow across Arizona. Southeast Arizona is currently in a region of localized differential AVA due to being in the wake of an MCV to the north. This is keeping mid level cloud cover at bay across the area and allowing the surface to heat freely. 

In addition, Southeast Arizona may get some marginal assistance from upper level divergence within the right entrance region of the aforementioned speed max in the Great Basin today.

12z GFS 250mb wind speed valid for 5PM MST this afternoon courtesy of Tropical Tidbits.

This region of upper level divergence is quite subtle and weak, so anticipating convection to be primarily thermodynamically driven today. Overall, the synoptics are neutral to slightly favorable for convection today.

Thermodynamics

The main issue for storm coverage and intensity today is the low and mid level lapse rates. There should be enough surface heating to mix out most of the CIN and any inversions, but both the 12z and 15z WRF-HRRR KTUS soundings forecast a minimum of between 40 and 70 J/kg of CIN later this afternoon.

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

The 15z has been a bit too aggressive lately, so am thinking the 12z will likely be more representative of the thermodynamic environment later today with maximum MLCAPE likely remaining below 500 J/kg. DCAPE is forecast to be around 1500 J/kg by this afternoon, so storms over the higher terrain should be able to produce strong(ish) outflow boundaries. 

What to Expect/Potential Impacts

Overall, expecting decreased storm coverage and intensity due to the atmosphere being worked over from overnight convection. Both the 12z and 15z WRF-HRRR as well as hourly runs of the HRRR this morning forecast convection in the Tucson vicinity later today. Am a bit skeptical of these CAM solutions considering they were too aggressive yesterday in Tucson when buoyancy was better than it is today.

Am expecting the higher terrain to do well though as differential heating should be able to initiate convection to the south and east of Tucson by mid afternoon.

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

By early evening, CAMs forecast robust outflow boundaries to move west/northwest toward Tucson and forecast convection to initiate in the vicinity.

12z (left) and 15z (right) WRF-HRRR 10-meter winds valid for 6PM MST this evening.

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

If outflows are able to move through the Tucson area, they would need to be strong enough to breach CIN. I would say the probability of storms in Tucson today is around 30% with greater chances over the surrounding higher terrain.

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

12z HREF 4-hour max wind speeds and probabilities of winds in excess of 30 knots where dbz>20 valid for 6PM MST this evening courtesy of the SPC.

Excessive rainfall is unlikely due to relatively brisk steering flow and a dry subcloud layer, and the 12z HREF only has 10% probabilities of 3-hour precipitation in excess of one inch in Southeast Arizona. However, isolated flash flooding is possible near recent burn scars and in poor drainage areas. 

Hazardous traveling conditions can be expected along the I10 corridor between Willcox and Tucson as well as along the I19 corridor between Tucson and Nogales with the main threat being reduced visibility from blowing dust.

Tomorrow's Forecast

I'm expecting storm coverage to be even more isolated tomorrow due to shortwave ridging developing over Southeast Arizona in response to an approaching inverted trough to the southeast.

12z GFS 500mb cyclonic vorticity valid for tomorrow at 11AM MST.

Differential anticyclonic vorticity advection will generate subsidence across Southern Arizona, and model soundings appear to be picking up on this.

12z KTUS WRF-HRRR sounding valid for 11AM MST tomorrow.