Risk of localized flooding over the next 48 h

July 28, 2024

Overview:

Upper-level troughing over the the Western US has pushed the upper-level anticyclone off to our east, turning the 500 hPa flow to southerly across southeastern Arizona. Further west, southwesterly flow associated with the upper-level trough has allowed dry air to push into western Arizona with a significant dewpoints gradient across western Maricopa and Pima counties. This surface dewpoint gradient should encroach on the Phoenix metro area during the course of the day and may be a focus for some very short lived convection developing off of the North Maricopa mountain range. Closer to Tucson, deep moisture is well established with precipitable water values this morning close to 1.7". Cloud cover this morning has been slow to burn off west and south of Tucson, although convective inhibition is dissipating fairly quickly as indicated by a developing cumulus over Cochise county as of 10:30 AM this morning. Fairly uniform but relatively weak southerly flow above the boundary layer should mean that storms should slowly propagate from south to north across the region. The slow movement, ample precipitable water, and recent rainfall will highlight the threat of localized flash flooding today. On the other hand, the deep moisture and surface dewpoints near 60 F will mean that the wind threat for today should be on the low side.

For tomorrow, the Western US trough will weaken, allowing upper-level heights to rise across southeastern Arizona. As the ridge builds westward, the flow should turn to the southeast. Moisture will still be in place, but I expect already mediocre lapse rates from today to continue to degrade a bit tomorrow.

Today:

Large Scale:

Weak, broad troughing in the 500 hPa flow across the NW quadrant of the United States has displaced the four-corners anticyclone to southern New Mexico, resulting in flow shifting to south-southwesterly over southern Arizona. Water Vapor imagery shows that southwesterly flow has allowed low-level dry air to become established in parts of western Arizona with the deepest axis of moisture shifting to south-central and southeastern portions of the state.

Low-level water vapor and 500 hPa Mesoanalysis Valid at 10 AM MST

Zooming In:

The axis of maximum precipitable water was situated over south central Pima county as of 11 AM this morning, with values ranging between 1.4" and 1.6" across eastern sections of the county with values rapidly diminishing to the northwest of this axis.

PWTR Mesoanalysis Valid at 11 AM MST

In the boundary layer, a dry line is situated across extreme western Pima county into north central Maricopa county

Lowest 100 hPa Mixing Ratio Mesoanalysis Valid at 11 AM MST

and is evident in the sharp dewpoint gradient across the Phoenix metro area as of 11 AM this morning.

Surface Observations Valid at 11 AM MST

On the thermodynamic side of the equation, this morning's sounding from Tucson indicates deep moisture with relatively minimal potential for evaporation with a DCAPE value of < 1000J/kg. Southerly winds of 10-20 knots in the mid-levels of the atmosphere should mean slow northward propagation of storms throughout the day. While lapse rates are somewhat pedestrian, this is offset by an unusually moist mixed layer with surface dewpoints unlikely to drop below the upper 50's F across the Tucson metro area. Given the moist mixed layer, expect CIN to burn off relatively early in the day with convective initiation already underway in Cochise county.

12Z KTUS Sounding

The Forecast:

Expect convection that is currently developing in Cochise county to increase and coverage and spread northward throughout the day, reaching the Tucson metro area by 3-5 PM MST.

12Z UAWRF-HRRR Reflectivity Valid at 1 PM (left) and 5 PM (right) MST

Expect CAPE values to be around 1000 J/kg as convection approaches allowing for significant rainfall rates in storms that do develop. The wind field is at best marginal for organization so I would not expect coherent propagation and instead will expect activity to pulse as it moves northward.

12Z UAWRF-HRRR Forecast KTUS Sounding Valid at 3:30 PM MST

Given the general southerly flow, expect the south facing slopes of local mountain ranges to be the most susceptible to excessive rainfall with rainfall amounts in excess of an 1" possible.

12Z UAWRF-HRRR 24 h Total Precipitation Valid at Midnight July 29th

This is supported by the HREF ensemble which indicates a 50-70% chance of 3 hr precipitation totals exceeding an 1" across the higher terrain of the greater Tucson area.

12Z HREF 3h Max Precip and Probability of > 1" Valid at 5 PM MST

These rainfall totals are likely to be problematic for some locations given rainfall over the past few days with local accumulations since Monday in the 1-3" range

Observed Rainfall Totals from July 22-27

Monday:

I don't expect much to change as far as the big picture goes for Monday. Heights in southern Arizona should rise slightly as the trough weakens and lifts northward with the upper-level flow shifting to southeasterly.

12Z HREF Mean 500 hPa Heights/Wind Valid 1 PM MST Monday

Precipitable water values in the southeastern part of the state will remain in the 1.4-1.6" range with higher values working their way back to the northwest as the southwesterly flow weakens.