Summary
Upper-level water vapor satellite imagery shows multiple Tropical Upper Tropospheric Troughs and a polar front influencing conditions south of a well-positioned but broad subtropical high in the Southwest. The 500 mb temperatures are too warm (above -5°C), and the 500mb heights are well over the 95th percentile. Moisture is present, especially in Arizona, but surface heating is insufficient for widespread thunderstorms in the valleys, given the current vertical temperature profile. Only mountainous regions are expected to see significant rainfall.
Forecasts indicate that convective inhibition (CIN) will limit storm activity today in Phoenix. Tucson has plenty of Convective Available Potential Energy (CAPE) if it can overcome the CIN present. Storms will initiate in the mountains, though, and most rain will remain confined to higher elevations in both regions.
The overall outlook is for a weak monsoon pattern, with limited thunderstorm development expected in southern Arizona through Tuesday.
Yesterday’s Weather
Thunderstorms developed on the Mogollon Rim in the early afternoon, propagating towards the south and west and reaching Phoenix by 6 pm. Strong winds and dust were associated with the storms, with the highest wind gusts between 45 and 50 mph. The storms swept west to the California border by midnight. Thunderstorms in southeast Arizona were slower to develop than forecast, with a few isolated storms in the late afternoon. A line of thunderstorms developed and moved west into Safford, Benson, and Nogales by dark. The storms weakened, moving into Tucson and the low deserts by 11 pm.
The 24-hour observed precipitation from the Maricopa County Flood Control alert system is below. Light precipitation fell in the Phoenix metro area (<0.2”) with heavier rain to the west and in the mountains north and east of Phoenix (>0.5”).
The Pima County Flood Control alert system observed precipitation shows heavier precipitation in the Catalina Mountains (>0.5”), although the 1.14” was at Summit, just south of the Tucson Airport. The general metro area got a trace to less than 0.25”.
The HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right) is shown below. The precipitation forecast from the Mogollon Rim and north was pretty good. Pima and western Cochise County precipitation was underestimated in the east and overestimated in the western part of the county, indicating storm propagation and downstream development were not forecast correctly. The missing weather balloon data for Southern Arizona is likely a factor in the problem forecast.
Analysis
The upper-level water vapor situation is quite complicated this morning, with three Tropical Upper Tropospheric Troughs (TUTTs) from the Gulf of Mexico to the Pacific Ocean all moving slowly west. A polar low is just west of Big Bend, Texas, and a polar front is approaching the Pacific Northwest.
There is still a large area of 60°F+ dewpoint temperatures in Arizona, offset by 50°F dewpoint temperatures in New Mexico. The thunderstorm outflow pushed all the way to the Colorado River in the west, aiding in moisture transport to the lower elevations of Arizona.
The HRRR-UA WRF 12Z initialization and observed sounding from Phoenix at 5 am (below) are very similar except for the area in the black box. The cooling in the layer below 850mb means that an extra 100 [J/kg] of energy from solar heating will be needed to get storms to form in the valleys. The convective temperature is 112°F while the forecast maximum temperature from the sounding is 102°F. NWS Phoenix forecasts a high of 109°F, and UA WRF is at 111°F. None of the forecasts get to the required convective temperature.
At 250mb, the monsoon high is in about the same position as yesterday, and the short-wave trough moved from the position marked in red yesterday to its present location in black.
The monsoon high is in a similar location as yesterday at 500 mb, with two short-wave troughs moving westward underneath the ridge.
At 700mb, flow is still weak below the monsoon ridge. The easterly wave south of Douglas is the remains of Bertha and could be a focal point for storms today along and south of the International border.
Today
The HRRR-UA WRF 12Z forecast soundings for 6 pm today show Phoenix has 290 [J/kg] and Tucson 800 [J/kg] for mixed-layer Convective Available Potential Energy (CAPE) and between -70 and -30 [J/kg] of Convective Inhibition (CIN), respectively. The Phoenix sounding does not appear to be able to support storms in the valley, while Tucson has plenty of CAPE if the CIN can be overcome. Anvils will be blowing to the west, which is a negative for both cities, as they will inhibit surface heating for continued inflows.
With a southeast wind flow aloft, thunderstorms will be confined to the mountains mainly in western Cochise and Santa Cruz counties, along the Mogollon Rim and Colorado Plateau. Heavy rain is the main threat, especially along the International border.
The 24-hour precipitation accumulation forecast shows rainfall is restricted to the mountainous areas of the Mogollon Rim and Santa Cruz and Cochise counties, where totals of 0.5″ are likely, with peaks getting more than 1.5″. None for the low deserts.
Tomorrow
A similar type of problem tomorrow, in terms of storms moving off the mountains, as there is still too much CIN. Another factor is the low-level northwest winds across southern Arizona, which may cool the boundary layer by a couple of degrees. Even with these negatives, some storms try to move into the valleys by the evening before dissipating.
Outlook
The Tuesday afternoon forecast shows almost a blank slate in the southern half of Arizona. The only thunderstorms predicted are in northeast Arizona and northwest New Mexico.
Global model ensemble forecasts indicate a low-grade monsoon pattern this week.
Discussion written by Pat Holbrook and Patrick Bunn