August 31st, 2024: Labor Day Weekend Forecast

 Synopsis

A weak mid level low will dominate the synoptic scale pattern across Arizona this weekend. Today will likely be the most active day this weekend as Arizona will be within the northeast quadrant of the upper level low where synoptic scale forcing is maximized. Poor moisture will limit strong buoyancy this afternoon, but a dry subcloud layer will promote a risk for locally strong outflow winds and blowing dust. 

There's quite a bit of uncertainty tomorrow regarding the extent/timing of synoptic scale forcing as well as the amount of instability. At this time, expect a chance for scattered showers and thunderstorms across mainly the higher terrain with isolated chances in the lower deserts.

By Monday (Labor Day), the upper level low is forecast to lift northeast with the 500mb anticyclone rebuilding across the Desert Southwest. This will lead to dry weather and increasing high temperatures next week, with some lower desert locations potentially reaching excessive heat criteria.

Current Conditions

This morning's 12z TUS observed sounding measured just north of an inch of pwat, a surface dew point of 50 degF, and very poor buoyancy.

12z TUS Sounding this morning courtesy of the SPC.

So for late August, thermodynamics are underwhelming.

Today's Forecast

Synoptics/Dynamics

The broad synoptic scale mid level pattern today is characterized by a low pressure system to the southwest, a diffused area of high pressure near the Four Corners, and weak easterly flow aloft across Arizona. 

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

At this time, today looks to be the best day for synoptic scale forcing due to Arizona being in the favorable northeast quadrant of this mid level low. Differential cyclonic vorticity advection and associated falling mid level heights will promote modest, synoptic scale ascent across the state this afternoon and evening. The mid level wind field is quite weak, so the magnitude of synoptic scale ascent is relatively small, but any assistance from synoptic scale dynamics is always welcomed.

With mid level easterlies being quite weak, organized storms/storm clusters are unlikely with storms remaining in more of a pulse mode today.

 

Thermodynamics

Our atmosphere is moisture starved this weekend, so buoyancy will be somewhat limited during the afternoon and evening hours. Strong surface heating will allow low and mid level lapse rates to steepen by this afternoon across the region. The 12z and 15z WRF-HRRR KTUS  model soundings forecast around 250 J/kg of Mixed-layer CAPE and minimal CIN by this afternoon.

12z (left) and 15z (right) KTUS WRF-HRRR soundings valid for 3PM MST this afternoon.

Similar thermos are expected in Phoenix as indicated by UA WRF KPHX model soundings.

12z (left) and 15z (right) KPHX WRF-HRRR soundings valid for 3PM MST this afternoon.

With large dew point depressions, DCAPE is expected to be sufficient enough (on the order of 1500 J/kg) to produce strong(ish) outflow boundaries. 

What to Expect/Potential Impacts

Even though there is some modest synoptic scale forcing induced by the mid level low to the southwest, storm development in the lower elevations will likely be dependent on outflow boundary induced lift this afternoon and evening. 

Both the 12z and 15z UA WRF-HRRR solutions forecast convection to initiate over the White Mountains, Mogollon Rim, and higher terrain of Southeast Arizona during the early to mid afternoon hours as shown below.

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

Storm coverage in the vicinity of the Tucson and Phoenix metros will likely be dependent on outflow boundaries from convection over the surrounding higher terrain. Both the 12z and 15z WRF-HRRR solutions are resolving convection this afternoon in the Tucson and Phoenix vicinities correlated to both runs forecasting outflows to move through those areas.

12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for this afternoon at 5PM MST.

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

Therefore, if efficient outflow boundaries can move through the Tucson and Phoenix vicinities this afternoon and evening, convection is likely to occur in those areas.

With minimal moisture and associated poor buoyancy, not expecting storms to produce excessive rainfall or large hail as the main threat will likely be locally gusty winds and blowing dust. The 12z HREF has 50 to 70% probabilities of 4-hour max wind speeds in excess of 30 knots (where dbz>20) across much of Central Arizona (including Tucson and Phoenix) this afternoon and evening.

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

U of A has a football game tonight at 7:30PM MST, so driving conditions in the Tucson metro as well as along the I10 could be extremely hazardous with the main impacts being reduced visibility from blowing dust. 

Tomorrow's Forecast

By tomorrow, the aforementioned mid level low will lift northeastward and become centered in extreme Northern Sonora with a diffused area of high pressure over the Rockies and a mid latitude trough deepening off the West Coast.

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

It appears Southeast Arizona will remain in somewhat of a favorable synoptic scale region of the low, but there's quite a bit of uncertainty regarding the amount of buoyancy to work with tomorrow afternoon and evening. 

At this time, expect scattered showers and thunderstorms across the eastern half of Arizona with activity likely to remain confined mainly to the higher terrain.

Labor Day Trough Next Week's Forecast

By Monday, the aforementioned mid level low will lift northeastward into the Southern Plains, causing mid level heights to rise and associated subsidence to develop across Arizona. High pressure aloft will also begin to rebuild into the Desert Southwest which will lead to increasing temperatures and suppressing convection Monday into the latter part of next week.

12z GFS 500mb cyclonic vorticity loop valid for 12z Monday through 12z Friday of next week courtesy of Tropical Tidbits.

It looks like September will begin high and dry, with daytime temperatures well above normal next week and potentially reaching excessive heat criteria in some of the lower desert locations. 

This will likely be the final discussion of the 2024 monsoon season as the semester has started and I don't really see any strong indication of moisture returning to the region after this weekend. If anything significant is expected during this month, I will write a discussion, but that doesn't seem likely at this time as I expect westerlies to begin to dominate our weather going forward. 

Eyad and I genuinely enjoyed writing these discussions, and I hope you all found them useful! Please don't hesitate to provide feedback, so we can continue to improve our discussions for next season!