Summary
A surge of tropical moisture is streaming north from Mexico into Arizona and on to the Great Plains. High surface dewpoints and precipitable water are widespread across eastern Arizona into New Mexico.
Forecast soundings indicate that although there is potential energy for storms, warm air aloft will limit thunderstorm development in much of western and central Arizona. The most significant storm activity is expected in SE Arizona this afternoon and overnight into Wednesday in central and southeast Arizona. Isolated storms are predicted during the day on Tuesday, with another overnight pulse of activity into Wednesday morning. Rainfall and lingering clouds are expected to suppress further storm development Wednesday afternoon. Long-range forecasts predict a few more days of active weather before storm activity diminishes by the weekend.
Yesterday’s Weather
The southwest part of Arizona had little thunderstorm activity while northern Arizona and most of New Mexico had scattered thunderstorms. The heaviest rain fell in the western portion of the Mogollon Rim north to the Grand Canyon, southeast Arizona, and across central New Mexico.
The HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right) are shown below. The forecast underestimated the areal coverage of light precipitation and, due to the synoptic circulation, missed some of the mountains where training was occurring.
Isolated showers and thunderstorms were in eastern Pima County yesterday afternoon and again this morning. In the Phoenix area, light showers occurred in San Tan Valley and north of Florence.
Analysis
The eastern tropical Pacific is relatively quiet, with only tropical storm Norbert and a depression in the eastern Pacific basin.
Moisture is flowing from the tropics northward into Arizona (black arrow), then northeast into the Great Plains; a broad trough and associated jet stream are drawing it north (light blue arrow). Several Tropical Upper Tropospheric Troughs (TUTT) are located from west of the Baja Spur east to south of Big Bend, TX, then to the Gulf of Mexico.
The visible satellite image overlaid with mosaic radar images from 8:30 am this morning shows showers and thunderstorms in eastern Arizona and western and northern New Mexico; the precipitation is light. The radar indicates showers in the northern Gulf of California are due to ducting of the radar beam, giving a false impression of precipitation where none exists.
Surface dewpoint temperatures from 7 am show it’s quite moist across the two-state region today, with 60° F dewpoint temperatures over the southern half of the region and 50°F dewpoint temperatures elsewhere.
The HRRR-UA WRF 12Z initialization (left) and observed (right) sounding for Phoenix at 5 am are below. The observed sounding has 3 mm more precipitable water than the initialization sounding. The satellite animation of yesterday’s activity (shown first) shows a strong west-to-east gradient in the low-level moisture field, with less cloud/moisture in southwest Arizona than farther east.
At 250mb, the subtropical ridge is near the boot heel of New Mexico with two low-pressure systems on either side: one southwest of Laredo and one west of the spur of the Baja Peninsula. The polar jet extends from northern California into Wyoming, with a shortwave trough from south-central Idaho to Las Vegas and Los Angeles. The flow between the ridge and shortwave trough is pulling monsoon moisture northward.
At 500mb, the jet flows across central California into southern Utah and then to southeast Wyoming. The same shortwave trough at 250mb is just a little further east, from southeast Idaho to northwest Arizona. Warmer air at 500mb is over western Arizona, which will inhibit thunderstorm activity, while the eastern half of Arizona still has colder air aloft. Once the shortwave passes by today, another trough will drop into northern Arizona by Wednesday, bringing cold air to 500 mb by mid-week.
At 700mb, two moisture flows are entering Arizona today, with the strongest across eastern Arizona into southwest New Mexico. The second, weaker flow is from the Gulf of California northward into southern Utah. Very dry air is in California, but this is about as far east as it will get due to the approaching West Coast trough.
Today
Below are four forecast soundings at the time of peak thunderstorm development in each location: Phoenix 4:30 pm (top, left), Gila Bend 4:30 pm (top, right), Tucson 3:30 pm (bottom, left), and Nogales 1 pm (bottom, right). The Gila Bend sounding has more Convective Available Potential Energy (CAPE) than Phoenix; however, neither sounding will produce thunderstorms because the warm air between 700 and 500mb will stop air from rising past that level.
The Nogales sounding has more CAPE than Tucson. Storms will move north from Santa Cruz County, then northeast as they move into Pima County. The question is how far west the storms will form because they won’t be able to push west using outflows due to southwesterly winds throughout the vertical profile.
The 12Z HRRR-UA WRF keeps storms mainly to the east of Tucson this afternoon, and the 15Z forecast is even further east in its prediction of storm location. What remains, though, is that the CAPE indicated on the sounding is ready to be tapped by the shortwave to the north. Overnight, showers and thunderstorms develop in Santa Cruz County. Storms are predicted to move or develop north into Pima, Pinal, and Maricopa counties. The precipitation should be light. This activity may lower chances of storms developing Tuesday afternoon in these areas.
Tomorrow
Only isolated storms are expected in southern Arizona on Tuesday afternoon, with activity focused in northeast Arizona and central New Mexico.
Overnight Tuesday to Wednesday is another peak in thunderstorm development across central and southeast Arizona. These storms will produce heavier rain totals than Monday to Tuesday night.
Outlook
Wednesday morning, storms form in southern and central Arizona, which will limit afternoon storm development in these areas due to the rain-cooled air and remnant clouds
Long-range global forecast ensembles show a few more days of active weather, then the storms will be winding down by the weekend.
Discussion written by Pat Holbrook and Patrick Bunn