Summary

Surface moisture remains elevated across southern Arizona, with the Gulf Surge bringing higher dewpoints northward. Today, convective conditions are even more favorable than yesterday, with forecasts consistently predicting afternoon thunderstorms for the Tucson and Phoenix areas. These storms are expected to start in the mountains and move into the valleys, bringing heavy rain and strong winds; Tucson by 7 pm and Phoenix by 10 pm. Recent ensemble forecasts show rainfall totals could range from half an inch to nearly two inches, depending on storm organization.

Tomorrow, storm activity will decrease in lower elevations, limited mostly to higher terrain, as is typical following a major storm day. Looking ahead, a quieter monsoon pattern will persist into the weekend, although a return of moisture may lead to renewed activity next week.

Yesterday’s Weather

It was a bust! We have been having a good run of accurate forecasts this season, but yesterday was not one of them. Below is the 12Z HRRR-UA WRF forecast radar for 6 pm Tuesday, and below is the Tucson Doppler radar observation at the same time. Why did the storms not develop as forecast? The clue was in the Phoenix observed sounding at 5 am, where we wrote the following comparing the model sounding to the observed sounding: 

“But what is missing from the initialization is the cooling below 700mb caused by the Gulf Surge and thunderstorm outflow. This has increased the Convective Inhibition (CIN) to over –500 [J/kg], which can’t be overcome with surface heating today.” 

Since we do not have an observed sounding in Tucson to compare to the model initialization, we don’t know if the same problem existed here at Tucson. So the state of the atmosphere that morning was one in which no amount of surface heating would allow storms to form. To make matters worse, the low-level winds were from the northwest advecting stable air into Tucson. Standing outside at 4 pm yesterday afternoon, the wind was relatively cool even though it was over 100°F, and no cumulus was in the sky…a bad omen, showing convective initiation was not possible, likely due to CIN.  

We were expecting severe weather in Tucson, but got none. We have been debating how important the observed sounding at 5 am is, since we don’t have any from the Tucson location at the University of Arizona. Well, it was a false alarm, so less of a big deal. But what would have happened if missing sounding data had resulted in a missed forecast where severe weather occurred in Arizona with no warning? Below is an animation of the zoomed-in view of the Pima County 12Z HRRR-UAWRF forecast (left), the most conservative in rainfall prediction from yesterday, and the radar observations (right).

The visible satellite image from 6:17 pm below, overlaid with mosaic radar and lightning strikes, shows thunderstorms from north-central New Mexico southwest to the White Mountains, then south into the eastern portion of Graham and Cochise County. Western Cochise County and most of Pima County had few altocumulus clouds. The Phoenix area also had mid-level clouds; remnants from leftover morning showers.  

The Rainlog data showed precipitation in the eastern portion of the state, and a couple of stations near Phoenix reported a few hundredths of an inch.

The HRRR-UAWRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right) are shown below. If we ignored the red area, it was a good forecast, and, as described above, a lack of a weather balloon launch in Tucson caused an error in understanding the vertical profile of the atmosphere that morning in the Tucson area. 

During the evening and overnight hours, a Mesoscale Convective System (MCS) developed south of Douglas and moved west to the Gulf of California. The infrared satellite image shows the location of the MCS at 2 am. 

The visible satellite image from this morning at 8 am shows the remains of the MCS over the Gulf of California with showers and thunderstorms associated with the MCS in the Casa Grande area. These lighter storms are caused by mid-level convergence and, like yesterday, should dissipate in the early afternoon.

Analysis

No significant change in surface moisture over southern Arizona, and the Gulf Surge continues with 50°F dewpoint temperatures moving northward to Flagstaff. In New Mexico, dewpoint temperatures increased in the southeast part of the state. In Tucson this morning, cumulus clouds are forming on the surrounding mountains as of 9:15am.

The 12Z HRRR-UA WRF forecast initialization and the observed sounding are similar today. The Convective Inhibition (CIN) is less than yesterday, but with all the clouds in the area this morning, afternoon storms will not develop in the valleys. That said, increased activity is expected on the Mogollon Rim as precipitable water is high, but the mid-lower atmosphere is worked over by the outflow from the MCS and its remnant clouds. 

At 250 mb, the center of the monsoon high-pressure has moved a little further east as the jet stream drops into the Midwest; now it’s centered over Pima County. The low-pressure center in Mexico, south of the high, was from the shortwave trough located underneath the high yesterday. The trough dropping south into Texas was missed yesterday, and the shortwave trough over Baja is from last night’s MCS.

The monsoon high-pressure ridge is in central Arizona at 500mb. This may be the last day for favorable temperatures aloft as warmer air will move into Arizona in the wake of stronger storms this afternoon and evening. 

At 700mb, the two shortwave troughs underneath the ridge are highlighted in red. The deep-moisture monsoon circulation is far to the south in Mexico, and Arizona will have to depend on Gulf of California moisture for storms today. 

Today

The 12Z HRRR-UA WRF forecast sounding for Phoenix (below, left) and Tucson (below, right) are explosive with Convective Available Potential Energy (CAPE) over 2000 [J/kg]. Is the model overdoing this? We’ve lost some confidence given yesterday’s bust, but the run-to-run consistency for today would point to the forecast being correct. Also, the latest 12Z GFS forecast also indicates a big day. The cumulus cloud developing on the mountains already this morning indicated convective initiation is possible today, unlike yesterday. 

Below are the 4 latest HRRR-UA WRF forecasts (6Z, 9Z, 12Z, and 15Z). All forecasts agree on thunderstorms in Tucson and Phoenix today, and unlike yesterday, there are no limiting factors like too much CIN. Storms form in the afternoon (3- 5 pm) over the mountains and push out into the valleys during the late afternoon and evening (5-7 pm) with the potential for heavy rain and wind gusts over 50 mph. 

The UA WRF ensemble mean shows a 0.5″ precipitation event, though the most recent forecasts range from 0.5″ to 1.75″. The higher totals are forecasts where the storms are more organized. Either way, it’s best to prepare for the worst scenario.

The 12Z and 15Z total precipitation forecasts through to 5 am tomorrow from HRRR-UA WRF are below. The 12Z forecast has more precipitation with storms moving further west away from the mountains. A blend of these forecasts would probably be more accurate, with both Phoenix and Tucson getting heavy precipitation today.

Tomorrow

Tomorrow, the lower elevations from Tucson and Phoenix west will be cooled by storms today, so won’t fire storms again. Thunderstorms will be limited to higher elevations with less precipitation and areal coverage; this is typical for a down day following an up day.

Outlook

After today’s storms, we will shift back to a low-grade monsoon pattern into the weekend, though moisture will return next week for more activity.

Discussion written by Pat Holbrook and Patrick Bunn