July 19th, 2024: Mid Grade Monsoon Pattern Today Through The Weekend

 Synopsis

The 500mb anticyclone will become anchored near the AZ/NV/UT Border region this afternoon through Sunday with northeasterly flow aloft over Arizona. The relatively favorable synoptic setup combined with near to slightly above average moisture will promote a Mid Grade Monsoon pattern through the weekend and into early next week with storm coverage, timing, and intensity dependent on day-to-day variability. Expecting a relatively similar outcome today compared to yesterday due to sufficient thermodynamics and northeasterly flow aloft. Storms will mainly remain confined to the higher terrain and in SE AZ (including Tucson) with a slight chance (< 20%) elsewhere. Main thunderstorm impacts today are lightning, gusty winds, blowing dust, and brief heavy rainfall.



 Yesterday's Cliff Notes


Scattered showers and thunderstorms developed over the Sky Islands, Mogollon Rim, and White Mountains yesterday afternoon with outflow boundaries initiating showers and storms in the Tucson vicinity during the early to mid evening hours. The Tucson International Airport received 0.57 inches of rainfall with gauges around the metro reporting between a few hundredths of an inch to above half an inch last night. A couple of the stronger cells in eastern Pima County produced 60 mph wind gusts reported via SPC storm reports. Both the 12z and 15z WRF-HRRR runs yesterday performed relatively well especially with storm coverage and timing in the Tucson vicinity. 

Unfortunately, strong outflow boundaries were unable to move through Phoenix last night to initiate convection.



Current Conditions


At 9:45 AM MST, visible satellite imagery displays mostly clear skies across Arizona so should no problem with destabilization this afternoon.


Visible satellite imagery as of 9:45AM MST courtesy of College of Dupage.


The 12z TUS sounding this morning measured 1.50 inches of precipitable water, around 1100 J/kg of CAPE, and a surface dew point of 65 degF.


12z TUS Sounding courtesy of the SPC.


Quite a bit of moisture in the mid to upper levels as indicated by the near saturated profile between 600mb and 300mb, but there's a noticeable dry slot near 850mb. In addition, mid level winds are beginning to turn more northeasterly as the 500mb anticyclone begins to shift west toward the AZ/NV/UT Border region today.



Today's Forecast


Synoptics


As mentioned above, the 500mb anticyclone will shift west toward the AZ/NV/UT Border region throughout the day with northeasterly flow aloft developing over eastern AZ.


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


Also, similar to yesterday, a subtle embedded shortwave in New Mexico diving southwestward along the eastern flank of the ridge is forecast to move through SE AZ today. This feature was indicated by 12z GFS and SPC 500mb analysis this morning, and this could once again aid in storm initiation over the higher terrain and in SE AZ.


Otherwise, main difference compared to yesterday is mid level northeasterly winds marginally increasing to between 15 and 20 knots this afternoon which favors storm activity to move toward the lower deserts later today.



Thermodynamics


CAMs forecasting only around 200 J/kg of CAPE late this afternoon in Tucson which I am almost certain will be a considerable underestimation. My reasoning is yesterday's 12z (and 15z) KTUS WRF-HRRR sounding forecast 321 J/kg of SBCAPE and a surface dew point of 49 degF at 00z which was well-below what was measured by the 00z observed TUS sounding as shown below.


Yesterday's 12z KTUS WRF-HRRR sounding valid for 00z (5:00PM MST). 


00z TUS sounding yesterday afternoon courtesy of the SPC.


In addition, this morning's 12z KTUS WRF-HRRR sounding forecast the surface dew point to be 53 degF at 10:30AM MST, but surface observations at the airport and around Tucson measuring dew points in the low 60s. Therefore, am expecting CAPE values to be well-above what is being forecast and likely near 1000 J/kg late this afternoon/early evening in Tucson.


Other than that, sufficient heating of the surface will allow low and mid level lapse rates to steepen by this afternoon with DCAPE likely approaching near 1500 J/kg again today.


12z KTUS WRF-HRRR sounding for this 6:00PM MST this evening.



What to Expect/Impacts


In general, expect a relatively identical scenario today compared to yesterday with scattered showers and thunderstorms across the higher terrain and in SE AZ (including Tucson) this afternoon and evening. Even with the aforementioned subtle embedded shortwave moving into SE AZ, not expecting much in the way of organized storm clusters. In fact, storm coverage will be primarily dependent on outflow boundary trajectories.


CAMs are in good agreement that storms will initiate over the White Mountains and Mogollon Rim by early afternoon with storms developing in Cochise and Santa Cruz counties by mid afternoon as outflow boundaries begin to move west/southwestward. 


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


Even with CAMs forecasting such limited buoyancy in Tucson, both the 12z and 15z WRF-HRRR show outflow boundaries moving through Pima County and initiating storms in the Tucson vicinity this evening around the same time frame as yesterday.


12z (left) and 15z (right) WRF-HRRR simulated reflectivity for this afternoon at 8:30PM MST.


Even though CAMs don't show storms in Phoenix today, I cannot completely rule out some isolated, short-lived cells in the vicinity if a potent outflow boundary can move through the metro. Otherwise, storms are expected to remain confined mainly to the higher terrain and in SE AZ (including Tucson) this afternoon and evening. Main impacts from storms will be brief heavy rainfall, lightning, and strong gusty winds and associated blowing dust. Traveling conditions will be hazardous along the I10 corridor between Tucson and the AZ/NM Border as  well as along the I19 corridor between Tucson and Nogales. 



Weekend Forecast


The 500mb anticyclone will become anchored near the AZ/NV/UT Border region through the weekend with precipitable water remaining near to slightly above normal. Also, a broad trough over the Plains will tighten the mid level height gradient acting to enhance mid level northeasterly flow aloft over AZ.


12z GEFS 500mb cyclonic vorticity loop for 12z tomorrow through 12z Monday courtesy of Tropical Tidbits.

 


12z GEFS precipitable water normalized anomaly loop for 12z tomorrow through 12z Monday courtesy of Tropical Tidbits.

Models are indicating a gradual uptick in activity this weekend into early next week due brisk mid level flow and potentially a few embedded shortwaves diving southward into the region. Hesitant to go too much into detail as day to day variability can have a significant impact on storm coverage, timing, and intensity. Overall expect a gradual uptick in storm coverage with increasing chances of storms in the lower desert this weekend into early next week.






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