Summary
A Tropical Upper Tropospheric Trough (TUTT) has shifted over southern Baja California, while Tropical Storm Elida is weakening as it heads northwest into cooler Pacific waters. The polar low over Texas is also weakening. Dry air is present in parts of New Mexico and Arizona, but most of Southern Arizona remains humid.
Forecasts indicate thunderstorms will develop in western New Mexico and southeast Arizona today, but a lack of instability will cause them to weaken before reaching lowland valleys like Phoenix and Tucson, which are expected to see little to no rainfall. Ensemble precipitation is <0.1″ for Tucson, though outlier forecasts have an isolated cell dropping more rain. Similar conditions are forecast for tomorrow, with storms forming along the Mogollon Rim and weakening as they propagate further west. By Tuesday, storm coverage in Arizona decreases, with some showers possible in Tucson. Looking ahead, temperatures are expected to rise, and dewpoint temperatures will drop by next weekend.
Yesterday’s Weather
Yesterday morning, showers and thunderstorms extended from Flagstaff south to the Mogollon Rim, and then to Phoenix and Tucson. Some heavy rain fell in the mountains of eastern Maricopa and Pinal counties. The precipitation moved slowly south and dissipated by early afternoon.
Overnight, more light showers developed from Phoenix down to Tucson. Both yesterday and this morning, the development was driven by subtle nighttime cooling atop the clouds.
The upper-level flow in southern Arizona was weak, and any storms that formed tended to shoot straight up, drop some local heavy rain, and die in place. Storms in northern Arizona that were pushed along had a northeast wind aloft at 10 to 15 mph. The storms shown on the satellite picture below formed at about 2 pm. By 8 pm, they had moved into Flagstaff, then into western and northwest Arizona. The storms in southeast Arizona propagated east as winds aloft were weak.
The early-morning visible satellite and radar images below show showers and clouds across the state. Clearing in northeast Arizona reflects dry air moving in.
Below is the HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right). The areal coverage is excellent, but precipitation intensity was underestimated northward from the Mogollon Rim.
The map of observed radiosonde locations below shows a large data gap over the Rockies, probably the reason the forecasts underestimated precipitation in northern Arizona.
Analysis
A Tropical Upper Tropospheric Trough (TUTT) to our south has moved slightly and is now over southern Baja California. Tropical Storm Elida has moved northwest and is heading towards the northern Pacific, and will dissipate as sea surface temperatures are too cool to support tropical systems. The polar low over Texas has moved and is weakening into an open wave.
Dry air has mixed down to the surface in northwest New Mexico and northeast Arizona, with dewpoint values < 60 °F there. It is also drier in far southwest Arizona. Otherwise, the atmosphere is almost tropical across the rest of the region with dewpoint temperatures between 60°F and 70°F.
The HRRR-UA WRF 12Z initialized and observed sounding for Phoenix at 5 am has similar precipitable water values (52mm). The forecast initialization is smoother than actual soundings because models have fewer vertical data points. The layer between 500mb and 600mb is saturated on the observed sounding, but icing is likely on the sensor in that layer rather than the atmosphere being saturated.
At 250 mb, the TUTT is further south than indicated on the map. The polar low in Texas has weakened, but it does have a broad circulation covering New Mexico and Texas. The monsoon high has shifted further west into California.
The polar low in Texas is almost an open wave this morning. The 500 mb high at this level is over Nevada, and the polar jet is farther north. The flow in northern Arizona is 15 knots from the east, weakening to light and variable over southern Arizona.
At 700mb, the polar low in Texas is an elongated open wave. Plenty of moisture is available in Arizona, except for the dry air in northeast Arizona and northern New Mexico.
Today
Today, looks like a Rim Runner day with one issue in the forecast. Thunderstorms will form in the early afternoon hours in western New Mexico and southeast Arizona and propagate westward, but the issue is that the boundary layer remains cool in the valleys of Maricopa, Pinal, and Pima counties. The storms will not have enough Convective Available Potential Energy (CAPE), as shown in the forecast soundings below. Without the necessary CAPE, storms will struggle to move any further west and will weaken/dissipate in the valleys. Forecasts do not indicate any rainfall in Phoenix; Tucson is predicted to get less than 0.1”.
The HRRR-UAWRF 12Z and 15Z forecast radar reflectivity shows the storms running out of CAPE as they enter the low valleys. The 12Z (left) shows Tucson Airport getting 0.7”, but it appears to be from an isolated storm, and the 15Z (right) shows 0.0”. The ensemble mean is less than 0.1″, which is likely a better estimate for the Tucson metro area. Overnight storms reform again, dropping some light rain from the Tucson area southeast to the Sierra Vista area. The Phoenix-to-Casa Grande region may also see some light morning showers.
Tomorrow
Tomorrow, storms form on the Mogollon Rim again and approach the valleys, but also weaken as they move further west. Valley precipitation in both Phoenix and Tucson will be around 0.1”, but similar to today in Tucson, an isolated thunderstorm could potentially produce up to an inch of localized rain.
Outlook
Comparing the forecast radar for tomorrow (above) to Tuesday evening (below), Tuesday has fewer overall storms in Arizona, with storms forecast around the edges of the low deserts and valleys. Some overnight showers may affect Tucson into Wednesday morning.
Global model ensemble forecasts for Tucson show afternoon high temperatures are up about ten degrees by next weekend, with dewpoint dropping. Until then, each day could have scattered afternoon and evening thunderstorms, but the global model is uncertain about which day.
Discussion written by Pat Holbrook and Patrick Bunn