August 2nd, 2024: Marginal Risk of Severe Wind Gusts in Southeast Arizona Today

Synopsis

The mid level subtropical ridge will continue to rebuild into the Intermountain West today with the 500mb anticyclone becoming centered near the Four Corners region and southeasterly flow aloft over Arizona. Rising mid level heights will lead to increasing surface temperatures across the lower deserts with some areas reaching excessive heat criteria this afternoon. Sufficient moisture combined with hot surface temps (100 to 105 degF) will allow MLCAPE to reach between 1000 and 1500 J/kg this afternoon and evening across Southeast Arizona. SPC has southeastern AZ (including Tucson) in a marginal risk for severe grade wind gusts (5%) due to forecast DCAPE values above 1500 J/kg this afternoon and evening. 

Storm activity is expected to decrease and high temperatures to increase this weekend due to a strong ridge overhead and dry air moving in from the east.

Current Conditions

Visible satellite imagery and lightning detection this morning displays scattered mid level clouds and isolated showers and thunderstorms across southwest Arizona, and an MCV propagating northward along the I10 corridor toward Phoenix.

Visible imagery overlaid with GLM flashes this morning courtesy of College of Dupage.

12z GFS 500mb analysis shows an upper level low moving northward along the Southern California coast and the anticyclone centered over southern Colorado with southeasterly flow aloft over Arizona.

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

SPC mesoanalysis of precipitable water this morning shows a moisture gradient (increasing from east to west) with precipitable water ranging from 1 to 2 inches across Arizona.

12z SPC mesoanalyis of total precipitable water.

The 12z TUS sounding shows an impressive thermodynamic profile which measured 1.84 inches of precipitable water, a surface dew point of 64 degF, and near 1400 J/kg of mixed-layer CAPE.

12z TUS sounding this morning courtesy of the SPC.

There's a lot to like about this sounding, especially the relatively high levels of buoyancy that aren't associated with ankle deep moisture. Also, there is some CIN (around -200 J/kg) which is a good thing because it will suppress convection throughout the late morning and early afternoon hours allowing the low and mid level environmental lapse rates to steepen.

Today's Forecast

Synoptics/Dynamics

The 500mb anticyclone will remain anchored near the Four Corner region with southeasterly flow aloft across Arizona today. Unlike the past 48 hours, we don't have any synoptic scale features (i.e. inverted troughs) to focus storm development and propagation. In fact, with the mid level ridge amplifying and associated 500mb heights rising, the synoptics are somewhat unfavorable for storm development today. However, the anticyclone remains well to our northeast so not expecting subsidence to completely suppress convection this afternoon and evening.

As mentioned above, an MCV is moving northward into Phoenix this morning which is leaving behind some localized subsidence in its wake across Southeast Arizona. This may seem like a bad thing, but this will keep midday storm development at bay and allow the surface to heat throughout the day today.

Thermodynamics

With relatively poor synoptics/dynamics, storm development and propagation will be primarily dependent on thermodynamic circulations this afternoon and evening. CAMs are in relatively good agreement that the combination of strong surface heating and sufficient low level moisture will allow mixed-layer CAPE to reach between 1000 and 1500 J/kg this afternoon and evening across southeast Arizona. The 12z KTUS WRF-HRRR sounding forecasts a well-mixed layer between the surface and 600mb, MLCAPE above 1000 J/kg, and minimal CIN late this afternoon.

12z KTUS WRF-HRRR sounding valid for 4:30PM MST this afternoon.

Also, CAMs forecast DCAPE above 1500 J/kg promoting a greater threat for strong outflow boundaries this afternoon. 

Lastly, upper level winds are forecast to be weaker than mid level flow, which would eliminate any threat of anvil shading (generates CIN).

Overall, thermodynamics are favorable for storm development and propagation in southeast Arizona this afternoon and evening.

What to Expect/Impacts

Both the 12z and 15z WRF-HRRR are in good agreement regarding storm coverage across southeast Arizona (including the Tucson metro) this afternoon and evening with the 12z being slightly more bullish. The synoptic pattern and associated dynamics will remain marginally unfavorable, and therefore will likely lead to a delayed convective initiation today. CAMs forecast convection to initiate over the higher terrain in Cochise, Santa Cruz, and Eastern Pima County by mid afternoon.

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

Once storms initiate over the higher terrain, propagation/additional cell development will be primarily dependent on outflow boundary trajectories and efficiency to produce lift, and considering DCAPE forecast to reach above 1500 J/kg this afternoon there is a good potential for strong outflow boundaries. Both the 12z and 15z WRF-HRRR forecast outflow boundaries to move through the Tucson metro from the south (associated with convection in Santa Cruz County) and southeast (associated with convection in Cochise County) late this afternoon/early evening initiating storms in the Tucson area as shown below. 

12z WRF-HRRR 10-meter winds valid for 5:30PM MST this afternoon.

12z WRF-HRRR simulated reflectivity valid for 6:30PM MST this evening.

12z WRF-HRRR 10-meter winds valid for 5:30PM MST this afternoon.

12z WRF-HRRR simulated reflectivity valid for 7:00PM MST this evening.

In general, if outflow boundaries can move into the Tucson metro late this afternoon/early evening, there should be plenty of buoyancy for storms to develop in the area. Outflows from the southeast would be favorable as diurnally driven northwesterly surface winds would maximize storm relative inflow and surface convergence. 

SPC has southeast Arizona (including Tucson) in a marginal risk (5%) for severe grade wind gusts today. 

SPC Day1 Wind Outlook.

This seems reasonable as the 12z HREF has 70% probabilities of 4-hour max wind speeds in excess of 30 knots (where dbz > 20) in Santa Cruz, western Cochise, and eastern Pima Counties (including Tucson) this evening.

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

The main risk will be isolated severe wind gusts, so not anticipating too much of an excessive rainfall threat today. However, isolated flash flooding is always a possibility especially near recent burn scars and in poor drainage areas. 

Traveling conditions could be extremely hazardous especially along the I19 corridor and along the I10 corridor between Willcox and Tucson as well as between Tucson and Casa Grande where poor visibility is possible due to blowing dust. 

Unfortunately, it will be much warmer today across the lower deserts due to the amplifying ridge overhead. High temps are expected to be between 100 and 105 degF in the lower deserts of southeast Arizona (including Tucson) and near 110 degF between Phoenix and Yuma.

12z WRF-HRRR 2-meter temperatures valid for 3PM MST this afternoon.

This Weekend

Models are in good agreement that the 500mb anticyclone will remain centered near the Four Corners through this weekend with weak easterly/southeasterly flow aloft over Arizona.

12z GEFS 500mb cyclonic vorticity loop from 12z Saturday through 12z Monday courtesy of Tropical Tidbits.

Even though the synoptic pattern is somewhat favorable based on climo, models forecast dry air to move into Arizona from the east with precipitable water dropping across the region Saturday and Sunday. 

12z WRF-HRRR total predictable water valid for Saturday at 5AM MST.

12z WRF-HRRR total predictable water valid for Sunday at 5AM MST.

With decreasing moisture and a strong ridge overhead, expect a lower grade monsoon pattern with storm coverage likely to remain confined mainly to the higher terrain and near the International Border. This also means high temperatures will increase with the forecast transitioning to highlight excessive heat rather than precipitation.