September 11th: 2023 Monsoon Farewell Event Today through Wednesday

Synopsis

A relatively weak Type IV Monsoon Transitional Pattern is expected today through Wednesday providing the region with cooler temps and an uptick in convection. Increasing moisture combined with a couple weak, embedded shortwaves will generate periods of showers and thunderstorms this afternoon through Wednesday afternoon across Arizona with some nocturnal/early morning activity possible through the period. The best activity will mainly remain confined to the higher terrain but some showers and storms will be possible in the lower deserts. Falling heights and added low level moisture will also provide the region with much cooler temperatures.

Current Conditions

Visible imagery this morning displays mostly clear skies across Arizona and an area of mid level clouds with embedded showers just off the SoCal coast moving rapidly eastward toward San Diego and Los Angeles.

Visible imagery as of 10:13 AM MST courtesy of the College of Dupage.

This area of mid level clouds and showers is associated with a weak, embedded shortwave that will eventually makes its way into Arizona later today.

The 12z GFS 500mb analysis this morning displays a couple key features of interest. First, the remnant circulation from former tropical cyclone Jova is centered several hundred miles southwest of San Diego. Also, a very subtle shortwave can be seen just off the SoCal coast. Lastly, the mid level anticyclone is centered well to the southwest of us with lower heights and brisk zonal flow across Arizona.

12z GFS 500mb analysis.

Moisture is also slowly increasing with SPC mesoanalysis displaying pwats a little over an inch in Tucson and pwats near 1.50 in in southwestern AZ.

SPC mesoanalysis of total precipitable water and 850mb moisture transport vectors as of 16:00z.

The 12z KTUS sounding still measuring an unimpressive thermodynamic profile with a surface dew point of 52 degF, 101 J/kg of CAPE, and 1.14 in of pwat.

12z TUS sounding courtesy of the SPC.

Today's Forecast

Synoptics

The overall synoptic pattern will continue to transport the remaining moisture from former tropical cyclone Jova into Arizona. Current guidance suggests that the shortwave near SoCal will be centered near the lower Colorado River Valley by late this afternoon, but moisture and instability will continue to increase throughout the day today across Southern and Central Arizona.

12z GFS 500mb absolute vorticity for this afternoon at 5:00 PM MST.

12z GFS total precipitable water for today at 5:00 PM MST.

This shortwave will move through Arizona late this afternoon into the late evening hours providing some positive vorticity advection and upper level divergence.

12z GFS 250mb divergence at 5:00 PM MST.

Moisture and Instability

Mid level moisture will continue to advect into Arizona throughout the day today with even a bit of low level moisture fanning out throughout the lower deserts thanks to the Gulf of California. I've plotted a 12z WRF-HRRR cross section of moisture transport between Yuma and Phoenix to show the depth of the moisture transport.

12z WRF-HRRR cross section of moisture transport between Yuma and Phoenix for this afternoon at 5:00 PM MST.

Continued low and mid level moisture advection combined with sufficient surface heating will aid in destabilization of our atmosphere throughout the day today. The 12z WRF-HRRR forecasts pwats to increase to between 1.25 in and 1.50 in across Southern Arizona with highest amounts to the west closer to the Gulf of California.

12z WRF-HRRR total precipitable water for today at 5:00 PM MST.

Unfortunately the 15z WRF-HRRR did not complete a full run this morning, so have to stick with the 12z guidance. The 12z KTUS WRF-HRRR sounding forecasting a maximum of around 500 J/kg of CAPE, over 1000 J/kg of DCAPE, and even some low level lift indicated by the omega profile by this afternoon.

12z KTUS WRF-HRRR sounding for today at 1:00 PM MST.

Current guidance suggests that the better activity will remain over the higher terrain to the south and east of Tucson today where slightly better instability combined with sufficient differential heating will be in place. Have plotted a 12z WRF-HRRR model sounding for Sierra Vista below for this afternoon.

12z KFHU WRF-HRRR sounding for today at 5:00 PM MST.

What to Expect

Current CAM guidance appears reasonable today since as I am writing this section there are already a couple cells over the higher terrain to the southeast of Tucson. Therefore, the best chances for storms this afternoon will remain in Cochise and Santa Cruz counties. Cannot rule out a cell or two in the Tucson vicinity this afternoon, but the chance is isolated based on current forecast guidance. What will likely keep storm activity to a minimum for areas in and west of Tucson is increasing mid level cloud cover throughout this afternoon which would limit surface heating and therefore destabilization. Below is the 12z WRF-HRRR forecast for the evolution of storms today.

12z WRF-HRRR simulated maximum reflectivity for this afternoon at 2:00 PM MST.

12z WRF-HRRR simulated maximum reflectivity for this afternoon at 4:00 PM MST.

12z WRF-HRRR simulated maximum reflectivity for this afternoon at 6:00 PM MST.

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

With DCAPE around 1000 J/kg today, any storm will be able to produce locally strong and gusty outflow winds. Severe grade hail appears unlikely but the chance is nonzero, and brisk steering flow and a dry sub cloud layer will likely keep excessive rainfall to a minimum today but isolated flash flooding is always possible.

Increasing low level moisture and cloud cover as well as lower mid level heights will provide some relief in high temperatures today with the Tucson vicinity around 100 degF and the lower deserts between Phoenix and Yuma in the 100 to 105 degF range.

12z WRF-HRRR 2-meter temps for this afternoon at 4:00 PM MST.

Overnight Forecast

The aforementioned shortwave near SoCal is forecast to move through Arizona overnight tonight increasing synoptic scale induced ascent due to positive vorticity advection.

12z GFS 500mb absolute vorticity for tonight at 11:00 PM MST.

The 12z KPHX WRF-HRRR sounding forecasts an impressive thermodynamic profile with 1800 J/kg of CAPE, 1.55 in of pwat, a surface dewpoint of 64 degF, and DCAPE around 1000 J/kg.

12z KPHX WRF-HRRR sounding for tonight at 11:00 PM MST.

The 12z WRF-HRRR forecasts showers and thunderstorms in the Phoenix vicinity overnight tonight.

12z WRF-HRRR simulated maximum reflectivity for tonight at 12:00 AM MST.

Interesting to point out however, that recent runs of the HRRR have been showing better activity farther south in the Tucson vicinity during the early morning hours rather than the Phoenix vicinity as shown below.

18z HRRR simulated reflectivity for early tomorrow morning at 4:00 AM MST.

Too easy to get fixated on the details, but in general, synoptic scale ascent combined with increasing moisture and instability will promote a chance of showers and thunderstorms across Central and Southern Arizona overnight tonight into early tomorrow morning.

Tuesday's Forecast

Considering that there is some uncertainty regarding the overnight/early morning forecast, am hesitant to provide significant details for tomorrow and Wednesday. Overnight/early morning convection could lead to mid level cloudiness and an overworked/stable atmosphere. Current guidance suggests that tomorrow could be the most active day, especially for Tucson eastward. Will need to reevaluate the forecast data tomorrow to provide a proficient forecast, but for now expect a chance of showers and storms in Southeastern Arizona tomorrow afternoon and evening. The current 12z WRF-HRRR forecast evolution of convection tomorrow is shown below.