The summer thunderstorm season in the desert Southwest arrives as a change in air mass, not as a single storm. Its onset is defined by moisture measurements.
The season is a wind reversal, not a rain event
Through late spring the region sits under dry air moving from the west. As land heats through summer, the circulation shifts and moist air is drawn northward.
That reversal is the actual seasonal phenomenon. Thunderstorms are the visible consequence of moisture arriving in a place with intense surface heating and high terrain.
Because the wind shift can occur without immediate rain, using the first storm as the marker would date the season inconsistently from year to year.
Dew point marks the arrival of the air mass
Dew point states how much water vapor the air actually contains, and it changes sharply when a different air mass moves in.
Desert spring air carries very low dew points. When surface dew points rise and stay elevated for several consecutive days, the moist regime has established itself.
A fixed date range is now used in official practice for consistency, but the underlying physical marker remains the sustained rise in moisture.
Terrain organizes where storms form first
Mountains heat faster than surrounding desert during the day, and air rising along their slopes converges near the ridges by early afternoon.
Storms therefore tend to initiate over high terrain and drift outward, which is why mountain communities see activity earlier in the day than valley cities.
Outflow from those storms spreads across lower ground and can trigger new cells far from the original mountain, extending activity into the evening.
Rain falls where the water does not stay
Desert soils, sun-baked and often thinly vegetated, absorb heavy rainfall poorly, so a large fraction of it runs off almost immediately.
Dry washes and canyons collect that runoff from a wide area, which is how a flash flood can arrive under clear sky miles downstream.
The disconnect between where rain falls and where water arrives is the central hazard of the season, and it does not require exceptional rainfall totals.
Dry storms carry their own hazards
Early in the season, moisture aloft can support thunderstorms while the lower atmosphere stays dry enough to evaporate the rain before it lands.
These storms produce lightning over dry fuels with little wetting rain, which is the combination behind many ignitions in the region.
Strong downdrafts from evaporating rain also reach the ground as blowing dust, cutting visibility on highways with very little warning.