July 26th, 2024: Delayed Initiation Today With Decreasing Storm Activity This Weekend

 Synopsis

Plenty of moisture remains in place for storm development today, but a delayed initiation and decreased coverage is likely due to an overworked atmosphere and a more unfavorable synoptic pattern. Showers and thunderstorms will be possible, mainly in SE AZ (including Tucson) later this afternoon through the late evening hours.  

Storm coverage is expected to decrease this weekend due to the 500mb anticyclone becoming displaced farther to the south and east with predominately drier westerly/southwesterly flow aloft across the region. Some isolated storm activity is possible, but will likely remain confined to the higher terrain and near the International Border Saturday and Sunday.

Yesterday's Cliff Notes

Another active afternoon and evening with numerous showers and thunderstorms across Arizona. Better storm coverage in SE AZ yesterday evening (including Tucson) with SPC storm reports showing damage to power lines and trees from severe grade wind gusts across Pima, Pinal, and Maricopa Counties. The main highlight was precipitation, especially in Tucson, where the International Airport measured a total of 1.83 inches! Other gauges in the Tucson metro reported precip between 1 and 2 inches so quite the soaker last night. 

Outflow from the southeast initiated numerous strong storms along the I10 corridor between Tucson and Phoenix as well as along the I8 corridor between Casa Grande and Gila Bend. This outflow boundary did initiate some showers and weak thunderstorms in the Phoenix Metro, but not much precipitation was reported (only a trace at Sky Harbor).

Current Conditions

Visible satellite imagery this morning displays mostly clear skies across Arizona, an MCV south of the Border, and another MCV over the White Mountains.

Visible satellite imagery this morning courtesy of College of Dupage.

These MCVs are associated with storm clusters last night and will continue to move southward throughout the morning hours.

The synoptic upper air pattern is beginning to change with a trough moving into California flattening the subtropical ridge and displacing the 500mb anticyclone farther to the south.

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

Still plenty of moisture with the 12z TUS sounding measuring 1.38 inches of precipitable water and a surface dew point of 68 degF.

12z TUS sounding this morning courtesy of the SPC.

However, the ample low level moisture is really shallow with a dry slot between 850 and 600mb. Over 1000 J/kg of CAPE this morning, but that's primarily due to the high dew point at the surface. There is also quite a bit of CIN to erode throughout the day today (-404 J/kg).

 Today's Forecast

Synoptics

Synoptics are more unfavorable for storm development today for a few reasons. First, mid level northerly/northeasterly flow is much weaker and messy due to the subtropical ridge weakening. Also, the 500mb anticyclone will slowly continue to slide south/southeast with mid level winds beginning to turn westerly in response. Mid level water vapor imagery shows a large source of dry air beginning to invade our atmosphere from the west as shown below.

Mid level water vapor imagery overlaid with RAP 500mb analysis as of 9:00AM MST this morning courtesy of College of Dupage.

In addition, the MCVs moving southward this morning are leaving subsidence in their wake which will have an impact on convective initiation over the terrain this afternoon. 

Overall, the synoptic pattern promotes a delayed convective initiation and less coverage of storms today.

Thermodynamics

CAMs are in pretty good agreement that it will take a few extra hours today for the atmosphere to destabilize over the higher terrain for convective initiation to occur. The 12z KTUS WRF-HRRR sounding forecasts an unimpressive thermodynamic profile this afternoon with only between 100 to 300 J/kg of CAPE and surface dew points dropping into the low 50s.

12z KTUS WRF-HRRR sounding for 5:00PM MST this afternoon.

Am expecting CAPE to be higher than what WRF is forecasting, likely will see MLCAPE around 500 J/kg this afternoon across the region which is much less than yesterday. DCAPE also forecast to be a tad lower (around 1300 J/kg), but still sufficient enough to produce outflow boundaries. 

The 12z and 15z WRF-HRRR as well as hourly runs of the HRRR this morning in good agreement that the best chance for storms in the lower deserts will be later in the evening. CAMs forecast dew points to rapidly increase in Pima, Pinal, and Maricopa Counties late this evening due to a Gulf of CA moisture surge and outflow boundaries acting to increase CAPE in the lower deserts.

12z WRF-HRRR 2-meter dew points valid for 10:00PM MST this evening.

12z WRF-HRRR maximum CAPE valid for 10:00PM MST this evening.

Model soundings forecasting between 500 and 1000 J/kg of Mixed-Layer CAPE during this time frame with the only caveat being the development of CIN (between -100 and -300 J/kg).

12z KTUS (left) and KPHX (right) WRF-HRRR soundings valid for 10:00PM MST this evening.

What to Expect/Impacts

A combination of an overworked atmosphere, poorer synoptic pattern, and less buoyancy will lead to a delayed convective initiation and less coverage of storms today. CAMs are in good agreement that storms will initiate over the higher terrain during the mid to late afternoon hours (between 2PM and 6PM MST). 

12z (left) and 15z (right) WRF-HRRR simulated reflectivity for this afternoon at 3:00PM MST.

After that outflow boundaries are forecast to (very) gradually move into the lower deserts. Both the 12z and 15z WRF-HRRR forecast cells pulsing around the lower deserts all night and even into the early morning hours with some differences in the coverage. 

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

12z (left) and 15z (right) WRF-HRRR simulated reflectivity for this evening at 11:00PM MST.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity for tomorrow morning at 2:00AM MST.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity for tomorrow morning at 5:00AM MST.

Not as concerned with the wind threat which is backed up by the 12z HREF which shows much lower probabilities of 4-hour max wind speeds (where dbz > 20) across the region this evening as shown below.

12z HREF 4-hour max wind speeds and probability of wind speeds in excess of 30 knots where dbz > 20 for tonight at 10:00PM MST courtesy of SPC.

Overall, scattered showers and thunderstorms are possible across the lower deserts of Pima (including Tucson), Pinal, and Maricopa County (including Phoenix) later tonight with probabilities increasing from northwest to southeast. Storm coverage and intensity in the major population centers (Tucson and Phoenix) will once again be primarily dependent on outflow boundary trajectories and efficiency to produce lift. 

Any storm is capable of producing lightning, gusty winds, blowing dust, and heavy rainfall. Considering the weak steering flow and the amount of precip in the Tucson area last night (higher soil moisture), am a bit concerned regarding an excessive rainfall threat. If cells anchor over a region, this could cause some flooding concerns so definitely something to keep an eye on. 

Expect hazardous traveling conditions this evening especially along the I10 corridor between Tucson and Casa Grande.

 Weekend Forecast

The synoptic upper air pattern will become increasingly more hostile into the weekend as the 500mb anticyclone becomes centered to the south and east of Arizona with drier westerly/southwesterly flow aloft over the region.

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

This sets up a low grade monsoon pattern with storm activity likely to mainly remain confined to the higher terrain of SE AZ and near the International Border. High temps aren't forecast to increase much due to enough low level moisture remaining in place and mid level heights staying below 594 dm.