A broad summer weather pattern is shifting across the Lower 48 as federal forecasters track an expanding ridge of high pressure expected to intensify heat over a large part of the country. In the West, that setup is expected to bring several days of dangerous heat beginning Monday, July 13, with the National Weather Service and the Weather Prediction Center highlighting the Intermountain West and adjacent states as a key part of the event.
NOAA says a strengthening ridge will drive the heat event
The Weather Prediction Center said in its July 10 extended forecast discussion that “dangerous heat” is expected to spread from the Intermountain West into the northern Plains and Upper Midwest through next week. The federal forecast described a “broad and anomalously strong upper-level ridge” stretching from the Intermountain West into the north-central United States, with model guidance showing strong agreement and high confidence in multiple days of hazardous heat.
That forecast matters because the Weather Prediction Center also said several days of above-normal daytime highs and warm overnight lows are likely, a combination that increases heat risk. The agency said widespread major to extreme HeatRisk is possible in parts of the affected region, and it noted that potentially record-breaking maximum and minimum temperatures could develop in some areas next week.
The current city forecasts already show the scale of the heat in some Western population centers. Las Vegas is forecast to reach 107 degrees on Tuesday, July 14, and 110 degrees on both Wednesday and Thursday, while Phoenix is forecast to remain near or above 110 through much of the coming week, according to National Weather Service-backed forecast data.
The western impact will not be uniform, but the heat footprint is broad. Sacramento is forecast to jump from the mid-90s early in the week to 103 degrees on Tuesday, July 14, and 104 on Wednesday, July 15. Las Vegas is already under extreme summer heat and is forecast to remain above 104 degrees every day through at least July 16, with little cooling at night.
Farther north, the same pattern is expected to push heat into areas that are not usually the hottest part of the West. Portland is forecast to reach the upper 80s and low 90s through the period, while Seattle is expected to warm into the 80s. Those numbers are lower than desert readings, but they can still raise public health concerns when warm conditions persist over several days, especially in homes without air conditioning.
What is not yet clear is which individual counties or cities will see formal heat advisories or excessive heat warnings several days in advance, because local National Weather Service offices issue those products closer to the event. The federal medium-range outlook identifies the broader risk zone, but a complete jurisdiction-by-jurisdiction list of alerts has not yet been finalized.
Forecasters attribute the heat to a classic midsummer ridge pattern, but this one is projected to be unusually expansive. The Weather Prediction Center said the upper-level ridge will remain in place across the Intermountain West into the north-central United States next week, creating a stable, sinking-air environment that favors stronger sunshine, suppressed cloud cover and hotter afternoon temperatures.
At the same time, the pattern has a second consequence for the Southwest. The Weather Prediction Center said increasing southerly flow around the western side of the ridge is expected to draw monsoonal moisture into the Southwest and the central Great Basin early next week. That means parts of Arizona, New Mexico and nearby areas could face both dangerous heat and scattered thunderstorms, with localized flash flooding possible near burn scars and steep terrain.
For residents across the West, the practical effect is likely to be multiple days of elevated heat stress rather than a one-day spike. Forecasts already point to prolonged triple-digit temperatures in the hottest inland areas and very warm nights in some cities, conditions that typically increase health risks because buildings and pavement do not cool efficiently after sunset. Federal forecasters have also signaled that the heat could continue through most of next week, even as storm chances increase in parts of the Southwest.