Summary
Tropical Storm Norbert is the latest storm to form in the eastern Pacific, but it is not expected to impact the mainland United States as it is forecast to move westward. In Arizona and New Mexico, tropical moisture has been pushed south, and dry air has advanced into the northern parts of both states, reducing storm potential, especially outside mountainous areas. Satellite imagery shows mostly clear skies this morning across AZ and NM, and surface observations show lower dewpoints in the north, while some moisture remains in the south.
Thunderstorm activity will be limited today, with Tucson and Phoenix likely staying dry; activity will be near Nogales. Moisture is expected to return on Friday, bringing another round of storms to the mountains, with activity weakening after sunset. With no major tropical systems affecting the region, the low-grade pattern will persist into next week.
Yesterday’s Weather
Visible satellite imagery overlaid with radar and lightning data from 5:30 pm yesterday shows showers and thunderstorms with more extensive areal coverage than on Wednesday and much more lightning. The storms tried to push into Tucson, but weakened before dropping rain in the metro area.
The 7 pm infrared satellite image overlaid with lightning shows the thunderstorms rapidly increasing in intensity up to this time, but with little Convective Available Potential Energy (CAPE) in the Tucson valley, the storms weakened as they moved into the metro area.
24-hour accumulated precipitation data from the Pima County Flood Control District rain gauge network shows the storms basically stopping in the Catalina and Rincon Mountains, with over an inch falling at a few gauges on the mountains to the east.
Similar data from Rainlog below shows the heavy rain in the western half of Cochise and southwest Graham counties. Some light rain did fall in the Tucson metro area.
The HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right) are shown below. The forecast areal coverage was correct, but it underestimated precipitation amounts.
Analysis
Tropical Storm Norbert is the newest in the succession of tropical systems in the eastern Pacific, and we will likely have a few more into October. Norbert does not pose any threat to the mainland United States, as it’s predicted to move west.
Moisture flow from the tropics has been pushed southward into Arizona and New Mexico. Tropical Storm Norbert is still weak, but will mature into a hurricane in a few days. The Tropical Upper Tropospheric Troughs (TUTTs) are in a similar location as on Wednesday.
The visible satellite image from 8 am shows only scattered clouds in central to northwestern Arizona and parts of eastern New Mexico.
Very dry air has moved into northern Arizona eastward into northern New Mexico, as shown by the < 40°F surface dewpoints. The southern part of the region remains moist, but the dry air to the north will slowly move south during the day.
The HRRR-UA WRF 12Z initialization (left) and observed (right) sounding for Phoenix at 5 am are below. The observed sounding shows a subsidence inversion at 450 mb, which the initialization smooths out. The initialization shows slightly more moisture between the surface and 900 mb, but precipitable water differs by less than 0.2 mm between the two soundings.
At 250mb, a strong short-wave trough extends from Nebraska to northern New Mexico. The subtropical high is centered over Pima County while a weak low-pressure system is west of Laredo.
At 500 mb, the short-wave trough in the Great Plains is just a little east of the 250 mb location, with the center of the ridge along the Colorado River south of Las Vegas.
Dry air has dropped into northern Arizona and New Mexico at 700mb, due to the circulation from the high centered in southwest Nevada and a front dropping southward in the Great Plains. Monsoon moisture into Arizona and New Mexico has been cut off.
Today
The Nogales forecast for 2:30 pm is the only sounding in the region that has a profile capable of producing thunderstorms today. The mixed-layer Convective Available Potential Energy (CAPE) is low at 210 [J/kg], and the motion of the storms should be to the southwest.
The 12Z HRRR-UA WRF forecast radar reflectivity shows the strongest storms in and near Santa Cruz County, with lighter storms extending eastward into southern New Mexico. Tucson and Phoenix should remain dry; dewpoints are low today, and strong winds are unlikely.
Tomorrow
Moisture moves back into Arizona on Friday with an increase in thunderstorm activity. Storms will remain near the mountains and weaken quickly after sunset.
Outlook
On Saturday, there’s not much change in activity compared to Friday, except that there are more mountain storms in New Mexico. The radar forecast below shows the thunderstorms are over and near the mountains in Pima, Santa Cruz, and Cochise counties.
Without any tropical activity, we should expect a low-grade monsoon pattern into next week.
Discussion written by Pat Holbrook and Patrick Bunn