Summary
Surface moisture has increased in Arizona due to recent thunderstorms, though vertical mixing is expected to lower the dewpoints today. Similar to yesterday, forecasts indicate that storms will primarily form in the mountains to the northeast in Arizona and move southwest, but Phoenix is unlikely to see much relief from the heat due to unfavorable wind patterns compared to Tucson. Expect similar rainfall totals to yesterday in higher-elevation areas.
Tomorrow, storm activity will intensify around Tucson and parts of southern Arizona, bringing the potential for strong outflow winds and possible dust storms. On Friday, activity is limited to Cochise County, though a larger influx of moisture on Saturday morning will likely lead to increased thunderstorm activity across the region over the weekend, including Phoenix.
Yesterday’s Weather
Yesterday, thunderstorms extended east of a line from the Grand Canyon to Flagstaff, then southeast to Globe, and then to Tucson and Nogales, as shown by the satellite, radar, and lightning composite animation below. Most storms were in the mountains, but some ventured into the valleys, especially south of Tucson. The wind gusts with the storms were generally in the 30 to 40 mph range at Ryan airfield, southeast of Tucson, reporting a gust of 41 mph. That said, no significant dust or severe weather events were reported by the Storm Prediction Center here.
The 24-hour observed precipitation graphic below is from the Pima County Regional Flood Control District Alert map. The rain gauges will only record data when .04 inches of rain is observed.
The two higher rainfall totals, over 0.5″ in Vail and Willow Canyon, are confirmed by the 24-hour precipitation total from MRMS (below, right).
The HRRR-UA WRF 12Z forecast radar reflectivity animation (below, left) compares well to the observed radar (below, right). A good forecast of convective initiation, storm intensity, and propagation, though the forecast missed some of the >0.5″ 24-hour precip totals (above).
The strongest storms were in Mexico, developing over the Sierra Madre, then moving west and dissipating before reaching the Gulf of California. The red/black temperatures are the coldest cloud tops, with some near ~80°C. Cloud tops were not as cold further north, likely due to lower surface dewpoint temperatures.
Analysis
Both low-pressure systems “L3” and “L4” are Tropical Upper Tropospheric Troughs (TUTTs). TUTT “L3” continues to move slowly west while TUTT “L4” has moved southwest and is along the south end of a trough that extends into southwest Mexico, as shown. “L5” is not a TUTT and has moved east. Yesterday’s position of “L5” was too far west and should have been near San Antonio, so its eastward movement is less than thought.
The 50°F isodrosotherm has moved further north into Arizona due to mixing from yesterday afternoon’s thunderstorms.
The Velocity Azimuth Display wind profile from the Yuma radar shows that the vertical extent of the mixing is only about 2000 feet (600m). The wind field near 13000 feet (4km) is at the top of the mixed layer. Between this level and 2000 feet, the atmosphere is very dry, so vertical mixing this afternoon will lower the surface dewpoint temperatures.
The observed weather balloon sounding in Phoenix at 5 am continues to show the increase in precipitable water, now at 36 mm (1.42 inches). The only question is whether Phoenix can get storms into the valley.
The jet at 250mb is from northern California to Colorado. TUTT “L3” is still providing a favorable upper-level wind environment along the Sierra Madre mountains of Mexico.
At 500 mb, the high-pressure system is over western New Mexico, with a ridge axis extending westward into the Pacific Ocean, as indicated. The “L5” low-pressure area is well south of the Big Bend with easterly winds between the two centers over northern Mexico. It is still cool aloft overall, except in western Arizona.
At 700 mb, the moisture and wind field is similar to yesterday, except the low has moved west from Eagle Pass to its present location. Again today, storms will form along the Sierra Madre mountains in Mexico, extend into Arizona, and move west towards the Gulf of California.
Today
The 12Z HRRR-UA WRF forecast soundings below are for Tucson (left) and Phoenix (right). The only real difference is that the thermal wind would have storms moving to the southwest in Tucson (favorable) and to the southeast in Phoenix (unfavorable). Thermal wind represents a change in geostrophic wind with height; look at the 0-6 km mean wind, the black arrow in the hodograph (bottom-right corner).
The HRRR-UA WRF 12Z radar reflectivity forecast below shows thunderstorms forming in the same area as yesterday, though a little later in the afternoon: the higher-terrain areas to the northeast are moving in the same general direction as yesterday (southwest). Once again, storms can’t move into the low deserts without more Convective Available Potential Energy (CAPE). Outflow winds may get up to 50 mph, but large areas of blowing dust is not expected. Phoenix will remain miserable, as there’s no thunderstorm forecast to provide any cooling.
Tomorrow
CAPE increases in Tucson, and storms form earlier in the afternoon. Phoenix’s CAPE does not change much, and the overall northeast-to-southwest flow is again not favorable for storms in Phoenix.
The HRRR-UA WRF 12Z radar reflectivity and 10-meter wind forecast show strong activity by 6 pm. Outflow from the storms then pushes the storm cells across all of eastern Pima County and southern Pinal County, but they don’t reach Casa Grande. Outflow winds should be stronger, prompting severe thunderstorm and dust storm warnings in Pima County.
Outlook
There is a subtle shift in the winds aloft on Friday. Storms form further east in Cochise County, though they mostly dissipate before reaching Pima County; we will need to see why in tomorrow’s and Thursday’s forecasts.
This is the precursor to a larger push of moisture into Arizona, from Friday night through Saturday, as shown in the wind and 2-meter dewpoint temperature forecasts below.
The global model ensemble, GEFS (below), shows storms will be quite active this weekend, even in Phoenix, with a possible down day on Monday.
Discussion written by Pat Holbrook and Patrick Bunn