Summary
Southerly winds from thunderstorm outflows have increased moisture in the low deserts, especially in southern Arizona, leading to higher dewpoints and favorable conditions for thunderstorms.
UA WRF forecasts indicate storms in the Tucson area if outflows aid convective initiation, with the possibility of dust storms and high wind gusts. Phoenix is less likely to see storms today due to unfavorable wind patterns. Accumulated precipitation forecasts suggest the most significant rainfall will occur in and around Santa Cruz County today.
Into the weekend, storm activity increases, gradually shifting northward towards Phoenix, arriving by Saturday evening and ramping up Sunday afternoon/evening.
The atmosphere remains moist and tropical into the middle of next week, keeping the chances of widespread rain high, though there looks to be a drying trend at the end of next week.
Yesterday’s Weather
Thunderstorms as of 6 pm were along and east of a line from Globe to Fort Huachuca, then extending south into a larger cluster in Mexico. These storms initiated in the White Mountains, moving mainly southward at first, but outflows forced a more westward motion by evening, with individual storms still moving southward. Strong wind gusts of 60+ mph occurred at Safford, Huachuca City, and the Hopkins RAWS weather stations. A few storms made it into Pima County, associated with the 40+ mph outflows, but little rain.
Only four rain gauges in Pima County reported measurable rain and were close to the Santa Cruz County line. Rainlog data below show that the heaviest rain was along the Mexican border south of Sierra Vista, which agrees with the UA WRF forecast precipitation.
Below is the HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right) for July 9th. The forecast missed some of the precipitation totals from storms in Graham and Cochise counties triggered by outflow boundaries.
An outflow reached the Tucson metro by 8 pm, as shown on radar, bringing gusts up to 45 mph, but it didn’t produce any rain, as there still wasn’t enough mixing to break the cap of CIN (Convective Inhibition) and tap into the CAPE (Convective Available Potential Energy), which probably wasn’t enough anyway.
Analysis
The Velocity Azimuth Display (VAD) wind profile from Yuma shows southerly winds from thunderstorm outflows, with much stronger winds than yesterday, extending up to 4000 feet (120 meters).
Comparing the analysis of observational weather-balloon data with the model initialization is the best way to determine whether the model is initialized correctly. The wind field looks correct at 250 mb in the GFS-UA WRF 00Z initialization, though there’s an additional low-pressure feature in the Gulf of California that was missing from the original analysis plot.
TUTT “L3” is still being shown on the map along with TUTT “L5”. Forecasts show that in about 72 hours, they will merge into a single larger TUTT. The tongue of dry air from Texas into northern Mexico is slowly being dissipated by thunderstorms along the Sierra Madre mountains.
Moisture has increased in the low deserts due to thunderstorm outflow, as indicated by the VAD wind profile at Yuma (shown earlier). Both northern Arizona and New Mexico are dry, and even eastern New Mexico does not have 60°F dewpoint temperatures. Southern Arizona has 60°F dew points this morning increasing from yesterday.
The observed sounding from Phoenix at 5 am below shows that precipitable water dropped to 32mm (1.15″) this morning, which was 2mm (0.2″) less than yesterday. The surface dewpoint temperature is higher, but this moisture is only reflected in the lowest 50 mb, since dry air aloft has reduced the total column precipitable water. Convective Inhibition (CIN) has lowered signaling that convective potential should increase this afternoon.
At 500 mb, the monsoon ridge is elongated from southern New Mexico and has shifted slightly farther north than yesterday. Winds are now more easterly over southeast Arizona, compared to the north winds from yesterday, and the easterly wave in Mexico is harder to find in the circulation below the ridge. A trough is pushing a cold front in Kansas and Nebraska.
The easterly wave extends from southwest New Mexico south into Mexico at 700 mb. The monsoon high has been displaced westward to the northern Gulf of California. A lot of moisture will move into most of Arizona as the monsoon high shifts to its mean position in the Four Corners.
Today
The forecast soundings for Phoenix at 6 pm (below, left) and Tucson at 4 pm (below, right) indicate good potential for thunderstorms, with wind gusts up to 60 mph in Tucson and southeast Arizona. Over 400 [J/kg] of CAPE in Tucson, good wind shear with southwest storm propagation not impeding northeasterly inflows, and sufficient precipitable water! In Phoenix, the predicted 120[J/kg] of CIN is unlikely to be overcome today, even with strong outflows from storms near Tucson. The thermal wind is from the west at Phoenix (unfavorable), but is more favorable from the northeast at Tucson.
The forecast radar reflectivity and wind animation below shows storms developing in Santa Cruz and Cochise counties this afternoon, moving westward like yesterday, arriving in Tucson during the evening hours, while additional storms continue to develop in Cochise County. High winds and dust storms are possible. Note that the storms die off by midnight, with the strongest winds in Tucson between 8 and 9 pm. The storms aren’t predicted to reach Pinal or Maricopa counties.
Successive HRRR-UA WRF forecasts at 12Z (below, left) and 15Z (below, right) show precipitation totals backing off, particularly in the Tucson metro area. This indicates that storms reaching the Tucson valley depend on outflow initiation. 12Z says yes, 15Z says no. Both models forecast less than 0.25″ in the Tucson metro, with totals up to 0.5″ to 1.5″ in Santa Cruz County and north to Green Valley.
Tomorrow
Tomorrow is rinse and repeat, except the storms are further north, as moisture encroaches northward with each successive day of thunderstorms and outflows. Tomorrow’s outflows are expected to be larger and capable of affecting Maricopa and Pinal counties as well as the low deserts. Saturday evening appears to be the best day for a haboob as well; look at the winds into Maricopa County between 8 and 10 pm.
Comparing the precipitation totals for today’s 24-hour period (below, left) to tomorrow’s 48-hour period (below, right) shows storms will be more widespread east of Pima County tomorrow, with a good chance of a downpour in the Tucson metro; totals of 0.25″ to 0.5″ are possible.
Outlook
Storms form earlier in the day on Sunday as precipitable water reaches over 40mm (1.5″) over the low deserts. Sunday is the day that good storms move into the Phoenix area and the low deserts. Dust storms and winds up to 60 mph are possible, and a haboob is again possible because wetting rain has not hit either Pinal or Maricopa counties in the past two days.
The atmosphere will be loaded with moisture next week. Although thunderstorms are unlikely to produce severe weather, heavy rain is possible as we transition to a more tropical atmosphere.
There are many members of the global GEFS model indicating heavy rain; we can’t argue with that, as the flow through the middle of next week remains positively tropical. Though there appears to be a drying trend later next week into the weekend.
Discussion written by Pat Holbrook and Patrick Bunn