WEATHERTrackCAST - Mesoscale Discussion WPC #1511 for Heavy Rainfall

Convection streaming northward across central/northern Arizona into southern Utah will pose an isolated to scattered flash flood threat through 15Z. While quick cell motions have generally kept hourly totals under 0.5 inches, high short duration rates over increasingly saturated ground will drive runoff concerns, with the potential for linear training bands to produce locally significant impacts in sensitive terrain.

Discussion

A broad plume of deep tropical moisture is streaming northward across the Southwest ahead of a mid/upper level low tracking into California. This moisture advection has pushed PW values to near or at record levels for mid to late October across a wide corridor. Despite being near the typical diurnal minimum in instability, thermodynamic profiles remain unusually buoyant, with CAPE exceeding 1000 J/kg across portions of Arizona and hovering near 500 J/kg into southern Utah.

Convective activity across Arizona is expected to persist and gradually increase in coverage and intensity through the early morning hours. This will be driven by strengthening large scale forcing for ascent from the west as the upper low approaches, combined with the eastward progression of the low level moisture transport axis into central Arizona. Convective elements developing within this corridor will continue to stream rapidly north northeastward into southern Utah.

To this point, fast storm motions to the north-northeast have largely kept hourly rainfall totals in check, generally below 0.5 inches. However, short duration rainfall rates remain quite high within the convective cores. Furthermore, antecedent rainfall over the past 24 hours has left conditions increasingly saturated and hydrologically sensitive across the region. Consequently, even quick moving cells will be capable of generating localized flash flood concerns. As convective coverage expands and instability gradually ticks upward this morning, cumulative rainfall totals will steadily mount over Arizona and southern Utah.

The deep unidirectional southerly flow supports localized training and backbuilding, particularly across central Arizona at the nose of the low level moisture transport axis and along the sharpest instability gradient. Several recent HRRR/RRFS runs have indicated this potential, highlighting a corridor between Phoenix and Flagstaff favored for training convection capable of producing rates exceeding 1"/hr. Even in southern UT where hourly rates remain under 1 inch, recent HRRR runs depict some FFG exceedance developing by 10-12Z, with spatial coverage expanding through mid morning. While the FFG exceedance footprint is somewhat delayed across Arizona, HRRR TLE exceedance probabilities climb by 12Z as linear training bands may begin to organize.

Overall, at least isolated to scattered flash flooding is considered likely through 15Z. Given the broader synoptic evolution and diurnal heating, this threat will likely continue beyond 15Z and may ultimately peak later in the day. Locally significant flash flood impacts are possible, especially across complex terrain, slot canyons, and sensitive drainage basins.

Chenard

River Forecast Centers

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