July 22nd, 2024: Down Day Today, Low Grade Monsoon Pattern Through the Work Week

 Synopsis

Expect somewhat of a down day today across much of Arizona due to poor thermodynamics. Also, a dry slot is forecast to invade the mid levels from the north throughout the day limiting convection over the higher terrain this afternoon. CAMs forecast some isolated/scattered activity over the Mogollon Rim and White Mountains, but struggle to allow outflows to initiate storms in the adjacent lower deserts. SPC has a marginal risk for severe wind gusts in extreme Western AZ due to better moisture and instability. Otherwise, expect a relatively quiet day in Phoenix and Tucson with scattered showers and storms mainly remaining confined to the higher terrain. Expect a low grade monsoon pattern through Friday with day to day variability impacting timing, coverage, and intensity of storms.

Yesterday's Cliff Notes

Thunderstorms refused to develop in the major metropolitan areas of Phoenix and Tucson yesterday, but had no problem developing pretty much everywhere else. Eyad will provide a detailed summary in tomorrow's zoom discussion, but I'll provide some cliff notes. 

Storms over the Rim moved rapidly south into Phoenix, but outflow boundaries became decoupled from the storm clusters and some downsloping adiabatic warming weakened the effectiveness of outflow boundaries in Phoenix. When outflow boundaries encountered terrain south of Phoenix this caused new cells to initiate and were able to maintain new storm clusters from I8 near Gila Bend southward toward the International Border.  

In Tucson, storms initiated around the metro with the strongest storms near the Tucson mountains and along the I19 corridor down to Nogales. Mt Lemmon disrupted the southward propagating outflow boundaries which caused a lack of development in the Tucson metro, so Tucson got "Lemmoned" yesterday. 

Otherwise, SPC storm reports had quite a few damaging wind reports on the outskirts of both the Tucson and Phoenix metros as well as over the higher terrain. The Tucson International Airport received 0.10 inches and Sky Harbor received 0.02 inches. A picture paints a thousand words which you can see in the precip estimate map below.

24-hour total precipitation estimates courtesy of the NWS.

Current Conditions

Still plenty of moisture this morning with the 12z TUS sounding measuring 1.50 inches of precipitable water, a near saturated 700mb - 500mb layer, and a surface dew point of 65 degF.

12z TUS sounding this morning courtesy of SPC.

However, lapse rates aren't great, around 6 to 7 degC/km as well as relatively dry layers between 850mb and 700mb and between 500mb and 300mb. 

The mid level water vapor imagery overlaid with 500mb RAP analysis this morning highlights the forecast for today with the 500mb anticyclone centered near Southern Nevada and a dry slot diving southward into Eastern Arizona.

Mid level water vapor imagery overlaid with SPC 500mb RAP analysis this morning courtesy of College of Dupage.

Today's Forecast

Synoptics

The 500mb anticyclone will remain anchored over Southern Nevada today with northerly/northeasterly flow aloft over Arizona. Also, the mid level trough over the Southern Plains begins to move east which puts Central and Eastern AZ in an unfavorable synoptic scale region.

12z GFS 500mb cyclonic vorticity for 11:00AM MST this morning courtesy of Tropical Tidbits.

As shown in mid level water vapor imagery above, dry air in the mid levels will invade our atmosphere throughout the day today promoting an even more unfavorable environment for storm development.

12z GFS 700-300 hPa relative humidity for 11:00AM MST this morning courtesy of Tropical Tidbits.

Thermodynamics

The 12z KTUS WRF-HRRR model sounding reflects the aforementioned mid level dry layer which shows a subsidence inversion near 500mb this afternoon and associated poor buoyancy.

12z KTUS WRF-HRRR sounding for 4:00PM MST this afternoon.

Expecting a well-mixed layer below 600mb this evening though due to sufficient surface heating. Forecast DCAPE above 1500 J/kg will promote the potential for strong outflow boundaries coming off the higher terrain, but updrafts will really struggle to breach the mid level subsidence inversion this afternoon and evening. 

What to Expect/Impacts

Overall, expect a significant decrease in storm activity today especially compared to yesterday.  Also, convective initiation will likely be delayed as it will take a few extra hours for thermodynamically driven circulations to produce enough ascent to breach the subsidence inversion. CAMs forecast storms to initiate over the Rim, White Mountains, and Sky Islands of SE AZ around mid afternoon.

12z WRF-HRRR simulated reflectivity for this afternoon at 3:00PM MST.

By early evening, southwestward moving outflow boundaries could initiate a few storms in Gila, Graham, Eastern Pinal, and Cochise Counties.

12z WRF-HRRR simulated reflectivity for this evening at 8:00PM MST.

By mid to late evening, outflow boundaries are forecast to move toward Phoenix and Tucson, but high values of CIN will make it difficult for sufficient updrafts to develop and be maintained.

12z WRF-HRRR simulated reflectivity for this evening at 10:00PM MST.

12z KTUS (left) and KPHX (right) WRF-HRRR soundings for 9:00PM MST this evening.

In general, expect a delayed convective initiation over the higher terrain, and storms remaining  mainly confined to the Mogollon Rim, White Mountains, and SE AZ Sky Islands today. Cannot rule out an isolated storm or two in the Tucson vicinity, but as I said the thermodynamics are quite hostile for updrafts today. 

SPC has a marginal risk for severe wind gusts in extreme Northwestern AZ today where better instability is available. 

SPC DAY 1 Wind outlook.

 

Therefore, traveling conditions will be hazardous along I10 and especially I40 this afternoon in Northwest AZ with the main hazard being reduced visibility from blowing dust.

Tomorrow Through Friday Overview

The trough over the Plains will continue to move east today and tomorrow allowing the 500mb anticyclone to rebuild into the Intermountain West. In response, mid level winds turn more easterly/northeasterly over Arizona and mid level heights will rise to around 594 dm. 

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

By the latter half of the work week, the GFS and ECMWF as well as their ensemble members are in good agreement a trough moving into the Pacific Northwest will flatten the ridge and displace the anticyclone over Southern Arizona. However there is some model disagreement regarding moisture with the GEFS forecasting moisture to plummet and the deterministic GFS forecasting moisture to remain near to slightly above normal. This will need to be monitored closely as less moisture would mean hotter temps and decreased storm chances.

Overall, expect a low grade monsoon pattern tomorrow through Friday with day-to-day variability having a significant impact on storm timing, coverage, and intensity.