Summary
A developing tropical system southeast of Hurricane Karina is being monitored as the only significant threat in the tropics, with global-scale weather models differing on its path. Across the southwestern United States, a low-pressure system and a strong jet stream brought overnight storms, especially affecting Arizona and New Mexico. Satellite and surface gauge data show abundant moisture in the region, fueling showers and thunderstorms, particularly around Tucson, where heavy rain is expected, but severe weather is unlikely.
Today, the heaviest rainfall is forecast from Tucson north to Pinal County from 11 am onward, with more storms developing in neighboring counties moving southwest, with additional storms in central New Mexico later today. Storm activity will decrease tomorrow, with only isolated strong storms in northeast New Mexico, and precipitation is expected to taper off into Tuesday. By midweek, scattered thunderstorms will mainly affect mountainous areas, while temperatures in Tucson are forecast to stay below 100°F as a weaker monsoon pattern settles in, with the potential for storm activity to increase again by the weekend.
Yesterday’s Weather
The visible satellite imagery from 5 pm yesterday didn’t look threatening, but two small clusters of thunderstorms were about to change that. The first cluster (green) was pushing eastward from west of Wickenburg with the heaviest storms along the south side of the cluster. The cluster to the south (red) was moving northward and did produce a 58-mph wind gust at Gila Bend.
By 9 pm the two clusters were merging near Phoenix. These storms produced heavy rain, with many locations recording over an inch, especially in the foothills from Wickenburg east to Cave Creek. There was blowing dust, and winds were below 50 mph. Overnight, the storms built further south, extending to Tucson. By 7 am, the radar shows storms redeveloping in Maricopa, Pinal, and Pima counties. These storms are not severe, but they will add to today’s rainfall totals.
The HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right) are shown below. The forecast was a little aggressive with precipitation amounts. The heavier rain near the Mogollon Rim was on the south side of the Rim and not the north side. Heavier rain fell in Cochise and parts of Graham County and in western New Mexico than was forecast. The areal coverage of the thunderstorms was just about right.
The Pima County Flood Control Alert gauges show heavy rain across Pima County, with the heaviest from Avra Valley down to three points. The Brawley Wash rose 5 feet at three points. Rainfall was generally lighter in the Tucson area, although a couple of gauges were near or over an inch.
In Maricopa County, the heaviest rain was in the foothills outside Loop 101, with many gauges reporting over an inch.
Analysis
Looking at the tropics, only the developing storm southeast of Hurricane Karina poses a threat. As is typical, the model forecasts differ, with the GFS bringing the storm toward the United States while the ECMWF model has it heading west.
Two low-pressure centers are along the jet stream. The first is in southern Canada and the second in southwest Nevada. The second low and associated jet stream are responsible for the overnight storms. A Tropical Upper Tropospheric Trough (TUTT) is in south-central Mexico, while a developing tropical system is in the northern Gulf of Mexico.
The visible satellite image from 8 am shows the showers and thunderstorms over Arizona and western New Mexico. Tucson will be in for more rain from storms just west of the area.
Plenty of surface moisture is across the region, with only a narrow strip of sub-50°F dewpoint temperatures in central New Mexico.
Both the HRRR-UA WRF 12Z initialization (left) and observed (right) sounding for Phoenix at 5 am are disrupted by the storms. Of note are the jet stream winds between 300 and 200mb, over 50 knots, which is something we’d expect in fall.
At 250mb, the monsoon ridge has been pushed away by the low and jet stream. Southeast Arizona is under an area of upper-level divergence, which is favorable for the development of storms.
The jet stream has moved into northwest Arizona at 500mb, with an upper low in central California. A short-wave trough extends from the low southeast to the northern Gulf of California. The monsoon ridge is in Oklahoma, and the flow around the high is bringing monsoon moisture into eastern Arizona.
At 700 mb, there’s plenty of moisture in Arizona, but dry air is still over eastern New Mexico. There’s no visible center to the monsoon high, but the boot heel of New Mexico is a likely spot.
Today
The forecast sounding for Tucson at 11 am (left) is for just before the thunderstorm cluster west of Tucson moves into the city. The Convective Available Potential Energy (CAPE) is still high at 800 [J/kg] with only minor Convective Inhibition (CIN). The downward DCAPE of 811 [J/kg] would indicate wind gusts less than 30 mph. Except for heavy rain, thunderstorms show no other potential for severe weather. The sounding at San Simon (right) shows thunderstorm potential in the late afternoon. Similar to Tucson, though, the main threat is heavy rain.
A cluster of storms is forecast to move northeast, with the bulk of the heavier rain from Tucson metro north to eastern and southern Pinal County. The HRRR-UA WRF 12Z (left) and 15Z (right) show that from 11 am onward, this area is predicted to receive heavy rain over the next few hours.
This afternoon, the cluster will weaken, and additional storms will develop in Santa Cruz and Cochise counties, building to the southwest. Additional storms develop in central and western New Mexico. Once again, severe weather is not expected, but heavy rain will occur.
Tomorrow
Tomorrow, thunderstorm coverage in Arizona will decrease significantly, and the only strong storms will be in northeast New Mexico.
The 24-hour total forecast precipitation accumulation (left) and 48-hour (right) show that the bulk of precipitation falls over the next 24 hours, with only lighter amounts on Tuesday.
Outlook
On Wednesday, southeast Arizona and the western two-thirds of New Mexico will see scattered thunderstorms, mainly near the mountains.
The long-range global model ensemble is not forecasting the temperatures to reach triple digits in Tucson as we return to a low-grade monsoon pattern for the remainder of this week. By the weekend, storms may increase, and Phoenix will see triple-digit high temperatures.
Discussion written by Pat Holbrook and Patrick Bunn