July 27th, 2024: Storm Activity Confined to Southeast Arizona This Weekend

 Synopsis

The 500mb anticyclone will be centered over South-Central/Southeastern Arizona today. This synoptic pattern is characteristic of decreased storm activity with coverage remaining confined to mainly the higher terrain of Southeast Arizona. The anticyclone is expected to shift east tomorrow and become anchored in Southern New Mexico with weak southerly/southeasterly flow aloft in Southeast Arizona. In addition, a subtle shortwave is forecast to move northward into Southeast Arizona which could allow an uptick in storm coverage tomorrow afternoon. The majority of activity will remain to the south and east of Tucson, but a few showers and storms are possible in the vicinity this weekend with better chances tomorrow. 

Current Conditions 

A trough moving into California continues to flatten the mid level subtropical ridge and is displacing the anticyclone to the south and east. 12z 500mb analysis this morning shows the center of the high over Southeast Arizona.

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

It is quite humid this morning with surface observations reporting dew points in the mid to upper 60s across the lower deserts thanks to the Gulf of California and overnight/early morning convection.

Surface dew point observations as of 9:30AM MST courtesy of the NWS.

The 12z TUS sounding this morning shows an impressive column of moisture with 1.85 inches of precipitable water, a surface dew point of 65 degF, and a near saturated profile in the 700 to 400mb layer.

12z TUS sounding this morning courtesy of SPC.

However, expecting the moisture in the mid to upper levels to dry out as westerly winds aloft will transport dry air upstream into the region. 

  

Today's Forecast



Synoptics

The 500mb anticyclone will remain centered over South-Central/Southeastern Arizona today generating messy, weak mid level flow across Arizona. Winds in the upper half of the troposphere are predominantly westerly allowing drier air from the west to invade our atmosphere. The combination of the anticyclone directly overhead and drier upper level air will promote subsidence across the region and therefore an unfavorable synoptic pattern.

Thermodynamics

Both the 12z and 15z WRF-HRRR are in good agreement that the best instability will remain confined to SE AZ this afternoon and evening. The 12z KTUS WRF-HRRR sounding forecasts the maximum in CAPE at noon with around 1000 J/kg.

12z KTUS WRF-HRRR sounding valid for 12PM MST this afternoon.

Models forecast buoyancy to plummet in Tucson into the late afternoon hours due to a subsidence inversion near 500mb, drying of the boundary layer, and relatively poor low to mid level lapse rates.

12z KTUS WRF-HRRR sounding valid for 5PM MST this afternoon.

CAMs forecast a moisture recovery in Tucson with dew points rising into the low 60s during the early evening hours. The 12z KTUS WRF-HRRR sounding forecasts between 500 and 1000 J/kg of Mixed-layer CAPE this evening due to this moisture surge.

12z KTUS WRF-HRRR sounding valid at 8PM MST this evening.

Also, the potential for strong outflow winds is lower today due to decreased values of DCAPE.  

What to Expect/Impacts

A relatively poor synoptic pattern will be in a tug of war with somewhat sufficient thermodynamics in SE AZ today. CAMs are in good agreement that scattered showers and thunderstorms will occur over the higher terrain in SE AZ during the afternoon hours.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid at 2:00PM MST.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid at 5:00PM MST.

I can't rule out a weakening cell or two coming off Mt. Lemmon into Oro Valley or the Catalina Foothills, but activity should overall remain outside the Tucson area this afternoon. The best chance for showers and storms in Tucson would be this evening when CAPE increases due to low level moisture recovery. CAMs aren't too excited with this scenario which seems likely as lift from outflow boundaries would need to be strong enough to breach -100 to -200 J/kg of CIN and a subsidence inversion in the mid levels. 

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid at 8:00PM MST.

With that being said, showers and thunderstorms will remain mainly confined to the higher terrain of SE AZ this afternoon and evening. I cannot rule out some activity in the Tucson vicinity, but it is completely dependent on outflow boundary trajectories and efficiency to produce ascent.

Any thunderstorm today will be able to produce lightning, gusty winds, blowing dust, and locally heavy rainfall. 

Traveling conditions could be hazardous along the I10 corridor from Tucson to the AZ/NM Border as well as along the I19 corridor between Tucson and Nogales.

Tomorrow's Forecast

Current guidance is suggesting that tomorrow could be marginally more active as the 500mb anticyclone shifts east into western New Mexico with weak southeasterly/southerly mid level flow across SE AZ.

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

Both the GFS and ECMWF show a very subtle shortwave coming northward across the Border tomorrow afternoon which could provide some synoptic scale forcing. Model soundings forecast around 1000 J/kg of Mixed Layer CAPE tomorrow afternoon in Tucson.

12z KTUS WRF-HRRR sounding valid for tomorrow at 3PM MST.

Based on current guidance, it looks like there should be a marginal uptick in storm coverage. Still some uncertainty regarding the timing, coverage, and intensity as day to day variability has had a significant impact on forecasts beyond 24 hours this month.