Summary
Arizona has abundant surface moisture while New Mexico remains drier due to air circulation around a Texas low. Forecasts for Phoenix and Tucson suggest that moisture levels may be overestimated, leading to higher forecast convective available potential energy (CAPE) than is likely. The morning cloud field will limit surface heating and prevent the valley from reaching convective temperatures.
Still, thunderstorms are expected to form over the mountains and will spread into the valleys, particularly around Phoenix and Tucson, with the heaviest rain expected in the evening (6 pm to midnight). Precipitation totals will likely accumulate gradually, reducing the risk of flash flooding despite ongoing runoff into washes. Somewhere in the 0.25″ to 0.5″ range is likely from recent forecasts, though older forecasts have totals over 1″ in both metro areas. After today’s storms, atmospheric energy will be depleted, resulting in fewer valley storms on Friday and Saturday, with most activity remaining confined to the mountains.
Yesterday’s Weather
GOES-19 (East) satellite imagery is currently unavailable because the satellite has entered safe mode, and recovery efforts are underway. Without the weather balloon launches at Phoenix, thanks to Salt River Project, we would have no upper-air data in Southern Arizona this morning. The composite radar images below were taken yesterday from 12 pm to 5 am this morning. Thunderstorms moved south and southwest, with the storms in southwest Arizona reforming again this morning.
The satellite image below is from GOES-18 (WEST) at 8 am this morning; clouds cover most of the state. When the atmosphere is this saturated, it only takes a little cooling aloft to initiate showers or light thundershowers, as is evident by the light showers currently on radar.
Below is the HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right). The forecast and MRMS observations seem to agree on the general pattern and intensity of rainfall, especially in Phoenix and Tucson; less so in Sonora.
Analysis
Arizona is very wet, while most of New Mexico has lower dew-point temperatures due to circulation around the Texas low, which is pulling dry air southward from Colorado.
The HRRR-UA WRF 12Z initialization (left) and observed sounding (right) for Phoenix at 5 am are below. The initialization has too much moisture in the layer between 800 and 900 mb. That makes the overall Convective Available Potential Energy (CAPE) values an overestimate. Because of the clouds, it’s unlikely we will get to the convective temperature of 102°F in the valley, so a different lifting mechanism is needed: likely outflows from mountain storms.
At 250mb, it’s a weird pattern for this time of year: a polar low in Texas and a trough off the West Coast with the monsoon ridge squeezed between the two. The TUTT over Baja is still blocking the monsoon ridge, too.
At 500 mb, the Polar low over west Texas dominates the weather pattern, along with the circulation around the monsoon ridge.
At 700mb, moisture is plentiful in Arizona, extending east and south to the low in Texas. Another easterly wave is in northern Mexico, south of Arizona.
Today
Given the observed soundings shown earlier, the forecast precipitable water in the near-surface layer at Phoenix and Tucson is likely too high in the HRRR-UA WRF 12Z soundings. If we can’t get to the convective temperature, the forecast surface-based CAPE is also too high. That said, there’s enough mid-level CAPE for thunderstorms to start on the mountains, and since the atmosphere is moist, they will be able to propagate off the mountains. The 0-6 km mean wind is more favorable in Phoenix (southwest) for propagation into the valley compared to Tucson (southeast), but there’s good wind shear at both locations, meaning additional downstream storm development is likely from outflow boundaries utilizing mid-level CAPE in the valleys.
Thunderstorms form in the mountains in the Mogollon Rim and along the International border and spread slowly southwest. Storms forecast for the Phoenix and Tucson areas will be later in the evening, 6 pm-midnight.
The 15Z (below right) has backed off the precipitation totals compared to the 12Z (below left); this could be because the cloud field is better represented by the later initialization.
The most recent 8 runs of the UA WRF weather forecast system are shown below, with greater weight given to more recently made forecasts. For Tucson, the ensemble mean is about 0.5″ with the most recent runs ranging from 0.1 to 0.4″. For Phoenix, the ensemble mean is 0.25″ with the most recent runs ranging from 0.03 to 0.6″. It’s best to expect all of those outcomes at various locations in the respective counties. Although the forecast is for heavy rain this evening, it will probably fall at a relatively slow rate, so these totals will incrementally accumulate over 6 hours. This lessens the likelihood of flash flooding; washes will still have running water, but not as deep as during a flash flood.
Tomorrow
The precipitation today removes most of the energy from the atmosphere for tomorrow. The radar forecast for tomorrow afternoon shows mountain thunderstorms with some storms moving off the mountains by evening before dissipating.
Outlook
Thunderstorms form again on the mountains on Saturday, but do not move into the valleys for the second day in a row.
There is considerable variability among global model ensemble forecasts, making the situation highly uncertain, even on the day of the event. One thing can be inferred: after today’s event, we will probably see less precipitation over the weekend.
Discussion written by Pat Holbrook and Patrick Bunn