Summary
Today, abundant surface moisture will fuel thunderstorm activity across southern Arizona. Moisture continues to flow into Arizona, with upper-level easterly winds supporting storm development, particularly around Phoenix and Tucson.
Phoenix has dewpoints above 60°F, 1.7″ of precipitable water, and is forecast to have 1200 [J/kg] of convective available potential energy this afternoon! Thunderstorms are expected to develop in the early afternoon and reach Phoenix by 6 pm, bringing high winds, heavy rainfall, and a risk of flooding, especially as the storms will move slowly. Forecasts indicate strong wind gusts, a possible haboob, and rainfall amounts of 0.5″ and greater for the Phoenix area.
Tomorrow, storms will form mainly in the mountains and may reach the valleys, but activity is expected to be less intense. By midweek, storm activity is forecast to increase again as upper-level winds shift direction.
Yesterday’s Weather
The visible satellite imagery loop with radar/lightning overlaid shows the initiation, development, and extent of the thunderstorms yesterday. The thunderstorm in Tucson had an 81mph wind gust at Davis-Monthan Air Force Base. Several other severe wind gusts were observed in the Tucson metro area, along with blowing dust. Heavy rain accompanied the storm, with one swift water rescue along the Alamo Wash. Tucson Electric Power reported 10,000 people without power at the peak of the storm. The storms weakened moving westward into central and western Pima County and all the way to Yuma.
The Rainlog data from yesterday indicates that the heaviest rain fell near the mountains, from east of Casa Grande into Tucson, and then south into Mexico (south of the green line). To the west of the black line, including the Phoenix metro area, rainfall was limited.
The HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and the corresponding observed precipitation from MRMS (right) for yesterday show that the forecast was good, except for the heavier rain in the mountains of eastern Pinal County, though the 15Z forecast had this feature.
The Pima County Flood Control gauges show the precipitation totals in eastern Pima County and Tucson. The heaviest rain was 2.2 inches near the Rincon Mountains. Tucson International Airport only had a trace of rain during the evening storms, but did get 0.02″ from showers overnight. More generally, most gauges received between 0.25 and 0.75″: the UA WRF ensemble mean forecast of 0.5″ provided a good estimate of the amount of rain, too. Great to see after yesterday’s false alarm forecast!
Over the next few days, TUTT “L3” and “L5” will combine near the southern tip of Baja and move into Baja, changing upper-level winds from east to south over Arizona.
Analysis
Plenty of surface moisture is available for storms again today, except for northwest New Mexico, west across far northern Arizona to Las Vegas.
The Velocity Azimuth Display wind profile from Yuma this morning at 1:49 am, then the radar went down. Moisture continues to stream into the state, and now upper-level easterlies are in place up to 5,000 feet (1,500m).
The people of Phoenix must have woken up this morning thinking they were in Miami, as precipitable water has increased to 1.84 inches (47mm), with a dewpoint above 60°F. It will take a little time for storms to develop, but this afternoon will be quite interesting. Thermal wind is towards the northwest. Thunderstorms will be similar to yesterday’s Tucson storms, with winds over 55 mph and, due to the slow storm motion, heavy rain.
At 250 mb, the monsoon ridge is firmly established just north of the Four Corners, with another center near Del Rio, TX. Two short-wave troughs are rotating under the ridge, and TUTT “L3” is on the edge of the map.
A third short-wave trough is approaching the Big Bend, and the monsoon flow at 500 mb is now moving into Canada by tomorrow as a long-wave trough approaches the west coast.
At 700 mb, moisture flow is exceptional, and even northern Arizona will receive moisture by Monday. The easterly waves are about 24 hours apart and will enhance thunderstorm activity today.
Today
The HRRR-UA WRF 12Z forecast soundings for Phoenix at 3 pm (left, below) and Tucson (right, below). Thunderstorms will form earlier than yesterday, with convective initiation by 1 pm over the mountains near Tucson. Storms will then move west-northwest at 10 to 15 mph. Though there is less Convective Available Potential Energy (CAPE) today, precipitable water is still 1.33″ (34mm), so heavy rain is possible. The storms are not as well organized initially as they were yesterday.
In Phoenix, storms have a greater potential for high winds, as CAPE is forecast to exceed 1200 [J/kg]! Heavy rain and flooding will accompany the thunderstorms, as storm motion is only 10mph.
By 4:30 p.m., thunderstorms will have already moved through Tucson and will be approaching Phoenix from both the southeast and the Mogollon Rim, with Phoenix squeezed between the two.
Wind gusts over 60 mph are possible, along with a haboob, given the 10-meter wind speed forecast below.
Note that Phoenix airport does not get precipitation in the HRRR-UA WRF 12Z (below, left), but as we saw yesterday, the randomness of thunderstorms and outflows makes the exact amount for a given location difficult to forecast; the 15Z forecast has Phoenix airport getting 0.75″ (below, right). With all the available precipitable water, Phoenix should be ready for the worst case!
24-hour precipitation totals are predicted to be less than 0.25″ for Tucson.
Tomorrow
The forecast radar reflectivity for tomorrow afternoon shows storms forming in the mountains, as the CAPE in the valleys will be wiped out from today’s activity, but storm motion will be fast enough that a few storms will move into valleys as they dissipate.
Outlook
Less intense activity is forecast for Tuesday, with storms forming on the Mogollon Rim and in the higher terrain of southeastern Arizona, moving west. We’ll need to see how today and tomorrow go before we can be confident of the situation on Tuesday.
From midweek into the weekend, storms should ramp back up as the winds aloft are more from the northeast.
Discussion written by Pat Holbrook and Patrick Bunn