Summary
Monsoon flow is present from Sonora to the Baja Peninsula into California around a well-established subtropical high-pressure system in the Southwest region. Recent forecasts initially suggested a potential Gulf Surge today, but current observations indicate that the increased humidity is largely due to outflow from thunderstorms in northwest Mexico, rather than a classic surge. High dewpoints are already present, and strong outflow winds have been observed, especially near Yuma.
The Phoenix weather balloon sounding and forecast show abundant mid-level moisture and unstable conditions, particularly over Arizona, supporting the development of thunderstorms. Today, storms are expected to form on the Mogollon Rim and northeastern Pima County, moving southwest toward Phoenix and Tucson, with threats of strong winds, localized rain, blowing dust, and a possible haboob. Additional storms will develop tonight and into tomorrow, mainly over and near mountainous areas. While more moisture is forecast to remain in place through next week, ensemble models suggest overall rainfall totals will remain modest, though daily storm chances will continue through the end of the month.
Yesterday’s Weather
Isolated thunderstorms developed in Arizona during the afternoon, and by evening the storms were more numerous in eastern Arizona and New Mexico. In Arizona, most of the storms were over the mountains. The high bases of the storms combined with excessive heat produced microburst wind gusts around 40 mph; Albuquerque had a wind gust of 78 mph.
An infrared satellite image from 6 am this morning shows dissipating deep convection in Mexico west of the spine of the Sierra Madre mountains. The low pressure south of the Texas Big Bend is the best estimate of the location of the remnant circulation from Tropical Storm Bertha. The storms west of Bertha are not related to Bertha, and all Bertha will do is bring additional moisture into Arizona. Thunderstorms continue propagating northward to just south of the International border.
The HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right) is shown below. A little more precipitation fell in eastern New Mexico, but overall, the forecast was accurate.
Monsoon flow is well established along the Baja Peninsula into California.
Analysis
Previous forecasts indicated what could have been interpreted as a Gulf Surge either this morning or evening. But today we see it is only outflow from the thunderstorm complex in Mexico. Since the dewpoints are already high, this outflow will only augment existing moisture. A typical Gulf Surge starts off with dewpoint temperatures in the 40°F range and then jumps up to the 60°F range with strong southerly winds.
Since dewpoint temperatures were already high, this is not technically a Gulf Surge, but outflow from the storms in northwest Mexico. Puerto Penasco had wind gusts up to 37 mph from the south and reached the radar site near Yuma at 7:15 am this morning (1412 on the graphic below, 10 kts and higher).
The HRRR-UA WRF 12Z initialization (left) and observed sounding (right) for Phoenix at 5 am are below. The observed sounding has greater vertical resolution dewpoint temperature data versus the smoothed forecast initialization, but both soundings have similar precipitable water. The dewpoint temperature in the green box is mid-level moisture and represents the layer of the atmosphere that is the Most Unstable (MU). The MU is at about 12,000 feet (3.6 km), and that is where the base of clouds will be initially. The thunderstorm outflow from Sonora was still well south of Phoenix during the 5 am sounding but will get there by this afternoon.
At 250 mb, the monsoon high-pressure ridge dominates the circulation over the region, with a couple of short waves moving west across northern Mexico. The wave to the west is related to the thunderstorm complex in northern Mexico, and the wave further east is probably from the remains of Bertha.
The monsoon ridge is in Colorado at 500mb; though it’s a broad ridge over the Southwest region, the location of the exact center is not as important as the circulation around it. The broad nature of the high means flow in most of Arizona is weak, though just south of the International border up to 30 kt easterly winds with embedded waves represent a promising monsoonal flow if the center of the high-pressure becomes more well defined and the stronger flow shifts north.
At 700mb, the wave furthest west is not visible at this level, and a different wave extends south from just east of the Big Bend. The wave between Chihuahua and Sonora is approaching southeast Arizona.
Today
The HRRR-UA WRF 12Z forecast soundings for 4 pm today show Phoenix has 475 [J/kg] and Tucson 550 [J/kg] for the most unstable layer, Convective Available Potential Energy (CAPE), and no Convective Inhibition (CIN). The downward CAPE indicates 45+mph winds at the surface from outflows and 60+mph at some locations. Storms will move to the southwest given the 0- 6 km mean wind direction.x
HRRR-UA WRF 12Z forecast soundings for Phoenix (left) and Tucson (right) at 4 pm.
The forecast radar reflectivity and winds show the storms will develop on the Mogollon Rim and northeastern Pima County, then propagate west towards Phoenix and southwest to Tucson. Storms don’t look well organized, so localized precipitation is most likely. Blowing dust and haboob in Phoenix is also possible. The storms will reach the Colorado River by midnight as they continue to develop and get more organized further downstream. Additional non-severe storms will develop in Santa Cruz and eastern Pima counties in the evening and move to the Phoenix area by the morning.
The heaviest rain will be on and near the mountains as shown in the total accumulated precipitation map below, and any flooding should be localized.
Tomorrow
Tomorrow, thunderstorms are forecast around the edge of the low deserts as the boundary layer will be cooled by today’s storms. Storms are predicted to develop in southeast Arizona, and southwest and northern New Mexico, though predominantly over and near the mountains.
Outlook
More moisture moves into Arizona on Monday from an outflow from a large thunderstorm complex in northwest Mexico on Sunday night. The storms in eastern Arizona will move west and may make it to Tucson Monday evening, but not much further west.
Global model ensemble forecasts show there is not a lot of accumulated precipitation through the end of the month, though storms are possible every day. Tucson and Phoenix forecasts are similar, with the ensemble mean less than 0.4” in both locations.
Discussion written by Pat Holbrook and Patrick Bunn