July 16th, 2026: Excessive Rainfall Threat in Southern Arizona

 Overview

Excessive rainfall with the potential for flash flooding is possible today due to a combination of synoptic scale ascent, high precipitable water, and weak steering flow. The large-scale pattern is characterized by a 500mb anticyclone ("monsoon high") centered well to the north of Arizona and an inverted trough/upper level low near El Paso. CAMs (Convective Allowing Models) continue to forecast numerous showers and thunderstorms across much of Southern Arizona today, but extensive cloud cover may inhibit sufficient instability across the region making today's forecast quite tricky. 

Current Conditions

Visible satellite imagery shows widespread mid level clouds with embedded showers and isolated thunderstorms across much of Arizona this morning with some patchy pockets of clearing.

Visible satellite imagery as of 9:10AM MST courtesy of the NWS.

These mid level clouds and embedded convection is associated with weak synoptic scale ascent induced by an inverted trough/upper level low over eastern New Mexico and western Texas. 

12z GFS 500mb absolute vorticity this morning courtesy of Tropical Tidbits.

The 12z PHX observed sounding shows plenty of available moisture with 1.86 inches of precipitable water and a near surface dew point of 65 degF.

12z PHX sounding this morning courtesy of SPC.

However, there's around -250 J/kg of CIN and lapse rates aren't great at around 6.5 degC/km. In addition the vertical wind profile is a mess with mainly weak northerly flow.

  
Today's Forecast

Synoptics

The synoptic scale pattern is a bit unique today with the 500mb anticyclone centered in Utah and an inverted trough/upper level low just east of the continental divide centered near El Paso as shown in the map earlier. This inverted trough will gradually continue to push westward toward Southeast Arizona today and tomorrow generating weak synoptic scale ascent across our area. In fact, SPC mesoanalysis indicates a few pockets of weak divergence at 300mb.

SPC 300mb analysis courtesy valid at 10:00 AM MST.

Overall, the synoptic scale pattern remains marginally favorable for convection today.

Thermodynamics and Wind Shear (The Good, The Bad, and The Ugly)

There is no shortage of moisture today as SPC mesoanalysis shows precipitable water ranging from 1.50 inches near Tucson to above 2 inches in the lower Colorado River Valley.

SPC mesoanalysis of precipitable water valid at 10:00 AM MST.

So that is the "good" I suppose. The "bad" are low and mid level lapse rates and the uncertain potential to reach convective temperatures today thanks to the pesky, extensive mid level cloud cover. These clouds are the main issue for today's forecast as they will have a major impact on the timing and location of convective initiation. What makes the situation even worse is the "ugly" vertical wind profile. Mean mid level flow across the state is incredibly weak and a bit all over the place as shown below.

SPC mesoanalysis of mean 850mb-300mb wind as of 10:00AM MST.

This not only is an issue with keeping these mid level clouds around, but also increases the risk of flash flooding due to slow moving thunderstorms. 

What to Expect and Potential Impacts

It's definitely a tricky forecast today. There's plenty of moisture and some buoyancy with 500 to 1500 J/kg of CAPE, but the extensive cloud cover is going to be an issue for eroding CIN, steepening low level lapse rates, and reaching convective temperatures. Any pockets of significant clearing should allow for thunderstorms to develop especially over the higher terrain. Early morning CAM solutions are definitely a bit bullish, but recent runs of the HRRR seem to be backing off storm intensity and coverage this afternoon and evening as they are picking up on the persistent cloud cover.

Overall, the threat of excessive rainfall is present due to deep moisture, modest synoptic scale ascent, and weak steering flow. Any storm that develops will be capable of localized flash flooding. WPC has issued a moderate risk for excessive rainfall in Santa Cruz and southeastern Pima County (including Tucson) for this afternoon and evening.

WPC Day 1 Excessive Rainfall Outlook (valid today).

This seems reasonable considering the 12z HREF shows 70 to 90% probabilities of 3-hour rainfall in excess of 1 inch and 10% probability of 3-hour rainfall exceeding 3 inches.

12z HREF 3-hour ensemble mean QPF and probabilities of 3-hour rainfall exceeding 1 and 3 inches valid at 5PM MST this afternoon courtesy of the SPC.

Interestingly, the 12z HREF did pick up on extensive mid level cloud cover persisting throughout the morning hours as shown below.

12z HREF ensemble mean mid level cloud cover (%) valid at 11AM MST courtesy of SPC.

The coverage and intensity of showers and storms today will heavily rely on if enough clearing can occur to allow for convective initiation over the higher terrain. We shall see how it pans out...