Summary
Surface moisture remains high across northern Arizona and New Mexico. Recent storms and cloud cover have cooled the low deserts, especially around Phoenix, where morning soundings show high precipitable water but significant convective inhibition.
Forecasts indicate that storms will develop over the mountains and move northwestward today, though weaker winds and lower CAPE may limit their intensity and organization, preventing them from reaching the valleys. Another easterly wave could enhance the storms today, though activity is still expected to reach Maricopa County by evening. Tomorrow, upper-level winds shift, favoring storms that form along the Mogollon Rim and move southwest into the valley, but these are expected to be weaker. Through mid-week, the monsoon pattern persists, with continued chances for mountain storms and occasional strong winds, particularly as upper-level troughs influence the region.
Yesterday’s Weather
Storms were active in southeast Arizona, with many severe weather events in Pima and Cochise counties; the NWS issued 12 severe thunderstorm/dust storm warnings. The easterly wave approaching southeastern Arizona yesterday aided the development and movement of the storms. Phoenix was ready to get hit by the storms, as seen in the late-afternoon satellite animation below at 5 pm. But strong outflows from storms on the Mogollon Rim swept into Phoenix, pushing the Convective Available Potential Energy (CAPE) out of the area. The UA WRF forecasts underestimated storms in Cochise, Santa Cruz, and Pima counties, but were better in Maricopa, Pinal, and Yavapai counties. Microscale structures that consequently affect the additional storm development are hard to represent in any weather forecast model.
The MRMS observation data for yesterday shows that the precipitation affected a large area of the state. The heaviest rain fell mainly in the mountains in northwest Arizona and in Pima and Cochise counties.
Pima County Flood gauges show there were several values over an inch In the Tucson area, although most of the metro area was less than a quarter inch.
Rainlog gauges show Phoenix metro only got a few drops, but the surrounding areas 0.25″ to over 1″
Below is the HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right). The forecast precipitation pattern was reasonable, but it certainly underestimated total rainfall amounts. Several flash flood warnings were issued in the southern half of Arizona.
Tropical Upper Tropospheric Troughs (TUTTs) “L3” and “L5” have combined off the west coast of the central Baja Peninsula and were initialized too far south by the weather models. The combined TUTT is expected to remain in the same general location over the next several days.
Analysis
There’s still plenty of surface moisture, with the 50°F isodrosotherm extending into northern Arizona and New Mexico. There are even a few 70°F dewpoint temperatures in Pinal and Maricopa counties, and the low deserts of Arizona have cooled significantly from yesterday’s outflows/storms.
The observed sounding at Phoenix from 5 am (below, right) still has very high precipitable water (PW) values, but a lot of Convective Inhibition (CIN). The circled area represents the surface-based CIN of –263 [J/kg]. The surface-based Convective Available Potential Energy (CAPE) is 957 [J/kg] when the convective temperature reaches 103°F. With the morning clouds, we’re not guaranteed to reach the convective temperature, though it is forecast, and another mechanism may be needed to get storms in the Phoenix valley. The HRRR-UA WRF 12Z initialization matches the observed PW, CAPE, and wind shear well, though the profile is a smoothed version of the observations, which is typical given the need for a discrete number of model levels.
At 250mb, the TUTT is south of the model position shown in red. In black is the actual position, and the difference will keep us in a general southeast flow today. The polar trough is off the west coast, and another is in the Mississippi Valley; together, these two troughs will put a squeeze on the monsoon ridge by mid-week. A strong upper-level low-pressure system will form over Midland by Thursday. It’s difficult to analyze the location of the short-wave troughs at this level.
The short-wave troughs were easier to find at 500 mb, and the days when the easterly waves moved into Arizona are noted. So, there’s one more easterly wave to influence today, then a break.
At 700 mb, monsoonal flow continues, although some drying is noted in Mexico west of the Sierra Madre. The monsoon ridge is in north-central New Mexico.
Today
The HRRR-UA WRF 12Z forecast soundings for Phoenix at 3:30 pm (left, below) and Tucson at 2 pm (right, below). By early afternoon, storms will form over the mountains and move westward to the northwest at 10 mph. CAPE is less than yesterday at both locations.
Thunderstorms form on the mountains in southeastern Arizona and the Mogollon Rim, then move northwest toward the edge of the desert. If the forecast is correct with the weaker thermal wind, it will be difficult for storms to move through the valleys, but the last of the easterly waves moving into the state may aid lifting potential, as yesterday. With lower CAPE values, strong winds are less likely. Storms will move into Maricopa County later in the evening at 5 pm.
Tomorrow
Tomorrow, upper-level winds shift to a more northeast flow, setting Arizona up for a Rim runner or Rim shot, where storms form in the mountains along the Mogollon Rim and then move southwest into the valley. Tomorrow’s storms appear weaker, as the valleys have not recovered temperature-wise from the weekend storms; less surface heating and lower surface dew points. Precipitable values remain high.
Outlook
On Wednesday, storms again form in the mountains in the afternoon and sweep southwest into the Phoenix area during the evening, accompanied by strong winds. Tucson storms will have already moved from the Catalinas across the metro by early afternoon. We’ll need to see if the precipitation amounts stay consistent at >0.5″ in tomorrow’s forecast before being more confident about whether it’s likely or not.
The global model ensemble is a typical-looking monsoon forecast, except for some more potential on Thursday 16th with another Rim runner.
Discussion written by Pat Holbrook and Patrick Bunn